摘要:

FEATURE ARTICLE Molecular magnetic semiconductors, metals and superconductors: BEDT-TTF salts with magnetic anions Peter Daya and Mohamedally Kurmoob aDavy Faraday Research L aboratory, T he Royal Institution of Great Britain, 21 Albemarle Street, L ondon, UK W1X 4BS bIPCMS-GMI, 23 rue L oess, BP 20/CR, 67037 Strasbourg Cedex, France Molecular charge-transfer salts that consist of alternating layers of organic donor molecules and inorganic anions oer the opportunity of bringing together in the same crystal lattice conduction electrons localised on the donor sublattice and magnetic moments localised on the inorganic anions.A wide range of such compounds is formed by the donor molecule BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene]. Some are semiconductors, others are metals while one is a superconductor, the first containing paramagnetic 3d moments. The structures, magnetic and conductivity properties of examples from each category are reviewed.Superconductivity was first found in molecular solids about Semiconducting tetrahalogenoferrate(iii ) salts 15 years ago1 and since then some hundred or so examples First attempts to insert 3d ions into BEDT-TTF charge- have been found, with the critical temperature Tc rising from transfer salts centred on tetrahalogeno anions, beginning with 1.4 to almost 13 K.2 All of them belong to the class of FeIII because of its large paramagnetic moment (S=5/2).7 compounds called charge-transfer salts, in which a conjugated Surprisingly, electrochemical synthesis under the same con- molecular cation is combined with a smaller inorganic anion, ditions yielded dierent chemical stoichiometries for the salts or vice versa.The majority are formed from organo-chalcogen containing FeCl-4 and FeBr-4 . The structure of (BEDT- donor cations such as tetramethyltetraselenofulvalene TTF)2[FeCl4] consists of dimerised stacks of BEDT-TTF (TMTSF) and bis(ethylenedithio)tetrathiafulvalene (BEDT- molecules separated by the sheets of tetrahedral FeCl-4 anions.TTF). The latter are particularly significant, not only because The anions are situated in an ‘anion cavity’ formed by the they provide the highest Tcs found so far,3 but because of the ethylene groups of the BEDT-TTF molecules, which are very wide range of structures that they form, which facilitates arranged in the sequence …XYYXXYYX …(Fig. 1). Adjacent the correlation of structure with properties. molecules of the same type (XX¾ and YY¾) stack uniformly on An important reason for synthesizing new molecular-based top of each other but with a slight displacement between materials is to bring together in the same crystal lattice physical neighbours along the long in-plane molecular axis.On the properties that are not normally found in continuous lattice other hand, the long in-plane molecular axes of adjacent compounds. It is a striking feature of the superconducting molecules of dierent type (XY¾ and YX¾) are rotated relative charge-transfer salts based on BEDT-TTF that the organic to one another. Furthermore, X and X¾ (or Y and Y¾) are cations and inorganic anions are spatially segregated into closer to each other (ca. 3.60 A ° ) than X and Y (3.81 A ° ). The alternating layers, which led us to cite them as examples of shortest distances between BEDT-TTF molecules are shorter ‘organic–inorganic molecular composites’4 or ‘chemically con- than the sum of the van der Waals radii of two sulfur atoms structed multilayers’.5 Because of this separation into well (3.60 A ° ), thus suggesting the possibility of a quasi-one-dimen- defined organic and inorganic components the BEDT-TTF sional interaction along the a direction.charge-transfer salts are attractive synthetic targets for attempts In (BEDT-TTF)[FeBr4], on the other hand, there are no to combine superconductivity in a molecular lattice with stacks and planes of closely spaced BEDT-TTF, in marked properties more often found in the inorganic solid state like cooperative magnetism. Establishing superconductivity in a crystal containing localized magnetic moments is an important objective because superconductivity and magnetism have long been thought inimical to one another: Cooper pairs are disrupted by external fields and by the internal fields generated in ferromagnets.6 This Feature Article summarises our eorts to make conducting molecular charge-transfer salts of BEDT-TTF containing anions consisting of transition-metal complexes of two types [tetrahalogeno- and tris(oxalato)].Some of the resulting phases are semiconductors and others are metallic without becoming superconducting. However, in one case we find superconductivity in the presence of paramagnetic FeIII centres.The latter seems to be the first example, (not only among molecular materials) of a superconductor containing localised Fig. 1 The XXYYXX stacking sequence of BEDT-TTF in (BEDTTTF) 2[FeCl4]7 paramagnetic 3d centres, in a stoichiometric lattice. J. Mater. Chem., 1997, 7(8), 1291–1295 1291contrast to most of the compounds containing this donor Metallic tetrahalogenocuprate(ii) salts molecule.The only short S,S distances (<3.50 A ° ) are between The anions CuX42- (X=Cl, Br) are unusual in the wide variety two BEDT-TTF molecules in dierent pairs but there is no of geometries they adopt, from square-planar to flattened continuous network of short S,S contacts through the lattice. tetrahedral.It turns out that the crystal structures (and hence The FeBr-4 form a three-dimensional lattice separated by the properties) of the BEDT-TTF salts with these anions are pairs of BEDT-TTF molecules. One FeBr-4 has short intermol- correspondingly varied. Amongst them is the first example of ecular distances to two donor molecules but no extended a metallic molecular charge-transfer salt in which magnetic interaction between BEDT-TTF molecules through the resonance is observed simultaneously from conduction and FeBr-4 is possible, which correlates with the insulating behav- localised electrons, (BEDT-TTF)3[CuCl4]·H2O.14 Other iour of this compound.examples containing CuBr42- and CuCl2Br22- have the largest The structure of the FeCl-4 salt is closely related to those of change of conductivity with pressure ever seen in conducting (BEDT-TTF)2[InBr4],8 a-(BEDT-TTF)2[PF6],9 b-(BEDT- organic solids.TTF)2[PF6]10 and (BEDT-TTF)2[AsF6],11 with the salts The crystal structure of (BEDT-TTF)3[CuCl4]·H2O consists containing octahedral anions exhibiting interstack side-by-side of stacks of BEDT-TTF parallel to the c-axis, with short contact distances nearly identical to those found in (BEDT- interstack S,S contacts leading to the formation of layers in TTF)2[FeCl4].There are no short contact distances between the ac-plane (Fig. 3).14 These layers are interleaved by the sulfur atoms along the molecular stacking direction in any of inorganic anions and the water of crystallization in the same these compounds and so their conduction properties are highly general arrangement as in the FeCl4 salt.However, in (BEDT- one-dimensional. For example, in b-(BEDT-TTF)2[PF6] the TTF)3[CuCl4]·H2O there are three crystallographically inde- ratio of the conductivities along the direction of side-by-side pendent BEDT-TTF molecules and two dierent stacks. contacts to that along the molecular stacking direction is Stack A contains only BEDT-TTF of type I, and stack B has 20051.9,10 (BEDT-TTF)2[FeCl4] is also semi-conducting, like an alternate arrangement of II and III.Within the ac-plane, the PF6 and AsF6 salts, with an activation energy of 0.21 eV. the stacks form an XYYXYYX array. The anions lie in planes Given that the formal charge per donor molecule is +1/2 parallel to the layers of the donor, within which pairs of in the FeCl-4 salt we would expect a contribution to the CuCl42- anions are connected by hydrogen bonds through susceptibility from unpaired spins on the organic cations.For the two water molecules to form discrete units. The CuMCl example, the molar susceptibility of (BEDT-TTF)2[GaCl4], bond lengths average 2.25 A ° with a trans-ClMCuMCl angle of which has a similar structure but a diamagnetic anion, has a 150°, both values being quite normal for copper(II) halides broad maximum near 90 K, which can be fitted from 70 to with a Jahn–Teller distortion.The bond lengths and angles of 300 K either by a one-dimensional (Bonner–Fisher) or quad- the three independent BEDT-TTF molecules are almost ident- ratic layer antiferromagnetic model to yield exchange param- ical while the two dierent overlap modes are the same in each eters J of 66.5(5) and 89.6(1) K, respectively, similar to those stack.(BEDT-TTF)3[CuCl4]·H2O is the only BEDT-TTF salt found in the semiconducting a¾-(BEDT-TTF)2X with X= with 352 charge stoichiometry that remains metallic down to AuBr2, Ag(CN)2 or CuCl2,12 where the absolute values of the very low temperatures at ambient pressure. In general, such susceptibility agree with expectations for localised moments salts undergo metal (or semimetal) to insulator (or semicon- corresponding to S=1/2 per pair of donor molecules.Of ductor) transitions, some of which are sharp {e.g. (BEDT- course, in the FeCl-4 salt, the susceptibility is dominated by TTF)3[ClO4]2},15 and others broad [e.g.(BEDT- the anion. Thus the magnetic response of the donor sublattice TTF)3Cl2·2H2O].16 is all but obscured, though with care it can be disentangled. Because it is metallic down to at least 400 mK (BEDT- From the low-temperature data, a value of the Weiss constant TTF)3[CuCl4]·H2O is an extremely interesting subject in can be extracted and the parameters of the fit (S=5/2, g=2, which to observe the interaction between localized moments h=-4 K) used to calculate the dierence between observed and conduction electrons, which manifests itself in two ways.17 and calculated at all temperatures to obtain the contribution First, there is a shift in g value and broadening of the EPR from the BEDT-TTF.The resulting dierence gives a good fit line, the former being distinctly larger than those found in b- to a singlet–triplet model (Fig. 2).13 or a¾-phase BEDT-TTF salts (2.003–2.010),15 which strongly suggests that there is some interaction between the localised and conduction electrons. A second indication of such interaction comes from the way that the spin susceptibility of the conduction electrons falls below its Pauli limit as that of the localised electrons increases at low temperature. Most striking Fig. 2 Temperature dependence of the magnetic susceptibility of Fig. 3 Crystal structure of (BEDT-TTF)3[CuCl4]·H2O projected (BEDT-TTF)2[FeCl4] (circles) and the BEDT-TTF contribution (squares). The solid lines are fits (see text).13 along the c axis14 1292 J. Mater. Chem., 1997, 7(8), 1291–1295of all, however, the product of the spin susceptibility of the Cu pressure of any known organic conducting solids: 25 S cm-1 kbar-1 up to 8 kbar, finally attaining a plateau at resonance and the temperature rises at low temperature, indicating a short-range ferromagnetic interaction.Fitting the data 22 kbar with 500 times the ambient pressure value. At low temperatures (<100 K) there are phase transitions that take to the Bleaney–Bowers18 model indicates a ferromagnetic exchange constant of 4(1) K.Since the shortest Cu,Cu dis- both salts from the semimetallic to a semiconducting re�gime at pressures above 3 kbar. tance is 8.5 A ° , direct exchange interaction between the Cu moments is very small. On the other hand, the CuCl42- are Clearly the two bromo salts are on the borderline between metallic and semiconducting behaviour.Their magnetic arranged as dimers bridged by H2O, which may provide an exchange pathway. Nor can we rule out the possibility that properties are also unusual in that the susceptibility associated with the conduction electrons on the BEDT-TTF stacks is exchange between Cu moments is mediated by the free carriers of the BEDT-TTF layers (the so-called RKKY mechanism).very high. The drop of 2.5×10-3 emu mol-1 in susceptibility at the low-temperature transition is due to the loss of the Whilst the interaction between the BEDT-TTF and the metal ion spins is weak though observable in (BEDT- contribution from these electrons (Fig. 5). To account for a contribution of this magnitude one must assume two spins per TTF)3[CuCl4]·H2O, in semiconducting (BEDT-TTF)2[FeCl4] and insulating (BEDT-TTF)[FeBr4], it is negligible.By con- formula unit which are antiferromagnetically coupled. These facts suggest strongly that the CuBr42- and CuCl2Br22- salts trast, in (BEDT-TTF)3[CuBr4] and (BEDT-TTF)3[CuBr2Cl2] the interaction between the two sublattices is strong, and are just on the insulator side of the Mott–Hubbard transition.Semiconducting behaviour is not due to a gap in the one- structural phase transitions transform the electrical and magnetic properties.19 The CuBr42- and CuCl2Br22- salts are very electron density of states, but arises because the holes on the BEDT-TTF stacks localise due to the strong Coulomb inter- dierent structurally from the CuCl42- salt, and their electronic properties are unrelated.Whereas the CuCl42- are distorted action. The 352 charge stoichiometry of these salts requires that two holes are accommodated per three BEDT-TTF sites. from square planar, as is usually expected for Jahn–Teller d9 ions (the trans-ClMCuMCl angle being 150°), the anions in As noted already, the bond lengths show that two of the BEDT-TTF molecules are charged and that one is close to the bromo salts are, very unusually, square planar. In fact, to our knowledge this is the first compound in which planar neutral. The transport properties are therefore those of a ‘magnetic’ semiconductor, with an activation energy Ueff/2, CuBr42- has been found.20 Nevertheless the stoichiometry of all three salts is 351.Two crystallographically independent where Ueff is the on-site Coulomb energy (Hubbard energy) associated with the transfer of a charge to place two charges BEDT-TTF molecules (X and Y) are found in both bromo compounds; comparing the bond lengths with salts containing on a single site. In contrast to metallic (BEDT-TTF)3[CuCl4]·H2O only a BEDT-TTF molecules with well defined charges one finds that they are 0 and +1.21,22 Both crystal structures consist of layers single EPR line is seen in the bromo salts, though the g values above the 55–60 K transition are intermediate between those of BEDT-TTF, stacked in XYYXYY sequence and adopting the a-phase mode of packing.They are separated by pseudo- expected for a CuII moment and for spins on BEDT-TTF sites.Hence the two spin systems interact significantly, in contrast square-planar tetrahalogenocuprate(II ) (Fig. 4). The room-temperature conductivities are very similar, being to the other magnetic anion salts.7,14 low for metallic organic conductors and high for semiconductors but they are aected dramatically by pressure.19,23 Superconducting and semiconducting In fact, they show the largest changes in conductivity with tris(oxalato)ferrate(iii) salts Recently instances have come to light of two-dimensional bimetallic layers containing uni- or di-positive cations and [MIII(C2O4)3]3- in which the oxalato ion acts as bridging ligand, forming infinite sheets of approximately hexagonal symmetry, separated by bulky organic cations.24,25 In view of the unusual magnetic properties of these compounds it is worth exploring the synthesis of compounds containing similar anion lattices but interleaved with BEDT-TTF molecules.Three such compounds have been fully characterised to date, (BEDT-TTF)4[AFe(C2O4)3]·PhCN (A=H2O, K, NH4).26 While the stoichiometry of BEDT-TTF to Fe is the same in Fig. 5 Magnetic susceptibilities of (a) (BEDT-TTF)3[CuBr4] and Fig. 4 Crystal structure of (BEDT-TTF)3[CuBr4] projected along the (b) (BEDT-TTF)3[CuCl2Br2]. Broken lines are fits to the quadratic layer antiferromagnetic model.23 c axis23 J. Mater. Chem., 1997, 7(8), 1291–1295 1293all three, the presence or absence of a monopositive cation not omeric neutral molecules (Fig. 7). Molecular planes of neighbouring dimers along [011] are ornted nearly orthogonal to only changes the electron count (and hence the band filling) in the organic layer but also drastically alters the packing of one another, as in the k-phase structure of (BEDT-TTF)2X,29 but the planes of the dimers along [100] are parallel.This the BEDT-TTF. Thus, the compounds with A=K or NH4 are semiconductors with the organic molecules present as (BEDT- combination of (BEDT-TTF)22+ surrounded by (BEDT-TTF)0 has not been observed before. The (BEDT-TTF)0 describe an TTF)22+ and (BEDT-TTF)0, while that with A=H2O has BEDT-TTF packed in the b arrangement27 and is the first approximately hexagonal network, while the (BEDT-TTF)22+ are positioned near the oxalate ions, with weak hydrogen example of a molecular superconductor containing a lattice of magnetic ions.28 bonding between the terminal ethylene groups and oxalate O (2.51–3.05 A ° ).Packing of the BEDT-TTF in the H2O salt is The crystal structures of all three compounds consist of alternate layers containing either BEDT-TTF or quite dierent: there are no discrete dimers but stacks with short S,S distances between them, closely resembling the b- [AFe(C2O4)3]·PhCN.The anion layers contain alternating A and Fe forming an approximately hexagonal network (Fig. 6). structure in metallic (BEDT-TTF)2AuBr227 and the pressureinduced superconductor (BEDT-TTF)3Cl2·2H2O.16 The Fe are octahedrally coordinated by three bidentate oxalate ions, while the O atoms of the oxalate which are not coordi- The K and NH4 salts are both semiconductors but the H2O salt is a metal with resistivity of ca. 10-2 Vcm at 200 K, nated to Fe form cavities occupied either by K or H2O. The benzonitrile molecules occupy roughly hexagonal cavities in decreasing monotonically by a factor of about 8 down to 7 K, at which temperature it becomes superconducting (Fig. 8). the AFe(C2O4)3 lattice and there can be little doubt that it performs an important ‘templating’ role.Chirality is a further In line with their contrasting electrical behaviour the magnetic properties of the K and H2O compounds are also quite unusual feature of the anion layers. The point symmetry of Fe(C2O4)33- is D3, and the ion may exist in two enantiomers. dierent. The susceptibility of the semiconducting A=K compound obeys the Curie–Weiss law from 2 to 300 K with the In fact alternate anion layers are composed exclusively either of one or the other.Fe dominating the measured moment. In particular, there is little contribution from the BEDT-TTF, including those mol- Although the anion layers are very similar, the molecular arrangements in the BEDT-TTF layers are quite dierent in ecules whose bond lengths suggest a charge of +1.Hence the (BEDT-TTF)22+ are spin-paired in the temperature range the K and NH4 salts from the H2O one. In the former there are two independent BEDT-TTF, whose central CNC bond studied (the singlet–triplet energy gap is expected to be >500 K), while the remaining BEDT-TTF contribute nothing lengths dier markedly, indicating charges of 0 and +1. The +1 ions occur as face-to-face dimers, surrounded by mon- Fig. 7 View of the BEDT-TTF layer in (BEDT-TTF)4 [KFe(C2O4)3]·PhCN projected along the central CNC bonds26 Fig. 6 Anion and solvent layers in (BEDT-TTF)4 Fig. 8 Temperature dependence of the resistance of (BEDTTTF) 4[(H2O)Fe(C2O4)3]·PhCN from 5 to 200 K26 [AFe(C2O4)3]·PhCN; top, A=K; bottom, A=H2O26 1294 J. Mater. Chem., 1997, 7(8), 1291–12957 T.Mallah, C. Hollis, S. Bott, M. Kurmoo, P. Day, M. Allan and to the paramagnetic susceptibility, in agreement with the R. H. Friend, J. Chem. Soc., Dalton T rans., 1990, 859. assignment of zero charge. On the other hand, the supercon- 8 M. A. Beno, D. D. Cox, J. M. Williams and J. F. Kwak, Acta ducting A=H2O salt obeys the Curie–Weiss law from 300 to Crystallogr., Sect. C, 1984, 40, 1334.ca. 1 K above Tc, though with a temperature independent 9 H. Kobayashi, R. Kato, T. Mori, A. Kobayashi, Y. Sasaki, G. Saito paramagnetic contribution. The measured Curie constant of and H. Inokuchi, Chem. L ett., 1983, 759. 10 H. Kobayashi, T. Mori, R. Kato, A. Kobayashi, Y. Sasaki, G. Saito 4.38 emu K mol-1 is close to that predicted for Fe3+ (6A1), and H. Inokuchi, Chem.L ett., 1983, 581. while theWeiss constant (-0.2 K) signifies very weak antiferro- 11 P. C. W. Leung, M. A. Beno, G. S. Blackman, B. R. Coughlin, magnetic exchange between the Fe moments. However, there C. A. Miderski, W. Joss, G. W. Crabtree and J. M. Williams, Acta is a strong diamagnetic contribution in the superconducting Crystallogr., Sect. C, 1984, 40, 1331. temperature range, returning to Curie–Weiss behaviour above 12 D.Obertelli, R. H. Friend, D. Talham, M. Kurmoo and P. Day, 10 K. Whilst the EPR spectrum of the semiconducting A=K J. Phys., Condens. Matter, 1989, 1, 5671. 13 M. Kurmoo, P. Day, P. Guionneau, G. Bravic, D. Chasseau, compound consists of a single narrow resonance, that of the L. Ducasse, M. L. Allan, I. R. Marsden and R. H. Friend, Inorg. A=H2O compound consists of two resonances: a narrow one Chem., 1996, 35, 4719.assigned to the Fe3+ by analogy with the A=K compound, 14 P. Day, M. Kurmoo, T. Mallah, I. R. Marsden, M. L. Allan, and a much broader resonance from the conduction electrons. R. H. Friend, F. L. Pratt, W. Hayes, D. Chasseau, G. Bravic and This situation is reminiscent of what we found in (BEDT- L.Ducasse, J. Am. Chem. Soc., 1992, 114, 10722. TTF)3[CuCl4]·H2O.14 15 H. Kobayashi, R. Kato, T. Mori, A. Kobayashi, Y. Sasaki, G. Saito, T. Enoki and H. Inokuchi, Chem. L ett., 1984, 179. In the series (BEDT-TTF)4[AFe(C2O4)3]·PhCN (A=H2O, 16 M. J. Rosseinsky, M. Kurmoo, D. Talham, P. Day, D. Chasseau K, NH4), the lattice is stabilized by PhCN molecules included and D. Watkin, J.Chem. Soc., Chem. Commun., 1988, 88. in the hexagonal cavities. The oxalato-bridged network of A 17 S. E. Barnes, Adv. Phys., 1980, 30, 801. and MIII provides an elegant means of introducing transition- 18 B. Bleaney and K. D. Bowers, Proc. R. Soc. L ondon A, 1952, 214, metal ions carrying localized magnetic moments into the lattice 451. of a molecular charge-transfer salt.In the case of the A=H2O 19 M. Kurmoo, D. Kanazawa, P. Day, I. R. Marsden, M. L. Allan and R. H. Friend, Synth. Met., 1993, 55–57, 2347. compound it led to the discovery of the first molecular 20 A. C. Massabri, O. R. Nascimento, K. Halvorson and R. D. Willett, superconductor containing localized magnetic moments within Inorg. Chem., 1992, 31, 1779. its structure, while the A=K, NH4 compounds are semicond- 21 S.Hebrard, G. Bravic, J. Gaultier, D. Chasseau, M. Kurmoo, ucting. We are now working to incorporate other transition- D. Kanazawa and P. Day, Acta Crystallogr., Sect. C, 1994, 50, metal ions at the A site to create a two-dimensional magneti- 1892. cally ordered array between the BEDT-TTF layers as well as 22 P. Guionneau, G. Bravic, J. Gaultier, D.Chasseau, M. Kurmoo, D. Kanazawa and P. Day, Acta Crystallogr., Sect. C, 1994, 50, introducing 3d metal ions other than Fe. 1894. Our group has been supported by the UK Engineering and 23 I. R. Marsden, M. L. Allan, R. H. Friend, M. Kurmoo, Physical Sciences Research Council and DGXII of the D. Kanazawa, P. Day, G. Bravic, D. Chasseau, L. Ducasse and European Union (Human Capital and Mobility Programme).W. Hayes, Phys. Rev. B, 1994, 50, 2118. 24 H. Tamaki, Z. J. Zhong, N. Matsamoto, S. Kida, M. Koikawa, N. Achiwa, Y. Hashimoto and H. Okawa, J. Am. Chem. Soc., 1992, References 114, 6974. 25 C. Mathonie`re, S. G. Carling, Y. Dou and P. Day, J. Chem. Soc., 1 K. Bechgaard, K. Carneiro, F. G. Rasmussen, K. Olsen, Chem. Commun., 1994, 1551. G. Rindorf, C. S. Jacobsen, H. J. Pedersen and J. E. Scott, J. Am. 26 M. Kurmoo, A. W. Graham, P. Day, S. J. Coles, M. B. Hursthouse, Chem. Soc., 1981, 103, 2440. J. M. Caulfield, J. Singleton, L. Ducasse and P. Guionneau, J. Am. 2 See for example Organic Superconductors, ed. J. M. Williams et al., Chem. Soc., 1995, 117, 12209. Prentice Hall, Englewood Clis, NJ, 1992. 27 M. Kurmoo, D. Talham, P. Day, I. D. Parker, R. H. Friend, 3 (a) H. Urayama, H. Yamochi, G. Saito, K. Nozawa, T. Sugano, A. M. Stringer and J. A. K. Howard, Solid State Commun., 1987, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto and J. Tanaka, 61, 459. Chem. L ett., 1988, 55; (b) J. M. Williams, A. M. Kini, H. H. Wang, 28 A. W. Graham, M. Kurmoo and P. Day, J. Chem. Soc., Chem. K. D. Carlson, U. Geiser, L. K. Montgomery, G. J. Pyrka, Commun., 1995, 2061. D. M. Watkins, J. M. Kommer, S. J. Boryschnk, A. V. Strieby 29 See for example, H. Yamochi, T. Komatsu, N. Matsukawa, Crouch, W. K. Kwok, J. E. Schirber, D. L. Overmyer, D. Jung and G. Saito, T. Mori, M. Kusunoki and K. Sakaguchi, J. Am. Chem. M-H. Whangbo, Inorg. Chem., 1990, 29, 3262. Soc., 1993, 115, 11319. 4 P. Day, Phil. T rans. R. Soc. L ondon A, 1985, 314, 145. 5 P. Day, Phys. Scr., 1993, T49, 726. 6 See for example, T op. Curr. Phys., 1983, 32, 34, passim. Paper 6/08508C; Received 19th December, 1996 J. Mater. Chem., 1997, 7(8), 1291–1295 1295

ISSN:0959-9428    

DOI:10.1039/a608508c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

FEATURE ARTICLE Chemical synthesis of ceramic materials David Segal AEA T echnology, F7 Culham, Abingdon, Oxfordshire, UK OX14 3DB A range of increasingly important chemical syntheses for ceramic materials are described. These syntheses are coprecipitation, molten salts, sol–gel processes, hydrothermal techniques, liquid-phase and gas-phase reactions, polymer pyrolysis, the Pechini and citrate gel methods, aerosols and emulsions.Common themes relating the syntheses are outlined and their advantages over conventional solid-state reactions are described. The scope for further chemical studies on ceramic synthesis is discussed. precursors. Thus for barium titanate, BaCO3 and TiO2 powders Introduction are mixed, milled and calcined. Repeated cycles of milling and Worldwide studies on ceramics, polymers and metals during calcination are carried out to achieve the solid-state reaction.this century have resulted in the establishment of materials Relatively high temperatures are required for solid-state reac- science as a scientific discipline. A feature of these studies, tions, typically around 1200°C because of limited diusion particularly for ceramics, is their interdisciplinary nature and during calcination, and this can result in decomposition of the at the present time chemistry is making an increasingly import- ceramic product.For example the phosphor host material3 ant contribution1 to the research, development and manufac- GdAl2B4O10.5 is unstable above 1050°C and conventional ture of ceramic materials. Chemistry has two major roles when synthesis using oxide powders yielded <5% of the product applied to ceramics.The first is the synthesis of novel materials, phase at 1000°C and only 15–30% at the same temperature usually in the form of powders. For example, the discovery in after pelletising the oxide reactants. Other disadvantages of 1986 of high-temperature superconductors such as the method are the formation of undesirable phases such as YBa2Cu3O7-x highlighted the role of chemical synthesis in the BaTi2O5 during the preparation of BaTiO3, large grain sizes preparation of novel ceramics.However, successful exploitation (detrimental for high-strength ceramic components) due to of ceramics requires not only methods for their synthesis but firing at high temperature and poor chemical homogeneity also techniques for their fabrication into useful shapes, e.g., particularly when dopant oxides are introduced in small coatings, fibres, monolithic ceramics and powders with a amounts during the synthesis of electroceramics. In addition, controlled particle size for a wide variety of applications.These particle size reduction by milling can introduce chemical applications include controlled-porosity coatings for ceramic impurities into the ceramic product. Carefully controlled membranes, coatings on window glass for selective trans- addition of dopants is important in ceramic synthesis.For mission and reflection of solar radiation, optical fibres and example, in the preparation4 of BaTiO3-based Y5V and Z5U fibres for lightweight thermal insulation, ceramic honeycombs dielectric compositions for use as multilayer ceramic capacitors, for use in automotive catalytic converters and unaggregated additives include CaZrO3 and CaSnO3 to reduce the Curie powders for high-strength structural ceramic components.point from 130°C to around 10°C, MgZrO3 and CaTiO3 to The second chemical role is in the development of fabrication techniques for ceramic shapes, for example coatings, monoliths smooth out the temperature variation of capacitance as well and fibres.Synthesis and fabrication fall within the subject as modifiers such as CeO2 and Nb2O5 as grain-growth inhibiarea of ceramic processing that occupies an interface between tors. However, the advantages of solid-state reactions are the conventional studies in chemistry and materials science.ready availability of oxide precursors and the low cost for Processing is presently an important feature of ceramic studies powder production on the industrial scale. These reactions are in universities and industrial environments while chemical also convenient for laboratory-scale preparations. methods for ceramic synthesis are under intense and increasing For non-oxide powders, conventional syntheses include the investigation.This is because, compared to conventional cer- direct reaction of a metal with a gas, e.g., TiN is produced5 by amic powder processing2 using solid-state reactions between nitridation of Ti metal in N2 at 1500°C. Carbothermal powder reactants, these methods have the potential to yield reduction is also a conventional preparation route5,6 for non- ceramics with tailored properties and with performance advan- oxides. Thus TiB2 is prepared by the carbothermal reduction tages over conventional materials.In this article the conven- of a mixture of TiO2 and B2O3 at 1000°C or by reduction6 of tional method for the preparation of ceramic powders is first TiO2 with boron carbide and carbon at 2000°C, while indus- described together with the advantages and disadvantages of trial SiC powder is made7 in the Acheson process by carbother- the technique.The materials requirements for improved cer- mal reduction of silica at temperatures higher than 1800°C. amics are then outlined. A selection of chemical routes for the The main disadvantage of conventional syntheses for these synthesis of ceramic materials in the form of powders, coatings, refractory non-oxides is the requirement for extensive grinding fibres and monoliths are then described together with examples for particle size reduction, for example to around 0.5–1.0 mm of the syntheses. The chemical principles involved are high- for TiN, that introduces chemical impurities into the powders.lighted along with areas where a greater chemical understand- Precipitation from solution is also a conventional preparation ing is required in order to exploit fully the application of for one-component oxides. In the Bayer process8 for manufac- chemistry to the processing of ceramic materials. ture of a-Al2O3, bauxite is hydrothermally dissolved in sodium hydroxide to form sodium aluminate solution.An aggregated Conventional synthesis of ceramic powders gibbsite (a-Al2O3·3H2O) powder is produced by seeding the solution with gibbsite crystals and it is converted to a-Al2O3 The conventional synthesis2 for multicomponent ceramic powders is solid-state reaction between oxide and/or carbonate at around 1500°C although this temperature can be lowered J.Mater. Chem., 1997, 7(8), 1297–1305 1297Table 1 Melting points for alkali metal nitrates and eutectic mixtures12 by the use of fluorine compounds as mineralisers that also modify the shape of a-Al2O3 crystals. metal nitrate melting point/°C Conventional syntheses produce powders which are not particularly suited for the fabrication of coatings and fibres.LiNO3 255 Chemical routes are attracting attention for ceramic synthesis NaNO3 307 KNO3 334 because some of them allow direct fabrication of coatings, 50 mol% NaNO3–50 mol% KNO3 220 fibres and monoliths without powder intermediates. These 43 mol% LiNO3–57 mol% KNO3 132 routes have the potential to achieve improved chemical homogeneity on the molecular scale which is very important for electroceramics whose electrical functions are determined by the addition of small amounts of dopant oxides.For structural Reactions in molten salts ceramics, improved mechanical properties such as strength can be obtained by removal of powder aggregates and chemical The term molten salt refers12 to the liquid state of compounds which melt to give liquids displaying a degree of ionic proper- syntheses allow preparation of unaggregated powders.In addition, when chemical routes are used diusion distances ties. Alkali metal nitrates have relatively low melting points (Table 1) whereas even lower melting points are obtained in are reduced on calcination compared to conventional preparations owing to mixing of components on the colloidal or their eutectic mixtures.A molten salt can behave as a solvent or as a reactant. Thus in a nitrate melt acid–base reactions molecular level that favours lower crystallisation temperatures for multicomponent ceramics. These potential advantages for can occur according to the Lux–Flood formalism13 whereby an acid is an oxide ion acceptor and a base is an oxide ion improving the performance of ceramic components have given rise to the increased application of chemistry, through ceramic donor; nitrate ions are bases in this formalism.Nitrite melts are more basic than nitrate melts whereas addition of Lux– processing, for the development of ceramic materials. Flood bases such as Na2O2, Na2O and NaOH to a nitrate melt increases its basicity. The starting materials for reactions in molten salts are inorganic compounds, in particular sulfates Coprecipitation and chlorides, that are blended with the alkali metal nitrates The aim in coprecipitation is to prepare multicomponent or nitrites as a powder mixture before heating to the reaction ceramic oxides through formation of intermediate precipitates, temperature.Thus when Zr(SO4)2 is used, the Zr4+ ions usually hydrous oxides or oxalates, so that an intimate mixture formed on dissolution of the zirconium salt in the melt behave of components is formed during precipitation, and chemical as acids according to the Lux–Flood formalism.homogeneity is maintained on calcination. This method has Early studies on the synthesis of ZrO2 investigated14 the been applied to BaTiO3 powder with a view to fabrication of reaction of molten Zr(SO4)2 in a LiNO3–KNO3 eutectic and improved multilayer capacitors.Thus a barium titanyl oxalate ZrO2 powder was produced for reaction temperatures up to precipitate was produced9 on addition of oxalic acid to a 430°C. For more basic melts made by additions of Na2O2, mixed barium and titanyl chloride solution under controlled Na2O and NaOH, the conversion to ZrO2 occurred at lower conditions of pH, temperature and reactant concentration.temperatures but favoured formation of alkaline zirconates, although the powder characteristics were not determined. In BaCl2+TiOCl2+2H2C2O4+4H2O� later work15,16 Zr(SO4)2 was reacted in a nitrite eutectic of 65 mol% NaNO2–35 mol% KNO2 (melting point 220°C) in BaTiO(C2O4)2·4H2O+4HCl (1) a one-stage reaction at 270°C according to the stoichiometric equation Dopants such as lanthanides were introduced by coprecipitation and the precipitate was calcined up to 700°C after Zr(SO4)2+6NO2-�ZrO2+2SO42-+2NO3-+4NO (2) collection by filtration, washing and drying to produce a BaTiO3 powder after milling of 0.4–1.0 mm.In coprecipitation Tetragonal ZrO2 (t-ZrO2) was obtained at 450°C with a mixture of amorphous and poorly crystalline ZrO2 at 300°C careful control of solution conditions is required to precipitate all cations and thus maintain chemical homogeneity on the which crystallised to t-ZrO2 at 500°C; ZrO2 had a crystallite size of 5 nm and specific surface area greater than 150 m2 g-1.molecular scale. Other examples of powders made by coprecipitation are LiNixCo1-xO2 with 0<x<1 for evaluation10 as Reaction of Zr(OCl)2 with a NaNO3–KNO3 eutectic at 250°C yielded17 amorphous ZrO2 which crystallised to tetragonal cathode materials in rechargeable lithium-based batteries and nanocrystalline11 yttria-stabilised zirconia (particle size ZrO2 on calcination, although the conversion temperature for the tetragonal phase decreased as the reaction temperature <100 nm) suitable for compaction and sintering to a nanostructured ceramic where the grain size is less than 100 nm.increased and it was produced directly at 450°C. For 3 mol% Y2O3–ZrO2 the reaction18 was carried out at 450°C between This fine-grained ceramic with potential improved mechanical properties due to the increased percentage of atoms in the Zr(OCl)2 and YCl3 in a NaNO3–KNO3 eutectic.The reaction product, t-ZrO2 powder, which had specific surface areas grain boundary region cannot be obtained by conventional synthesis owing to the high reaction temperature. The method around 110 m2 g-1 and a crystallite size of 6–10 nm, could be pressed and sintered at 1500°C to 98.5% of the theoretical is also used for the manufacture of high-performance ceramic powders; thus unaggregated Y2O3-stabilised ZrO2 (Tosoh density, although the extent of powder aggregation was sensitive to washing and drying procedures.Alumina–zirconia solid Corporation) with a particle size of around 0.1 mm is produced by coprecipitation of hydroxides from mixed yttrium and solutions have also been produced19 in powder form from Zr(OCl)2 and AlCl3 in a NaNO3–KNO3 eutectic, and reactions zirconyl chloride solutions after which the hydroxide precipitate is dried, calcined and milled.Washing and drying pro- in molten salts have not been restricted to zirconia-based systems. In a recent study20 cubic Ba0.75Sr0.25TiO2 was pre- cedures (e.g. water-washing, solvent-washing, azeotropic distillation) that are used for coprecipitated hydroxides can pared by the addition of a BaCO3–SrCO3–TiO2 mixture to a eutectic melt of 49 mol% NaOH–51 mol% KOH at 300°C have a drastic eect on the mechanical properties of a sintered powder as they aect the degree of powder aggregation and for 12 h.For this reaction the eutectic was a solvent in which the reaction kinetics were considerably faster than in the need to be considered when developing a coprecipitation route to a ceramic powder.Another precipitation technique, not as conventional solid-state reaction. Reactions in molten salts can be considered to fall within widely reported as coprecipitation, involves the use of molten salts. the general class of reactions in non-aqueous liquids and have 1298 J.Mater. Chem., 1997, 7(8), 1297–1305not been widely applied for ceramic powder preparation even form can be introduced as an electrolyte solution or oxide powder. Spherical sol–gel powders can be made by the follow- though the reactants are readily available. The nucleation, growth and aggregation of ceramic powders during preparation ing processes.(1) Dispersing a sol to an emulsion in an immiscible organic is of fundamental importance to the usefulness of the powders and is an area of continuing debate within activities in ceramic solvent capable of extracting H2O from the sol, for example 2-ethylhexanol. Gelation occurs during the dehydration pro- processing. If unaggregated powders can be obtained in melts for a wide range of compositions then molten salt chemistry cess and this approach was used for thoria-based nuclear fuels.(2) External gelation in which a sol is dispersed to an could, in the future, oer an attractive route for synthesis of ceramic powders. emulsion in a water-immiscible solvent and gelation is eected by addition of a long-chain amine or NH3(g) to the solvent. (3) Internal gelation in which an ammonia donor such as Sol–gel processing of colloids hexamethylenetetramine or urea is added to the sol before emulsification and gelation occurs by release of NH3(g) on Colloid science is important for the successful application of chemistry to ceramic synthesis.This is because powder prep- warming solvent. Spray-drying24 sols also produces spherical powders.aration involves nucleation and growth of particles to a size often less than 1 mm and thus the powders are colloidal Examples1,7 of sol–gel powders are electrically conducting ceramics such as Ni0.3Zn0.7Fe2O4 and 3% SnO2–In2O3, systems.21 In addition powders are often handled in the form of colloidal dispersions and this is illustrated by the synthesis plasma-spray Cr2O3–ZrO2 powders around 10 mm diameter and catalyst supports.Advantages of this sol–gel technique method known as sol–gel processing of colloids. A sol–gel preparation can be divided7 into five stages. The starting are good chemical homogeneity due to mixing components at the colloidal level and lower reaction temperatures; thus high- material, for example a metal salt, is converted in a chemical process to a dispersible oxide, which s a colloidal disper- density ThO2–UO2 spheres were obtained at 1150°C, considerably lower than for the conventional powder mixing method sion (sol) on addition to dilute acid or H2O.Removal of H2O and/or anions from the sol produces a sti gel in the form of (ca. 1700°C). In addition, because it involves handling liquid feeds, small amounts of dopants can be readily introduced spheres, fibres, fragments or coatings and this transition is usually reversible. Calcination of the gel in air yields an oxide while lower crystallisation temperatures enable preparation of phases that are unstable at high temperature.product after decomposition of the salts. For example, ceria sols have been made7 by first adding NH3(aq)–H2O2 to The versatility of sol–gel processing is illustrated by its use for fabrication of ceramic shapes other than powders.Thus cerium(III ) nitrate solution. After careful washing of the CeIV hydrate to remove entrained electrolyte the precipitate was thin oxide coatings around 1 mm can be prepared7 on substrates by first applying a sol by spinning or dip-coating after which peptised with HNO3 to sols with a particle size of around 8 nm.While peptisation involves breaking up coarse precipi- the liquid layer is dried to a gel coating that is calcined to oxide. As an example,25 silica coatings for improved oxidation tates, deanionisation of metal salt solutions involves growing molecular species to colloidal units in the form of polynuclear resistance of stainless-steel wire mesh were prepared by dipcoating.Although the technique is low cost and simple to ions. For example, treatment22 of chromium(III ) nitrate solution with a long-chain primary amine removes hydrolytic HNO3 carry out, cracking and loss of adhesion can occur on drying. Fibres can be drawn26,27 from sols with controlled rheological from the aqueous solution with the resultant formation of polymeric colloidal species.Flame-hydrolysed powders23 such properties and calcined to oxide. Whereas aluminosilicate fibres are conventionally prepared from a melt which limits as SiO2 also form sols on dispersion in water. An example of a sol is shown in Fig. 1. It is useful to note that many of the the Al2O3 content to around 65 mass% owing to the melt viscosity, the use of sol–gel yields a much wider range of chemical techniques attracting attention for ceramic synthesis are not new and often date back many years.Thus sol–gel compositions such as 95% Al2O3–5% SiO2 and a range of polycrystalline ceramic fibres are manufactured27 from sol–gel processing of colloids was first exploited in the late 1950s for fabrication of ThO2–UO2 ceramics for use7 as spherical fuel materials.Preparation of sol–gel fibres is an example of powderless processing that avoids the synthesis of a ceramic powders in high-temperature thermal nuclear reactors. For preparation of multicomponent oxides, sols are blended powder with a well defined particle size and its subsequent consolidation into a shape. Powderless processing is an attract- together before gelation and a component unavailable in sol ive method for fabrication of ceramic monoliths as it can produce near-net size and shape components in a few processing steps.For example, SiO2 sols in combination with potassium silicate and a hydrolytic agent such as formamide were cast28,29 directly to monolithic silica gels with a controlled pore size in the range 100–300 nm.After leaching out K+ ions, the gels were dried by using microwaves and sintered to silica monoliths. A key feature of the synthesis was that large pore sizes reduced the capillary forces on drying the gel, thus preventing cracks in the ceramic. Powderless processing also includes freeze casting.30 For example, SiO2 sols were cast into a mould, frozen in liquid nitrogen and the mould was then removed.After thawing, the intact gel was calcined to a nearnet size and shape component. An important aspect of these fabrication processes is colloidal processing31 in which the tendency of sol particles to flocculate and coagulate with the resultant eect on porosity is controlled, but a greater understanding of changes in sol structure taking place during fabrication is required to fully utilise these powderless processing techniques for a wide range of oxide ceramics.The hydrolysis of cations32–34 and inorganic polymerisation in aqueous solution are of fundamental importance to wet Fig. 1 Transmission electron micrograph of a silica sol chemical methods of synthesis such as sol–gel in which sols J. Mater.Chem., 1997, 7(8), 1297–1305 1299are formed from cations that can undergo hydrolysis. alkoxide solutions in order to relate the solution properties of the precursors to the ceramic properties. Techniques35 have Monovalent hydrolysis products whose formation1 is represented by the equation included light scattering, small-angle X-ray scattering, NMR spectroscopy, liquid chromatography and this characterisation [M(H2O)n]z+�[M(OH)p(H2O)n-p](z-p)++pH+ (3) is an active area of sol–gel studies.As for all chemical syntheses the success of a sol–gel synthesis is ultimately judged by the where n is the number of bound water molecules, p is the number of protons removed from the cation on hydrolysis and performance of the chemically derived ceramic. Sol–gel processing of metal alkoxides has been widely used z is the valency of the cation M, can condense to polyvalent or polynuclear ions which can be colloidal, for example, for the preparation of submicrometre oxide powders. For example, Y3Fe5O12 was prepared38 by hydrolysis of a mixture [AlO4Al12(OH)25(H2O)11]6+; polynuclear ions contain OH bridges, MMOHMM (olation) or oxygen bridges, MMOMM of Fe(OC2H5)3 in C2H5OH and Y(OC4H9)3 in xylene.The resulting gel was, after drying, converted at 700°C to Y3Fe5O12 (oxolation) and are precursors for particle growth. A quantitative approach for predicting the products of cation hydrolysis with a mean diameter of 9 nm. Alkoxides have diering hydrolysis rates and the benefits of improved homogeneity can as a function of the experimental conditions, known as the partial charge model,33,34 has been developed.In this thermo- thus be lost during hydrolysis of mixed alkoxides. Modification of hydrolysis rates which aect particle formation, growth and dynamic model, proton exchange (and the associated electron transfer) between metal ions and solution occurs until the aggregation can be made by complexing the alkoxide with a chelating ligand.For example, Zr(OC3H7)4 can be complexed mean electronegativity of the hydrolysed species becomes equal to the mean electronegativity of the surrounding aqueous with acetylacetone and hydrolysis of this alkoxide with39 toluene-p-sulfonic acid in the presence of acetylacetone yielded solution. The ability to predict solution species and their condensation behaviour is a very useful tool for understanding ZrO2 particles around 3 nm in diameter after ageing the reactants at 60°C.Monodispersed spherical powders, usually the chemistry of sol–gel processes. submicrometre, can be obtained by controlled hydrolysis of alkoxide solutions and are known as Sto�ber spheres.40,41 An Sol–gel processing of metal–organic compounds example of a powder made by controlled hydrolysis (also known as homogeneous nucleation) is shown in Fig. 2 and the The phrase sol–gel has been used inaccurately in the scientific literature and is often assigned to all wet chemical methods mechanism of particle formation and growth is of relevance to other ceramic syntheses. While powder formation by homo- for ceramic synthesis even though there are only two sol–gel processing techniques.The second35 sol–gel technique involves geneous nucleation has often been considered to involve nucleation from supersaturated solutions at a critical particle ceramic synthesis by hydrolysis of metal–organic compounds, in particular metal alkoxides,36 M(OR)z where R is an alkyl size followed by diusion of molecular species onto growing nuclei, recent studies42–44 suggest an anative mechanism in group, and at the present time publications on sol–gel dominate the literature on ceramic synthesis.This method is particularly which nucleation followed by aggregation of small particles takes place during particle growth. associated with non-fusion routes to oxide glasses. For preparation of multicomponent oxides, alkoxides are mixed Many electroceramics are used in the form of thin coatings, around 1 mm, and the development of sol–gel coatings is an together in alcohol and a component unavailable as an alkoxide is introduced as a salt, for example an acetate, so that the area of expanding activity; a sol–gel PbTiO3 coating is shown in Fig. 3.Alkoxide-derived coatings include Pb1-xLa2x/3TiO3 resulting solution has the required ceramic composition.Hydrolysis is carried out under controlled conditions of pH, where x is between 0 and 0.2 for pyroelectric applications,45 and La0.5Sr0.5CoO3 electrode materials46 while the ability of added H2O, alkoxide and alcohol concentration. The reaction conditions can result in the solution forming a solid monolithic chemical methods to tailor the properties of ceramics is highlighted by fabrication of ceramic membranes.47–49 While gel (known as an alcogel) which is dried and calcined to an oxide powder, or precipitation of powders directly from solu- polymer membranes are widely available, ceramic membranes have operating advantages, in particular superior chemical and tion.The reactions occurring in solution are complex32 but involve hydrolysis and condensation to polymeric species as thermal durabilities.Nanofiltration membranes have pore diameters in the separation layer around 2 nm and have been represented by the equations prepared48 by controlled hydrolysis of Zr(OC5H11)4 to ZrO2 M(OR)z+H2O�M(OR)z-1(OH)+ROH (4) particles less than 5 nm in diameter. These particles were then deposited from a dispersion onto a substrate to produce the 2M(OR)z-1(OH)�M2O(OR)2z-2+H2O (5) membrane layer.Alcogels can be dried and sintered to crack- M(OR)z+M(OR)z-1(OH)�M2O(OR)2z-2+ROH (6) Metal–oxygen–metal (MMOMM) bonds are formed in solution by self-condensation or by cross-condensation when dierent alkoxides are used, thus M¾MOMM. Sol–gel processing with alkoxides shares the same versatility and benefits as the use of colloids but has some additional advantages.Because many alkoxides are liquids or volatile solids they can be purified to form extremely pure oxide sources which is important for electroceramic synthesis, while improved chemical homogeneity is obtained owing to mixing of components at the molecular level. The improved homogeneity is associated with lower crystallisation temperatures than for use of colloids, often between 400 and 800°C for gel�oxide conversion.However, alkoxides are relatively expensive compared to precursors for sol–gel processing of colloids, a limited range of them are commercially available and their use involves a solvent-based rather than a water-based process. There are certainly requirements37 to develop economical syntheses for large quantities of alkoxides to fully exploit the process.Many Fig. 2 Titania spheres by controlled hydrolysis of an alkoxide chemical studies have been carried out on characterising 1300 J. Mater. Chem., 1997, 7(8), 1297–1305of ceramic powders, as measured by the number of publications, is much less than in the two sol–gel processes.This is surprising because hydrothermal synthesis oers a low-temperature, direct route to submicrometre, oxide powders with a narrow size distribution avoiding the calcination step required in sol–gel processing. When applied to ceramic powders hydrothermal techniques often involve heating metal salts, oxides or hydroxides as a solution or suspension in a liquid at elevated temperature and pressure up to about 300°C and 100 MPa.Syntheses that oer a lower temperature route57 than solidstate reactions to compounds are frequently described as soft chemistry routes or chimie douce reactions, although these terms have not been widely used in the ceramic literature. The advantage of hydrothermal synthesis is illustrated by the preparation of BaFe12O1958 from a suspension of Ba(OH)2 and a-FeOOH.Single-phase submicrometre ferrite particles were obtained at 325°C compared with a temperature between 1150 and 1250°C in the conventional solid-state synthesis by firing Fig. 3 Sol–gel lead titanate coating a mixture of a-Fe2O3 and BaCO3. For SrFe12O19, coprecipitated mixed strontium(II ) and iron(III ) hydroxides were heated in the temperature range 160–220°C for up to 5 h, yielding free monolithic components.Silica alcogels were prepared50 from hydrolysed Si(OC2H5)4 solutions in the presence of a strontium hexaferrite59 powder, about 2 mm in diameter with a platelet morphology. This reaction temperature is low com- drying control chemical additive, formamide, which modified the pore size distribution in the alcogel producing an average pared to 1250°C for the conventional solid-state reaction between SrCO3 and Fe2O3 and 800°C for coprecipitation.The pore size around 5 nm. This modification allowed conversion of the alcogel to crack-free silica components. Alcogels can hydrolysis reaction represented in eqn. (3) can become more pronounced under hydrothermal conditions compared to also be converted by supercritical drying to aerogels51,52 which are transparent, porous solids with high surface area and ambient temperatures leading to direct formation of oxide powders from electrolyte solutions.This type of reaction is densities as low as 0.003 g cm-3. In supercritical drying, pore fluid is removed directly above its critical point. For C2H5OH, known as forced hydrolysis.As an example, monoclinic ZrO2 powder with particle diameters in the range 1–10 nm was the common solvent in sol–gel processing, the critical temperature and pressure are 243°C and 6.4 MPa respectively, but produced60 by heating a 0.2 mol dm-3 solution of ZrO(NO3)2 at pH 0.5. Hydrothermal techniques have also been used for recently a low-temperature method has been introduced52 in which ethanol is replaced by CO2 and the latter is removed fabrication of thin films, for example BaTiO361 on Ti substrates.Understanding the reaction mechanism of hydrothermal above its critical point, 31°C and 7.2 MPa; the lower critical temperature is a processing advantage. syntheses and the nature of solution species can help in the production of powders with well defined properties.Ceramics are hard, brittle solids whereas polymers are tough, flexible and ductile. Hybrid polymer–ceramic materials can Dissolution–recrystallisation processes are considered important in particle nucleation and growth under hydrothermal share the advantages of these two types of materials while minimising the disadvantages. These materials have been called conditions while polynuclear ions that are precursors for sol–gel processes are important for forced hydrolysis reactions.ceramers53 to indicate a material that has a unique combination of ceramic and polymer properties, ormosils54 which refers For example a tetrameric species [Zr(OH)2·4H2O]48+ has been considered60 to form particles of critical size which specifically to an organically modified silicate for a SiO2-based system but are now more generally known as ormocers55 nucleated from solution and grew by aggregation. Thermodynamic modelling of hydrothermal processes has been (organically modified ceramics).Alkoxides have a central role in the synthesis of ormocers. While the use of alkoxides for carried out, particularly for BaTiO3,62 and these calculations indicate the stability of the desired product under dierent oxide synthesis involves precursors where all attached alkoxy groups can be replaced on hydrolysis, preparation of ormocers conditions, namely pH, temperature and reagent concentrations.Although many chemical syntheses are very dierent uses precursors in which some attached groups are retained on reaction. These hybrid materials contain an inorganic from a praiewpoint, they are often related by underlying chemical themes.Hence a greater knowledge of the formation backbone and organic components that produce chemical or structural modification of the inorganic network. Low-tem- and structure of solution species will be useful in increasing understanding of powder formation in sol–gel and hydrother- perature curing (ca. 150°C) produces the ormocer solid in contrast with high crystallisation temperatures required for mal processes. oxide ceramics. As an example, an ormocer was prepared54 by reaction of a silanol-terminated polydimethylsiloxane (molecu- Polymer pyrolysis lar mass 1750) with acidified Si(OC2H5)4 at 80°C and gelation occurred at ambient temperature. Potential applications of Polymer pyrolysis refers to the synthesis of a polymeric compound, sometimes called a preceramic polymer, which is then ormocers are many,56 including optoelectronics, sensors, coatings, elastomers and porous materials.Development of ormo- fabricated into a shape and pyrolysed to the ceramic. Polymer pyrolysis is particularly associated with the synthesis of high- cers is an exciting area of materials science where there is considerable scope for innovative sol–gel chemistry to produce tensile strength b-SiC fibre as a result of pioneering work63,64 carried out in the 1970s.This work involved thermal rearrange- novel materials with tailored properties. ment of polysilanes to polycarbosilanes. For example, (CH3)2SiCl2 was converted by reaction with Li to dodeca- Hydrothermal synthesis methylcyclohexasilane, [(CH3)2Si]6 which underwent ringopening and polymerisation at 400°C under Ar to yield a Hydrothermal techniques are widely used in industrial processes for the dissolution of bauxite prior to precipitation of polycarbosilane.Continuous polycarbosilane fibre could be prepared65 either by drawing from solution or by melt-spinning gibbsite in the Bayer process and for the preparation of aluminosilicate zeolites.Interest in this method for synthesis and was pyrolysed to b-SiC fibres although the molecular J. Mater. Chem., 1997, 7(8), 1297–1305 1301mass distribution of the polycarbosilane aected the fibre example cubic vanadium nitride with a particle size of 0.1–1 mm is formed by decomposition of the product of the reaction of quality.These studies also led to a process for the manufacture7 of continuous polycrystalline b-SiC fibre and an important VCl4(l) and NH3(l) at 1500°C in NH3. For borides, Ti(BH4)3, obtained from the reaction6 of B2H6 with Ti(OC4H9)4, decom- step in the process is curing of polycarbosilane fibres in O2 which cross-links molecular chains of the polymer preventing posed at 140°C in xylene according to the equation melting during polymer decomposition.Conversion of polycar- 2Ti(BH4)3�2TiB2+B2H6+9H2 (7) bosilanes to fibre is a further example of powderless processing. Besides their use in the preparation of fibres, preceramic to give an aggregated TiB2 powder of high purity with a polymers can be applied as films and moulded into shapes particle size of 0.1–0.2 mm, while cubic ZnS was obtained74 on that can be cured and pyrolysed to near net-shape ceramic passing H2S through a diethylzinc solution at ambient tempera- components with high purity and compositional homogeneity ture.In an extension of sol–gel processing to non-oxides, at relatively low temperature.They are also used to build-up amorphous GeS2 was obtained75 by reaction of H2S with the matrix, by impregnation, of fibre-reinforced ceramic matrix Ge(OC2H5)4 at ambient temperature and crystallised on heat- composites. These benefits for fabrication have led to increased ing, although it was easily contaminated with GeO2. Low- interest in the chemical design of preceramic polymers that temperature decomposition is an advantage of non-aqueous can be converted directly to ceramic fibres and monoliths.reactions that have been extended to oxides and include Another preceramic polymer is polysilastyrene,66 a soluble condensation between metal chlorides and metal alkoxides, phenylmethylpolysilane made by reaction of (CH3)2SiCl2 and which has been referred76 to as a non-hydrolytic sol–gel C6H5CH3SiCl2 with Na in toluene.Solubility, melting point, reaction represented by the equation the curing process (whether by heat treatment or by ultraviolet MClz+M(OR)z�Clz-1MOM(OR)z-1+RCl� irradiation) and, in particular, ceramic yield on pyrolysis are all important properties for successful use of preceramic poly- 2MOz/2+zRCl (8) mers, and further examples of the range of materials developed For example, TiCl4 and Ti(OC3H7)4 were heated together at are shown67 in Table 2.This area of ceramics where synthesis 110°C and the resulting monolithic gel was decomposed to has a crucial role is under increasing68,69 investigation. anatase at 500°C. Non-hydrolytic reactions can be carried out Polymer pyrolysis is not restricted to Si-based systems. Thus, by reaction of a metal chloride with an ether77 which forms reaction of B10H14 with diamines such as NH2CH2CH2NH2 an intermediate adduct and then a metal chloroalkoxide.This yielded a solid preceramic polymer70 which was pyrolysed in approach has a processing advantage as the use of alkoxides, NH3 at 1000°C to a BN-containing powder. For AlN, anodic which may be dicult to synthesise, is avoided.For example, dissolution of Al in acetonitrile containing a primary amine VOCl3 was reacted with (C3H7)2O at 110°C and the resulting and a tetraalkylammonium salt (for increasing the solution gel was decomposed at 500°C to orthorhombic V2O5 powder. conductivity) produced71 a liquid with composition Al(NHR)3 that underwent polymerisation to a solid gel with composition Al2(NR)3.The latter was pyrolysed above 800°C in NH3 to Aerosol-derived powders AlN powder with a crystallite size of 25 nm. Although this last example involves polymer pyrolysis it can also be classified as Two aerosol routes are used for powder preparation. The first involves generation of a supersaturated vapour from a reactant a non-aqueous liquid-phase reaction.followed by homogeneous nucleation, while the second involves generation of liquid droplets by various methods which Non-aqueous liquid-phase reactions undergo a heat treatment to solid particles. For example,78 the silazane precursor [CH3SiHNH]n where n=3 or 4, which was Liquid-phase reactions take place in a non-aqueous solvent, which may be inert or one of the reactants, and are associated made by ammonolysis of CH3SiHCl2, was decomposed to a vapour at about 1000°C in NH3 . Rapid condensation of the with the synthesis of non-oxide powders, particularly Si3N4.An interfacial reaction7,72 occurs between SiCl4(l) in a mixed gaseous product produced nanometre-sized ceramic powders in the Si(N,C) system. Liquid droplets can be generated by cyclohexane–benzene solvent and NH3(l) at-40°C producing an amorphous solid, silicon diimide Si(NH)2, which decom- use of pneumatic jet atomisation or ultrasonic atomisation after which the droplets are dried and transported by a carrier poses on heating to 1400°C to a-Si3N4.This reaction forms the basis for a commercial route to Si3N4 powder (Ube gas to a furnace where they are decomposed to oxide powders.Decomposition of droplets in this way is often referred79 to as Industries) which consists of equiaxed particles, about 0.1–0.2 mm in size, containing 95% of the a-phase and 5% of spray-pyrolysis. Compared to conventional powder synthesis, aerosol methods have the ability to produce spherical particles b-Si3N4; this powder contains lower levels of impurities than those from other syntheses for Si3N4 such as solid-state reac- with sizes ranging from micrometres down to 100 nm with high purity and chemical homogeneity for complex metal tion, as the reactants can be readily purified.Other metallic nitrides have been73 made by similar interfacial reactions, for oxides. An example80 of an aerosol-derived cathodoluminescent phosphor, Y2.82Tb0.18Ga2.5Al2.5O12, made by ultrasonic atomisation is shown in Fig. 4. Other materials made by aerosol Table 2 Ceraoduct and yield from preceramic polymers67 methods include submicrometre SrFe12O19,81 while electrostatic atomisation has been used for synthesis of ZrO2 temperature/°C; yield powder.82 precursor atmosphere product (mass%) Spray-drying24 is used in industrial processes for converting polymethylvinylsilane 1000; Ar SiC 83 a liquid feed into a dry powder.When applied to ceramic polytitanocarbosilane 1300; N2 SiCxOyTiw 75 powders the feed can consist of oxide colloids or salt solutions polyhydridosilazane 1200; N2 Si3N4 74 and when these precursors are used spray-drying is analogous polymethylsilazane 800; NH3 Si3N4 85 to aerosol techniques. Spray-dried powders are often larger polyvinylsilazane 1200; N2 SiCxNy/C 85 than aerosol-derived powders, around 10 mm in diameter.This polyvinylphenylsilazane 1000; N2 Si3N4 85 polycyclomethylsilazane 1000; Ar Si3N4/SiC 88 technique is a convenient way to prepare multicomponent polyborosilazane 1000; Ar BN/Si3N4 90 oxides with good chemical homogeneity. For example, polyboronsiliconimide 1250; NH3 SiBxNy 72 GdAl2B4O10.5, which could not be obtained3 in high yield by polymethylsiloxane 1000; He SiOxCy 85 solid-state reactions between oxides owing to its decomposition polyphenylsilsesquioxane 1400; Ar SiCxOy 78 above 1050°C, was readily prepared as a single phase by 1302 J.Mater. Chem., 1997, 7(8), 1297–130520 nm were obtained from a mixture of hexamethyldisilazane, (CH3)3SiNHSi(CH3)3 and NH3.Direct current argon plasmas have been used to prepare87 TiN powders with diameters around 10 nm by heating a mixture of TiCl4 and NH3 at 1100°C. Plasma-derived powders which had a much finer particle size than commercial TiN (size around 0.5–1.0 mm) prepared by nitridation of Ti could be pressureless sintered at 1400°C whereas the conventional powder is dicult to sinter and requires hot-pressing at temperatures higher than 1800°C.Pechini and citrate gel methods In the Pechini method,88 polybasic chelates are formed between a-hydroxycarboxylic acids containing at least one hydroxy group, for example citric acid, HOC(CH2CO2H)2·CO2H with metallic ions. The chelate undergoes polyesterification on Fig. 4 Aerosol-derived Y2.82Tb0.18Ga2.5Al2.5O12 powder heating with a polyfunctional alcohol, for example ethylene glycol, HOCH2CH2OH.Further heating produces a viscous resin, then a rigid transparent, glassy gel and finally a fine spray-drying mixed electrolyte solutions followed by calci- oxide powder. Advantages of the Pechini method are the nation at 950°C. Spray-roasting or spray-calcination involves ability to prepare complex compositions, good homogeneity converting a liquid feed to droplets that are fed directly to a through mixing at the molecular level in solution and control furnace and powder sizes obtained from this technique and of the stoichiometry.Low firing temperatures are required for spray-drying are similar. Spray-roasting has been used to decomposition of the resin to the oxide, thus 650°C for BaTiO3 manufacture83 multicomponent ceramics, for example ferrites compared to 1000°C for the conventional solid-state reaction.such as MnFe2O4. This is a versatile method for the synthesis of multicomponent oxides, for example Pb3MgNb2O9.89 The citrate gel90,91 Gas-phase reactions method shares the advantages of the Pechini method with respect to chemical homogeneity and compositional control.Reactions between gases have been used to prepare both For the synthesis92 of YBa2Cu3O7-x metal nitrate solutions of oxides and non-oxide powders on the laboratory and industrial Y, Ba and Cu were added to citric acid solution and the pH scale. These reactions are characterised1 by the use of a variety raised to between 6.5 and 7.0 in order to dissolve insoluble of heating techniques including furnace heating, lasers, gas barium citrate but not to precipitate metal hydroxides.The plasmas and flame propagation, while the underlying chemical solution, which contained polybasic chelates, was concentrated principle that determines particle formation is homogeneous to a viscous resin and dried to a transparent gel that was nucleation from supersaturated vapours.In flame-hydrolysis,84 pyrolysed to a fine powder. The citrate gel method is also a volatile compounds such as SiCl4 are passed through an versatile method for synthesis of multicomponent oxides, for oxygen–hydrogen stationary flame. Molten primary oxide par- example SrCo0.8Fe0.2O3-x.93 ticles formed by nucleation grow by coalescence to larger A claimed advantage of many chemical syntheses is their droplets.As particles solidify they stick together on collision, improved homogeneity although evidence for this is not always forming solid aggregates which then associate to loosely bound available. However, a recent study using Raman and 13C NMR agglomerates. Flame-hydrolysed silicas have been used23 in spectroscopy indicated that during the preparation94 of BaTiO3 sol–gel processing while examples of other flame-hydrolysed by the Pechini method the coordination of Ba and Ti in the oxides are shown in Table 3.Pioneering work85 on the use of mixed metal complex remained almost unchanged on polym- high-powered lasers to heat gases yielded Si3N4 with a particle erisation and that molecular-level mixing was retained at the size between 10 and 25 nm and nitrogen surface area of 117 m2 resin stage and probably in the pyrolysed resin.There is further g-1 from a mixture of SiH4 and NH3. Silane has a strong scope for using characterisation techniques for monitoring absorbance band at 10.6 mm, near the wavelength of CO2 changes in homogeneity at dierent stages of the wide range lasers, and was decomposed to a supersaturated Si vapour of chemical syntheses.which reacted with NH3. These reactions are characterised by fast heating and cooling rates (106 K s-1 and 105 K s-1 respectively) and reaction times around 10-3 s. Dierent precursors have been used in laser-driven reactions; for example,86 Emulsion-derived powders mixed Si3N4–SiC powders with an average particle size of Emulsions have been used widely in aqueous sol–gel processes for converting sols to gel powders by dehydration, internal Table 3 Oxides made by flame hydrolysis84 gelation and external gelation as was described earlier.Powders oxide raw material boiling point/°C made by these methods have sizes in the range of 1 mm or larger and emulsion methods have been extended to include SiO2 SiCl4 57 use of microemulsions, enabling preparation of ceramic par- Al2O3 AlCl3 180a ticles in the nanometre size range.Thus Fe3O4 powders were TiO2 TiCl4 137 made95 by precipitation from Fe3+/Fe2+ salts with NH3 (aq) Cr2O3 CrO2Cl2 117 in microemulsions of sodium bis(2-ethylhexyl)sulfosuccinate Fe2O3 Fe(CO)5 103 GeO2 GeCl4 84 (Aerosol OT)–heptane–H2O while TiO2 particles with diam- NiO Ni(CO)4 42 eters less than 10 nm were prepared96 by controlled hydrolysis SnO2 SnCl4 114 of Ti(OC3H7)4 in reverse micelles of Aerosol OT produced in V2O5 VOCl3 127 an Aerosol OT–hexane–H2O solution; the TiO2 nanoparticles ZrO2 ZrCl4 331a aggregated into sols with sizes of 20 to 200 nm, eventually forming gelatinous precipitates.aSublimation temperature. 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They also allow direct Processing of Advanced Ceramics, ed. J. D. Mackenzie and fabrication of ceramics as coatings, fibres and monoliths.For D. R. Ulrich, Wiley, New York, 1988, p. 901. the syntheses under discussion, molten salt reactions oer a 7 D. L. Segal, Chemical Synthesis of Advanced Ceramic Materials, straightforward route to ceramic powders using readily avail- Cambridge University Press, Cambridge, 1989. able precursors, although their extension to a wide range of 8 G. Huiras, in Ceramic T echnology International 1994, ed.I. Birkby, compositions requires further study. Sol–gel processing with Sterling Publications Limited, Hong Kong, 1993, p. 37. 9 J. M. Bind, T. Dupin, J. Schafer and M. Titeux, J. Metals, 1987, both aqueous oxide colloids and alkoxides has attracted 54, 60. tremendous attention as measured by the numbers of publi- 10 D. Caurant, N. Baer, B.Garcia and J. P. Pereira-Ramos, Solid cations. Aqueous sol–gel processing methods avoid the use of State Ionics, 1996, 91, 45. organic solvents, which is a practical advantage, and involve 11 C. D. Sagel-Ransijn, A. J. A. Winnubst, A. J. Burgraaf and low-cost precursors. Full exploitation of the alkoxide route, H. Verweij, in Ceramic T ransactions, volume 51: Ceramic particularly for powders, requires economical synthesis of Processing Science and T echnology, ed.H. Hausner, G. L. Messing and S. Hirano, The American Ceramic Society, Westerville, OH, metal alkoxides but this method is very useful for fabrication 1995, p. 33. of thin oxide coatings where the cost of the precursor is not a 12 D. H. Kerridge, in Chemistry of Non-Aqueous Solvents, ed. critical factor. A major development of sol–gel processing has J.J. Lagowski, Academic Press, New York, 1978, vol. 5B, p. 269. been on organic–inorganic hybrids and there is considerable 13 H. Lux, Z. Elektrochem., 1939, 45, 303. potential here for innovative chemical synthesis of new mate- 14 D. H. Kerridge and J. Cancela Rey, J. Inorg. Nucl. Chem., 1977, rials with tailored properties.Other gel methods, the Pechini 39, 405. 15 H. Al Raihini, B. Durand, F. Chassagneux, D. H. Kerridge and and citrate gel techniques, have been shown to be very useful D. Inman, J.Mater. Chem., 1994, 4, 1331. for synthesis of multicomponent electroceramic powders. 16 Y. Du and D. Inman, J.Mater. Sci., 1996, 31, 5505. Sol–gel processing has been applied mainly to oxide cer- 17 B.Durand and M. Roubin, Mater. Sci. Forum, 1991, 73–75, 663. amics. There is scope for extending sol–gel processing of 18 M. Descemond, C. Brodhag, F. Thevenot, B. Durand, M. Jebrouni alkoxides to non-oxide ceramics which links sol–gel to non- and M. Roubin, J.Mater. Sci., 1993, 28, 2283. aqueous liquid-phase reactions. The latter have been widely 19 D. Hamon, M. Vrinat, M. Breysse, B.Durand, L. Mosoni, M. Roubin and T. des Couriers, Eur. J. Solid State Inorg. Chem., applied to non-oxide powders, in particular for the manufacture 1993, 30, 713. of silicon nitride. Hydrothermal synthesis and sol–gel pro- 20 S. Gopalan, K. Mehta and A. V. Virka, J. Mater. Res., 1996, 11, cessing, although very dierent from a practical viewpoint, 1863. involve hydrolysis of solution species.Increased knowledge 21 D. H. Everett, Basic Principles of Colloid Science, Royal Society of about the formation of hydrolysis products in solution would Chemistry, London, 1988. enhance understanding of the processes involved in particle 22 D. L. Segal and J. L. Woodhead, Proc. Br. Ceram. Soc., 1986, formation from solution. Hydrothermal synthesis is an estab- 38, 245. 23 E. M. Rabinovich, in Sol–Gel T echnology for T hin Films, Fibres, lished industrial process and oers a direct route to oxide Preforms, Electronics and Specialty Shapes, ed. L. C. Klein, Noyes powders, but it has not attracted the attention given to sol–gel Publications, New Jersey, 1988, p. 260. methods. There is potential for extending hydrothermal 24 K. Masters, Bull. Am. Ceram.Soc., 1994, 73, 63. methods to a wide range of compositions for both oxide and 25 R. L. Nelson, J. D. F. Ramsay, J. L. Woodhead, J. A. Cairns and non-oxide powders. Control of powder aggregation, for J. A. A. Crossley, T hin Solid Films, 1981, 81, 329. example in coprecipitation and molten salt reactions, is import- 26 J. D. Birchall, in Concise Encyclopaedia of Advanced Ceramic Materials, ed.R. J. Brook, Pergamon Press, Oxford, 1991, p. 236. ant for ceramic applications and needs to be considered when 27 H. G. Sowman, Bull. Am. Ceram. Soc., 1988, 67, 1911. carrying out a synthesis. A key advantage of polymer pyrolysis 28 R. D. Shoup and W. J.Wein, US Pat., 4059658, 1977. over other methods is its ability to yield high-strength non- 29 R. D. Shoup, in Ultrastructure Processing of Advanced Materials, oxide fibres, and there is considerable scope for chemical ed.D. R. Uhlmann and D. R. Ulrich, John Wiley and Sons, New design of polymers suitable for conversion to fibres, coatings York, 1992, p. 291. and monolithic ceramics. Chemical methods have the ability 30 J. Laurie, C. M. Bagnall, B. Harris, R. W. Jones, R. G. Cooke, to produce powders with an exceptionally small size in the R.S. Russell-Floyd, T. H.Wang and F. W. Hammett, J. Non-Cryst. Solids, 1992, 147–148, 320. nanometre range; gas-phase reactions, microemulsions and 31 L. Bergstrom, in Ceramic T ransactions, volume 51: Ceramic aerosol methods are particularly suited for preparation of Processing Science and T echnology, ed. H. Hausner, G. L.Messing nanoparticles. However, the success of chemical syntheses of and S. Hirano, The American Ceramic Society, Westerville, OH, ceramic materials can be judged primarily by whether they 1995, p. 341. result in improved materials properties and performance. In 32 C. F. Baes Jr. and R. E. Mesmer, T he Hydrolysis of Cations, John order to fully exploit chemical methods now and in the next Wiley and Sons, New York, 1976. 33 J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, century a close interaction is required between ceramic studies 18, 259. carried out in the disciplines of chemistry and materials science. 34 J. Livage, M. Henry and J. P. Jolivet, in Chemical Processing of Advanced Materials, ed. L. L. Hench and J. K. West, John Wiley and Sons, New York, 1992, p. 223. References 35 C. J. Brinker and G. W. Scherer, Sol–Gel Science. T he Physics and Chemistry of Sol–Gel Processing, Academic Press, New York, 1990. 1 D. L. Segal, inMaterials Science and T echnology: A Comprehensive 36 D. C. Bradley, R. C. Mehrotra and P. D. 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ISSN:0959-9428    

DOI:10.1039/a700881c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATION Spontaneous helix formation in smectic liquid crystals comprising achiral molecules Tomoko Sekine,a T. Niori,a J. Watanabe,a T. Furukawa,b S. W. Choib and H. Takezoeb aDepartment of Polymer Chemistry, T okyo Institute of T echnology, O-okayama, Meguro-ku, T okyo 152, Japan bDepartment of Organic and Polymeric Materials, T okyo Institute of T echnology, O-okayama, Meguro-ku, T okyo 152, Japan Spontaneous formation of two types of helical structure is observed in two smectic phases consisting of banana-shaped achiral molecules for the first time.Local optical resolution occurs to form spatially resolved helices of both handednesses. Since the discovery of the first liquid crystalline material in to have helical arrangements of their constituent molecules.More recently, the helical smectic A phase, the so called twisted 1888, helicity has proven to be the most fascinating topic of research in orientationally ordered mesophases.1 The choles- grain boundary (TGB) phase in which a discontinuous layer twist exists, has been found. In these helical phases, at least a teric mesophase was the first liquid crystal with helical structure to be found.Other phases, such as blue phases and the fraction of the constituent molecules have an asymmetric carbon. This is the first report of spontaneous helix formation ferroelectric and antiferroelectric smectic phases, were shown Fig. 1 Optical micrographs observed in a 5 mm thick homogeneously aligned cell of 18: (a) SX1 , 0 V; (b) SX1 , 50V; (c) SX2 , 0 V; (d) SX3 , 0V J.Mater. Chem., 1997, 7(8), 1307–1309 1307Table 1 Transition temperatures of compounds 1n originates from the characteristic layer packing of bananashaped molecules in which their bend direction is uniformly compound transition temperature/°C aligned within a layer, and the spontaneous polarization arises along the bend direction of molecule [see later, Fig. 3(a)]. SX3 SX2 SX1 Iso Thus, the fringe pattern results from the helical structure with 16 $ 144.9 $ 158.5 $ 173.1 $ the in-layer polarization precessing along the layer normal, 17 $ 141.3 $ 155.2 $ 171.7 $ and its disappearance upon field application is due to the helix 18 $ 139.7 $ 151.9 $ 173.9 $ unwinding, as in the chiral SC* phase.4 19 $ 137.7 $ 145.9 $ 173.5 $ There is no indication of helical structure in the SX2 phase 110 $ 138.4 $ 144.4 $ 172.7 $ [see Fig. 1(c)], but the SX3 phase looks like a blue phase,5 as 112 $ 140.9 $ 169.9 $ can be seen from the texture of Fig. 1(d), also suggesting a helical structure. In order to explore the origin of the blue colour in the SX3 phase, the spectrum of the scattered light in fluid smectic systems comprised of achiral banana-shaped was measured. As shown in Fig. 2(a), a sharp reflection band molecules. was clearly observed. The peak wavelength around 430 nm The materials used were compounds 1n (n=6, 7, 8, 9, 10, 12 corresponds to the blue reflection colour, which originates and 16), which show three smectic phases, SX1 , SX2 and SX3 , as from a Bragg (selective) reflection due to a helical structure.shown in Table 1. According to X-ray diraction, the smectic To assess the handedness of the helix, the reflection measure- layer spacings are approximately the same as the lengths of ments using right- and left-circularly polarized lights were the molecules in bent configurations.2,3 X-Ray patterns also made after heating the cell up to the isotropic phase and show the liquid-like association of molecules within a layer in cooling it down to SX3 .The results are shown in Fig. 2(b). the highest temperature SX1 , whereas there is a two-dimen- While the wavelength of the selective reflection peak falls into sionally-ordered packing of molecules in SX2 and SX3 . a range of about 4 nm, the absolute value and the sign of the Figs. 1(a) and (b) show optical micrographs observed for dichroic ratio are randomly distributed around zero.These the SX1 phase of 18. Very strikingly, a fringe pattern character- facts indicate a delicate unbalance in the existence probability istic to a helical structure is observed [Fig. 1(a)]. By applying of two domains of right- and left-handed helices within the an electric field of 50 V, the fringe pattern immediately viewing spot; it is noteworthy that a significant unbalance can disappears to give a fan-shaped texture [Fig. 1(b)].As already be produced by the addition of chiral dopants. Thus, we can reported, the SX1 phase shows ferroelectricity with a spon- safely conclude that a helical structure is spontaneously formed taneous polarization of ca. 60 nC/cm-2.3 This ferroelectricity and its generation is determined by chance development, with nearly equal probability, of right- or left-handed helix.It is interesting that the helical structure is formed along the dierent axes between the SX1 and SX3 phases. In SX1 , the fringe patterns are superimposed on the fan-shaped texture. Hence, the molecules and the polarizations precess along the layer normal [see Fig. 3(b)]. On the other hand, SX3 exhibits Fig. 2 (a) Spectra of scattered light measured for the blue SX3 phase of Fig. 3 Schematic illustrations of the molecular packing and the smectic layer structures. (a) Banana-shaped molecules stack with the same 18 using right- and left-circularly polarized light. A detector was placed in front of the sample surface (see inset) so that the specular reflection bend direction to form a layer.The spontaneous polarization appears along the bend direction parallel to the layer. (b) The helical structure of the obliquely incident light was avoided. (b) Peak wavelength and dichroic ratio, (IR-IL)/(IR+IL), obtained for repeated measurements. in the SX1 phase is formed along the layer normal, as in the SC* phase, while (c) the helix axis in the SX3 phase is parallel to the layer, similar Here IR (IL) stands for scattered light intensity of right- (left-)circularly polarized light.to the TGB phase. 1308 J. Mater. Chem., 1997, 7(8), 1307–1309a blue reflection colour when it is prepared from the homo- considered to arise as a two-dimensional escape from the macroscopic polarization.9,10 At present, we have no decisive geneously aligned SX1 , but no colour can be recognized when it is generated from the homeotropically aligned SX1 .This data to select one of above reasons. In conclusion, two types of helical structure are observed in means that the helical twisting of molecules takes place along a layer [see Fig. 3(c)], but not along the layer normal. The two smectic phases consisting of banana-shaped achiral molecules.The local optical resolution occurs to form spatially same result was obtained from the X-ray observations, as will be reported soon. This specific axis of helix along the layer is resolved helices of both handednesses. This is the first observation of spontaneous generation of helical structures in achiral similar to that observed in the twisted grain boundary (TGB) phase.6 The TGBA (TGBC) phase appears above the SA (SC*) liquid crystalline systems.phase. In this context, the SX3 phase definitely diers from the normal TGB phase. References Finally we considered the origin of the helices. Two reasons can be inferred. One is due to the conformational chirality. 1 J.W. Goodby, J. Mater. Chem., 1991, 1, 307. 2 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe, This is based on the ground-state conformation in which the Mol. Cryst. L iq. Cryst., in the press. planes of the two aromatic rings joined by an ester (or amide) 3 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe, linkage are twisted.7,8 If the conformational ground state is J.Mater. Chem., 1996, 6, 1231.maintained in the liquid crystal state for any reason, the two 4 R. B. Meyer,Mol. Cryst. L iq. Cryst., 1977, 40, 33. benzylideneaniline groups in the mesogenic part may be twisted 5 P. P. Crooker, L iq. Cryst., 1989, 5, 751. 6 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and to each other like propera (see the chemical structure of 1n). J. S. Patel, Nature, 1989, 337, 449.Two kinds of propera-like molecules with right- and left- 7 P. Coulter and A. H. Windle, Macromolecules, 1989, 22, 1129. handed twist are expected to be generated, and if the molecules 8 D. Casarini, L. Lunazzi, E. Pasquali, F. Gasparrini and C. Villani, are segregated with like twist sense and stack to form a layer, J. Am. Chem. Soc., 1992, 114, 6521. a twisting power arises in each segregated domain. As a second 9 A. G. Khachaturyan, J. Phys. Chem. Solids, 1975, 36, 1055. possibility, the twisting power can be induced from the dipole- 10 A. Yoshimori, J. Phys. Soc. Jpn., 1959, 14, 807. dipole interaction.9 Particularly in this ferroelectric system, the interaction may be significant and the helical twisting is Communication 7/02026K; Received 24thMarch, 1997 J. Mater. Chem., 1997, 7(8), 1307–1309 1309

ISSN:0959-9428    

DOI:10.1039/a702026k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATION New organic metals based on a BEDT-TTF derivative with steric hindrance Jun-ichi Yamada,*a† Satoru Tanaka,a Hiroyuki Anzai,a Tatsuo Sato,b Hiroyuki Nishikawa,b Isao Ikemotob and Koichi Kikuchi*b‡ aDepartment of Material Science, Faculty of Science, Himeji Institute of T echnology, 1479-1 Kanaji, Kamigori-cho, Akogun, Hyogo 678-12, Japan bDepartment of Chemistry, Faculty of Science, T okyo Metropolitan University, Hachioji, T okyo 192-03, Japan Table 1 Conducting behaviour of the DOET salts The conducting behaviour of radical cation salts based on the anion solvent D5Aa srt/S cm-1b dioxane-fused BEDT-TTF derivative is reported, two of which show metallic conducting properties.Cl2Br- TCEc 351 2.9d (Ea=89 meV) Br2Cl- TCE 451 2.4d (Ea=83 meV) Br3- TCE 351 <10-6d Br2I- TCE 553 <10-6d I2Br- TCE 553 1.2d (Ea=30 meV) I3- THF 151 2.2e (Ea=37 meV) For the electrical conductivity of one-dimensional organic Au(CN)2- 5% EtOH–TCE 251 13e(TMIf=30 K) metals composed of p-electron donor and acceptor molecules, AuCl2- TCE 151 12d(Ea=76 meV) it is well known that intermolecular donor–donor interactions AuI2- TCE 151 0.033d (Ea=220 meV) (face-to-face interactions) in the segregating donor stack pro- BF4- TCE 251 27e(TMI=100 K) vide the pathway for electron conduction.1 Therefore, one ClO4- TCE —g 1.0d (Ea=79 meV) important aspect of the hitherto known molecular designs for ReO4- TCE 351 0.43d (Ea=110 meV) p-electron donors is an attempt to reduce the steric hindrance, PF6- TCE 251 0.95d (Ea=110 meV) which would prevent face-to-face interactions, on a donor AsF6- TCE 251 0.35d (Ea=120 meV) molecule.2 On the other hand, we have already reported the SbF6- 5% EtOH–PhCl 251 0.63e (Ea=23 meV) synthesis of a BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] derivative condensed with a 1,4-dioxane ring by cis fusion aDetermined by elemental analysis.bRoom temperature conductivity measured by a four-probe technique.c1,1,2-Trichloroethane. dMeasured [(1,4-dioxanediyl-2,3-dithio)ethylenedithiotetrathiafulvalene, on a compressed pellet. eMeasured on a single crystal. fTemperature DOET].3 If such a BEDT-TTF derivative with steric hindrance of metal–semiconductor transition. gNot determined because this could give metallic radical cation salts, further extension of the complex may explode during analysis.molecular designs for the development of p-electron donors leading to new organic metals would be possible.4 In this paper, we disclose the conducting behaviour of DOET-based organic superconductor b-(BEDT-TTF)2I3 (Fig. 1). The radical cation salts and the crystal structure of (DOET)2BF4. DOET molecules are stacked face-to-face to form a column along the [110] direction. In the b-structure, molecular dimerization is usually observed in a stack.In the case of (DOET)2BF4, the dimerization is rather smaller than that in b-(BEDT-TTF)2I3 [the overlap ratios of p1 to p2 are 1.6 and 2.1 in (DOET)2BF4 and b-(BEDT-TTF)2I3, respectively], and interestingly, the bulky dioxane ring exists within a hollow As reported previously,3 the room temperature conductivity † Crystal data for (DOET)2BF4: (C12H10O2S8)2BF4, M=968.19, of the TCNQ complex of DOET was <10-6 S cm-1 for a triclinic, space group P1� , a=6.630(4) A ° , b=8.995(3) A ° , c=16.638(6) A ° , compressed pellet.So, in order to explore the metallic radical a=91.96(3)°, b=96.14(4)°, c=111.17(3)°, V=917.2(7) A ° 3, Z=1, Dc= 1.782 g cm-3, m=9.589 cm-1, F(000)=493.The data were collected cation salts derived from DOET, the preparation of the DOET on a Mac Science MXC18 diractometer equipped with graphite salts with suitable anions was examined by electrochemical monochromated Mo-Ka (l=0.71073 A ° ) radiation using the v–2h scan oxidation with a controlled current5 in 1,1,2-trichloroethane technique to a maximum 2h of 60°.Data were corrected for Lorentz (TCE), THF, 5% EtOH–TCE, or 5% EtOH–PhCl containing and polarization eects. The structure was solved by direct methods the corresponding tetra-n-butylammonium salt. The con- and refined by full-matrix least-squares analysis (anisotropic for C, O ducting behaviour of the resulting DOET salts is summarized and S atoms and isotropic for B and F atoms) to R=0.058 and Rw= 0.062 with GOF=1.46 for 2763 observed [I2s(I)] reflections from in Table 1.In a series of the DOET salts with linear anions, 4203 independent reflections (Rint=0.063). After the refinement of the (DOET)2Au(CN)2 showed high room temperature conduc- DOET molecule, three large peaks remained near the position (0.5, tivity (13 S cm-1) for a single crystal, and the temperature 0.5, 0) in the dierence Fourier map, so BF4 anions were considered dependence of its resistivity revealed that this salt was metallic to be orientationally disordered and these peaks were assigned to F down to about 30 K.In addition, the salt with BF4- exhibited atoms with 2/3 occupancy. The maximum and minimum peaks in the metallic conductivity with a metal to semiconductor transition.final dierence Fourier map corresponded to 0.58 and -0.51 e A ° -3. All calculations were performed using CRYSTAN (MacScience, Japan). The X-ray analysis of (DOET)2BF4† shows that the mode Atomic coordinates, bond lengths and angles, and thermal parameters of the molecular overlap is very similar to that in a famous have been deposited at the Cambridge Crystallographic Data Centre (CCDC).See Information for Authors, Issue 1. Any request to the CCDC for this material should quote the full literature citation and † E-mail: yamada@sci.himeji-tech.ac.jp ‡ E-mail: kikuchi-koichi@c.metro-u.ac.jp the reference number 1145/39. J. Mater. Chem., 1997, 7(8), 1311–1312 1311Fig. 2 The energy band structure and the Fermi surface of (DOET)2BF4 In conclusion, we found two kinds of new organic metals based on a p-electron donor with steric hindrance which is anomalous from the viewpoint of the ordinary molecular design.This finding may be not only useful for new molecular designs of p-electron donors, but also contributable to structural factors enhancing the dimensionality of the conduction in molecular-based organic metals.Investigations on X-ray analysis of the other DOET salts and preparation of radical cation salts using the DOET analogues as p-electron donors are in progress. We would like to thank Professor T. Mori of Tokyo Institute of Technology for giving us his program and fruitful suggestions for the calculation of the band structure of (DOET)2BF4. References 1 For examples, see: Synthesis and Properties of L ow-dimensional Materials, Special Issue of Ann.N. Y. Acad. Sci., ed. J. S. Miller and A. J. Epstein, 1978, 313; Highly Conducting One-Dimensional Solids, ed. J. T. Devreese, R. P. Evrard and V. E. van Doren, Plenum Press, New York, 1979; A. E. Underhill, J. Mater. Chem., 1992, 2, 1; Fig. 1 Donor arrangement of (DOET)2BF4. (a) Intermolecular S,S J.M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, contacts (<3.70 A ° ) are indicated by dotted lines: d1=3.543(3) A ° , d2= H. H. Wang, A. M. Kini and M-H. Whangbo, Organic 3.695(3) A ° , d3=3.486(3) A ° , d4=3.455(3) A ° , d5=3.654(3) A ° . Superconductors (Including Fullerenes), Prentice Hall, Englewood (b) Intermolecular overlap integrals (×10-3) a, p1, p2, q1 and q2 are Clis, NJ, 1992.-4.66, 23.81, 15.17, 7.05 and 5.34, respectively. 2 J. S. Miller, Ann. N. Y. Acad. Sci., 1978, 313, 25. 3 J. Yamada, Y. Nishimoto, S. Tanaka, R. Nakanishi, K. Hagiya and H. Anzai, T etrahedron L ett., 1995, 36, 9509. Our X-ray diraction space of the donor stack. There are many S,S contacts study of DOET has confirmed that the dioxane ring was condensed between stacks shorter than the van derWaals distance (3.70 A ° ) by cis fusion.Detailed data will be reported in a full paper. instead of no short S,S intermolecular contact within a stack. 4 For the bis(dioxane)- and bis(oxathiane)-fused BEDT-TTF deriva- Therefore, the anisotropy of the interaction is small in the ab tives, see: A. M. Kini, U. Geiser, H. H. Wang, R. Lykke, J. M. Williams and C. F. Campana, J. Mater. Chem., 1995, 5, 1647, which plane and the conduction band is 3/4 filled, leading to its 2D reported the crystal structure of the I3- salt based on the bis(diox- band structure with a nearly isotropic closed Fermi surface ane)-fused BEDT-TTF derivative; J. Hellberg, K. Balodis, M. Moge, (see Fig. 2). This band structure suggests metallic behaviour P. Korall and J-U. von Schu�tz, J.Mater. Chem., 1997, 7, 31, in which down to low temperatures, but this salt becomes a semicon- the synthesis of a bis(oxathiane)-fused BEDT-TTF derivative was ductor around 100 K. According to the X-ray analysis, the described. orientation of BF4 anions is completely disordered, so this 5 H. Anzai, J. M. Delrieu, S. Takasaki, S. Nakatsuji and J. Yamada, J. Cryst. Growth, 1995, 154, 145. random potential of BF4 anion layers might disturb the electron conduction and make this salt a semiconductor at Communication 7/02531I; Received 14th April, 1997 low temperature. 1312 J. Mater. Chem., 1997, 7(8), 1311–13

ISSN:0959-9428    

DOI:10.1039/a702531i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Synthesis and characterization of an extended tetrathiafulvalene derivative coordinated to copper iodide and its charge-transfer and radical ion salts J. Ramos,a V. M. Yartsev,a,b S. Golhen,c L. Ouahabc and P. Delhae ` sa* aCentre de Recherche Paul Pascal, CNRS et Universite � Bordeaux I, Av. Albert Schweitzer, 33600 Pessac, France bCentro de Fý�sica, IVIC, Apartado 21827, Caracas 1020-A, Venezuela cL aboratoire de Chimie du Solide et Inorganique Mole � culaire, URA 1495 CNRS, Universite � de Rennes I, 35042 Rennes Cedex, France A new bisfunctional TTF type donor molecule (D–s–D) with a copper iodine bridge has been synthesized and characterized.Its salts have been prepared using standard crystallization techniques. The structural, electrical, optical and low-temperature magnetic properties of these new compounds have been investigated.It has been shown in particular that the compound (D–s–D)(TCNQ)3 presents a low-temperature magnetic ground state which is connected to the antiferromagnetic interactions of the bis-radical cation. This result opens the way to prepare new extended-hybrid TTF molecules containing paramagnetic metals showing related magnetic and electrical properties.Intensive work on extended tetrathiafulvalene (TTF) type Chemical preparation and structural donors in recent years (see ref. 1 and 2) has resulted in the characterization synthesis of conducting salts with 151 stoichiometry. This is Synthesis of products the most obvious consequence of an increased molecular size and a corresponding decrease of the Coulomb correlation To prepare the unsymmetrical compound 1, the first step is energy between two electrons occupying the same MO with the synthesis of the precursor thiones following a known opposite spins.Extended p-electron conjugated bis-fused3 and method.13 Then one has to couple them through the usual tris-fused4 tetrathiafulvalenes as well as non-conjugated sys- procedure with trimethylphosphite, followed by chromatogra- tems (D–s–D type) are found to exist; both types of system phy on a silica gel column to separate the symmetrical and are potential donors for organic conductors and ferromagnets unsymmetrical products. Compound 1 (unsymmetrical) is as well as being multiredox systems.1 In radical ion salts, obtained in a total yield of ca. 5%. extended donors usually form segregated stacks along one In the next step 1 is dissolved in dry warm acetonitrile, then crystallographic axis but the presence of many side-by-side mixed with a stoichiometric amount of copper(I) iodide in the short S,S contacts leads to the formation of conducting two- same solvent. The complex precipitated immediately but the dimensional sheets even if some compounds present alternate mixture is left overnight to complete the process.The final stacks.5 Spectroscopic6,7 and magnetic8 studies provide useful product 2 was recrystallized in acetonitrile and orange single information about electronic correlations and also electron– crystals were obtained in a yield of 80%. molecular vibration (EMV) coupling revealed by the appear- The mass spectrometry (FAB technique) has evidenced both ance of so-called vibronic modes: these eects are especially the presence of copper and the organic ligands.The elemental influenced by a modification of the molecular size. The decrease analysis (found: C, 21.11; H, 1.37; I, 21.78; Cu, 10.92; S, 44.38%. of the Coulomb repulsion energy in extended ions results in a Calc.: C, 20.81; H, 1.75; I, 21.99; Cu, 11.01; S, 44.44%) confirms shift of the charge-transfer band to lower energy and more the proposed structure.This compound shows no EPR signal, eective coupling between p electrons and intramolecular indicating the presence of CuI and that 2 is diamagnetic. vibrations. A complementary approach9,10 is based on the idea of X-Ray crystal structure introducing transition metals connected with TTF derivatives The crystal data for a single crystal has been obtained on an as for example in copper iodide coordination polymers.In Enraf-Nonius CAD4 diractometer. The crystal acquisition such a hybrid compound, it is also possible to tune the valence data are summarized in Table 1 and the crystal structure is state of the introduced metal for intervalence electronic transitions. 11,12 Following this track we have been interested in the synthesis of bis(ethylenedithiodimethylthiotetrathiafulvalene copper iodide) 2, the extended donor obtained by connecting two BEDT analogues (ethylenedithiodimethylthiotetrathiafulvalene, EDT-DMT-TTF, 1) by a Cu–I2–Cu bridge (D–s–D type) (Fig. 1). This compound represents an improvement over similar polymeric complexes already described10 since it is soluble and has allowed us to prepare crystalline radical ion salts as 2(BF4) and charge-transfer salts such as 2(TCNQ)3.Here, we report the synthesis, chemical characterization and crystal structure of the new hybrid p donor molecule 2. We study the spectroscopic, electric and low-temperature magnetic properties of the obtained charge-transfer and radical ion salts, and finally we propose some explanations and correlation Fig. 1 Unsymmetrical ligand 1 and complex 2 between them. J. Mater. Chem., 1997, 7(8), 1313–1319 1313Table 1 Crystal data and refinement for compound 2 bond lengths, comparable with those determined in previous work on a similar molecule.10 The most striking feature is the formula C20H20Cu2I2S16 coplanarity of both donor molecules, symmetrically with formula mass 1154.20 respect to the central bridging iodine atoms.The equivalence crystal system monoclinic of both donor units, coupled with the long bridging element, space group C2 a/A° 25.326(13) will aect the properties of 2, which behaves in some respects b/A° 8.7161(11) as two isolated molecules of 1.c/A° 34.868(13) b/degrees 111.30(2) Radical ion salts V /A° 3 7171(5) Z 8 We have carried out cyclic voltammetry on 2 under standard Dc/g cm-3 2.138 conditions, in acetonitrile containing 0.1 M NBun4PF6 crystal dimensions/mm 0.5×0.1×0.1 employing a glassy carbon working electrode, a Ag/AgCl radiation (l/A° ) Mo-Ka(0.710 73) reference electrode and a Pt counter electrode, sweeping temperature 293 K scan method h–2h between -1.5 and +1.2 V at a rate of 100 mV s-1.h range/degrees 1.25–25.97 Two reversible oxidation waves are detected, with ED values range of hkl -31/28, 0/10, 0/42 of 0.51 and 0.77 V. A reduction wave is found at -0.78 V m(Mo-Ka)/mm-1 3.857 which is not reversible owing to some chemical reaction. From collected reflections 7496 these results we attribute the two oxidation waves to the independent reflections (Rint) 7374(0.0335) formation of bis-monocationic and bis-dicationic species; absorption correction y-scan transmission (max., min.) 0.999, 0.91 indeed these values are quite similar to those already found in R indices [I>2s(I)] R1=0.0575, wR2=0.1698 BEDT-TTF.14 The reduction wave could be due to the S, on F2 1.188 reduction of CuI to Cu0 which causes decomposition of the Dr/e A ° -3 (max., min.) 1.167, -1.233 complex.It is interesting that we do not observe any oxidation wave of CuI to CuII between 0.5 and 1.5 V as might be expected for an intervalence charge transfer. shown in Fig. 2. Selected bond distances and angles including We have also prepared salts of 2 by electro-oxidation and those of the copper–iodine–copper bridge between the two direct oxidation in the liquid phase.In the first case we used donor molecules are listed in Table 2.† a classical two-compartment electrochemical cell with Pt elec- The unit cell is based on two independent molecules A and trodes under usual conditions (0.1 M NBun4BF4 or NBun4PF6 B forming dimers as shown in Fig. 2 with slightly dierent in dry CH3CN under an electric current of 1 mA) and obtained black–green single-crystal needles of 2(BF4) and 2(PF6). In the second method we used a U-shaped diusion cell in which 2 and neutral TCNQ are stored in dierent compartments (c#10-3 M). When equimolecular quantities were used there remained an excess of the neutral m2. After three or four weeks we obtained black crystalline fibres of 2(TCNQ)n (with TCNQF4 a blue–black powder was obtained). Crystal structure and stoichiometry of the charge-transfer salts The quality of the single-crystals of both compounds allowed Fig. 2 X-Ray structure of a (D–s–D) molecular crystal us only to obtain the unit-cell parameters. For the BF4 salt, using an Enraf-Nonius CAD4 diractometer, we found a Table 2 Selected bond distances (A ° ) and bond angles (degrees) with standard deviations in parentheses monoclinic structure (P2/c) with the following parameters: a= (3.75 A °)n, b=22.62 A ° , c=11.60 A ° , b=97.60°; the crystal struc- Cu(1)MS(2) 2.398(8) Cu(3)MS(18) 2.392(8) ture of the PF6 salt is isomorphous.Cu(1)MS(1) 2.430(8) Cu(3)MS(17) 2.408(8) Because the TCNQ charge-transfer compound showed many Cu(1)MI(2) 2.571(4) Cu(3)MI(4) 2.576(4) defects, only the Laue photographic technique was employed.Cu(1)MI(1) 2.626(4) Cu(3)MI(3) 2.625(4) This compound crystallizes in the triclinic (P1 or P1�) system Cu(2)MS(9) 2.351(8) Cu(3)MCu(4) 2.734(5) Cu(2)MS(10) 2.371(8) Cu(4)MS(26) 2.351(8) with proposed parameters a20 A ° , b14.8 A ° , c=4.03 A ° , Cu(2)MI(1) 2.586(4) Cu(4)MS(25) 2.360(8) c=80° and V1211 A ° 3 (Z=1).Cu(2)MI(2) 2.635(4) Cu(4)MI(3) 2.590(4) In both compounds we observe one large unit-cell length as Cu(1)MCu(2) 2.732(5) Cu(4)MI(4) 2.633(4) found in the neutral compound (Fig. 1) which indicates the probable location of the long axis of the extended TTF S(2)MCu(1)MS(1) 88.1(3) S(18)MCu(3)MS(17) 88.7(3) molecule.S(9)MCu(2)MS(10) 90.7(3) S(26)MCu(4)MS(25) 91.3(3) I(2)MCu(1)MI(1) 117.1(2) I(4)MCu(3)MI(3) 117.1(2) After experimental measurements of the powder density and I(1)MCu(2)MI(2) 116.2(2) I(3)MCu(4)MI(4) 116.28(14) electronic microprobe analysis (which confirmed the presence S(2)MCu(1)MI(2) 115.6(2) S(18)MCu(3)MI(4) 115.3(2) of N, S, Cu and I) it turns out that the stoichiometries are 151 S(1)MCu(1)MI(2) 109.5(2) S(17)MCu(3)MI(4) 109.3(2) for the BF4 salt and 153 or possibly 152 for the TCNQ S(2)MCu(1)MI(1) 110.4(2) S(18)MCu(3)MI(3) 110.2(2) compound.The formulation 2(TCNQ)3 is proposed to explain S(1)MCu(1)MI(1) 112.7(2) S(17)MCu(3)MI(3) 112.9(2) the magnetic properties discussed below. S(9)MCu(2)MI(1) 115.5(2) S(26)MCu(4)MI(3) 115.0(2) S(10)MCu(2)MI(1) 114.5(2) S(25)MCu(4)MI(3) 113.7(2) S(9)MCu(2)MI(2) 109.5(2) S(26)MCu(4)MI(4) 110.1(2) Results and discussion of physical properties S(10)MCu(2)MI(2) 107.5(2) S(25)MCu(4)MI(4) 107.6(2) Dc conductivity measurements † Atomic coordinates, thermal parameters, and bond lengths and The dc conductivity has been determined on single crystals angles have been deposited at the Cambridge Crystallographic Data using a standard four-probe method with Pt paste. The BF4 Centre (CCDC).See Information for Authors, J. Mater. Chem., 1997, and PF6 salts show semiconducting behaviour with a room- Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/38. temperature absolute conductivity s290#10-4–10-5 S cm-1 1314 J.Mater. Chem., 1997, 7(8), 1313–1319and an activation energy Ea#0.1 eV just below 290 K. The processes arising from other combinations of charge distribution in neighbouring (D–s–D)+, considering each EDT– TCNQ charge-transfer salt is much more conducting and the room-temperature dc conductivity value s#0.2 S cm-1 is DMT–TTF (D) as an individual molecule. The low energy charge-transfer band is strongly coupled to the intermolecular almost constant down to 250 K.Below this temperature a progressive semiconducting regime is observed and an insulat- vibrations (see band attribution in Table 3) and in order to explain the experimental results we use a standard molecular ing phase is observed down to 50 K (Fig. 3). We propose the presence of a metal–insulator transition (TMI#K) as observed dimer model as in the case of complex TCNQ salts.16 The associated complex conductivity has the form: in similar compounds.15 Optical properties s(v)=iv e2R2 4V A 1 x(v)-.a ga2va va2-v2-ivcaB-1 (1) VIS–IR spectra (15000–400 cm-1) have been measured at room temperature with a Nicolet 750 instrument for absorp- where V is the volume per molecule, R denotes the distance tion measurements on compressed KBr pellets and with a between neighbouring molecules and the electronic polariz- Nicolet 740 instrument (6000–400 cm-1) equipped with a ability is Nic-Plan microscope and a SeZn polarizer for reflectivity experiments on single crystals (wirr ca. 100 mm). x(v)= 2M2vCT vCT2-v2-ivC (2) In a preliminary experiment the reflectance spectra were measured from the naturally grown surface of the crystal of a In eqn.(1), va, ga and ca are, respectively, the wavenumber, neutral molecule 2 and it appears to be practically independent the EMV (electron–molecular vibration) coupling constant and of the polarization. The major absorption bands are reported the damping factor for the ath totally symmetric mode of in Table 3 with their assignments.We associate the scale shaped intramolecular vibration.In eqn. (2), C is the phenomenological surface and the observed in-plane isotropy with a two- natural width of the originally uncoupled charge-transfer exci- dimensional organization in agreement with the crystal struc- tation with energy vCT and M is the matrix element for the ture (Fig. 2).charge-transfer transition. The fitting procedure which repro- The absorption spectra of KBr pellets of 2(BF4) and 2(PF6) duces the reflectance spectrum [Fig. 5(a)] has as parameters (Fig. 4) are dominated by a broad charge-transfer band with vCT=1700 cm-1, C=1500 cm-1, M=1, R=4.4 A ° , V=708 A ° 3, a first maximum about 1800 cm-1, a weak peak around e2=3 and three intramolecular vibrations at va=1496, 1468 6000 cm-1 and another peak at 10500 cm-1.The reflectance, and 500 cm-1 and ga=40, 60 and 20 cm-1, respectively. These measured parallel to the needle axis [Fig. 5(a)] shows a strong parameter values correspond well to those found for similar dispersion below 4000 cm-1 attributable to a charge-transfer compounds.6 excitation involving the extended p donor system.In the If we compare the spectra of 2(TCNQ)3 shown in Fig. 5 polarization perpendicular to the needle axis [Fig. 5(b)], we and 6, with those of the simple salt of the same donor, 2(BF4), observe at the same wavenumber a much less pronounced we observe that the charge-transfer band in the complex salt feature which could be associated with a charge transfer along is situated at a higher wavenumber (ca. 3200 cm-1). This the long molecular axis of 2. In the simple salt of a giant means that all antiresonances in the parallel polarization derivative of TTF: D1(ClO4) (D1 consists of a dihydro-TTF [Fig. 6(a)] may be attributed to the excitation of totally core with two conjugated 1,4-dithiafulvalen-6-yl side-arms) the symmetric intramolecular vibrations of TCNQ, which demon- CT band is at 5600 cm-1.In our case, there is no conjugation strates that the charge-transfer excitation takes place along the between D fragments of the D–s–D molecule and in a simple stack composed only of the TCNQ molecules (Table 3). To a salt of D–s–D with BF4 or PF6 the hole is localized on one first approximation, we can assume that 2 is present as a of the fragments D.Therefore, we ascribe the charge-transfer dication and does not contribute to the charge transfer. The band in the case of parallel polarization to transitions shown position of the charge-transfer band at 3200 cm-1 agrees well by arrows in the following scheme, even if intramolecular with that observed for the mixed-valence state stack of TCNQ charge transfer cannot be excluded.stacks15 and is in agreement with the relatively high dc conductivity value of the salt. The stoichiometry suggests a complete charge transfer of two electrons from 2 to a trimer D0–s–D+ U U D+–s–D0 of TCNQ molecules, so the appropriate model is a trimer with two radical electrons.17 In this model there are twallowed Following this scheme, two absorption bands at wave- optical transitions: numbers of 6000 and 10500 are intermolecular charge-transfer TCNQ-+TCNQ0+TCNQ-�TCNQ0 +TCNQ-+TCNQ- TCNQ-+TCNQ-+TCNQ0�TCNQ0 +TCNQ2-+TCNQ0 In our case, the latter transition has much less intensity, and can be disregarded.Therefore, the expression for the complex conductivity takes the form s(v)=iv e2R2 V AvCT2-v2-ivC 2M2vCT -. a ga2va va2-v2-ivcaB-1 (3) where V is the volume per molecular trimer and R is the distance between neighbouring molecules.A fit of this model to the experimental data for (D–s–D)(TCNQ)3 is shown in Fig. 6(a). The CT excitation energy found from this fit vCT= 2500 cm-1, may be used to calculate the transfer integral t: Fig. 3 Temperature dependence of the dc conductivity of a 2(TCNQ)3 single crystal measured along the long side vCT=1.5t (for a typical value of U/4t=1) giving t=0.2 eV.J. Mater. Chem., 1997, 7(8), 1313–1319 1315Table 3 IR absorption bands (n�/cm-1) for 2, 2(BF4) and 2(TCNQ)3 and molecular vibration wavenumbers for bis(ethylenedithio)tetrathiafulvalene (ET) and tetracyanoquinodimethane (TCNQ) molecules and ions. Antiresonance minima are marked * mode ET0a ET+b 20 2(BF4) 2(TCNQ)3 TCNQ0c TCNQ-d mode 2223* 2225 2206 2ag 2189* 1596* 1600 1615 3ag 1557 1522 1506 2ag 1552 1465 1517 1485* 3ag 1494 1427 1492 1441* 1440* 1454 1391 4ag 1416 1414 1388 1366* 5ag 1285 1287 1311 1323* 29b1u 1287 1282 1273 1258 1195* 1206 1196 5ag 1173 1175 1124 1124 1056 1011 1013 6ag 990 979 980 962* 1003 978 6ag 959 922 922 887 881 836* ?b3u 823* ?b3u ??b1u 771 770 721* 713 725 7ag 33b1u 672 687 616* 596 613 8ag 9ag 508 474 475 aM.E. Kozlov, K. I. Pokhodnia and A. A. Yurchenko, Spectrochim. Acta, 1987, 43, 323. bR. Swietlik, C. Garrigou-Lagrange, C. Sourisseau, G. Pages and P. Delhae`s, J. Mater. Chem., 1992, 2, 857. cT. Takenaka, Spectrochim. Acta, 1971, 37, 1535. dR. Bozio, I. Zanon, A. Girlando and C. Pecile, J. Chem. Soc., Faraday T rans., 1978, 74, 235.Fig. 5 Room-temperature reflectance of a 2(BF4) single crystal in Fig. 4 Absorption spectra of 2(PF6) in KBr pellet at room polarization parallel (a) and perpendicular (b) to the needle axis, and temperature theoretical calculations (dotted line) This fit confirms that 2 does not play any role in the chargetransfer processes. At room temperature we have measured the angular dependences of the EPR g-factor and the peak-to-peak linewidth (DH) for both salts and calculated xs, the spin susceptibility, Magnetic properties in comparison with a stable free radical, diphenylpicryl hydrazyl (DPPH) as reference.The compounds have been studied by EPR on single crystals in a Bruker ESP 300 E apparatus, equipped with an Oxford The g-factor and the linewidth values exhibit classical anisotropy, fitting a cosine square law, which allowed us to liquid-helium temperature accessory, in the temperature range 3.2–300 K.The bulk dc susceptibility and the field dependence determine the principal components (Table 4). Two essential observations are found from these room- of the magnetization have been measured in a SQUID magnetometer (Quantum MPMS-5), in the temperature range temperature experiments.(i) The relaxation mechanisms and therefore the linewidth values are a function of the electronic 1.7–300 K, with a magnetic field strength up to 50 kG (5 T). 1316 J. Mater. Chem., 1997, 7(8), 1313–1319Here D corresponds to the radical donor and Q to the TCNQ- (gQ=2.0026). In this case, (D–s–D)2+ is considered as a pair of radical cations, similar to BEDT +. If we assume the same g � -factor we can estimate that less than one quarter of the spin susceptibility is due to the TCNQ entities.Another important point is that the maximum g-factor is not in the same direction for the two types of salt. For the BF4 salt, the maximum is found when the long axis of the crystal is parallel to the magnetic field.However for the TCNQ salt the long axis is perpendicular to the field. That indicates that the structural organisation of the donor molecules is very dierent in both compounds. Temperature dependence for the BF4 salt. The EPR temperature variation of this salt shows some similiarity in behaviour with (EDT–DMT–TTF)2PF6, 12PF6, which shows a constant decrease in xS with temperature, and decreases to 40% of its room temperature value.19 This is similar to the thermal behaviour of 2(BF4) at temperatures >100 K [Fig. 7(a)]. At 100 K, however, there is an abrupt change in the slope of the spin susceptibility, which almost disappears at ca. 50K. We propose that the EPR signal below 50 K is a Curie tail corresponding to a small fraction of localized spins.However, the variation of the linewidth, DH, [Fig. 7(b)] is smooth, with no sudden variation. This type of transition, with a sharp variation in xS but a constant decrease of DH, has been observed in some phases of (BEDT)4[M(CN)4] (M= Ni, Pt)20,21 where the electrical charge in the BEDT moiety is the same as in our compound (r=1/2). The increase in the value of the g-factor is associated with the localization of the charge on the donors, approaching that of the isolated radical cation.Fig. 6 Room-temperature reflectance of a 2(TCNQ)3 single crystal in The behaviour is reminiscent of a spin Peierls type transition, polarization parallel (a) and perpendicular (b) to the needle axis, and theoretical calculations (dotted line). The reflectance has been calcu- posibly associated with a large fluctuation regime.In the lated for two electrons per trimer model with vCT=2500 cm-1, C= absence of low-temperature structural information, we assume 2200 cm-1, M=0.78, R=3.4 A ° , V=1736 A ° 3, e2=3 and seven intramolecular vibrations at va=2210, 1630, 1395, 1180, 958, 718 and 567 cm-1, ga=350, 540, 500, 300, 85, 190 and 50 cm-1, respectively.Table 4 Room-temperature EPR characteristics for the two chargetransfer salts (DMsMD)(BF4) (DMsMD)(TCNQ)3 gmin 2.0011 2.0023 gint 2.0044 2.0047 gmax 2.0084 2.0090 DHmin 66.0 1.43 DHint 68.6 1.95 DHmax 71.4 2.25 xs(300) 9.7×10-5 3.3×10-3 dimensionality.5 In a quasi-one-dimensional electronic system the linewidths are narrower than in a two-dimensional one, in general being about an order of magnitude less.Therefore it seems that the TCNQ complex behaves as a quasi-onedimensional system with a preferential stacking direction. The BF4 salt shows a broad signal, with a DH value close to those of k-phase materials, which are lamellar-like. (ii) g-Factor values are molecular characteristics more or less constant for a given radical ion. For instance, the mean gvalue gRT=2.0072 for TTF + and 2.0063 for BEDT +.5 For 2(BF4) gRT=2.0046, a rather low value which could arise from the large molecular size and a smaller spin–orbit coupling interaction.For the TCNQ complex we find a rather similar value, gRT=2.0053. This EPR signal is attributed to an exchange narrowing eect commonly observed in charge- Fig. 7 (a) Temperature dependence of the normalized spin susceptibil- transfer compounds such as TTF-TCNQ.18 In such a case the ity for a 2(BF4) single crystal.(b) Linewidth and g-factor temperature averaged g � -factor is given as: dependences corresponding to their minimum values for a 2(BF4) single crystal. g � =(g � DcD+g � QxQ)/(xD+xQ) (4) J. Mater. Chem., 1997, 7(8), 1313–1319 1317that when the temperature is lowered the salt evolves from (D–s–D) +2(BF4-)2 to (D–s–D)2+(D–s–D)0(BF4-)2.This hypothesis rationalizes the change of the g-factor towards the value for BEDT + at very low temperature. Temperature dependence for the TCNQ complex. The gfactor shows an increase with a decrease in temperature, and reaches the value for the isolated BEDT + radical cation (Fig. 8).This shows that only the donor moiety is responsible for the magnetic characteristics at low temperatures, and that the contribution of TCNQ - becomes negligible.The linewidth however shows a more notable phenomenon, with a clear maximum at low temperature, corresponding also to a weak maximum in the g-factor. Generally these eects are related to the onset of an interld owing to the appearance of magnetic fluctuations.The EPR spin susceptibility shows a Curie–Weiss behaviour with a Weiss constant (h=5 K) charac- Fig. 10 Temperature and field dependences of the magnetic susceptibil- teristic of antiferromagnetic interactions. ity for bulk polycrystalline 2(TCNQ)3 in the range 1.7–4 K ($, 10 The bulk magnetism, as measured in a SQUID magnet- kG; 1, 20 kG; &, 30 kG; ', 40 kG; +, 50 kG) ometer, confirms these results and gives more information.A plot of xT vs. T after diamagnetic core corrections is shown in Fig. 9. The temperature dependence can be divided into two with a maximum in xT , that corresponds with the EPR observation, and with further decrease in temperature the regimes. At rather high temperature a Curie term is still present magnetic susceptibility diminishes quickly. This is a clear sign of a magnetic phase transition.When we measured the mag- below 150 K which corresponds to a little more than two independent electrons (C=0.750). This indicates that the donor netic susceptibility at dierent field strengths (Fig. 10), we observe a magnetic field dependence which indicates a phase is wholly as the (D–s–D)2+ species, behaving as a biradical.Also, there is a small contribution due to the TCNQ stacks, transition at TC#4 K. It should be remembered that for lowdimensional compounds in an insulating state, the spin with localized spins below the Mott–Hubbard localization (Fig. 3). ordering at low temperature can usually occur with two possible ground states;22 an antiferromagnetic (AF) or a spin– At low temperature (T<50 K) a dierent situation arises Peierls (SP) state.In both cases the 3d ordering is due to significant interchain (or interplane) magnetic interactions with a structural change for a SP transition. Our preliminary data are rather in favour of an AF state below 4 K but we were not able to detect any spin–flop transition up to 50 kG, contrary to what is usually observed in this class of solid.Nevertheless in the precursor regime below 50 K we have evidenced AF interactions between the bis-cation radical of 2 which should not involve the anion radical species as indicated by the g-factor temperature dependence (Fig. 8). This picture is clearly dierent from the behaviour described by Iwasa et al.15 who evidenced an AF ground state for one phase of (BEDT-TTF)(TCNQ) where the TCNQ stacks are responsible for the magnetic phase transition.The picture that emerges is a charge-transfer complex with a degree of ionicity r#2/3, i.e. (D–s–D)2+(TCNQ)32-. On one hand the bis-functional donor 2 behaves as a biradical since the two TTF-type moieties are connected by an inert link and on the Fig. 8 Linewidth and g-factor temperature dependences corresponding other hand one should expect TCNQ trimers with two elec- to their maximum values for a 2(TCNQ)3 single crystal. The error trons to be able to give rise to a singlet ground state at low bars are small and not indicated. temperature. We clearly need more accurate structural information, with better crystals, to advance further explanations of the lowtemperature magnetic properties.It is however possible to speculate that we have a mixed situation where the donor part takes part in AF intermolecular couplings while the supposed TCNQ stacks are involved in a transition associated with the high-temperature electronic localization observed around 250 K. Conclusion A bis-functional TTF type donor containing a transition metal has been synthesized and characterized.This new diamagnetic (D–s–D) molecule is robust and shows two degenerate redox functionalities in solution. We have therefore been able to prepare new charge-transfer salts either by electrocrystallization (BF4 salt) or by diusion (TCNQ complex). These Fig. 9 Plot of xT vs. T for bulk polycrystalline 2(TCNQ)3 between 2 and 150 K compounds have been thoroughly investigated and both 1318 J.Mater. Chem., 1997, 7(8), 1313–13197 H. Tajima, M. Arifuku, T. Ohta, T. Mori, Y. Misaki, T. Yamabe, appear to present specific ground states. In particular, H. Mori and S. Tanaka, Synth. Met., 1995, 71, 1951. (D–s–D)(TCNQ)3 exhibits optical and electrical prop- 8 A. Graja, A. Lapinski, M. Salle� and A.Gorgues, Synth. Met., 1995, erties associated with the TCNQ moieties, whereas the 67, 1903. low-temperature magnetic behaviour is attributed to the 9 X. Gan, M. Munataka, T. Kuroda-Sawa and M. Maekawa, Bull. bis-functional donor. Chem. Soc. Jpn., 1994, 67, 3009. 10 M. Munakata, T. Kuroda Sowa, M. Maekawa, A. Hirota and S. This study illustrates the inter-relation between electrical Kitagawa, Inorg.Chem., 1995, 34, 2705. and magnetic properties. It will be of interest to prepare a 11 R. D. McCullough and J. A. Belot, Chem.Mater., 1994, 6, 1396. hybrid molecule containing a transition metal showing 12 N. Le Narror, N. Robertson, E. Wallace, J. D. Kilburn, A. unpaired spins where intermolecular magnetic exchange and E. Underhill, P. N. Bartllett and N.Webster, J. Chem. Soc., Dalton intramolecular charge transfer of p electrons can occur. T rans., 1996, 823. 13 N. Sventrup and J. Becher, Synthesis, 1995, 215. 14 P. Fre` re, R. Carlier, K. Boubekeur, A. Gorgues, J. Roncali, We thank J. Amiell (CRPP), and Drs. I. Bravic and B. Desbat A. Tallec, M. Jubault and P. Batail, J. Chem. Soc., Chem. Commun., (University of Bordeaux I) for their help in EPR spectroscopy, 1994, 2071.the Laue experiments and IR reflectivity measurements, 15 Y. Iwasa, K. Mizuhashi, K. Toda, Y. Tokura and G. Saito, Phys. respectively. J. R. acknowledges a EU fellowship associated Rev. B, 1994, 49, 3580. with Network contract ERBCHRXCT930271. 16 M. J. Rice, Solid State Commun., 1979, 31, 93; M. J. Rice, V. M. Yarsev and C. S. Jacobsen, Phys. Rev. B, 1980, 21, 3437. 17 V. M. Yartsev, Phys. Status Solidi (B), 1982, 112, 279. 18 Y. Tomkiewicz, A. R. Taranko and E. M. Engler, Phys. Rev. L ett., References 1976, 37, 1705. 1 M. Adam and K. Mu�llen, Adv.Mater., 1994, 6, 439. 19 A. Otsuka, G. Saito, T. Sugano, M. Kinoshita and K. Honda, T hin 2 T. Otsubo, Y. Aso and K. Takimiya, Adv. Mater., 1996, 8, 203. Solid Films, 1989, 179, 259. 3 Y. Misaki, H. Nishikawa, T. Yamabe, T. Mori, H. Inokuchi, 20 M. Tanaka, H. Takeuchi, M. Sano, T. Enoki, K. Suzuki and H. Mori and S. Tanaka, Chem. L ett., 1993, 729. K. Imaeda, Bull. Chem. Soc. Jpn., 1989, 62, 1432. 21 C. Garrigou-Lagrange, J. Amiell, E. Dupart, P. Delhaes, 4 H. Nishikawa, S. Kawauchi, Y. Misaki and T. Yamabe, Chem. L. Ouahab, M. Fettouhi, S. Triki and D. Grandjean, Synth. Met., L ett., 1996, 43. 1991, 41–43, 2053. 5 J.M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geisen, 22 C. Coulon, in Organic and inorganic L ow-dimensional Crystalline H. H. Wang, A. M. Kini and M.-H. Whangbo, Organic Materials, ed. P. Delhae`s and M. Drillon, NATO AISI series 1987, Superconductors (including Fullerenes), Prentice Hall, Englewood vol. 168B, p. 201. Clis, 1992. 6 A. Graja, V. M. Yartsev, C. Garrigou-Lagrange, M. Salle� and A. Gorgues, Phys. Status. Solidi (B), 1992, 174, 119. Paper 6/08575J; Received 23rd December, 1996 J. Mater. Chem., 1997, 7(8), 1313–131

ISSN:0959-9428    

DOI:10.1039/a608575j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Synthesis of bis(ether nitrile)s and bis(ether acid)s from simple aromatic diols by meta-fluoro displacement from 3-fluorobenzonitrile† Georey C. Eastmond* and Jerzy Paprotny Donnan L aboratories, Department of Chemistry, University of L iverpool, PO Box 147, L iverpool, UK L 69 3BX There is interest in synthesizing bis(ether acid)s and related compounds to use as monomers in order to prepare processable aromatic polymers, such as poly(ether amide)s and poly(ether ester)s.We have now found that it is possible to prepare eciently bis(3-cyanophenoxy)phenylenes by fluoro displacement from 3-fluorobenzonitrile and phenylene diols or their derivatives, such as substituted catechols, and to convert those bis(ether nitrile)s to bis(3-carboxyphenoxy)phenylenes which can be used in the synthesis of poly(ether amide)s and poly(ether ester)s.The meta-fluoro displacement is performed at elevated temperatures in N-methylpyrrolidinone. Following the development of processable poly(ether imide)s the same substitution pattern, there are nine possible isomers: ppp(X)2, pmp(X)2, pop(X)2, mpm(X)2, opo(X)2, etc. I,1 we recently demonstrated that major enhancement in processability can be achieved by modifying the substitution Where substitution patterns of the terminal rings are para or ortho, formation of the ether linkages by aromatic nucleo- pattern of the aromatic unit Ar between the ether linkages in I, especially by inclusion of 1,2-linked units derived from philic displacement (SNAr) reactions between a diol (to form the central ring) and a suitably activated nitro- or halo-benzene catechol or substituted catechols.2 It is logical to extend these concepts to the synthesis of poly(ether amide)s and poly(ether (Scheme 1) is easily achieved; Evers et al.5 used chloro displacement reactions while we used fluoro displacement.3 In these ester)s to achieve similar improvements in processability over conventional aromatic polyamides.3 reactions an activating group X may be a precursor of a desired X in structures II.Thus, MCN or MNO2 groups are activating groups and are also precursors to MCOOH and MNH2, respectively. The leaving group Y is preferably NO2 or F and is strongly activated if it is ortho- or para- to X. In this way we have prepared a variety of popX2 and oooX2 bis(ether acid)s and bis(ether amine)s from catechol and substituted catechols and have used these in the synthesis of poly(ether amide)s and poly(ether ester)s.3 These syntheses are relatively trivial. pop(CN)2 and pop(COOH)2 have also recently been prepared by others and used in the synthesis of poly(ether amide)s.7,8 A problem has been to prepare such materials in which the substitution pattern in the outer rings is meta, e.g. as mpmX2, mmmX2 or momX2, eciently because, according to the rules of SNAr reactions, if Y is meta to X it is not suciently activated to leave.Evers et al. recognised this problem and synthesized mpm(COOH)2 by a two-stage process, reacting the sodium salt of meta-cresol with dibromobenzenes in pyridine To achieve this end, poly(ether amide)s can be prepared in the presence of copper(I ) chloride and oxidizing the resulting from either/or both bis(ether acid)s or bis(ether amine)s of dialkyl species with potassium permanganate in pyridine to structure II, with dierent substitution patterns at each aro- obtain the diacid in 38% overall yield.5 Synthesis of the matic residue.Thus, the synthesis of substances with structure mpm(CN)2 was a five-stage synthesis with a yield of 8.5%.II is of interest. A number of bis(ether nitrile)s IIa, used in We have now demonstrated that, when X is MCN, meta- poly(benzoxazole) synthesis,4 bis(ether acid)s IIb, a bis(ether fluoro displacement is easily achieved in high yield according acid chloride) IIc5 and a bis(ether amine) IIc6 have long been to Scheme 2 at elevated temperatures when fluorobenzonitrile known. Evers et al.5 used an aromatic nucleophilic displace- III is reacted, under suitable conditions, with a diol IV ment reaction between p-chlorobenzonitrile and various diols (Table 1).Thus, mom(CN)2-type bis(ether nitrile)s become to prepare bis(ether nitrile)s directly, some of which were immediately available and, after hydrolysis, bis(ether acid)s of hydrolysed to the bis(ether acid)s. Thus, Evers et al.prepared bis(ether nitrile)s and bis(ether acid)s and devised a nomenclature based on the substitution patterns of the rings, e.g. ppp(COOH)2, pop(COOH)2.5 We have adapted this system to allow for substituents on the rings, thus we have, for example, p(3Me)op(COOH)2 for the diacid in which the terminal rings are para-linked and unsubstituted while the central ring is ortho-linked and carries a 3-methyl substituent.For unsubstituted species II which are symmetrical, i.e. the outer rings have † Presented in part at ‘Polycondensation ’96’, International Symposium Scheme 1 on Polycondensation, Paris, 1996. J. Mater. Chem., 1997, 7(8), 1321–1326 1321Scheme 2 Table 1 Diols used in fluoro displacement reactions with mnemonics Synthesis of 1,2-bis(3-cyanophenoxy)benzene [mom(COOH)2] used in bis(ether nitrile) and bis(ether acid) nomenclature Procedure A.Catechol (>99%) (44 g, 0.4 mol) and 500 ml N-methylpyrrolidone (NMP) were placed in a flask equipped diol code mnemonic with magnetic stirrer bar, thermometer, nitrogen inlet, Dean Stark trap and reflux condenser.The mixture was thoroughly deoxygenated with a stream of nitrogen. Anhydrous potassium carbonate (100 g) and xylene (100 ml) were added. Under a stream of nitrogen, the mixture was brought to the boil and dried azeotropically. 3-Fluorobenzonitrile (100 g) was added and reflux continued for 6 h with a flask temperature of 178–184°C. Then, over a period of 2 h, most of the xylene was distilled o while the temperature rose to 204°C.The dark brown mixture was precipitated into 4 l of ice–water mixture, the solid was filtered o and washed with deionized water until neutral. The brown crystalline mass was recrystallized from 1.5 l of methanol and 200 ml of water to yield 109 g (crude yield 87.3%) of brown crystals. The crude dinitrile was treated with Norit decolourizing agent and recrystallized from 1 l of methanol and 80–100 ml of water three times.The final yield of white crystals was 83 g (66.5% theoretical), mp 99–100°C, with a further 6.4 g of slightly coloured crystals; details are recorded in Table 2. Procedure B. This procedure was similar to Procedure A except that toluene was used to remove water azeotropically; flask temperature was 140°C.The product was isolated as in Procedure A and purified by two recrystallizations with decolourizing charcoal from methanol–water (551); results are presented in Table 2. Procedure C. This procedure was identical to Procedure B the type mom(COOH)2 etc. and, subsequently, bis(ether acid except that dimethylformamide (DMF) was used as solvent; chloride)s, including their derivatives from substituted cat- the reaction temperature was 120°C and results are presented echols, are also available (Scheme 2).Bis(ether nitrile)s can be in Table 2. used directly in the synthesis of poly(benzobisoxazole)s;4 bis(ether acid)s can be used in the synthesis of poly(ether amide)s and poly(ether ester)s. We report here the synthesis of Preparation of 1,2-bis(3-carboxyphenoxy)benzene [mom the bis(ether nitrile)s and bis(ether acid)s from catechol, and (COOH)2]. Bis(ether nitrile) mom(CN)2 (80 g) was placed in 2,3-dihydroxynaphthalene, the bis(ether nitrile) from 3,5-di- a round-bottomed flask with a solution of 80 g of potassium tert-butylcatechol and other related materials.Diols VIa–f hydroxide in 130 ml of water, followed by 100 ml of methanol.used are identified in Table 1 where their mnemonics used in The mixture was refluxed for 10 h, after which evolution of the above nomenclature are defined. Polymers were prepared ammonia could not be detected. The mixture was diluted to from some of the monomers to demonstrate their use in 2.5 l and acidified to pH 1.5. The product was filtered o and polymer synthesis.dispersed in 2.5 l of deionized water, warmed to 80–90°C, filtered and washed three times with deionized water. The wet Experimental cake was dissolved in 2 l of acetic acid and tetrahydrofuran (THF) (451). Other diacids were prepared similarly from the Materials corresponding bis(ether nitrile)s, and data are given in Table 3; the discrepancy between melting points of mmm(COOH)2 3-Fluorobenzonitrile was obtained from Fluorochem.Other found in this work and ref. 4 is unexplained but our analytical, reagents were obtained from Aldrich Chemical Company and NMRand melting point data were confirmed from independent were used without further purification (catechol was recrystallized from toluene) or were general laboratory reagents. syntheses by two workers. 1322 J. Mater. Chem., 1997, 7(8), 1321–1326Table 2 Synthesis of bis(ether nitrile)s elemental analysis (%) melting yield designationa procedure C H N point/°C (%) mom(CN)2 calc. 76.91 3.87 8.97 A found 99–100 87 (crude) B 77.1 3.81 8.93 99.2–99.7 70 C 20 mpm(CN)2 calc. 76.91 3.87 8.97 143.4–143.7 78 A found 76.91 3.87 8.90 139.5–141b mmm(CN)2 calc. 76.91 3.87 8.97 90.2–90.6 49 A found 76.76 3.84 8.89 81–82c m(2,3N)om(CN)2 calc. 79.54 3.89 7.73 157–158 51 B found 79.77 3.88 7.71 m(3,5dtB)om(CN)2 calc. 79.21 6.64 6.60 118–119 52 B found 79.38 6.67 6.58 pop(CN)2 C calc. 76.91 3.87 8.97 116.8–117.3 96 found 76.90 3.84 8.94 116.5–117.5b 117–118d 116.5–118e ooo(CN)2 C calc. 76.91 3.87 8.97 105.4–105.8 85 found 77.07 3.85 9.02 aSystematic names: mom(CN)2=1,2-bis(3-cyanophenoxy)benzene; mpm(CN)2=1,4-bis(3-cyanophenoxy)benzene; mmm(CN)2=1,3-bis(3-cyanophenoxy) benzene; m(2,3N)om(CN)2=2,3-bis(3-cyanophenoxy)naphthalene; m(3,5dtB)om(CN)2=1,2-bis(3-cyanophenoxy)-3,5-di-tert-butylbenzene; pop(CN)2=1,2-bis(4-cyanophenoxy)benzene; ooo(CN)2=1,2-bis(2-cyanophenoxy)benzene. bRef. 5. cRef. 9. dRef. 7. eRef. 8. Table 3 Synthesis of bis(ether acid)s and a bis(ether acid chloride) elemental analysis (%) melting yield designation C H Cl point/°C (%) mom(COOH)2a calc. 68.57 4.03 265–267 99 found 68.49 3.99 mpm(COOH)2 calc. 68.57 4.03 318.9–319.2 found 68.57 4.03 305–313b 70 mmm(COOH)2 calc. 68.57 4.03 205.5–206.1 found 68.40 4.04 269–274b 80 m(2,3N)om(COOH)2 calc. 72.00 4.03 260–262 89 found 72.14 3.98 pop(COOH)2 calc. 68.57 4.03 257–258 84 found 68.57 4.02 257–258c 255–256d ooo(COOH)2 calc. 68.57 4.03 184–185 90 found 68.60 3.97 mom(COCl)2 calc. 62.03 3.12 18.31 74–76 63 found 61.89 3.12 17.47 aSystematic names: mom(COOH)2=1,2-bis(3-carboxyphenoxy)benzene; mpm(COOH)2=1,4-bis(3-carboxyphenoxy)benzene; mmm(COOH)2= 1,3-bis(3-carboxyphenoxy)benzene; m(2,3N)om(COOH)2=2,3-bis(3-carboxyphenoxy)naphthalene; pop(COOH)2=1,2-bis(4-carboxyphenoxy) benzene; ooo(COOH)2=1,2-bis(2-carboxyphenoxy)benzene; mom(COCl)2=1,2-bis[3-(chlorocarbonyl)phenoxy]benzene.bRef. 5. cRef. 7. dRef. 8. Syntheses of 1,2-bis(4-cyanophenoxy)benzene [pop(CN)2], Preparation of polymers by the phosphorylation technique. Anhydrous calcium chloride (0.6 g) and lithium chloride (0.2 g) 1,2-bis(2-cyanophenoxy)benzene [ooo(CN)2] and their corresponding diacids pop(COOH)2 and ooo(COOH)2.Fluoro dis- were dissolved in anhydrous NMP (13 ml), under a nitrogen atmosphere. Anhydrous pyridine (4 ml) was added, followed placements from p-fluorobenzonitrile and o-fluorobenzonitrile were performed with catechol according to procedure C to by 1,2-bis(3-carboxyphenoxy)benzene (0.70 g, 2 mmol) and para-phenylenediamine (PPD; 2 mmol).The mixture was yield pop(CN)2 and ooo(CN)2, respectively. The bis(ether nitrile)s were hydrolysed to pop(COOH)2 and ooo(COOH)2 stirred under nitrogen and triphenyl phosphite (2 ml) was added. The temperature was raised to 105°C for 5 h when as described above for mom(COOH)2. Details are given in ref. 3 and results in Tables 2 and 3. additional triphenyl phosphite (1 ml) and pyridine (1 ml) were added and the mixture was stirred at 110°C.The resulting viscous liquid was poured into 300 ml of methanol–water Preparation of 1,2-bis(3-chlorocarbonylphenoxy)benzene [mom(COCl )2]. 1,2-Bis(3-carboxyphenoxy)benzene (10 mmol) (80:20). The polymer was filtered o and extracted with boiling methanol for 1 h. The yield of polymer was 0.78 g. Other was boiled under reflux, protected with a calcium chloride guard tube, in a nitrogen atmosphere with ca. 20 mmol of poly(ether amide)s were prepared similarly from pop(COOH)2 and m-phenylenediamine (MPD) and from m(2,3N)om thionyl chloride for 2 h. Excess thionyl chloride was distilled o under vacuum which was finally reduced to 0.2–0.5 Torr. (COOH)2 with MPD. The crude acid chloride was dissolved in 50 ml of cyclohexane, about one third of which was then distilled o, and the residual Polymers from 1,2-bis(3-chlorocarbonylphenoxy)benzene [mom(COCl)2] and MPD. MPD (1 mmol) was dissolved in solution left to crystallize.The crude acid chloride was sublimed from a thoroughly degassed melt at ca. 200°C at 0.5 Torr 7 ml of N,N-dimethylacetamide (DMAC) containing 8.7% CaCl2 and 0.7 ml of pyridine. The mixture was cooled to pressure (Table 3).J. Mater. Chem., 1997, 7(8), 1321–1326 1323-14°C, mom(COCl)2 (1 mmol) was added and the mixture converted to mom(COCl)2 by reaction with thionyl chloride (Table 3). was stirred vigorously for 2.5 h during which period it became very viscous. The mixture was kept overnight at ca. 0°C in a Thus, it is now possible to prepare readily a series of monomers, bis(ether nitrile)s, bis(ether acid)s and bis(ether refrigerator, followed by 24 h at room temperature.The polymer was precipitated into MeOH–H2O, twice extracted with acid chloride)s of type II in which the terminal aromatic rings are meta-substituted. These monomers can be, and have been, boiling MeOH and dried. used in the synthesis of poly(ether amide)s or poly(ether ester)s.The synthesis of representative poly(ether amide)s is described Syntheses of poly(ether ester) by transesterification. mom- in the preceding section. (COOH)2 (3.0 mmol), hydroquinone diacetate (3.0 mmol), In contrast, synthesis of equivalent dinitro species IId, pre- dibutylin oxide (0.002 g) and m-terphenyl (3–4 g) were melted cursors to diamines IIe, by fluoro displacement from meta- and stirred under nitrogen.Within 1–1.5 h the temperature fluoronitrobenzene is not eective under comparable con- was raised to 340°C. The reaction was run for 5–8 h, gradually ditions. When the corresponding fluoro displacement reaction raising the temperature to 370–380°C. After cooling to 110°C was performed with catechol under reaction conditions equival- the viscous melt was poured into acetone or into toluene– ent to Procedures A or B only black tars were recovered and methanol (40560).The polymer was filtered o and extracted we failed to extract pure dinitro compound. with boiling acetone or methanol and dried. Yields of polymer It is of interest to consider the eectiveness of the displace- were ca. 80%. Polymers were subjected to preliminary testing ment reaction with meta-activated species. Electron-with- for solubility and for ability to pull fibres from the melt. drawing groups meta to a leaving group are less activating than the same group in an ortho- or para-position. Even so, Instruments meta-nitro and nitrile groups do have some activating propensity. Terrier has commented that, ‘while two nitro groups may 1H and 13C NMR spectra were recorded on a Bruker AMX400 provide sucient activation to allow clean SNAr processes to spectrometer.Accurate molar masses were determined on a occur, single meta-nitro groups are too weakly activated for VG Analytical 7070E mass spectrometer. SNAr processes and there is a high tendency to react by alternative pathways’.10 In addition, it is usual to consider that Results and Discussion the nitrile group is less activating than the nitro group.10 However, in this work, we find that, at elevated temperatures, Synthesis of monomers and precursors a single nitrile group in a meta-position is sucient to activate fluoro displacement and there is no reason to believe that Experiments demonstrated that the ecient synthesis of bis(ether nitrile)s, precursors to bis(ether acid)s, where CN is reaction proceeds by other than a normal SNAr process.meta-Nitro displacement reactions are uncommon but are meta to the ether linkage is very sensitive to reaction conditions. Conditions for fluoro displacement from meta-fluorobenzo- known in, for example, nitro displacement from meta-dinitrobenzene by methoxide or thiophenoxide anion in hexamethyl- nitrile were optimised for the parent mom(CN)2 as Procedure A.These reaction conditions were also applied to the syntheses phosphoramide (HMPA) at room temperature,11 between the same reagents in chlorobenzene (80°C) under phase transfer of mpm(CN)2 and mmm(CN)2 with the results presented in Table 2.conditions12 and by potassium fluoride in HMPA at 180°C.13 Idoux et al. have shown that m-NO2 and m-CN will activate Bis(ether nitrile)s were also prepared from 2,3-dihydroxynaphthalene and 3,5-di-tert-butylcatechol to give m(2,3N) nitro displacement and m-NO2 fluoro displacement by 2,2,2- trifluoroethoxide anion at 25°C.14 Yoshikawa et al. have now om(CN)2 and m(3,5dtB)om(CN)2, respectively; the syntheses of these materials were not optimised, and conditions used demonstrated that it is possible, by reacting m-dinitrobenzene in aprotic solvents with various bisphenols, to achieve m-nitro were Procedure B.Results are presented in Table 2. Data for mom(CN)2 show that success in these reactions, in terms of displacement in the presence of a base such as potassium carbonate at elevated temperatures.They achieved good yields yield and cleanliness of product, is very dependent on the reaction temperature. We previously found, when synthesizing (>90%) with many bisphenols but <60% with hydroquinone and did not report results with other phenylene diols.15 The pop(CN)2 and ooo(CN)2, that cleaner products were obtained by performing the nitrodisplacement reaction in DMF, use of diols without prior formation of the phenoxide anion is important with catechol and its derivatives because of oxidative Procedure C (Table 2); purity of the bis(ether nitrile) intermediates assists in producing pure bis(ether acid)s necessary for side reactions and there is no prior report of catechols being used in m-nitro displacement or m-fluoro displacement synthesis of high molecular mass polymers.However, in the synthesis of mom(CN)2-type species Procedure C was unsatis- reactions. Kirst and Una showed that fluoro displacement between factory; reaction yields were low (ca. 20%) and products black. It was only with diculty that a satisfactory sample of nitrofluorobenzenes and methoxide with a para-nitro group is faster than with a meta-nitro group by a factor of ca. 103 at mom(CN)2 was obtained by Procedure C, despite giving a 96% yield of pure pop(CN)2. Optimum conditions for the 100°C; this factor decreases at higher temperatures.16 Taking values of s- for para-CN and meta-CN as 0.88 and 0.56, synthesis of mom(CN)2 are NMP as solvent with a reflux temperature of ca. 180°C. Thus, conditions required to produce respectively, it is estimated that, at 100°C, para-CN is less reactive than para-NO2 by a factor of ca. 100 and that meta- good yields of pure products from meta-activated species dier significantly from those where ortho- or para-activation is used. CN is less reactive than para-CN by a factor of ca. 80.17 These data still predict that meta-NO2 would be more reactive than It is also necessary to perform the reactions under an atmosphere of nitrogen to minimise side reactions which lead to meta-CN.Despite our observation that meta-CN-activated fluoro displacement is more eective than the meta-NO2- dark products. Nevertheless, it is now clear that pure materials of type mom(CN)2 can readily be achieved (Table 2). activated reaction, there is no obvious reason to invoke a pathway other than normal fluoro displacement.Hydrolyses of all mxm(CN)2 species (x represents any substitution pattern of the central aromatic ring or other aromatic The exact compositions of species with meta-outer rings were confirmed by accurate mass determinations by mass species) are readily achieved and a typical set of reaction conditions is described above.Hydrolysis produces pure spectrometry. The calculated molar mass for mxm(COOH)2 species is 350.07904 and observed masses were 350.07889, bis(ether acid) in high yield (Table 3). Further, to prepare high molecular mass polyamides it is most convenient to react acid 350.07855 and 350.07889 for mpm(COOH)2, mmm(COOH)2 and mom(COOH)2 respectively; the calculated value for chlorides with diamines and, to this end, mom(COOH)2 was 1324 J.Mater. Chem., 1997, 7(8), 1321–1326Table 4 1H NMR parameters for bis(ether acid)s in [2H7]DMF; chemical shifts and coupling constants for designated protons and proton pairs diacid chemical shifts and coupling constants mpm d H(2,3,5,6)7.24;H(2¾,5¾)7.60;H(4¾)7.80;H(6¾)7.35 J/Hz (2¾,4¾)1.50;(2¾,6¾)2.54;(4¾,5¾)7.69;(4¾,6¾)1.07(5¾,6¾)8.24 mmm d H(2)6.86;H(4,6)6.93;H(5)7.50;H(2¾)7.63;H(4¾)7.82;H(5¾)7.60;H(6¾)7.39 J/Hz (2,4)2.35;(4,5)8.21;(2¾,4¾)1.54;(2¾,6¾)2.58;(4¾,5¾)7.70;(4¾,6¾)0.98;(5¾,6¾)8.18 mom d H(3,4,5,6)7.36;H(2¾)7.45;H(4¾)7.73;H(5¾)7.50;H(6¾)7.18 J/Hz (2¾,4¾)1.49;(2¾,6¾)2.67;(4¾,5¾)7.64;(4¾,6¾)1.03;(5¾,6¾)8.22 pop d H(3,4,5,6)7.34;H(2¾)7.01;H(3¾)8.0 J/Hz (2¾,3¾)8.98 ooo d H(3)7.01;H(4,5)7.19;H(6)7.01;H(3¾)7.90;H(4¾)7.25;H(5¾)7.56;H(6¾)7.10 J/Hz (3¾,4¾)7.78;(3¾,5¾)1.48;(4¾,5¾)7.33;(4¾,6¾)0.95;(5¾,6¾)8.22 m(2,3N)om(COOH)2 was 400.09518 compared with an exper- and ooo(COOH)2 were soluble in NMP.An example high molecular mass poly(ether amide) was prepared from imental value of 400.09518; errors were all less than 0.0005. Any alternative pathway might be expected involve isomeriz- mom(COCl)2 and MPD.The low molecular mass poly(ether amide)s prepared from ation and formation of isomers during reaction but no such change of substitution pattern occurs. NMR evidence is quite mom(COOH)2 and PPD or MPD by the phosphorylation technique were fusible and short fibres could be pulled from clear that meta-fluoro displacement with diols gives rise to meta-nitrile ethers.Splitting patterns for mom, mpm, mmm, ppp, the melts. The higher molecular mass poly(ether amide) prepared from mom(COCl)2 and MPD was also fusible and long pmp, pop and ooo bis(ether nitrile)s and bis(ether acid)s are all dierent and entirely consistent with the products of the fibres could (>10 cm) be drawn from the melt. The poly(ether ester) prepared from mom(COOH)2 and hydroquinone diacet- various species being obtained by normal aromatic nucleophilic displacement reactions with retention of substitution patterns. ate by the transesterification procedure described above was also fusible and short, brittle fibres could be drawn from Thus, for all species VIII with meta-substitution in the outer rings, 1H NMR spectra show distinctive spectra with very the melt.similar chemical shifts and coupling constants for those protons (Table 4); in Table 4 primes are used to denote protons in the Conclusions outer aromatic rings, absence of primes refers to protons on the central ring. This pattern is quite dierent from those for We have demonstrated the feasibility of performing ecient para- and ortho-substitution in the outer rings.In addition, meta-displacement reactions between m-fluorobenzonitrile and the splitting pattern for protons on the central rings for phenylene diols, including catechol and its derivatives, to mom(COOH)2 and pop(COOH)2 are identical complex mul- produce bis(ether nitrile)s in NMP at elevated temperatures tiplets characteristic of an AA¾BB¾ system with a small coupling and, from them, bis(ether acid)s which can be readily incorpor- constant; the pattern for ooo(COOH)2 is somewhat dierent ated into poly(ether amide)s or poly(ether ester)s. This develop- and is resolved into two multiplets but still characteristic of ment completes the possibilities of making bis(ether acid)s and an AA¾BB¾ system with a slightly increased coupling constant.related compounds based on structure II with all possible Further, the calculated chemical shifts for all protons in the substitution patterns at all aromatic rings. outer rings and for all carbons in all rings are almost identical with those observed from 1H and 13C NMR.18 Thus, there is The authors wish to thank E. I. du Pont de Nemours and Co no doubt that initial substitution patterns of the rings are Inc. for financial support which enabled this work to be maintained during the SNAr process.undertaken, Professor D. Bethell for useful discussion on nucleophilic substitution reactions and Dr M. Gibas (Silesian Technical University, Gliwice) for help with NMRspectroscopy and calculation of theoretical chemical shifts. References 1 R.O. Johnson and H. S. Burhlis, J. Polym. Sci., Polym. Symp., 1983, Polymer syntheses 70, 129. 2 G. C. Eastmond and J. Paprotny, Polymer, 1994, 35, 5148; One of the most convincing proofs of producing bis(ether G. C. Eastmond and J. Paprotny,Macromolecules, 1996, 29, 1382. acid)s of high purity is the ability to produce high molecular 3 G. C. Eastmond and J. Paprotny, US Pat., filed 1995, WO mass polymers.In order to identify soluble poly(ether amide)s 97/00903, 1997. which might be made to high molecular mass in solution we 4 R. C. Evers, F. E. Arnold and T. E. Helminiak, US Pat. 4,229,566, undertook preliminary syntheses using the phosphorylation 1980. 5 R. C. Evers, F. E. Arnold and T. E. Helminiak, Polym. Prepr., Am. technique.19 For those polymers found to be soluble in conven- Chem.Soc., Polym. Div. Inc., 1990, 21(1), 88;Macromolecules, 1981, tional solvents, example polymers were also synthesized using 14, 925. bis(ether acid chloride). In addition, an example poly(ether 6 T. P. Gannett and H. H. Gibbs, US Pat. 4,576,857, 1986. ester) was prepared by a transesterification procedure, as 7 L.-S. Tan and N. Venkatasubraian, US Pat. 5,514,769, 1996. described above. Thus, polymers were prepared from 8 S.-H. Hsiao and C.-F. Chang, Macromol. Chem. Phys., 1996, 197, ppp(COOH)2, pmp(COOH)2, pop(COOH)2, mpm(COOH)2, 1255. 9 R. K. Bartlett, G. O’Neill, N. G. Savill, S. L. S. Thomas and mmm(COOH)2, mom(COOH)2 and ooo(COOH)2 with PPD W. F.Wall, Brit. Polym. J., 1970, 2, 225. and MPD by the phosphorylation procedure described above. 10 F. Terrier, in Nucleophilic Aromatic Displacement: T he Influence of Poly(ether amide)s prepared from ppp(COOH)2, the Nitro Group, VCH,Weinheim, 1991, p. 10. pmp(COOH)2, mpm(COOH)2 or mmm(COOH)2 with PPD or 11 N. Kornblum, L. Cheng, R. C. Kerber, M. M. Kestner, MPD by the phosphorylation technique were only soluble in B. N. Newton, H. W. Pinnick, R. G. Smith and P. A. Wade, J. Org. concentrated H2SO4 or methanesulfonic acid. Poly(ether Chem., 1976, 41, 1560. 12 F. Montanari, M. Pelosi and F. Rolla, Chem. Ind., 1982, 412. amide)s similarly prepared from pop(COOH)2, mom(COOH)2 J. Mater. Chem., 1997, 7(8), 1321–1326 132513 G. Bartoli, A. La Trofa, F. Naso and P. F. Todesco, J. Chem. Soc., 17 D. Bethell, personal communication. 18 M. Gibas, personal communication. Perkin T rans. 1, 1972, 2671. 14 J. P. Idoux, M. L. Madenwald, B. S. Garcia and D-L. Chu, J. Org. 19 N. Yamazaki, F. Higashi and J. Kawabata, J. Polym. Sci., Polym. Chem. Ed., 1974, 12, 2149; F. Higashi, S.-I. Ogata and Y. Aoki, Chem., 1985, 50, 1876. 15 Y. Yoshikawa, K. Yamaguchi, K. Sugimoto, Y. Tanabe and J. Polym. Sci., Polym. Chem. Ed., 1982, 20, 2081. A. Yamaguchi, Eur. Pat. Appl., A1 0192480, 1986; S. Tamai, A. Yamaguchi and M. Ohta, Polymer, 1996, 37, 3683. Paper 7/01127J; Received 18th February, 1997 16 J. Hirst and S. J. Una, J. Chem. Soc. B, 1971, 2221. 1326 J. Mater. Chem., 1997, 7(8), 1321–1326

ISSN:0959-9428    

DOI:10.1039/a701127j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

High–yield reactive extraction of giant fullerenes from soot Frank Beer, Andreas Gu�gel,* Kai Martin, Joachim Ra�der and Klaus Mu�llen* Max-Planck-Institut fu� r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany By the common Soxhlet extraction with 1,2,4-trichlorobenzene a mere 8 mass % of virgin fullerene soot can be dissolved. The extracted soot was subjected to a reactive extraction with 5-hexadecanamido-1,3-dihydro-2-benzothiophene 2,2-dioxide (4), an ortho-quinodimethane precursor.Through an irreversible Diels–Alder cycloaddition an additional 12 mass %was solubilized. Mass spectrometry, vapour pressure osmometry and elemental analysis indicate that the soluble material consists of multiple adducts of fullerenes C60–C418. Since the discovery of the fullerenes by Kroto et al.1 and the Results and Discussion preparation of macroscopic quantities of C60 by Kra�tschmer All experiments described in the following were performed et al.,2,3 the scientific community has been interested in fullerene with fullerene soot produced by the Kra�tschmer–Human soot.This material is a new kind of amorphous carbon process.2,3 Prior to the experiments the soot was exhaustively consisting of globular and irregularly shaped carbon structures Soxhlet extracted with either toluene (8.1 mass % soluble as well as stacks of bent and planar carbon sheets with dierent material) or successively with toluene and 1,2,4-trichloro- sizes and curvatures.4 Considerable amounts of C60 and larger benzene (8.4 mass % soluble material).carbon clusters are embedded in this insoluble black fullerene Our first experiments were aimed at the reactive extraction matrix.Consequently, many reports have dealt with the extrac- of soot with modifying agents which form thermally unstable tion of fullerene soot using high-boiling solvents.5–12 With adducts with fullerenes. This approach would allow us to 1,2,4-trichlorobenzene as solvent Diederich et al.isolated fuller- cleave the modifying agent after the extraction and thus enes up to C216.5 Ruo and co-workers reported that up to to obtain unsubstituted fullerenes. Unfortunately, both the 37% of virgin fullerene soot could be extracted using 1,2,4- extraction with anthracene-9-methanol and with 2- trichlorobenzene.6 However, the latter results are not backed trimethylsilyloxybutadiene which are both known to form up by elemental analyses so it is unclear whether the extracted reversible Diels–Alder adducts15 with fullerenes in boiling material consists only of all-carbon molecules.Nevertheless, toluene yielded only negligible amounts of soluble material. these studies5,6,7,12 agree on the distribution of the fullerenes Realizing that the thermal stability of the adducts is obvi- in the extracts, and carbon clusters up to a molecular mass of ously a crucial condition for successful reactive extractions we 2500 have been detected by time of flight mass spectrometry.turned towards ortho-quinodimethanes 2 as modifying agents. Several questions remain to be answered: (i ) How much of ortho-Quinodimethanes can be generated by thermal the fullerene soot is actually soluble? (ii ) What is the fullerene extrusion of sulfur dioxide from 1,3-dihydro-2-benzothiophene content of the virgin fullerene soot? (iii ) Which fullerenes are 2,2-dioxide (1) and its derivatives above 200°C (Scheme 1).most abundant in the soot? (iv) Can the catalytic activity of They are very reactive enophiles, which eagerly form [4+2] extracted fullerene soot (e.g.towards the conversion of methane cycloaddition products 3 with fullerenes giving high yields as into higher hydrocarbons) be traced back to residual (not well.16,17 These adducts are both thermally stable and highly extracted) fullerenes or is it actually a property of the pure soluble in common solvents due to their conformational soot?13 mobility (cyclohexene ring inversion).Answering these questions requires a method for the com- Refluxing 1,2,4-trichlorobenzene-extracted fullerene soot plete extraction of the soot. Reactive extraction seems to be (500 mg) together with 175 mg (1.04 mmol) 1,3-dihydro-2- especially well suited to this purpose. In this method, the benzothiophene 2,2-dioxide (1) (Scheme 1) in 1,2,4-trichloro- extractable compounds are solubilized via reaction with a benzene for 24 h (Table 1, Entry 1) followed by filtration from suitable partner in the liquid phase and are thus easily transinsoluble material aorded a black solution.This solution was ferred into the liquid phase.14 again filtered through a glass frit (pore size 10–16 mm) to avoid Herein, we describe the reactive extraction of fullerene soot contamination with small, insoluble particles of soot.with ortho-quinodimethanes as modifying agents. The influence Afterwards the solution was evaporated and the residue sus- of the structure of the modifying reagent on the extraction pended in ethanol and sonicated for 5 min. Unreacted 1, yield and on the composition of the extracted material will be byproducts and residual solvent were dissolved in ethanol and addressed and the results will be compared with common extraction methods. could be easily separated by filtration.The black powdery Scheme 1 Generation of ortho-quinodimethane 2 from 1,3-dihydro-2-benzothiophene 2,2-dioxide (1) and its cycloaddition reaction with C60 J. Mater. Chem., 1997, 7(8), 1327–1330 1327Table 1 Entries; compounds 1, 4 used for reactive extraction or solvent used for extraction; amount of extracted soot; extraction yields; elemental analyses of extracted material; fullerene distributions determined by LD–TOF MS; Mn (g mol-1) determined by vapour pressure osmometry yield of extracted extraction/ fullerene vapour reactive adducts or elemental analysis yield yield pressure extraction carbon fullerenes CFull CFull fullerenes LD–TOFd tosmometry entry with soot /mg C (%) H (%) N (%) H/N (%) /mg (%)c MS Mn [g mol-1]e 1a 1 (175 mg) 500 mg 45 92.36 2.74 <0.2 >13 59.7 27 5.4 C60–C344 2590 2a 1 (88 mg) from 1 9 90.34 2.63 <0.3 >8 59.0 5 1.0 C60–C396 — 3a 4 (440 mg) 500 mg 164 83.40 7.59 2.70 2.82 27.8 46 9.2 C60–C400 6610 4a 4 (220 mg) from 3 46 82.84 6.91 2.70 2.56 27.2 13 2.6 C60–C418 6480 5a 4 (330 mg) 52 mg 24 83.33 6.85 2.84 2.41 24.9 6 11.5 C60–C418 — 6a 4 (100 mg) 500 mg 51 85.72 6.40 2.50 2.56 27.7 14 2.8 C60–C348 3770 7a 4 (100 mg) 1000 mg 56 85.46 7.04 2.50 2.82 34.0 19 1.9 C60–C308 4010 8a 4 (440 mg) 10 g 113 90.79 4.08 1.45 2.81 61.0 69 0.7 C60–C350 1930 9 toluene 10 g 810 99.10 0.27 0.22 1.23 99.1 810 8.1 C60–C180 — soxhlet 10b 1,2,4- 30 g 81 93.68 0.71 <0.1 >7.1 93.7 76 0.25 C60–C192 — trichlorobenzene soxhlet 11b 1,2,4- 10 g 26 96.46 0.40 <0.1 >4 96.5 25 0.25 C60–C196 — trichlorobenzene reflux 12b quinoline 25 g 389 87.63 2.09 2.28 0.92 70.0 272 1.1 C60–C260 — soxhlet a1,2.4-Trichlorobenzene-extracted soot.bToluene-extracted soot. cCalculated with respect to soot.dPositive ion LD–TOF MS. eVapour pressure osmometry in tetrahydrofuran (measured at 30°C). filtration residue was washed several times with ethanol and mass ratio can be determined by elemental analysis and must dried under vacuum. Finally, a yield of 45 mg of mat- have a value of 2.8 for the formed adducts assuming that pure erial was obtained, which readily dissolved in CHCl3 or addition products of 4 and carbon clusters are formed.Again tetrahydrofuran (THF). According to its H5C ratio, which the sums of proportions of elements C, H, N and S determined was determined by elemental analysis, this corresponds to a by elemental analyses are only 96–98%. As mentioned above yield of fullerenes of 5.4 mass % (calculated with respect to we assign the missing amount of 2–4% to oxygen resulting soot).The sum of the proportions of elements C, H, N and S from epoxidation products of fullerenes. determined by elemental analysis is only 95%. We assume that Treatment of 1,2,4-trichlorobenzene-extracted fullerene soot the residual amount consists of oxygen which has reacted w with 4 (440 mg, 1.04 mmol) according to the above the fullerenes.It is well known that epoxidation of fullerenes procedure led to the isolation of 164 mg of soluble material in solution is induced by UV irradation18 or heating19 in the (Table 1, Entry 3). The elemental analysis of this material presence of oxygen. showed a H5N ratio of 2.8 which proved that the sample Laser desorption time-of-flight (LD–TOF) mass spectra of consisted entirely of modified carbon clusters. The total amount this sample showed a distribution of unmodified fullerenes from of carbon clusters extracted from the soot was calculated to C60 to C344, due to retro-Diels–Alder reaction of the formed be 9.2 mass %.This is considerably higher than the 5.4 mass % adducts during the ionization process. Vapour pressure which was achieved with the less flexible 1.From this it follows osmometry which gave a number-average molecular mass of that the higher the solubility of the adducts the higher the Mn=2590 g mol-1 also indicated that the soluble material extraction yields. consisted of Diels–Alder adducts of high molecular mass As described before for 1, a second extraction of the soot fullerenes (giant fullerenes).Additional soluble fullerene mate- with 4 yielded another 2.6 mass % of soluble material (Table 1, rial was obtained when the once-extracted soot was subjected Entry 4). It follows that approximately 12 mass % of the soot to a second reactive extraction. This yielded another 1 mass % can be extracted by this approach. In comparison, the total of soluble material. The LD–TOF mass spectra of this sample extraction of a small amount of soot (52 mg) within one step showed slightly higher fullerenes up to C396.with an extremely large excess of 4 (330 mg, 0.69 mmol) yielded This experiment clearly showed that ortho-quinodimethanes 11.5% soluble material (Table 1, Entry 5). are very well suited for reactive extraction. Obviously, the The soluble material which was received from the first solubility of the adducts is the crucial point for the additional reactive extraction (Table 1, Entry 3) was fully characterized yield and the distribution of extractable material.We therefore by means of mass spectrometry, vapour pressure osmometry synthesized the ortho-quinodimethane precursor 5-hexadecan- and thermogravimetric analysis.Positive as well as negative amido-1,3-dihydro-2-benzothiophene 2,2-dioxide (4) carrying ion LD–TOF mass spectra again show, due to retro-Diels– a long flexible alkyl chain. Alder reactions of the adducts during the ionization process, a distribution of even numbered unmodified carbon clusters (Fig. 1). The positive ion mass spectrum shows fullerenes up to mass ca. 4800 corresponding to C400 (Fig. 1, top panel). 170 peaks can be assigned to fullerenes with good signal to noise ratio. The molecular mass of fullerenes in the negative ion mass spectrum is somewhat lower compared to that in the positive ion mass spectrum. Strong signals of C60, C70 and C84 The fixed H5N mass ratio of 2.8 in 4 allows a more precise can be observed (Fig. 1, bottom panel). 125 peaks can be calculation of the product composition and can serve as an indicator for the purity of the extracted products. The H5N counted for fullerenes with good signal to noise ratio. Ruo 1328 J. Mater. Chem., 1997, 7(8), 1327–1330evident that the soluble material consists of fullerenes C60–C400 which are multiply functionalized with ortho-quinodimethanes. However, isolation of single adducts from the adduct mixture with HPLC was not achieved due to the unavoidable formation of multiple adducts and the corresponding regioisomers; the soluble material consists of more than 1000 compounds.Further experiments with dierent stoichiometries show that both the yields of fullerenes and the composition of the extracted material can be influenced. Reaction of 500 mg of soot with 100 mg (0.23 mmol) of 4 (Table 1, Entry 6) yielded 51 mg of fullerene adducts (fullerene content: 27.7%) which corresponds to 2.8 mass % pure fullerenes (calculated with respect to soot).In comparison, reaction of 1000 mg of soot with 100 mg (0.23 mmol) of 4 (Table 1, Entry 7) yielded 56 mg of fullerene adducts (fullerene content: 34%) which is equivalent to 1.9 mass % pure fullerenes.These results show that increasing soot to reagent ratios lead both to diminished yields of fullerenes and increasing fullerene content of the extracted material. With that, the number of attached ortho-quinodimethanes is also decreasing. The latter result is supported by the reaction of 440 mg (1.04 mmol) of 4 with a very large excess of soot (10 g) (Table 1, Entry 8) since this extraction yields a highly fullereneenriched sample with a fullerene content of 61 mass %.It can be concluded that the reactive extraction proceeds in three separate steps: (i ) fullerenes which are firmly embedded Fig. 1 (a) Positive and (b) negative ion LD–TOF MS of fullerene sample from reactive extraction with 5-hexadecanamido-1,3-dihydro- in the soot matrix and are not extractable by conventional 2-benzothiophene 2,2-dioxide 4 Soxhlet extraction react with ortho–quinodimethanes, (ii ) the modified and therefore soluble fullerenes dissolve in 1,2,4- trichlorobenzene, and (iii ) by further reaction of excess ortho- et al.discuss some reasons for dierences in negative and quinodimethanes with dissolved fullerene adducts multiple positive ion mass spectra.5 They suggest that dierent ioniz- adducts are formed.ation probabilities for positively and negatively charged carbon The advantage of the described reactive extraction becomes clusters in LD–TOF MS experiments lead to significant dier- evident by comparing it with conventional Soxhlet extraction. ences in the distributions of fullerenes. Field-desorption (FD) The yield of fullerenes from conventional Soxhlet extraction MS is a well-known technique for detecting fullerenes and of virgin soot with toluene is found to be 8.1% (Table 1, Entry fullerene adducts without fragmentation during the ionization 9).A subsequent extraction with 1,2,4-trichlorobenzene yields process. FD MS of this sample showed, apart from multi- only additional 0.25% fullerenes (Table 1, Entry 10) and with adducts of C60 and C70, mono- and bis-adducts of the recently quinoline 1.1% (Table 1, Entry 12).Furthermore, elemental isolated and characterized C80 (see Experimental).20 analysis of the quinoline extract indicates that the sample There are also significant dierences in the fullerene distri- contains large, not removable amounts of impurities due to butions of samples received from conventional and reactive decomposition of the solvent.Hence, the yields of reactive extractions. Fig. 2 shows the positive ion LD–TOF mass extraction are superior to those of conventional extraction. spectrum of pure fullerenes from 1,2,4-trichlorobenzene extract (Table 1, Entry 10). Fullerenes up to mass 2100 (C192) can be Conclusions detected.The FD mass spectrum of this sample indicates the existence of fullerenes with more than 100 carbon atoms too Giant fullerenes which are barely soluble in common solvents (see Experimental). and are firmly embedded in the solid fullerene soot matrix can It follows that reactive extraction enables the extraction of be eciently functionalized by means of reactive extraction fullerenes with approximately double the molecular mass com- with ortho-quinodimethanes. The fullerenes are thus made pared to conventional extraction. soluble in 1,2,4-trichlorobenzene and are extractable in very The number-average molecular mass Mn of the sample from high, so far unprecedented, yields. entry 3 was determined by vapour pressure osmometry as Commercially available fullerene soot which yields 8.4mass% 6610 g mol-1.Thermogravimetric analysis showed that these of soluble fullerenes by entire extraction with common solvents fullerene adducts are thermally stable up to at least 310°C aords additional 11.8 mass % of soluble material (funcwhich is in accordance with results obtained for ortho-quinodi- tionalized fullerenes C60–C418) by reactive extraction with an ortho-quinodimethane precursor (4).The above experiments methane adducts of C60.21 From the analytical results it is indicate that the conformational flexibility of the ortho-quinodimethanes is the key factor for the amount of extracted material. Therefore, further experiments will make use of ortho-quinodimethanes substituted with highly branched (extremely flexible) groups. Our studies show that the amount and the composition of the extracted material can be significantly influenced by diering the ratio of fullerene soot to modifying agent.The entirely extracted soot will be utilized to study if the catalytic activity of fullerene soot is due to the soot itself or comes from included redox active fullerenes.Experimental Carbon soot: for all experiments, samples of the same soot were used. The soot was prepared by arc synthesis according Fig. 2 Positive ion LD–TOF MS of 1,2,4-trichlorobenzene-extracted fullerenes to the Kra�tschmer–Human2,3 process and was provided by J. Mater. Chem., 1997, 7(8), 1327–1330 1329Hoechst AG. The soot was previously extracted with toluene ethanol and sonicated for 5 min.After filtration, washing with ethanol and drying a black powder was obtained. to remove C60 and C70 (approximately 8.1 mass %). LD–TOF MS was performed using a Bruker Reflux mass Extraction of the soot under reflux (Table 1, Entry 11) spectrometer with a 337 nm N2 laser. Laser power was adjusted to be the lowest level at which an ion signal was observed.The soot was placed in a flask and 200 ml of the solvent was A VG Instruments ZAB 2–SE–FPD mass spectrometer was added. After refluxing for 24 h, the suspension was filtered. used for recording FD mass spectra. The solution was evaporated and the residue suspended in 150 ml ethanol and sonicated for 5 min. Filtration, washing 1,3-Dihydro-2-benzothiophene 2,2-dioxide (1) with ethanol and drying gave a black powder.The reagent was synthesized as described by Cava and Deana.22 Positive ion FD MS of crude material from entry 10 dH (300 MHz, [2H6]acetone, 21°C) 4.35 (s, 4H, CH2), 7.29–7.40 (m, 4H, CH). dC (75 MHz, CDCl3, 28°C) 47.3 (CH2), arom. C C60 720.6 (95%), C70 840.7 (100%), C76 911.7 (15%), C78 935.6 atoms: 126, 129 and 131.5. Positive ion FD MS: 168 ([M+], (31%), C82 983.7 (13%), C84 1009.9 (14%), C86 1031.6 (21%), 100%). Mp 150–152°C.C88 1056.5 (22%), C90 1079.9 (21%), C92 1104.5 (10%), C94 1127.0 (8%), C96 1152.9 (20%), C98 1177.9 (5%), C100 1201.7 (11%), C106 1272.2 (11%), C110 1321.7 (7%), C112 1343.9 (8%), 5-Hexadecanamido-1,3-dihydro-2-benzothiophene 2,2-dioxide (4) C114 1367.7 (17%), C116 1392.4 (16%), C118 1418.9 (9%), C120 1,3-Dihydro-5-amino-2-benzothiophene 2,2-dioxide23 (2.87 g; 1439.1 (22%), C124 1489.1 (22%), C136 1633.5 (12%). 15.7 mmol) and triethylamine (2.1 g; 20 mmol) were dissolved in 1,4-dioxane (250 ml). Hexadecanoyl chloride (4.73 g; This research was supported by the ‘Bundesministerium fu�r 17.3 mmol) was added dropwise at room temp. over 20 min. Bildung und Forschung’ (grant number: 13N6665/8). We thank The solution was stirred under reflux for 20 min.The solution the Hoechst AG for providing fullerene soot. was cooled to room temp. and then added to H2O (300 ml), giving a white precipitate. The precipitate was collected by References filtration, washed with H2O and Et2O and dried in vacuo. Yield: 5.75 g (13.6 mmol; 87%). dH (500 MHz, C2D2Cl4, 100°C) 1 H. W.Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. E. Smalley, Nature, 1985, 318, 162. 0.87 (t, 3H, CH3), 1.26 (br, 24H, CH2), 1.66 (m, 2H, CH2), 2 W. Kra�tschmer, K. Fostiropoulos and D. R. Human, Chem. Phys. 2.34 (t, 2H, CH2), 4.25 (s, 2H, CH2), 4.27 (s, 2H, CH2), 7.13 (s, L ett., 1990, 347, 167. 1H, CH), 7.18 (d, 1H, CH), 7.3 (d, 1H, CH), 7.65 (s, 1H, NH). 3 W. Kra�tschmer, L.D. Lamb, K. Fostiropoulos and D. R. Human, dC (125 MHz, C2D2Cl4, 100°C) 14.2 (CH3), aliph. CH2: 22.80, Nature, 1990, 347, 354. 25.64, 29.63 (several signals overlapping), 32.08, 37.85, 57.08 4 H. Werner, D. Herein, J. Blo�cker, B. Henschke, U. Tegtmeyer, T. Schedel-Niedrig, M. Keil, A. M. Bradshaw and R. Schlo�gl, (CH2) and 57.63 (CH2), arom. CH: 117.71, 120.64 and 126.76, Chem.Phys. L ett., 1992, 194, 62. quart. arom. C atoms: 126.78, 132.0 and 139.03, 171.0 (CNO). 5 F. Diederich, R. Ettl, Y. Rubin, R. L. Whetten, R. Beck, M. Alvarez, Positive ion FD MS: 421.2 ([M+] 100%). Elemental analysis: S. Anz, D. Sens-harma, F. Wudl, K. C. Khemani and A. Koch, C24H39NO3S, calc. C 68.52%, H 9.29%, N 3.29%, S 7.47%. Science, 1992, 252, 548. Found C 68.37%, H 9.32%, N 3.32%, S 7.6%.Mp 152–154°C. 6 C. Smart, B. Eldridge, W. Reuter, J. A. Zimmerman, W. R. Creasy, N. Riviera and R. S. Ruo, Chem. Phys. L ett., 1992, 188, 171. 7 W. R. Creasy, J. A. Zimmerman and R. S. Ruo, J. Phys. Chem., General procedure for reactive extractions with 1 and 4 1993, 97, 973. (Table 1, Entry 1–8) 8 H. Shinohara, H. Sato, Y. Saito, M. Takayama, A. Izuoka and T.Sugawara, J. Phys. Chem., 1991, 95, 8449. All reactions were carried out under a dry, oxygen-free argon 9 H. Shinohara, H. Sato, Y. Saito, A. Izuoka, T. Sugawara, H. Ito, atmosphere. Soot and 1 or 4 were added to 150 ml of 1,2,4- T. Sakurai and T. Matsuo, Rapid Commun. Mass. Spectrom., 1992, trichlorobenzene. Refluxing for 24 h followed by filtration from 6, 413. 10 D. H. Parker, P.Wurz, K. Chatterjee, K. R. Lykke, J. E. Hunt, insoluble material aorded a black solution. The solution was M. J. Pellin, J. C. Hemminger, D. M. Gruen and L. M. Stock, evaporated and the residue suspended in 150 ml of ethanol J. Am. Chem. Soc., 1991, 113, 7499. and sonicated for 5 min. Unreacted 1 or 4 and residual solvent 11 D. H. Parker, K. Chatterjee, P. Wurz, K. L. Lykke, M.J. Pellin were dissolved in ethanol while a suspension of the reaction and L. M. Stock, Carbon, 1992, 30, 1167. products was formed. Filtration, washing with ethanol and 12 K. R. Lykke, D. H. Parker and P. Wurz, Int. J.Mass Spectrom. Ion drying gave a black powder. Processes, 1994, 138,147. 13 A. S. Hirschon, H. -J. Wu, R. B. Wilson and R. Malhotra, J. Phys. Chem., 1995, 99, 17483.Positive ion FD MS of crude material obtained from entry 3 14 E. Schlichting, W. Halwachs and K. Schu�gerl, Chem. Eng. Commun., 1987, 51, 193. C60-monoadduct 1078.4 ([M+], 25%), C60-bisadduct 1435.8 15 Yi. -Zh. An, G. A. Ellis, A. L. Viado and Y. Rubin, J. Org. Chem., ([M+], 35%), C60-trisadduct 1791.9 ([M+], 45%), C60-tetra- 1995, 60, 6353. adduct 2151.4 ([M+], 100%), C60-pentaadduct 2508.6 ([M+], 16 P.Belik, A. Gu�gel, J. Spickermann and K. Mu�llen, Angew. Chem., 45%), C70-monoadduct 1197.6 ([M+], 35%), C70-bisadduct 1993, 105, 95; Angew. Chem., Int. Ed. Engl., 1993, 32, 78. 17 B. Illescas, N. Martin, C. Seoane, P. de la Cruz, F. Langa and 1555.5 ([M+], 70%), C70-trisadduct 1912.5 ([M+], 55%), C70- F.Wudl, T etrahedron L ett., 1995, 36, 8307. tetraadduct 2270.2 ([M+], 45%), C70-pentaadduct 2627.3 18 J. M. Wood, B. Kahr, S. H. Hoke, L. Dejarme, R. G. Cooks and ([M+], 10%), C80-monoadduct 1317.1 ([M+], 20%), C80- D. Ben-Amotz, J. Am. Chem. Soc., 1991, 113, 5907. bisadduct 1674.8 ([M+], 15%). 19 A. M. Vasallo, L. S. K. Pang, P. A. Cole-Clarke and M. A. Wilson, J. Am. Chem. Soc., 1991, 113, 7820. 20 F. H. Hennrich, R. H. Michel, A. Fischer, S. Richard-Schneider, Thermogravimetric analysis of the extract from entry 3 S. Gilb, M. M. Kappes, D. Fuchs, M. Bu�rk, K. Kobayashi and S. Nagase, Angew. Chem., 1996, 108, 1839. (Mass 7.5 mg, heating rate 10 K min-1, N2 atmosphere) 21 A. Gu�gel, A. Kraus, J. Spickermann, P. Belik and K. Mu�llen, 313–560°C (-54.23%, maximum at 408.3°C, residue 3.43 mg). Angew. Chem., 1994, 106, 601; Angew. Chem., Int. Ed. Engl., 1994, 33, 559. Soxhlet extractions of the soot (Table 1, Entries 9, 10, 12) 22 M. P. Cava and A. A. Deana, J. Am. Chem. Soc., 1959, 81, 4266. 23 M. Walter, A. Gu�gel, J. Spickermann, P. Belik, A. Kraus and The soot was placed in the thimble of a Soxhlet extractor. The K. Mu�llen, Fullerene Sci. T echnol., 1996, 4, 101. extractions were carried out for 24 h. After filtration, the solution was evaporated and the Paper 6/08186J; Received 10th February, 1997 1330 J. Mater. Chem., 1997, 7(8), 1327–1330

ISSN:0959-9428    

DOI:10.1039/a608186j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Study of cetyltrialkylammonium bromide and tribromide salts in the solid phase R. Caminiti,*a M. Carbone,b G. Mancinic and C. Saduna aDept. of Chemistry, Istituto Nazionale di Fisica della Materia, University of Rome ‘L a Sapienza’, P.le A. Moro, 5 00185 Rome, Italy bDept. of Chemical Sciences and T echnologies, University ‘T or Vergata’, V ia della Ricerca Scientifica 1, 00133 Rome, Italy cCentro CNR di Studio sui Meccanismi di Reazione, C/O Dept.of Chemistry, University of Rome ‘L a Sapienza’, P.le A. Moro, 5 00185 Rome, Italy Some cetyltrialkylammonium tribromide salts have been studied in the solid phase using Raman spectroscopy and energy dispersive X-ray diraction. The first technique detected the presence of asymmetric tribromides while the latter technique revealed the degree of asymmetry of the tribromide units (dierence in the bond lengths between external and central Br atoms), by application of a subtraction method.In this method the spectra of the corresponding monobromide salts were recorded and dierences between tribromide and monobromide curves isolated the tribromide contributions. The existence of asymmetrical and symmetrical tribromide ions has been established and the degree of asymmetry was then correlated to the steric hindrance and electronegativity of the ammonium substituents. The values of the BrMBr bond distances have been deduced; the most asymmetric tribromide has BrMBr distances at 2.38 and 2.66 A° , while the symmetric tribromide has both BrMBr bond lengths equal to 2.52 A° .A linear geometry is confirmed for the tribromide ions.The study of polyhalides is important from an applied and a Cetyltrymethylammonium bromide (CTAB) is one of the most studied cationic surfactants, used in many fields such as fundamental point of view. Polyhalides are typically used in oxidation reactions of organic molecules as well as for doping micellar catalysis, medicine and detergency.This surfactant forms micellar aggregates in aqueous solution which are polymers in order to change their conductivity.1 The number of atoms in a polyhalogen unit can be quite responsible for its useful physico-chemical properties. The corresponding tribromide salt as well as the others we investi- large and depends on the nature of the halogen atom as well as on the hosting structure.Units containing up to seven2 and gated are surfactants used in bromination reactions.11–15 The studied quaternary ammonium salts all contain the eight3 atoms have been found for polyiodides and up to ten for polybromides.4 cetyl (C16H33) fragment but dier in the head group so that we investigated the eect of head group variation on the In all cases a better knowledge of the polybromides structure helps in the understanding of their properties and possible polyhalogen asymmetry. Study of the polyhalogen asymmetry is usually carried out applications.As far as tribromides are concerned, structural studies showed that the tribromide ions usually have linear by a combination of Raman spectroscopy and X-ray diraction. The stretching vibration frequencies are correlated to the geometry which can be either symmetric, with the same bond lengths between the external and the central Br atoms number of atoms in the polyhalogen unit as well as to their structure.For a tribromide the Raman spectra can be used to [Br(1)MBr(2)=Br(2)MBr(3)], such as in [(CH3)3NH+]2 Br-Br3-5 or asymmetric with dierent lengths between the deduce whether the ion is symmetric or asymmetric.The X-ray diraction technique can give information about external and the central Br atoms [Br(1)MBr(2) Br(2)MBr(3)], such as in CsBr3,6 with a dierence of 0.33 A° ). the structure, thus, in the present case, an exact evaluation of the degree of asymmetry can be achieved. The degree of asymmetry is even higher in PBr7 where the Br3- units show bond lengths of 2.39 and 2.91 A° .7 The deg- In the present study we employed both techniques.In particular the degree of asymmetry of the tribromide was ree of asymmetry (the dierence between the BrMBr bond lengths) is not constant in dierent tribromides, but depends determined by large angle X-ray scattering (LAXS). LAXS is a powerful technique for determining structural parameters of on the counter ion and on the host structure. Several reports, based on Raman spectroscopic evidence, non-crystalline systems, since it can provide information about the short-range order.11–26,30 In particular energy dispersive support the existence of tribromide and pentabromide anions.4,8–10 X-ray diraction (EDXD)18–20,22,25,26,29–31 has been found to be a suitable tool in the investigation of such systems due to Studies on cetyltrimethylammonim tribromide (CTAB3) are reported in ref. 4. The authors indicate the presence, in this its high speed and reliability compared to traditional angular scanning X-ray diraction (ADXD). molecule, of two Raman bands at 153 and 205 cm-1, which they attribute to BrMBr stretching vibrations, concluding that For this purpose, X-ray diraction patterns of the corresponding monobromide samples were collected, and dierences the Br(1)MBr(2) and Br(2)MBr(3) distances are dierent.No structural studies on powdered CTA bromide or tribromide evaluated between the curves relative to the tribromide salts and the corresponding monobromide ones. In this way the have been reported. We investigate here the degree of asymmetry of the tribrom- contribution of the tribromide unit was isolated and analysed.ide ion in CTAB3 (sample G1) and in three other quaternary ammonium salts; cetylquinuclidinium tribromide (CQB3, sample G3), cetyltripropylammonium tribromide (CTPAB3, Experimental sample G5) and cetyldimethyl(2-hydroxyethyl)ammonium Materials tribromide (CDEAB3, sample G7). The CTAB (sample G2), which is commercially available (Fluka) was purified by crystallization from absolute alcohol * Email: r.caminiti@caspur.it J.Mater. Chem., 1997, 7(8), 1331–1337 1331and diethyl ether. The cetylquinuclidinium bromide (sample fall outside of the used energy range. The stability of the voltage power supply for X-rays is better than 0.1%. G4) was prepared as reported in ref. 27. The cetyltripropylammonium bromide and the cetyldimethyl(2-hydroxylethyl)am- The X-ray tube operates at 50 kV and 40 mA for recording the spectra of all samples while conditions of 45 keV and monium bromide (samples G6 and G8) were prepared by standard quaternization procedures. 35 mA were also used for sample G5. X-Ray detection was accomplished using an EG&G liquid- The corresponding yellow–orange tribromides were prepared through reaction of the monobromides with Br2, as nitrogen cooled ultrapure Ge SSD (ORTEC, model 92X), connected to a PC 286 via ADCAM hardware and Maestro reported in ref. 31.Elemental analyses for H, C, N and Br were performed by II software, which performs the necessary analogue-to-digital conversions and amplifications.In order to obtain the relation the Microanalytical Service of the Area della Ricerca di Roma of the CNR, giving results in agreement with the formulae of between the photon energy vs. the channel number in the multichannel analyser (MCA), the absorption edge of Ba, In, the materials (Table 1). Pd and Eu were used. The linear relationship between the photon energy and the channel number was good.Techniques Raman spectra. FT Raman spectra (resolution ±4 cm-1) of Measurements the tribromide salts were recorded on a FRA 106 FT-Raman accessory, mounted on a Bruker IFS 66 FT-IR vacuum The transmission geometry has been employed, since it allows an easier correction for the sample absorption.29,30 For instrument, operating with an exciting frequency of 1064 nm (Nd5YAG laser) and with a germanium diode detector cooled measurement of the incident beam spectrum, which is necessary in the energy dispersive method, we used the tube at 50 or at liquid N2 temperature.Power levels of the laser source varied between 20 and 100 mW. The solid samples were packed 45 kV and 2 mA, while the dimension of the slits was 100 mm×160 mm. into a suitable cell and then fitted into the compartment designed to use 180° scattering geometry.No sample decompo- Fig. 1 shows the primary beam spectrum as well as transmission spectra of samples G7 and G8 measured under the sition was observed during the experiments. same conditions. Each of the three measurements was recorded over 15000 s. In order to exclude the fluorescence lines from X-Ray diraction patterns.The dispersive X-ray diraction experiments were carried out by employing a non-commercial the spectrum as well as regions where the intensity is strongly absorbed by the sample, the range used in the data analysis X-ray energy scanning diractometer,25,26,30 with a solid-state detector, constructed at the Department of Chemistry, was restricted between channel 330 and 680, which corresponds to an energy range of 22.123–45.735 keV.In order to cover a University of Rome ‘La Sapienza’, Powder Diraction Laboratory.29 In the design of the machine the goniometer is suciently wide region of s-space (s=1.014Esin h) the diracted X-ray photons from samples were collected at dierent scat- substituted by two rotating arms, which can be moved independently by a motor in the range of 2h -5° to 120°, by a tering angles.Measurement angles and used energy ranges are listed in Table 2. program from our group written in BASIC. The X-ray optical path is defined by four Huber 20 mm variable slits mounted The scattering intensity was obtained over the range s= 0.2–16.2 A° -1. The sample (a pellet of thickness 2 mm) was on the arms.The distance between the X-ray source and the sample, equal to that between the sample and the detector is 20 cm. The geometry of our machine is such that both reflection and transmission intensities can be measured. Advantages of the energy dispersive method over the conventional angle dispersive method have been described18–20,22,29–31 and the applicability of the method has already been widely described in the literature.18–20,29,31–33 A more detailed description of the apparatus has been given in ref. 29 and 30.The X-ray source is a Seifert tube (3 kW) with a tungsten target, which provides X-radiation in the energy range 0–60 keV. The W L lines in the energy range 8–11 keV and the fluorescent X-rays from Br (11.91, 13.29 keV) Table 1 Elemental analysisa Fig. 1 Energy spectra of the primary beam (a), the transmittance of the G8 (b) and the G7 (c) samples vs. channel number in the MCA sample C(%) N(%) H(%) Br(%) G1 (CTAB3) 42.70 2.8 8.23 42.97 Table 2 Scattering parameters associated with the minimum (43.53) (2.67) (8.07) (45.72) (22.123 eV) and maximum values (45.735 eV) of the energy for each G2 (CTAB) 61.57 3.99 12.02 24.07 measurement angle (62.62) (3.84) (11.62) (21.92) G3 (CQB3) 47.81 2.50 7.55 42.14 h/degrees smin smax (47.93) (2.43) (8.04) (41.59) G4 (CQB) 65.61 3.21 10.07 21.11 21 8.04 16.62 (66.32) (3.36) (11.13) (19.18) 15.5 5.99 12.39 G5 (CTPAB3) 50.31 2.12 8.55 38.69 10.5 4.08 8.45 (49.35) (2.3) (8.95) (39.40) 8.0 3.12 6.45 G6 (CTPAB) 66.01 3.21 11.97 18.81 5.0 1.96 4.04 (66.93) (3.12) (12.13) (17.81) 3.5 1.37 2.83 G7 (CDEAB3) 43.01 2.48 7.88 43.45 3.0 1.17 2.43 (43.34) (2.53) (8.00) (43.25) 2.0 0.78 1.62 G8 (CDEAB) 61.02 3.49 11.09 20.39 1.5 0.59 1.21 (60.9) (3.55) (11.24) (20.26) 1.0 0.39 0.80 0.5 0.19 0.41 aCalculated values in parentheses. 1332 J. Mater. Chem., 1997, 7(8), 1331–1337placed at the centre of the goniometer. The measuring time at with each angle was set so as to obtain a minimum of 50000 counts per channel.I(E,h)=ICoh(E,h)+ E¾I0(E¾)P(E¾, h)AInc(E, E¾, h)IInc(E¾, h) EI0(E)P(E, h)ACoh(E, h) Fig. 2 shows, as an example, the diracted intensity vs. the channel number at h=21, for sample G7. The intensity is (2) plotted in logarithmic scale in order to show also the Br fluorescence lines. Restricting the channel ranges means that where h is the scattering angle, E is the photon energy revealed the Br fluorescence lines can be excluded from the data by the detector and E¾ is the initial energy of a photon analysis range.inelastically scattered at the observed energy E; K is the scale Fig. 3 shows diracted intensities for sample G7 at dierent factor between the intensity reaching the detector and the collection angles.The transmittance of the sample at h=0° intensity scattered by a stoichiometric unit of sample; I0(E) is has been made under the same experimental conditions as that the energy spectrum of the primary beam measured at h=0°; for the primary beam. The two measurements are necessary to P(E,h) is the polarization factor by a scattering of primary determine the sample linear absorption coecient, which varies radiation with polarization P(E), ICoh(E,h) is the total scattered to a significant extent with the X-ray energy.From the relation elastic intensity and to which three dierent terms contribute [eqn. (3)], where Sicifi2(s) is the self-scattering intensity; ci is It(E)/I0(E)=exp[-m(E)t] the concentration of the dierent species, i1(s) and i2(s) are the we obtain the experimental values of exp[-m(E)t] used in intensities of interfering waves scattered by atom pairs belong- eqn.(1) and (2) for absorption corrections.30,31 ing to the same and dierent domains respectively; s is the The total intensity I¾ scattered by a sample and observed by scattering parameter and is defined by eqn. (4), the energy dispersive detector in approximation of single ICoh(E,h)=.n cifi2(s)+i1(s)+i2(s) (3) scattering and transmission geometry25,30,31 can be expressed as: s=4p sin h/l=1.014 E sin h (4) I¾(E,h)=KI0(E)P(E,h)ACoh(E,h)I(E,h) (1) where E is expressed in keV and s in A° -1. A more detailed description of the terms reported in eqn. (1) and (2) has been given in ref. 25 and 29–31. Data treatment After correction of the collected experimental data, for escape peak suppression, the intensity data were handled, as described by Nishikawa and Iijima,31 by means of our DIF1 program written in Fortran IV.Normalization to a stoichiometric unit of volume containing one Br atom was performed. Radial distribution functions D(r), were calculated from the static structure functions i(s) [eqn.(5)], according to eqn. (6). i(s)=Icoh(E,h)-.icifi2(s) (5) D(r)=4pr2r0+2rp-1Psmax 0 s.i(s).M(s) sin(rs) ds (6) Fig. 2 EDXD profile of the sample G7 obtained at h=21 In this equation r0=[Sinifi(0)]2V -1, where V is the stoichiometric unit of volume chosen, ni=number of atoms i per unit volume, and fi the scattering factor per atom i. The sharpening factor is given by eqn.(7). M(s)={fBr2(0)/fBr2(s)} exp(-0.005 s2) (7) In order to determine the BrMBr bond lengths in the tribromomide ions, the peaks referring to the BrMBr interactions were isolated by a subtraction method.34–36 By subtracting the radial distribution function of the monobromide ammonium salts from the corresponding tribromide salts, we have obtained a dierence radial distribution function that contains information selectively about the Br first neighbours in the range 0–3 A° .The resulting dierence curves are indicated as D(r)Br3 -D(r)Br. The subtraction operation is valid only if the organic structure is unaltered by the presence of two extra bromine atoms, so that its contribution to the radial distribution function remains the same both in the mono- and in the tribromide salts.The absence, in the dierence curve, of extra oscillations with respect to the original curves or of negative peaks indicates the correctness of this hypothesis. Theoretical peaks were calculated, by a corresponding Fourier transformation of the theoretical intensities for pairs of interactions between atoms p and q (Debye functions) Fig. 3 EDXD profiles of the sample G7 obtained at dierent collection angles [eqn.(8)], using the same sharpening factor and the same smax J. Mater. Chem., 1997, 7(8), 1331–1337 1333value as for the experimental data and assuming the root mean in (C6H5)4As+Br3-,39 and the resulting constant force is lowered significantly from that in Br2. square deviation to be spq. The Raman spectrum of Br2 in benzene,8 yields a funda- ipq(s)=.fpfq sin(rpqs) (rpqs)-1 exp(-0.5 s2pqs2) (8) mental stretching frequency at 306 cm-1, while, for symmetrical Br3- units (with equal BrMBr distances) such as in (n- Since our goal was the determination of BrMBr distances within the tribromide ion we considered the Debye functions C4H9)N+Br3-, the actual BrMBr stretching frequency is 179 cm-1, and is the average of symmetrically and antisym- only for BrMBr first neighbour interactions.metrically coupled normal modes.39 The vibration frequencies dier for asymmetrical Br3- units Data analysis and the two stretching modes are observed separately. In CsBr3,6 which shows two dierent BrMBr bond distances of Raman spectra 2.44 and 2.77 A° , the vibration frequencies were observed at The Raman spectra of the studied samples are shown in Fig. 4 140 and 208 cm-1. In a more asymmetrical tribromide such and observed vibration frequencies of the Raman-active bands as PBr4+Br3-,7 where the BrMBr bond lengths are in the are collected in Table 3, where also a comparison to literature range 2.39–2.91 A° , the stretching frequencies are observed40 at data for symmetric tribromides is made.Large shifts in both 249–135 cm-1. the wavenumber and intensity of the bands are observed and Apparently, the splitting between the vibration frequencies appear to correlate with the degree of asymmetry in the two can be correlated to the degree of asymmetry and can be BrMBr bond lengths. considered a qualitative method to deduce whether the tribro- The vibrational spectra of tribromides can be viewed in mide ion is symmetrical or asymmetrical.terms of interaction of Br2 with Br- with the latter acting as Within this simple picture, samples G1 and G7, with stretch- a Lewis base and Br2 as a Lewis acid. Upon complexation of ing vibration frequencies of 153, 205 and 140, 223 cm-1, Br2 with Br-, BrMBr antibonding molecular orbitals are respectively, contain asymmetrical Br3- units, with most prob- populated and the BrMBr bonds weakened; hence the BrMBr ably, a higher degree of asymmetry in sample G7 (larger distance is increased from 2.3 A° in Br2 37 to 2.53 A° , for instance, splitting between the vibration frequencies).Sample G3 might contain a symmetrical Br3- unit since a coupled vibration frequency is observed at 163 cm-1.This value is somewhat dierent from the value of 179 cm-1 reported for (n-C4H9)N+Br3-,39 but this may simply depend on the BrMBr bond distance, which we have calculated for the sample G3, but is not reported for (n-C4H9)N+Br3-. Sample G5 shows somewhat unusual behaviour in that the two vibration frequencies of 168 and 226 cm-1 do not follow the usual trend, of a lower stretching frequency for the n1 vibration and higher frequency for n3 compared to the average frequency for symmetric units. Furthermore at increasing asymmetry the intensity of the peak corresponding to the n3 vibration increases as does as the intensity ratio n3/n1 (see ref. 4 Fig. 1). For sample G7, instead, the n1 vibration is more intense. To be sure that both the peaks refer to the sample and not, for instance, to a symmetric Br3- (with a stretching vibration at 168 cm-1), with an impurity yielding a peak at higher vibration frequency a newly prepared sample was analysed, which yielded a similar Raman spectrum and diraction pattern.In this case we consider the Raman spectrum is not sucient to discriminate whether the tribromide unit is asymmetrical. The splitting of the frequencies for sample G7 hints rather at resonance eects, which can occur when two vibrations have closely spaced frequencies X-Ray diraction patterns Observed structure functions, in the form s.i (s).M(s) are shown in Fig. 5 for samples G1 and G2. Below 3 A° -1 the curves for the monobromide and tribromide salts show similar structures, Fig. 4 Raman spectra of the tribromide salts.Sample G3 shows one peak only and hints at symmetric tribromide. The samples G1, G5 and G7 show two peaks, which suggest the presence of asymmetric tribromides, though the splitting and the intensity ratios are not the same Table 3 Stretching frequencies of the tribromide Raman-active vibrations sample cation n1/cm-1 n3/cm-1 ref. G1 CTA 153 205 CTA 153 205 4 G3 CQ 163 G5 CTPA 168 226 Fig. 5 Observed structure function s.i (s).M(s) of samples G2 and G1. G7 CDEA 140 223 The structure function of the tribromide sample shows much wider Cs+ 138 213 8 oscillations compared to the corresponding monobromide one. 1334 J. Mater. Chem., 1997, 7(8), 1331–1337whereas much wider oscillations are seen for the tribomide in the contribution of a higher scattering species, namely a BrMBr interaction.The literature data related to BrMBr distances in the range 3–15 A° -1, hinting at the presence of extra interactions not present in the monobromides, most probably due tribomide ions, both for symmetric and asymmetric units are in agreement with such a value, therefore we consider that the to the first neighbour BrMBr interactions. No significant dierence in the reduced intensities is displayed by dierent comparison of this peak to theoretical peak shapes yields the BrMBr bond distances.samples. The radial distribution functions in the Di form [Di(r)= Another typical feature in the tribromides RDF is a peak at ca. 5.1 A° , which we ascribe to Br(1),Br(3) interactions, since D(r)-4pr2r0] are shown in Fig. 6 for the monobromides in and Fig. 7 for the tribromides in the range 0–15 A° . However it is rather intense and has no correspondence in the monobromide curves. The position of this peak confirms that the anion we only analysed the region 0–3 A° where we can distinguish the first-neighbour BrMBr interactions. The distribution func- Br3- has a linear geometry, as suggested in a previous EXAFS study.1 tions of the monobromide samples show two peaks at ca. 1.5 and 2.5 A° arising from C,C, and NMC bond distances (ca. Fig. 8(a) shows the radial distribution function of samples G1 and G2 in the range 0–5.5 A° , compared to the theoretical 1.5 A° ), and the non-directly bound C,C and NMC distances, with sp3 hybridization (ca. 2.5 A° ) calculated for the organic peak shapes calculated for the monobromide.In this calculation we have considered interactions of the type CMX, with substituents of the ammonium cation. In the radial distribution functions of the tribromides, the X=C, N, O between all the first and second neighbours. Particularly for the C,C interaction we considered the CTAB area of the peak at ca. 2.5 A° is much larger, compared to the corresponding monobromide curves, therefore cannot be structure determined by Campanelli and Scaramuzza41 in which the CMX distances used in the calculations are reported. simply ascribed to the C,C interaction, but it must contain The experimental curve is well reproduced by the calculated peak shapes in the 0–2.7 A° range, indicating that the considered interactions are sucient to simulate the RDF of the monobromide ammonium salts.Therefore the subtraction of the monobromide RDF from the corresponding tribromide RDF simply yields the peaks related to BrMBr interactions. If any change in the cation geometry occurs when coordinated to a tribromide, instead of a monobromide ion or if some interactions are present in the monobromide ion, which disappear for the tribromide, this would result, either in a residual peak and/or in a negative peak.The peak at ca. 1.5 A° can be used to determine if the subtraction operation is valid. As shown in Fig. 8(b) for sample G1 the dierence radial distribution function is fairly flat in the range 0–2 A° and the dierence radial distribution functions for the other samples show the same characteristics.We can, therefore, consider that the subtraction of the monobromide Fig. 6 Radial distribution functions of the form D(r)-4pr2r0 of the monobromide samples Fig. 8 (a) Dotted line, D(r) of the sample G2 in the range 0–5.5 A° ; continuous line, D(r) of the sample G1; dashed line, theoretical peak shape calculated by introducing in the Debye formula the first- and second-neighbour interactions of the ammonium cation, which is common to both samples.(b) Dierence curve between the radial distribution functions of sample G1 and G2 [D(r)G1-D(r)G2]. The dierence curve in the range 0–2 A° is rather flat, therefore all Fig. 7 Radial distribution functions of the form D(r)-4pr2r0 of the contributions of the ammonium cation have been removed by subtraction. tribromides samples J.Mater. Chem., 1997, 7(8), 1331–1337 1335curve isolates only the BrMBr contribution to the tribromide RDF and that the calculation of theoretical peak shapes, which reproduces the peak at ca. 2.5 A° in the dierence curve should give exact BrMBr distances. This operation was performed for all the samples and the corresponding BrMBr distances are reported in Table 4, together with mean square root used in the calculation.In Fig. 9 the experimental and theoretical dierence curves have been reported for all the sample pairs, and show good agreement. The presence of a peak at ca. 5.1 A° in the tribromide RDF allows us to infer the Br3- geometry.1 This peak, which we consider correlated to a Br,Br interaction, corresponds to the sum of the BrMBr distances in both Fig. 10 Comparison between observed (solid lines) and calculated (dotted lines) structure functions s.i(s).M(s) for G1 and G3 symmetric and asymmetric units. The only possible geometry that would yield peaks at a distance twice the fundamental one, is linear.1 The theoretical s.i(s).M(s) curves have been calculated by introducing only interactions related to the Br3- unit, in the Debye formula [eqn.(8)], using the parameters For instance the dierence between the Br(1)MBr(2) and Br(2)MBr(3) distances is 0.00 A° in sample G3 and 0.28 A° in reported in Table 4. From this we can see how, despite the similarities between the samples (they are all ammonium salts sample G7. Furthermore the latter value is comparable to the situation where Cs+ is the counter ion.with a long aliphatic chain) symmetric (G3), slightly asymmetric (G5), and highly asymmetric (G1, G7) structures are all The degree of asymmetry in the tribromide ions is correlated to their geometry with respect to the cation as well as to its observed. Finally, theoretical curves are compared to experimental electronegativity. The asymmetry can be viewed as a preferential interaction of one of the external Br atoms with the cation, ones in Fig. 10 for samples G1 and G3. It is of interest how the use of these interactions is already which can induce a polarization of the BrMBr bonds. The eect of the electronegativity is quite evident for instance in sucient to reproduce the oscillations of the structure functions in the entire range, that is, by contrast quite flat for the CsBr3,6 where an electropositive atom such as Cs+, causes a high degree of asymmetry.corresponding monobromides, clearly indicating that the Br3- ion gives the main contribution to the scattered intensity. In quaternary ammonium salts with hydrocarbon chains, electronegativity is not particularly important for the BrMBr bond polarization and the asymmetry is simply connected to Results and Discussion the orientation of the linear Br3- unit relative to the nitrogen atom.A completely symmetrical surrounding of the nitrogen In the previous sections we showed that the studied samples display dierent degrees of asymmetry of the tribromide ion. atom leads to a symmetric tribromide such as in (n- C4H9)N+Br3-.39 Table 4 BrMBr bond lengths in Br3- and the corresponding root The investigated samples all contain a long aliphatic chain mean square deviation.s1, s2 and s3 refer to the Br(1)MBr(2), (the cetyl fragment), which introduces an element of asymmetry Br(2)MBr(3) and Br(1)MBr(3) bond lengths respectively around the nitrogen atom, since the other substituents around the nitrogen are small. In the cetyltrimethylammonium salt sample Br(1)MBr(2) Br(2)MBr(3) Br(1)MBr(3) s1=s2 s3 this degree of asymmetry is high and, correspondingly, the /A° /A° /A° tribromide asymmetry is high.In the cetyltri-n-propyl G1 2.45 2.66 5.11 0.08 0.15 ammonium salt the length of the substituents is already G3 2.52 2.52 5.04 0.11 0.15 sucient to prevent a preferential interaction of one Br end G5 2.48 2.55 5.03 0.10 0.14 with the nitrogen and this tribromide is almost symmetric.G7 2.38 2.66 5.04 0.08 0.15 The behaviour of the cetylquinuclidinium ammonium salt is interesting, in that despite its apparent geometrical asymmetry a symmetric tribromide is observed. Most probably the aliphatic chains are forced into a ring geometry, leaving space for the nitrogen atom to adopt a similar interaction with both external Br atoms.The high degree of asymmetry for the cetyldimethyl(2- hydroxyethyl)ammonium is not surprising, since the geometrical asymmetry around the nitrogen atom is accompanied by the presence of an electronegative element, oxygen, which can polarize the BrMBr bonds. It is interesting to note the good correspondence between the Raman spectra and the X-ray diraction patterns, with a single stretching vibration frequency for symmetric tribromides and split vibration frequencies for highly asymmetric tribromide units.The slight asymmetry in sample G5 might have induced, as previously suggested, a Fermi resonance. In summary we studied the dependence of the degree of asymmetry of tribromides on the substituents of ammonium salts, by Raman spectroscopy and large angle scattering.The results show that the electronegativity of the ammonium substituent can polarize one Br external atom, yielding BrMBr bonds of dierent length. However even with non-polar substituents it is possible to obtain asymmetric tribromides, depending on the steric hin- Fig. 9 Dierence radial distribution curves: solid lines, experimental curves; dotted lines, theoretical curves drance and on the possibility for the tribromide units to 1336 J.Mater. Chem., 1997, 7(8), 1331–1337G. Paschina, G. Piccaluga and G. Pinna, J. Mater. Sci. L ett., 1988, interact with the nitrogen atom either asymmetrically or 7, 407. symmetrically. In all cases the tribromide geometry was linear. 18 T. Egami, J.Mater.Sci., 1978, 13, 2587. 19 T. Egami, in Glassy metals I, ed. H. J. Gunterodt and H. Beck, Springer Verlag, Berlin, 1981, p. 25. Dr. Luca Antonio Petrilli and Mr. Franco Dianetti of the 20 G. Fritsch and J. Keimel, Mater. Sci. Eng., A, 1991, 134, 888. Servizio Microanalisi, Area della Ricerca di Roma of the CNR 21 F. Haydu, Phys. Status Solidi A, 1980, 60, 365. are acknowledged for their kind contribution. 22 T. Egami, J. Appl. Phys., 1979, 50, 1564. 23 A. Mosset, J. Galy, E. Coronado, M. Drillon and D. Beltran, J. Am Chem. Soc., 1984, 106, 2864. 24 R. Caminiti, C. Munoz Roca, D. Beltran-Porter and A. Rossi, Z. References Naturforsch., T eil A, 1988, 43, 591. 25 A. Capobianchi, A. M. Paoletti, G. Pennesi, G. Rossi, R. Caminiti 1 H. Oyanagy, M. Tokumoto, T. Ishiguro, H.Shirakawa, H. and C. Ercolani, Inorg. Chem., 1994, 33, 4635. Nemoto, T. Matsuita, M. Ito and H. Kuroda, J. Phys. Soc. Jpn., 26 D. Atzei, D. De Filippo, A. Rossi, R. Caminiti and C. Sadun, 1984, 53, 4044. Spectrochim. Acta, Part A, 1995, 51, 11. 2 F. Demartin, P. Deplano, F. A. Devillanova, F. Isaia, V. Lippolis 27 R. Germani, P. P. Ponti, T. Romeo, G. Savelli, N. Spreti, and G.Verani, Inorg. Chem., 1993, 32, 3694. G. Cerichelli, L. Luchetti, G. Mancini and C. A. Bunton, J. Phys. 3 E. E. Having, K. H. Boswijk and E. H. Wiebenga, Acta Org. Chem., 1989, 2, 553. Crystallogr., 1954, 7, 487. 28 G. Cerichelli, G. Mancini and L. Luchetti, T etrahedron, 1994, 50, 4 G. R. Burns and R. M. Renner, Spectrochim. Acta, Part A, 1991, 3797. 47, 991. 29 (a) R. Caminiti, C.Sadun, V. Rossi, F. Cilloco and R. Felici, XXV 5 C. Romers and E. W. M. Keulemans, Proc. K Ned. Akad. Wet., Ser National Congress of Chemical Physics. Cagliari, 1991; (b) Pat. B, 1958, 61, 345. 01261484, June 23, 1993. 6 G. L. Breneman and R. D. Willet, Acta Crystallogr., Sect. B, 1969, 30 M. Carbone, R. Caminiti and C. Sadun, J. Mater. Chem., 1996, 25, 1073. 6, 1709. 7 G. L.Breneman and R. D. Willet, Acta Crystallogr., Sect. B, 1967, 31 K. Nishikawa and T. Iijima, Bull. Chem. Soc. Jpn., 1984, 57, 1750. 23, 467. 32 K. Nishikawa and N. Kitagawa, Bull. Chem. Soc. Jpn., 1980, 53, 8 D. W. Kalina, J. W. Lyding, M. T. Ratajack, C. R. Kannewurf and 2804. T. J. Marks, J. Am. Chem. Soc., 1980, 102, 7854. 33 V. Petkov and Y. Woseda, J. Appl. Crystallogr., 1993, 26, 295. 9 J. C. Evans and G. Y. S. Yo, Inorg. Chem., 1967, 6, 1483. 34 R. Caminiti, G. Johansson and I. Toth, Acta Chem. Scand., Ser. A, 10 A. Schnittke, H. Stegemann, H. Fuellbier and J. Gabrusenoks, 1986, 40, 435. J. Raman Spectrosc., 1991, 22, 627. 35 G. Johansson and R. Caminiti, Z. Naturforsch., T eil A, 1986, 41, 11 G. Cerichelli, C. Grande, L. Luchetti, G. Mancini and C. A. 1325. Bunton, J. Org. Chem., 1987, 52, 5167. 36 R. Caminiti, F. Cilloco and R. Felici,Mol. Phys., 1992, 75, 681. 12 M. T. Bianchi, G. Cerichelli, G. Mancini and F. Marinelli, 37 C. Andreani, F. Cilloco, L. Nencini, D. Rocca and R. N. Sinclair, T etrahedron L ett., 1984, 25, 5205. Mol. Phys., 1985, 55, 887. 13 G. Cerichelli, L. Luchetti and G. Mancini, T etrahedron L ett., 1989, 38 G. Ollis, V. J. James, D. Ollis and M. P. Boogard, Cryst. Struct. 30, 6209. Commun., 1976, 5, 39. 14 G. Cerichelli, C. Grande, L. Luchetti and G. Mancini, Org. Chem., 39 W. Gabes and H. Gerding, J.Mol. Struct., 1972, 14, 267. 1991, 56, 3025. 40 W. Gabes and H. Gerding, Recl. T rav. Chim. Pays-Bas, 1971, 90, 15 G. Cerichelli, L. Luchetti and G. Mancini, T etrahedron, 1996, 52, 157. 41 A. R. Campanelli and R. Scaramuzza, Acta Crystallogr., Sect. C, 2465. 1986, 42, 1380. 16 J. J. Del Val, L. Esquivias, P. L. Olano and F. Sanz, J. Non- Crystalline Solids, 1985, 70, 211. 17 A. Corrias, G. Ennas, G. Licheri, G. Marongiu, A. Musinu, Paper 6/04726B; Received 5th July, 1996 J. Mater. Chem., 1997, 7(8), 1331–1337 1337

ISSN:0959-9428    

DOI:10.1039/a604726b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Synthesis and characterization of micrometre-sized, polypyrrole-coated polystyrene latexes† Stuart F. Lascelles and Steven P. Armes* School of Chemistry, Physics and Environmental Sciences, University of Sussex, Falmer, Brighton, UK BN1 9QJ Near-monodisperse, micrometre-sized poly(N-vinylpyrrolidone)-stabilized polystyrene latexes have been coated with polypyrrole by in situ deposition of the conducting polymer from aqueous solution.If the conducting polymer overlayer is suciently thin, it lies inside the steric stabilizer layer and the coated latexes retain reasonable colloid stability. The polypyrrole loading on the latex particles was systemically varied over a wide range (1–50 mass%) simply by changing the initial latex concentration. Pressed pellet conductivity measurements on the dried composites indicated an anomalously low percolation threshold of 1–2 vol.%, which is consistent with the conducting polymer component lying on the surface of (rather than within the interior of ) the latex particles. IR spectroscopy studies of the composites confirmed the presence of several strong absorption bands due to the polypyrrole component.However, scanning electron microscopy (SEM) studies showed no evidence for the polypyrrole overlayer, which suggests that it must be very smooth and uniform. Disk centrifuge photosedimentometry (DCP) studies on the coated latexes revealed the presence of doublets and higher aggregates which indicates that these dispersions are weakly flocculated. However, if the polypyrrole overlayer is very thin (<10 nm) the coated particles exist in solution mainly as singlets with relatively few doublets or larger floccs.Polystyrene latex can also be coated with other conducting polymers such as polyaniline and poly(3,4-ethylenedioxythiophene). Finally, both polyelectrolytes and physically adsorbed poly(N-vinylpyrrolidone) can provide an eective steric barrier to allow controlled deposition of the conducting polymer overlayer.Polypyrrole is a relatively air-stable organic conducting poly- With regard to conducting polymer deposition onto latex substrates, Garnier’s group have described the synthesis of mer which suers from limited processability.1 In its doped, polypyrrole-coated polystyrene latexes of sub-micrometre conductive form it is usually insoluble in all organic solvents, dimensions11 whilst Yamamoto and co-workers reported coat- probably due to some degree of cross-linking.2‡ In order to ing styrene–butadiene latexes with polypyrrole using the improve this material’s poor processability various groups H2O2–HBr–Fe3+ oxidant system.12 In both cases the authors have reported the synthesis of colloidal dispersions of polypyrsuggested that the presence of acidic surface functional groups role particles.3–6 Such particles are easily prepared by carrying (sulfonic or carboxylic acids) were important for successful out the chemical polymerization of pyrrole in the presence of composite formation.Both groups claimed that their composite a suitable water-soluble polymer such as methyl cellulose or particles were colloidally stable but no experimental evidence poly(vinyl alcohol).The water-soluble polymer adsorbs onto was provided in either case to support these claims. At Sussex the precipitating polypyrrole nuclei and prevents further aggrewe have spent considerable time and eort13 attempting to gation via a steric stabilization mechanism.7 Thus the ‘core’ of repeat these syntheses using the identical latex substrates (same these particles contains the conducting polymer, which is manufacturer, particle size, surface charge, etc.) described by surrounded by an outer layer of adsorbed, solvated waterthe French and Japanese groups.Invariably we observed soluble polymer. significant flocculation/precipitation of the composite particles: Several groups have reported using inorganic oxide disper- these observations are consistent with the relatively high sions as colloidal substrates for the deposition of conducting Hamaker constant reported for polypyrrole by Vincent’s polymers.For example, Partch et al. have shown that colloidal group.14 It seems, at the very least, that these syntheses are inorganic oxide particles containing transition metal oxidants not particularly robust with respect to the colloid stability of such as Fe3+ or Ce4+ can act as particulate oxidants for the the latex composite particles.15 Indeed, Beadle et al.have polymerization of pyrrole monomer at elevated temperatures.8 shown that the chemical polymerization of aniline in the However, these composite particles are not normally colloidally presence of chlorinated copolymer latex particles always led stable unless a water-soluble polymer such as poly(vinyl to the macroscopic precipitation of the polyaniline–copolymer alcohol) or poly(N-vinylpyrrolidone) is added to provide a latex composites.16 In this latter study the conducting polymer steric barrier to particle aggregation.9 Similarly, Armes et al.loading in the composite was conveniently controlled by have chemically polymerized pyrrole (and aniline) in the pres- varying the initial copolymer latex concentration at a fixed ence of micrometre-sized silica particles using soluble oxidants: concentration of aniline monomer and oxidant. conducting polymer–silica composites were obtained as macro- In 1993 Yoshino et al.reported the deposition of a thin scopic precipitates.10 Scanning electron microscopy (SEM) (10–20 nm) polypyrrole overlayer onto 10 mm diameter poly- studies confirmed that the conducting polymer component had ethylene spheres.17 These coated particles were then mixed a globular morphology and that its deposition was inhomo- with the original uncoated polyethylene spheres and hot- geneous: some silica particles were thickly coated with con- pressed at 90–130°C to form semiconductive composites.ducting polymer while others remained substantially uncoated. These composites exhibited very low percolation thresholds at polypyrrole loadings of 0.1–0.2 vol.%. However, the Japanese † ©British Crown Copyright 1996/DERA. Published with the per- group’s study emphasized the electrical properties of the hot- mission of the controller of Her Brittannic Majesty’s Stationery Oce.pressed composite materials and potential applications; the ‡ Very recently a Korean group have reported that, under carefully polypyrrole-coated polyethylene particles themselves were not controlled synthesis conditions, it is possible to obtain non-cross- extensively characterized.For example, no attempt was made linked polypyrrole chains which can be rendered soluble in m-cresol using surfactant-type dopant anions. to examine the morphology of the polypyrrole overlayer by J. Mater. Chem., 1997, 7(8), 1339–1347 1339electron microscopy and no spectroscopic studies were reported. Recently Wiersma et al. at DSMResearch have demonstrated that submicrometre-sized sterically stabilized latex particles can be coated with polypyrrole (or polyaniline) in aqueous media to form conducting polymer composite latexes with good colloidstability. 18 Theseworkers emphasisethat usinganadsorbed non-ionic polymeric stabilizer such as poly(ethylene oxide) or hydroxymethylcellulose in these syntheses is critical for producing stable colloidal dispersions at high Fe3+ concentrations: all control experiments carried out in the absence of such non- Fig. 1 Schematic formation of polypyrrole-coated latex particles.Note ionic stabilizers resulted in macroscopic precipitation.18a that the deposited conducting polymer overlayer lies inside the solvated steric stabilizer layer. Transmission electron microscopy (TEM) studies on the coated particles provided direct evidence for a ‘core–shell’ morphology and both aqueous electrophoresis and dielectric measurements supported this observation.18b It was suggested that the con- and a magnetic stirrer bar.After heating to 70°C under a ducting polymer is formed as a thin layer at the surface of the nitrogen blanket, the reaction vessel was purged with a stream latex particles without significantly interfering with the steric of nitrogen for 1 h at 70°C.Then a solution of azoisobutyron- stabilization mechanism conferred by the solvated outer layer of itrile initiator (0.50 g) predissolved in styrene monomer (50 g) non-ionic polymer (see Fig. 1). The Dutch group have focused was added to the reaction vessel with vigorous stirring.The on coating low Tg latexes of 50–500 nm diameter based on styrene polymerization was then allowed to proceed for 24 h polyurethane, poly(vinyl acetate) or alkyd resins.§ Unlike the before cooling to room temperature. The resulting latex par- sterically-stabilized polypyrrole dispersions described above, ticles were then purified by repeated centrifugation–redisper- such composite particles exhibit remarkably good film formation sion cycles (replacing successive supernatants with deionized properties at room temperature, despite encapsulation of the low water).The latex syntheses were carried out three times under Tg latex component by an outer layer of high Tg conducting the above conditions (or similar) using PVP stabilizers having polymer. Solid-state conductivities were reported to lie in the molecular masses of either 44000 or 360000.range 10-5–101 S cm-1, depending on the conducting polymer loading.However, thereis some evidencethat thecoating process Polystyrene latex characterization may be somewhat inhomogeneous and/or inecient: depending The latexes were sized using disk centrifuge photosedimento- on the reaction conditions, a substantial fraction of the latex metry (DCP).All measurements were carried out using a substrate can remain uncoated.19 Brookhaven BI-DCP instrument, operating in the line start In a recent communication20 we reported that the DSM mode. Samples for DCP analysis were prepared by adding a coating protocol can be easily adapted in order to coat few drops of the aqueous latex mixture to 3 ml of a 152 v/v% micrometre-sized, poly(N-vinylpyrrolidone)-stabilized poly- methanol–water mixture.This solution was immersed in an styrene latex particles with a polypyrrole overlayer (see Fig. 1). ultrasonics bath for 1–10 min just prior to DCP analysis. The Polystyrene was selected as a ‘model’ substrate since it has a centrifugation rate was adjusted to between 2000 and 3000 rpm, relatively high Tg (rigid, non-deformable particles) and latexes depending on the size of the particles being measured.A can be readily synthesized with narrow size distributions over particle density of 1.05 g cm-3 was assumed for the polystyrene a wide size range (50 nm–10 mm).21,22 Potential applications latex particles.¶ DCP confirmed that the three latexes had for these micrometre-sized coated latexes include an improved relatively narrow size distributions, yielding mass-average par- stationary phase for electrochromatography23 and novel ticle diameters of 1.57±0.12, 1.80±0.06 and 1.64±0.03 mm.‘marker’ particles for visual agglutination diagnostic assays.24 These sizes were confirmed by TEM (Hitachi 7100 instrument) In the present study we describe our preliminary findings in and SEM studies (Leica Stereoscan 420 instrument) on gold- more detail and include our latest results.coated dried latexes. One of the latexes was analyzed by FTIR spectroscopy (KBr disk) using a Nicolet Magna Series II Experimental spectrometer (64 scans, 4 cm-1 resolution). Materials Polypyrrole coating protocol Styrene (Aldrich) was purified by passing through a column of activated neutral alumina to remove the inhibitor.Two grades PVP-stabilized polystyrene latexes. FeCl3·6H2O oxidant of poly(N-vinylpyrrolidone) (PVP), with molecular masses of (0.91 g) was dissolved into an aqueous solution of the PVP- 44000 and 360000 respectively, were obtained from BDH and stabilized polystyrene latex (0.10–12.40 g dry mass), in a screw- used without further purification.Aliquat 336 (Aldrich) and cap bottle with magnetic stirring. Pyrrole (0.10 ml) was added azoisobutyronitrile (BDH) were used as supplied. Pyrrole was via a syringe and the polymerization was allowed to proceed kindly donated by BASF; it was purified by passing through for 24 h. The coated latex particles were then purified by a column of activated basic alumina and stored at -15°C repeated centrifugation–redispersion cycles (successive super- prior to use.FeCl3·6H2O, (NH4)2S2O8, H2O2 (27.5 mass% natants being replaced by deionized water) in order to remove soln.) and HBr (48% soln.) were all purchased from Aldrich the inorganic by-products of the pyrrole polymerization. CHN and used without further purification.elemental microanalyses were determined for each oven-dried coated latex. The polypyrrole loadings of each of the coated Polystyrene latex synthesis ¶ The DCP technique is seldom applied to sterically stabilized latex The PVP steric stabilizer (7.5 g) and tricaprylylmethylammon- particles because the solvated stabilizer layer leads to considerable ium chloride (Aliquat 336; MeN[(CH2)7Me]3Cl) surfactant uncertainty in their eective particle density, which is an important co-stabilizer (2.0 g) were dissolved in isopropyl alcohol input parameter for DCP.However, for the micrometre-sized latexes (400 ml) in a 1 l round-bottomed flask fitted with a condenser studied here, the stabilizer layer d is so thin relative to the particle diameter D (typically d is 10–20 nm and D is at least 1570 nm) that this potential source of error was considered to be negligible.Indeed, § DSM Research are currently marketing a commercial latex for antistatic and anticorrosion applications under the tradename excellent agreement was always obtained between the DCP data and our electron microscopy studies. ConQuest. 1340 J. Mater.Chem., 1997, 7(8), 1339–1347latexes were determined by comparing their nitrogen contents Results and Discussion to that of the corresponding uncoated polystyrene latex (N ca. Three poly(N-vinylpyrrolidone)-stabilized polystyrene latexes 0.17–0.21%), and conventional polypyrrole ‘bulk powder’ (N= were synthesized according to protocols described in the 16.5±0.5%) synthesized in the absence of any latex particles.literature.21,22 Disk centrifuge and electron microscopy studies Polystyrene latexes were also coated with polypyrrole using confirmed that these three latexes were in the micrometre size two alternative oxidant systems: (NH4)2S2O8 (0.38 g) and range and that each latex had a reasonably narrow size H2O2 (0.20 ml, 27.5% soln.). With the latter oxidant system distribution.A typical TEM of one of the latexes is shown in both HBr (0.30 ml, 48% soln.) and a catalytic amount of FeCl3 Fig. 2. FTIR spectroscopy studies confirmed the presence of a were added, as described by Yamamoto and co-workers.12,25 weak feature at 1660 cm-1 attributable to the poly(N-vinyl- Finally, in some experiments the polystyrene latexes were pyrrolidone) stabilizer.The nitrogen microanalyses for these coated with thin overlayers of either polyaniline or poly(3,4- micrometre-sized latexes were typically 0.17–0.21%, which is ethylenedioxythiophene) using (NH4)2S2O8 or Ce(SO4)2 consistent with a stabilizer content of ca. 1.5 mass% (this may respectively. be an upper limit value since it was assumed that the concentration of the nitrogen-containing Aliquat 336 surfactant was Charge-stabilized polystyrene latex.The procedure for coat- negligible). Using these data the adsorbed amount of PVP ing the 1 mm diameter charge-stabilized polystyrene latex stabilizer on the latex particles was calculated to be approxi- (kindly donated by Polymer Laboratories, UK) was essentially mately 4.5 mg m-2. Recent X-ray photoelectron spectroscopy the same as that described for the sterically stabilized poly- (XPS) studies have confirmed the presence of the poly(N- styrene latexes, except that PVP was physically absorbed onto vinylpyrrolidone) stabilizer at the surface of our polystyrene the latex prior to the addition of the oxidant.The PVP was latex particles.27 present in significant excess to ensure complete coverage of For the conducting polymer coating experiments the concen- the charge-stabilized latex.Thus, PVP (94.6 mg, MW 44000) trations of pyrrole monomer and FeCl3 oxidant were kept was dissolved in 5.85 g of an aqueous dispersion of the latex constant. The polypyrrole loading was controlled simply by (10 mass% solids content). This reaction solution was then varying the initial concentration, and hence the available stirred for 24 h prior to addition of the FeCl3 oxidant (0.566 g), surface area, of the polystyrene latex particles.The polypyrrole followed by injecting the pyrrole monomer (62.5 ml) via a loadings on the latex composites determined from nitrogen micropipette. Latex clean-up was carried out as described microanalyses were in very good agreement with the theoretical earlier.As a control experiment, deposition of polypyrrole values (Table 1). This result indicates that the pyrrole was onto the charge-stabilized polystyrene latex particles was also quantitatively polymerized under the latex coating conditions. attempted in the absence of PVP. Beadle et al. also reported reasonable control over conducting polymer loading for the deposition of polyaniline onto low Tg chlorinated latexes.16 However, the latex composites were Other steric stabilizers. A polystyrene latex of 670 nm diam- obtained in the form of macroscopic precipitates in this latter eter was synthesized in alcoholic media using a poly[(2- study and larger dierences between the theoretical and actual dimethylamino)ethyl methacrylate–block–methyl metha- conducting polymer loadings were observed.crylate] (PDMAEMA–PMMA) copolymer stabilizer as In the present study, there were no signs of gross precipi- described previously.26 The PMMA block acts as an anchor tation except for sample 1 (Table 1), which contained a very and adsorbs onto the surface of the polystyrene particles while high polypyrrole loading (ca. 51%). Subsequent inspection of the PDMAEMA block acts as the solvated stabilizer layer. After centrifugation clean-up, this latex was coated with polypyrrole using an FeCl3 oxidant in 1.2 M HCl. Since the PDMAEMA block is highly protonated in this acidic solution this steric stabilizer behaves as a cationic polyelectrolyte during the pyrrole polymerization. Characterization of the polypyrrole-coated polystyrene latexes Chemical composition. FTIR spectra of latex composites dispersed in KBr disks were recorded using the instrument described above.After clean-up, all uncoated and coated latexes were oven-dried overnight at 60°C prior to CHN elemental microanalyses at an independent laboratory (Medac Ltd at Brunel University, UK). Particle size analysis and degree of dispersion.Disk centrifuge studies were carried out as described earlier. It was assumed that the polypyrrole-coated polystyrene latexes had the same scattering characteristics as carbon black. The densities of the dried composite latexes were determined using helium pycnometry (Accupyc 1330, Micromeritics). Conductivity and conductivity stability measurements.The conductivities (s) of compressed pellets of the polypyrrolecoated polystyrene latexes were determined using standard four-point probe techniques at room temperature. These measurements were repeated at regular intervals over a period of several months. Random errors associated with these measurements are estimated to be approximately 10%, with Fig. 2 Typical transmission electron micrograph of an uncoated, micrometre-sized poly(N-vinylpyrrolidone)-stabilized polystyrene latex an additional systematic error of ca. 5–10%. J. Mater. Chem., 1997, 7(8), 1339–1347 1341Table 1 Eect of varying the total surface area of latex available for the deposition of polypyrrole (PPY) on the polypyrrole loading and layer thickness, the colloid stability and the electrical conductivity of the coated latex particlesa latex surface theoretical PPY actual PPY calculated PPY layer colloid stability of sample area/m2 loading (mass%) loading (mass%)b thickness/nm PPY-coated latexc sd/S cm-1 1 0.4 52.7 51.1 164.3 unstable 3 2e 1.2 25.6 25.1 59.7 flocc’d 4 3 1.8 18.2 18.4 41.1 flocc’d 6 4e 2.3 15.4 16.6 36.5 flocc’d 3 5 3.6 10.0 9.9 20.5 stable 2 6 5.4 6.9 6.1 12.3 stable 2 7 7.1 5.3 5.6 11.2 stable 2 8 7.1 4.7 4.6 10.3 stable 0.8 9 10.0 3.4 3.5 7.8 stable 0.2 10 15.0 2.3 2.1 4.6 stable 6×10-2 11 27.4 1.3 1.2 2.6 stable 3×10-3 12 39.4 0.9 1.0 2.2 stable <10-6 aAll polystyrene latexes were synthesized using PVP of MW=44000 unless indicated otherwise.bDetermined by reduced nitrogen content relative to polypyrrole ‘bulk powder’ using CHN elemental microanalyses. cDetermined by DCP; flocc’d=flocculated.dDetermined by the four-point probe method on compressed pellets at room temperature. eStabilized using PVP of MW 360000. The 1.57 mm diameter polystyrene latex was used for samples 1, 3, 5, 6 and 7. The 1.64mm diameter latex was used for samples 2 and 4 and the 1.80 mm diameter latex was used for samples 8–12.this destabilized latex using SEM revealed both the original onto latex particles which are already well coated with a micrometre-sized polystyrene latex particles and also the dis- solvated, adsorbed layer of PVP stabilizer. In this regard, we tinctive, submicrometre-sized globular morphology of poly- note that the elegant XPS and time-of-flight secondary ion pyrrole ‘bulk powder’ formed as a separate sub-phase [see mass spectroscopy (TOF-SIMS) measurements on micrometre- Fig. 3(a)]. In contrast, as the latex surface area available for sized polystyrene latexes reported by Deslandes et al. are polypyrrole deposition was increased, little or no precipitation consistent with a relatively ‘patchy’, rather than continuous, was observed and SEM studies showed no evidence for any layer of PVP stabilizer.28 Even when the surface concentration separate polypyrrole subphase.At a polypyrrole loading of ca. of PVP is relatively high the stabilizer chains are only anchored 25% (sample 2 in Table 1) the polystyrene particles are uni- via a small number of monomer residues, with the rest of the formly coated with a globular polypyrrole overlayer [Fig. 3(b)]. chain being solvated as loops and tails, rather than adsorbed For polypyrrole loadings lower than 10% (e.g., sample 6) the as trains at the particle surface. Thus, a substantial fraction of morphology of the coated latexes was remarkably smooth and the total surface area of the sterically stabilized latex particles uniform [Fig. 3(c)], with no direct evidence for the deposited is actually available for the deposition of polypyrrole.Indeed, polypyrrole overlayer. It is interesting to compare this uniform in our layer thickness calculations using eqn. (1) it is assumed polypyrrole morphology with the globular, inhomogeneous that all of the latex surface becomes coated with conducting morphology reported by Armes et al. for micrometre-sized polymer.silica particles coated with polypyrrole or polyaniline overlay- The colloid stability of the polypyrrole-coated polystyrene ers at similarly low loadings.10 It appears that the addition of latexes was assessed using DCP. Typical mass average particle a suitable polymeric steric stabilizer to maintain the dispersion size distributions of a coated and an uncoated latex are shown stability of the colloidal substrate during conducting polymer in Fig. 4. Since the polypyrrole overlayer is relatively thin (see deposition substantially aects the morphology of the above), the main peak in the size distribution of the coated overlayer. latex approximately coincides with that of the uncoated precur- The bulk densities of polystyrene and polypyrrole were sor latex.However, there is both a secondary peak and also a determined to be 1.05 and 1.46 g cm-3 by helium pycnometry. distinct ‘tail’ to higher particle diameter in the size distribution Since SEM studies confirm that the polypyrrole overlayer is curve of the coated latex. A simple calculation confirms that relatively smooth and uniform, for a given polystyrene latex the position of the secondary peak corresponds to that expected diameter it is possible to calculate the average thickness, x, of for doublets (aggregation of two latex particles), with the tail the deposited polypyrrole overlayer using eqn.(1): corresponding to larger flocc structures (triplets, multiplets, etc.). Given the relatively high Hamaker constant reported for x=RCAM2r1 M1r2 +1B1/3-1D (1) polypyrrole,14 it is perhaps not surprising that the polypyrrole overlayer can interfere with the steric stabilization mechanism responsible for maintaining the colloid stability of the latex where R is the radius of the uncoated latex particles, M1 and particles.However, provided that the polypyrrole overlayer is r1 are the mass fraction and density of the polystyrene compo- suciently thin (relative to the stabiliser layer thickness), it is nent and M2 and r2 are the mass fraction and density of the nevertheless possible to minimize destabilization of the coated polypyrrole component, respectively.For mass loadings of latex particles. For example, DCP size distribution data for a 1–10% the polypyrrole overlayer thicknesses lie in the range 1.6 mm polystyrene latex synthesized using a high molecular 2–20 nm (Table 1).According to Yamamoto and co-workers, mass (360000) PVP stabilizer coated with ca. 10% polypyrrole PVP chains with a molecular mass of 360000 (i.e. samples 2 by mass is depicted in Fig. 5(a). This size distribution contains and 4 in Table 1) have an adsorbed layer thickness of ca. very little evidence for flocculation and is very similar to that 20–30 nm.25 Obviously PVP chains of lower molecular mass of the original uncoated latex.On the other hand, a 1.6 mm would give rise to thinner layers of steric stabilizer. Thus it is latex with a similar polypyrrole loading synthesized using a apparent that, at lower mass loadings (<10%), the conducting low molecular mass (MW 44000) PVP stabilizer (10% poly- polymer overlayer is either smaller than, or comparable to, the pyrrole by mass) is clearly appreciably flocculated [Fig. 5(b)]. thickness of the adsorbed PVP layer. On the other hand, at It is well known that increasing the molecular mass of a loadings of 16.6% (sample 4 in Table 1) or higher, the polypyrhomopolymer stabilizer provides a thicker steric barrier role overlayers become much less uniform and the assumptions towards particle aggregation.Clearly, the additional layer made in deriving eqn. (1) are no longer valid. It is interesting that polypyrrole should deposit so readily thickness provided by a higher molecular mass PVP stabilizer 1342 J. Mater. Chem., 1997, 7(8), 1339–1347Fig. 4 Mass-average particle size distribution curves obtained for (a) an uncoated polystyrene latex and (b) a polypyrrole-coated latex (polypyrrole loading of 4.6%; sample 11 in Table 1) obtained using the disk centrifuge Fig. 5 Mass-average particle size distribution curves obtained for two polypyrrole-coated polystyrene latexes obtained using the disk centrifuge: (a) PVP stabilizer molecular mass=360000 and (b) PVP stabilizer molecular mass=44000.The polypyrrole loading in each case is approximately 10% by mass. of dispersion, i.e. the proportion of aggregates decreased rela- Fig. 3 Scanning electron micrographs of a polypyrrole-coated poly- tive to the singlet peak. styrene latex with (a) a relatively high polypyrrole loading (51.1%); The relationship between log conductivity and conducting (b) an intermediate polypyrrole loading (25.1%); (c) a relatively low polypyrrole loading (6.1%).Note the absence of any globular polymer volume fraction for the polypyrrole-coated poly- polypyrrole morphology at the lowest loading styrene particles is shown in Fig. 6(a). A remarkably low percolation threshold of ca. 1–2 vol. % is observed for these latex composites, presumably since the conductive component is located exclusively on the outside of the latex particles.SEM can lead to a considerable improvement in the degree of dispersion of the coated latex particles. A more quantitative studies on fractured pellets have confirmed that, although somewhat deformed by the pelletisation process, the latex assessment of the influence of the polypyrrole overlayer on the latex colloid stability would require accurate measurement of particles remain intact.Thus, the coated particles eectively behave like micrometre-sized polypyrrole particles and ecient the thickness of the adsorbed PVP stabilizer layer. Unfortunately, given the relatively large particle size of the charge transport through the material can occur without significant interference from the underlying electrically insulat- polystyrene latexes, this is an experimentally inaccessible parameter since the PVP layer thickness is considerably less than ing polystyrene component (Fig. 7). This conductivity data stands close comparison with the best (lowest) percolation the observed standard deviation of the latex size distribution. Finally, it was noted that ultrasonication of the coated latexes thresholds claimed for other conducting polymers such as polyaniline.29,30 Similar results have been reported by just prior to DCP analysis temporarily improved their degree J.Mater. Chem., 1997, 7(8), 1339–1347 1343Fig. 6 Variation of conductivity with polypyrrole loading for (a) polypyrrole- coated polystyrene latex composites and (b) heterogeneous mixtures of the dried polystyrene latex mixed with polypyrrole bulk powder.Note the much lower percolation threshold observed in the former system. Fig. 8 FTIR spectra of (a) uncoated polystyrene latex particles; (b) heterogeneous mixture of 6.0% polypyrrole mixed with 94.0% polystyrene latex particles; (c) polypyrrole-coated polystyrene latex particles (polypyrrole loading 5.6%); (d) polypyrrole chloride bulk powder.The absorption bands due to polypyrrole are strong in spectrum (c) but much weaker in spectrum (b). This is consistent with a core–shell particle morphology for the latex composites. IR spectrum of a heterogeneous mixture comprising 94% uncoated polystyrene latex and 6% polypyrrole ‘bulk powder’ was recorded. The polypyrrole bands are barely detectable in this latter spectrum, even though the polypyrrole content is actually slightly higher than that of the coated latex.This suggests that the ‘core–shell’ morphology of the polypyrrole- Fig. 7 Schematic representation of charge transport between polypyr- coated polystyrene latex particles leads to enhanced IR absorp- role-coated polystyrene latex particles at the microscopic level.tion by the conducting polymer component. This observation Interparticle charge carrier transport can occur via surface conduction in the polypyrrole overlayer with minimal interference from the is probably related to the latex particle size being similar to underlying electrically insulating polystyrene cores. the wavelength of the IR radiation. We have observed similar ‘enhanced absorption’ eects in the Raman spectra of these composite particles; these results are presented in the follow- Yoshino’s group for non-colloidal 35 mm polyethylene spheres, although somewhat lower ‘plateau’ conductivities were ing paper.32 It is well known that the environmental stability of polypyr- obtained by these workers.17 As a control experiment, heterogeneous mixtures of pre-weighed quantities of dried uncoated role doped with chloride anions is relatively poor: significant conductivity decay over a period of a few days is not uncom- polystyrene latex combined with polypyrrole ‘bulk powder’ (synthesized as a precipitate in the absence of latex) were mon.33 On the other hand, much better conductivity stability is usually found for polypyrrole doped with aromatic sulfonate prepared. These control samples gave ‘classical’ percolation behaviour [Fig. 6(b)], with a conductivity threshold of ca. anions. In particular, Kuhn et al. at the Milliken Research Corporation have developed polypyrrole syntheses involving 20 vol. %.31 Thus, there is no doubt that the ‘core–shell’ morphology of the polypyrrole-coated polystyrene latexes is aromatic sulfonate dopant anions which have been optimized for coating high surface area textile fibres.34 This Milliken responsible for the unusual conductivity behaviour of these materials. coating protocol was utilized in the present study in order to coat the polystyrene latex particles.Similar polypyrrole load- IR spectra of a dried polystyrene latex and a polypyrrolecoated polystyrene latex are shown in Fig. 8. The spectrum for ings and conductivities were obtained, but the coated latexes exhibited somewhat poorer colloid stabilities as measured by the uncoated latex is typical for that of polystyrene, with an additional weak feature at ca. 1660 cm-1 attributable to the the DCP technique. However, the long-term conductivity stability of the aromatic sulfonate-doped polypyrrole overlayer pyrrolidone carbonyl of the PVP stabilizer.The coated latex (polypyrrole loading ca. 5.6% by mass) contains several is somewhat improved compared to that of the chloride-doped polypyrrole material (Fig. 9). The eect of using chemical additional strong bands at 1549, 1316, 1186 and 910 cm-1 due to the doped polypyrrole component.1,3 Even allowing for oxidants other than FeCl3 was also examined.Both (NH4)2S2O8 and H2O2–HBr–Fe3+ were used to synthesize polypyrrole being a strong IR absorber, these bands seem rather intense given the relatively low concentration of the polypyrrole-coated polystyrene latexes (Table 2). Similar conducting polymer loadings were obtained, but conductivities conducting polymer component. As a control experiment, an 1344 J.Mater. Chem., 1997, 7(8), 1339–1347Fig. 9 Typical conductivity decay curves for pressed pellets of Fig. 10 Mass-average particle size distribution curve obtained using polypyrrole-coated polystyrene latex particles: (a) aromatic sulfonate the disk centrifuge for a polypyrrole-coated, polyelectrolyte-stabilized dopant anions according to ref. 32 and (b) chloride dopant anions.polystyrene latex (the polypyrrole loading is 5.0% by mass). The The polypyrrole loadings are 5.4% and 4.4% by mass respectively. polyelectrolyte stabilizer is a cationic PDMAEMA–PMMA block copolymer (see main text). were somewhat lower, probably due to over-oxidation of the polypyrrole chains.35 any steric stabilizer layer, significant aggregation of this latex occurred soon after addition of the pyrrole monomer Finally, the deposition of conducting polymers other than polypyrrole has also been investigated. Both polyaniline- and [Fig. 12(a)]. However, physical adsorption of an excess of PVP (MW=44000) onto this charge-stabilized latex, followed by poly(3,4-ethylenedioxythiophene)-coated latexes have been synthesized with conducting polymer loadings of 5–10 mass%.addition of the FeCl3 and pyrrole reagents, allowed the preparation of a polypyrrole-coated polystyrene latex with reason- DCP analysis confirmed that these coated latexes also have reasonable degrees of dispersion. Pressed pellet conductivities able colloid stability [Fig. 12(b)] and a conducting polymer loading of ca. 8.8 mass%. This result confirms that a chemically were as high as 1 S cm-1 for the polyaniline-coated latexes and 10-2 S cm-1 for the poly(3,4-ethylenedioxythiophene)- grafted stabilizer layer is not essential: a merely physically adsorbed stabilizer is sucient to allow good control over the coated latexes.All of the examples of conducting polymer-coated latexes polypyrrole deposition process. Moreover, it suggests a more general coating protocol, which in principle could be extended claimed in the DSM patent12 involve the use of a non-ionic polymeric stabilizer [either poly(ethylene oxide) or cellulosic to include colloidal substrates other than polymer latexes, e.g.inorganic oxides, silicates, sulfides, etc. derivatives] which is chemically grafted onto the surface of the latex particles. Two obvious questions which arise are the following: (1) Can polyelectrolytes act as eective steric stabiliz- Conclusions ers for the deposition of polypyrrole? (2) Is a chemically grafted stabilizer essential for the successful deposition of polypyrrole In summary, it has been shown that sterically stabilized, micrometre-sized polystyrene latexes can be readily coated without significant loss of colloid stability? With regard to the first question, the successful coating of a PDMAEMA–PMMA- with a very thin conducting polymer overlayer.In the case of polypyrrole, good control over the conducting polymer loading stabilized polystyrene latex with ca. 5% polypyrrole by mass (Fig. 10) confirms that this cationic polyelectrolyte block has been demonstrated simply by varying the initial concentration of latex particles used in the coating protocol.At low copolymer acts as a reasonably eective steric stabilizer. Thus, a non-ionic steric stabilizer does not appear to be an essential loadings (<10 mass%) the deposited polypyrrole layer is very smooth and uniform. The intensity enhancement of IR bands pre-requisite. In most of the present work we have utilized poly(N-vinyl pyrrolidone): according to Deslandes et al.this attributable to polypyrrole and the observation of an anomalously low conductivity percolation threshold are both consist- steric stabilizer probably becomes grafted onto the latex particles during the free-radical dispersion polymerization of styr- ent with a ‘core–shell’ morphology for the composite particles.The disk centrifuge has been shown to be an excellent technique ene.28 Thus, in order to address the second question we attempted to coat an aqueous dispersion of ‘naked’, charge- for assessing the colloid stability of the coated particles, which are usually weakly flocculated. However, if the polypyrrole stabilized, 1 mm polystyrene latex (Fig. 11). In the absence of Table 2 Polypyrrole (PPY) mass loadings, colloid stability and pellet conductivities for polypyrrole-coated polystyrene latexes synthesized using alternative oxidant systems oxidant PVP molecular PPY loading colloid stability of sample system mass/g mol-1 (mass%)a PPY-coated latexb sc/S cm-1 13 H2O2–HBr–Fe3+ 44000 7.3 stable 0.1 14 (NH4)2S2O8 44000 8.9 stable 0.2 15 ‘Milliken’ 360000 8.1 flocc’d 2.0 aDetermined by reduced nitrogen content relative to polypyrrole ‘bulk powder’ using CHN elemental microanalyses.bDetermined by DCP. cDetermined by the four-point probe method on compressed pellets at room temperature. Samples 13 and 14 were prepared with the 1.57mm diameter polystyrene latex and sample 15 with the 1.64 mm diameter latex. J. Mater. Chem., 1997, 7(8), 1339–1347 1345overlayer is relatively thin compared to the steric stabilizer layer then a reasonable degree of dispersion can be achieved.Higher polypyrrole mass loadings generally result in higher electrical conductivity but poorer colloid stability. Nevertheless, it is possible to obtain conductivities in the 10-1–100 S cm-1 range and colloid stabilities at polypyrrole loadings of 3–6%.Incorporation of aromatic sulfonate anions into the pyrrole polymerization leads to improved conductivity stability but more flocculated dispersions. Other conducting polymers such as polyaniline and poly(3,4-ethylenedioxythiophene) can also be deposited onto polystyrene latexes without significant loss of colloid stability. With regard to the nature of the steric stabilizer layer, cationic polyelectrolytes such as PDMAEMA–PMMA can also act as eective steric stabilizers.Thus, a non-ionic steric stabilizer is not a prerequisite for the successful synthesis of conducting polymer-coated latexes. Finally, it has been shown that a chemically grafted steric stabilizer is not essential: under appropriate conditions a physically adsorbed poly(N-vinylpyrrolidone) stabilizer can prevent significant particle aggregation and hence allow good control over the conducting polymer deposition process.We wish to acknowledge the Defence Research Agency, Fort Halstead, UK for financial support in the form of a PhD studentship to S.F.L. The EPSRC is acknowledged for capital equipment funds for the purchase of the disk centrifuge (GR/H93606).Both the Defence Research Agency and DSM Fig. 11 Schematic representation of the synthesis of polypyrrole- Research are acknowledged for partial funding of the FTIR coated polystyrene latexes using a physically adsorbed poly(N- spectrometer. vinylpyrrolidone) stabilizer References 1 See, for example, Proceedings of the 1992 International Conference on Synthetic Metals (ICSM ’92), Synth.Met., 1993, 55–57. 2 (a) F. P. Bradner, J. S. Shapiro, H. J. Bowley, D. L. Gerrard and W. F. Maddams, Polymer, 1989, 30, 914; (b) J. Y. Lee, D. Y. Kim and C. Y. Kim, Synth.Met., 1995, 74, 103. 3 R. B. Bjorklund and B. Liedberg, J. Chem. Soc., Chem. Commun., 1986, 1293. 4 S. P. Armes and B. Vincent, J. Chem. Soc., Chem. Commun., 1987, 288. 5 N. Cawdery, T. M. Obey and B.Vincent, J. Chem. Soc., Chem. Commun., 1988, 1189. 6 S. P. Armes, M. Aldissi, G. C. Idzorek, P. W. Keaton, L. J. Rowton, G. L. Stradling, M. T. Collopy and D. B. McColl, J. Colloid Interface Sci., 1991, 141, 119. 7 (a) Dispersion Polymerisation in Organic Media, ed. K. E. Barrett, J. Wiley, New York 1975; (b) D. H. Napper, Polymeric Stabilization of Colloidal Dispersions, Academic, London, 1983. 8 R. Partch, S. G. Gangolli, G. Matijevic, W. Cai and S. Arajs, J. Colloid Interface Sci., 1991, 144, 27. 9 R. Partch, personal communication, 1994. 10 S. P. Armes, S. Gottesfeld, J. G. Beery, F. Garzon and S. F. Agnew, Polymer, 1991, 32, 2325. 11 (a) A. Yassar, J. Roncali and F. Garnier, Polym. Commun., 1987, 28, 103; (b) A. Yassar, J. Roncali, F. Garnier, M. J. Michel and C.Bonnebat, Fr. Pat., 2616790. 12 C. F. Liu, T. Maruyama and T. Yamamoto, Polym. J., 1993, 25, 363. 13 P. H. Beadle, D. Phil. T hesis, University of Sussex, UK, 1995. 14 G. Markham, T. M. Obey and B. Vincent, Colloids Surf., 1990, 51, 239. 15 T. Yamamoto, personal communication, 1994. 16 P. H. Beadle, S. P. Armes, S. Gottesfeld, C. Mombourquette, R. Houlton, W. D. Andrews and S.F. Agnew, Macromolecules, 1992, 25, 2526. 17 K. Yoshino, X. H. Yin, S. Morita, Y. Nakanishi, S. Nakagawa, Fig. 12 Mass-average particle size distribution curves obtained using H. Yamamoto, T. Watanuki and I. Isa, Jpn. J. Appl. Phys., 1993, 32, 979. the disk centrifuge for (a) a polypyrrole-coated, charge-stabilized polystyrene latex of 1 mm diameter coated in the absence of any PVP 18 (a) A.E. Wiersma and L. M. A. vd Steeg, Eur. Pat. 589529; (b) A. E. Wiersma, L. M. A. vd Steeg and T. J. M. Jongeling, Synth. stabilizer and (b) the same latex coated with a thin overlayer of polypyrrole using physically adsorbed PVP to provide a steric Met., 1995, 71, 2269. 19 T. J. M. Jongeling, personal communication, 1995. stabilizer layer. The polypyrrole loading on both these coated latexes is approximately 11% by mass. Note that a much higher degree of 20 S. F. Lascelles and S. P. Armes, Adv.Mater., 1995, 7, 864. 21 (a) A. R. Goodall, M. C. Wilkinson and J. Hearn, J. Polym. Sci., dispersion is obtained in the presence of the PVP stabilizer. 1346 J. Mater. Chem., 1997, 7(8), 1339–1347Polym. Chem. Ed., 1977, 15, 2193; (b) C. W. A. Bromley, Colloids 27 C. Perruchot, M. M. Chehimi, M. Delamar, S. F. Lascelles and S. P. Armes, L angmuir, 1996, 12, 3245. Surf., 1986, 17, 1. 22 (a) A. J. Paine, W. Luymes and J. McNulty, Macromolecules, 1990, 28 Y. Deslandes, D. F. Mitchell and A. J. Paine, L angmuir, 1993, 9, 1468. 23, 3104; (b) C. K. Ober, K. P. Lok and M. L. Hair, J. Polym. Sci., 29 C. Y. Yang, Y. Cao, P. Smith and A. J. Heeger, Synth. Met., 1993, Polym. L ett., 1985, 23, 103. 53, 293. 23 H. Ge, P. R. Teasdale and G. G. Wallace, J. Chromatogr., 1991, 30 P. Banerjee and B. M. Mandal, Macromolecules, 1995, 28, 3940. 544, 305. 31 R. Zallen, T he Physics of Amorphous Solids; John Wiley, New 24 (a) H. Kawaguchi, Microspheres for Diagnosis and Bioseparation in York, 1983, ch. 4. PolymerMaterials for Bioanalysis and Bioseparation, ed. T. Tsuruta 32 S. F. Lascelles, S. P. Armes, P. A. Zhdan, S. J. Greaves, et al., CRC Press, London, 1993; (b) P. J. Tarcha, D. Misun, A. M. Brown, J. F. Watts, S. R. Leadley and S. Y. Luk, J. Mater. M. Wong and J. J. Donovan, Polymer L atexes: Preparation, Chem., following paper. Characterisation and Applications, ed. E. S. Daniels, E. D. Sudol, 33 S. P. Armes and M. Aldissi, Polymer, 1990, 31, 571. M. S. El-Aassar, ACS Symp. Ser. no. 492, 1992, 22, 347. 34 H. H. Kuhn, W. C. Kimbrell, J. E. Fowler and C. N. Barry, Synth. 25 C. F. Liu, D. K. Moon, T. Maruyama and T. Yamamoto, Polym. Met., 1993, 57, 3707. J., 1993, 25, 775. 35 S. Maeda and S. P. Armes, J.Mater. Chem., 1994, 4, 935. 26 (a) F. L. Baines, S. P. Armes and N. C. Billingham,Macromolecules, 1996, 29, 3096; (b) F. L. Baines, S. Dionisio, S. P. Armes and N. C. Billingham,Macromolecules, 1996, 29, 3416. Paper 7/00237H; Received 10th January, 1997 J. Mater. Chem., 1997, 7(8), 1339–1347 1347

ISSN:0959-9428    

DOI:10.1039/a700237h

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Surface characterization of micrometre-sized, polypyrrole-coated polystyrene latexes: verification of a ‘core–shell’ morphology† Stuart F. Lascelles,a Steven P. Armes,*a Peter A. Zhdan,b Stephen J. Greaves,b Andrew M. Brown,b John F.Watts,b Stuart R. Leadleyc and Shen Y. Lukc aSchool of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton, UK BN1 9QJ bDept. of Materials Science and Engineering, University of Surrey, Guildford, Surrey, UK GU2 5XH cAnalytical Divison, Courtaulds Research, P.O.Box 111, 101 L ockhurst L ane, Coventry, UK CV 6 5RS Micrometre-sized, polypyrrole-coated polystyrene latexes with various conducting polymer loadings have been extensively characterized using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), Raman and UV–VIS reflectance spectroscopy, scanning force microscopy (SFM) and scanning electron microscopy (SEM). Both XPS and TOF-SIMS studies are consistent with relatively uniform, chloride-doped polypyrrole overlayers.Raman studies also indicated a ‘core–shell’ morphology since only bands attributable to polypyrrole were observed; no evidence was found for the underlying polystyrene component even at the lowest polypyrrole loadings.This is most likely due to remarkably ecient attenuation of the polystyrene bands by the highly absorbing polypyrrole overlayer. UV–VIS reflectance spectroscopy studies confirmed that a coated latex had a much lower reflectance (higher absorbance) than a heterogeneous admixture of polypyrrole and polystyrene with a similar polypyrrole content.High-resolution images of the polypyrrole overlayer nanomorphology were obtained using SFM. At low polypyrrole loadings (1.0 mass%) the overlayer was relatively smooth and uniform, but higher loadings (8.9 mass%) resulted in a rougher, more globular morphology. Finally, the underlying polystyrene latex ‘core’ was quantitatively removed by solvent extraction.SEM studies of the polypyrrole residues revealed a ‘broken egg-shell’ morphology, thus providing irrefutable evidence for the ‘core–shell’ morphology of the original polystyrene/polypyrrole particles. Recently we reported1,2 coating micrometre-sized, sterically stabilized polystyrene latex particles with ultrathin overlayers of polypyrrole (Fig. 1). Polystyrene was selected as a ‘model’ colloidal substrate since it has a relatively high Tg (i.e. the particles are rigid and non-deformable) and latexes can be readily synthesized with narrow size distributions over a wide particle size range (50 nm–10 mm).3,4 Variation of the polystyrene particle concentration (and hence total surface area of latex) at fixed pyrrole polymerization conditions proved to be a very eective method for controlling the conducting polymer loading on the latex particles.Scanning electron microscopy (SEM) studies confirmed that, at polypyrrole mass loadings of <10%, the conducting polymer overlayer had a relatively smooth and featureless morphology. FTIR spectroscopy studies on the dried composite particles suggested that the absorption bands due to the polypyrrole overlayer component were Fig. 1 Schematic representation of an isolated, micrometre-sized, poly- significantly enhanced compared to those of heterogeneous pyrrole-coated polystyrene latex particle mixtures of polypyrrole ‘bulk powder’ and polystyrene latex. Similarly, conductivity measurements on pressed pellets of the thickly coated latex (comprising 8.7 mass% polypyrrole) was dried, coated latexes indicated an anomalously low conduc- predominantly polypyrrole-rich, with surprisingly little evi- tivity percolation threshold.Both spectroscopic and conduc- dence for either the underlying polystyrene component or the tivity data were consistent with a ‘core–shell’ latex morphology. poly(N-vinylpyrrolidone) stabilizer.It was concluded that the Potential applications for these micrometre-sized polypyrrole- polypyrrole overlayer on the polystyrene latex particles was coated polystyrene composite latexes include new stationary relatively uniform, rather than ‘patchy’. phases for electrochromatography5 or possibly novel ‘marker’ In the present work we describe a detailed study of particles for visual agglutination diagnostic assays.6 In the surface composition and nanomorphology of these addition, polypyrrole-coated polystyrene latexes of submicro- micrometre-sized, polypyrrole-coated polystyrene latexes metre dimensions have recently proved7 to be a useful model using six experimental techniques, namely XPS, time-of- system for understanding the behaviour of polypyrrole-coated flight secondary ion mass spectroscopy (TOF-SIMS), film-forming latexes such as those currently being developed8 Raman spectroscopy, UV–VIS reflectance spectroscopy, by DSM Research for antistatic and anticorrosion applications.scanning force microscopy (SFM) and SEM. Recently, in collaboration with Chehimi and co-workers, one of our micrometre-sized polypyrrole-coated polystyrene latexes was extensively examined by X-ray photoelectron spec- Experimental troscopy (XPS).9 This study confirmed that the surface of this Polystyrene latex synthesis and polypyrrole coating protocol The general latex synthesis procedure is outlined in the preced- † British Crown Copyright 1996/DERA.Published with the permission of the controller of Her Brittannic Majesty’s Stationery Oce.ing paper.2 The two latexes were sized by disk centrifuge J. Mater. Chem., 1997, 7(8), 1349–1355 1349photosedimentometry (DCP) using a Brookhaven BI-DCP preparation was achieved in a similar manner to that used for the TOF-SIMS experiments. instrument, operating in the line start mode as described previously.2 DCP studies confirmed that the two latexes had narrow size distributions and were of very similar size: the Raman spectroscopy. Raman spectra were recorded using a Bruker FRA 106 spectrometer.Excitation was provided by an mass-average particle diameters were 1.57±0.12 and 1.80±0.06 mm, respectively. Throughout this paper these lat- Adlas Nd5YAG laser at a wavelength of 1064 nm, operating at 30 mW for the polypyrrole-containing samples and 300 mW exes are referred to as the ‘1.6 mm’ and the ‘1.8 mm’ latexes. These DCP sizes were confirmed by transmission electron for the uncoated polystyrene latex.Data were acquired at a resolution of 4 cm-1 and spectra were averaged over 2000 microscopy (TEM) (Hitachi 7100 instrument), and SEM (Leica Stereoscan 420 instrument) on gold-coated dried latexes. A scans.portion of each sample was oven-dried at 60 °C overnight prior to CHN elemental microanalyses at an independent UV–VIS reflectance spectroscopy. A diuse reflectance accessory (PE 60 mm integrating sphere) was used in conjunc- laboratory (Medac Ltd at Brunel University, UK). The general polypyrrole coating protocol is also outlined in the preceding tion with a Lambda 9 Perkin-Elmer UV–VIS spectrophotometer. The dried powders were mounted onto double-sided paper.2 adhesive tape. Control experiments on uncoated, micrometresized polystyrene latex indicated ca. 100% reflectance for this Solvent extraction experiments system in the range 350–800 nm, and also confirmed that there Excess THF (20 ml) was added to ca. 100 mg of a dried was no significant contribution to the UV–VIS spectrum from polypyrrole-coated polystyrene latex (1.6 mm polystyrene latex; the underlying adhesive tape.Single-scan spectra were recorded 6.5% polypyrrole loading by mass) at room temperature and at a scan speed of 60 nm min-1. Reflectance measurements this solution was left to stand overnight. The resulting black were performed against a barium sulfate standard, which had residues were filtered, washed with THF and dried in an oven been previously referenced to an NPL-calibrated opal overnight at 60°C.The residues yield was consistent with the standard. mass loss expected for the polystyrene component. The chemical composition and morphology of the residues were analyzed Scanning force microscopy. Samples for SFM studies were by FTIR spectroscopy and SEM respectively (see later). prepared by drying a small drop of the aqueous latex dispersion onto a very flat Si(100) substrate.This procedure produced mechanically stable deposits which were analyzed by SFM in Characterization of polypyrrole-coated polystyrene latexes the contact mode. Standard microfabricated Si3N4 cantilevers Chemical composition and conductivity measurements.The (100 mm in length with a spring constant of 0.40 N m-1) with polypyrrole loadings of each of the coated latexes were deter- integrated pyramidal Si3N4 4 mm tips were employed as force mined by comparing their nitrogen contents to that of the sensors for SFM imaging. The apparatus used in this research corresponding uncoated polystyrene latex (the nitrogen con- was a Nanoscope II SFM (Digital Instruments Inc, Santa tents were 0.17 and 0.18% for the 1.6 and 1.8 mm latexes, Barbara, CA, USA), which has been described in detail else- respectively), and conventional polypyrrole ‘bulk powder’ where.11 The samples were scanned at constant force in the (average nitrogen content of 16.5±0.5%) synthesized in the range 10-7–10-8 N.SFM images from uncoated latex particles absence of latex particles.The conductivities (s) of compressed demonstrated good reproducibility due to a relatively strong pellets of the polypyrrole-coated polystyrene latexes were attractive interaction between the deposited particles. On the determined using standard four-point probe techniques at other hand, SFM imaging of the polypyrrole-coated latex room temperature.It is estimated that the random error particles was much more dicult, probably because of repulsive associated with such measurements is ca. 10%, with a system- interaction between the particles as a result of electric charge atic error of ca. 5–10%. accumulation during scanning. Such charging could arise from direct mechanical interaction between the insulating Si3N4 tip Time-of-flight secondary ion mass spectrometry.Spectra were and the electrically conductive particles mounted on the Si acquired using a VG Scientific Type 23 system. This instrument wafer, covered by a thin insulating surface layer of oxidized was equipped with a single stage reflectron time-of-flight silicon. It should be noted that the image analysis software is analyser and a MIG300PB pulsed liquid-metal (69Ga) ion designed for planar substrates and is not well suited to features source.Static SIMS conditions10 were employed for the analy- with high curvature such as latex particles. In the present ses (i.e. <1013 ions cm-2 per analysis) which was accomplished study relative morphological dierences between samples are using a primary ion beam pulsed at 20 kHz and 25 ns.The emphasized and no attempt has been made to quantify surface beam was rastered over an area of ca. 1 mm2 at a rate of 50 roughness. frames s-1, and both positive and negative spectra were acquired over a mass range of 5–800 u. Specimens were FTIR spectroscopy. FTIR spectra (KBr disk) were recorded prepared by decanting a small amount of the colloidal disper- using a Nicolet Magna 550 Series II research-grade instrument.sion onto 1 cm diameter sample stubs and allowing the aqueous Spectra were typically averaged over 64 scans at 4 cm-1 phase to evaporate; this provided a uniform coverage of the resolution. latex particles on the stub and no signals from the underlying stub were evident in either the SIMS or XPS spectra.Scanning electron microscopy. Morphology studies were carried out using a Leica Stereoscan 420 instrument operating at 30 kV. The samples were mounted on a double-sided X-Ray photoelectron spectroscopy. XPS spectra were collected using a VG Scientific ESCALAB Mk II spectrometer. adhesive carbon disc and sputter-coated with a thin layer of gold to prevent sample charging problems.This was operated in the constant analyzer mode (CAE) using pass energies of 50 eV for survey spectra and 20 eV for the high-resolution spectra. Al-Ka radiation was employed at an Results and Discussion anode power of 420 W. For spectral acquisition and subsequent data processing the spectrometer was controlled by a VGS The latex syntheses via dispersion polymerization using a poly(N-vinylpyrrolidone) stabilizer in alcoholic media proved 5000 data system based on a DEC PDP 11/73 computer. Quantification was achieved using peak areas and Wagner to be reasonably repeatable. Two polystyrene latexes of ca. 1.6 and 1.8 mm diameter were obtained in high yield. For the sensitivity factors and the manufacturer’s software. Specimen 1350 J. Mater. Chem., 1997, 7(8), 1349–1355conducting polymer coating experiments the synthesis conditions (i.e.the concentrations of the pyrrole and FeCl3 reagents) were held constant and the latex particle concentration was systematically varied in order to control the final polypyrrole loading.1,2 The particle size, conducting polymer loadings and electrical conductivities of the four polypyrrolecoated polystyrene latexes examined in this study are summarized in Table 1.The polypyrrole overlayer thicknesses were calculated as described previously.2 The overlayer was assumed to be uniform and the polypyrrole and polystyrene densities were taken to be 1.46 and 1.05 g cm-3, respectively. Most of the characterization work described in the present study was carried out on samples 1 and 4, which are coated with relatively thick and relatively thin polypyrrole overlayers, respectively.The polypyrrole loading in sample 1 is above the ‘knee’ in the conductivity percolation threshold curve for these composite particles,2 and therefore the conductivity of this material is comparable to that of polypyrrole bulk powder (1 S cm-1). In contrast, the polypyrrole loading in sample 4 is below the percolation threshold and this sample has a much lower conductivity (<10-6 S cm-1).X-Ray photoelectron spectroscopy The survey spectrum of an uncoated 1.8 mm diameter poly(Nvinylpyrrolidone)- stabilized polystyrene latex is shown in Fig. 2. An N 1s peak is clearly visible and is most likely due to the poly(N-vinylpyrrolidone) stabilizer at the surface of the latex particles.However, it is impossible to exclude the possibil- Fig. 2 X-Ray photoelectron survey spectra of: (a) an uncoated, poly(Nvinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter; (b) ity of contributions to the N 1s signal from AIBN-initiated and (c) polypyrrole-coated, poly(N-vinylpyrrolidone)-stabilized poly- polystyrene chains and/or the nitrogen-containing cationic styrene latexes of 1.8 mm diameter with polypyrrole mass loadings of surfactant (Aliquat 336) used in the polystyrene latex syntheses. 1.0 and 8.9% respectively (see samples 4 and 1 in Table 1) If we assume that these species do not significantly aect the microanalytical nitrogen content of the latex, a stabilizer content of ca. 1.5% can be calculated. This is consistent with the observation of a very weak feature due to the pyrrolidone latexes would be expected to be higher than that of the uncoated latex. This is indeed the case.The other survey carbonyl group at ca. 1660 cm-1 in the IR spectrum of this material.2 Applying appropriate sensitivity factors we estimate spectra depicted in Fig. 2 are for two polypyrrole-coated polystyrene latexes with polypyrrole mass loadings of 1.0 and an N/C ratio of 0.031 from the XPS spectrum of the uncoated latex.This value is lower than that determined by XPS for the 8.9% and N/C ratios of 0.052, and 0.085, respectively (samples 4 and 1 in Table 1). It is noteworthy that the reduced N/C poly(N-vinylpyrrolidone) stabilizer alone, which suggests that the surface coverage of the latex particles by the stabilizer is ratio found for the lower mass loading is consistent with the polypyrrole overlayer thickness being less than the XPS sam- incomplete.This conclusion is in agreement with studies reported by both Deslandes et al.12 and Chehimi and co- pling depth of 2–5 nm for this sample. Close inspection of the XPS spectra of the coated latexes workers.9 According to their respective structural formulae the theor- reveals additional signals due to Cl 2p.This indicates that the cationic polypyrrole chains are doped with chloride anions etical N/C ratio in polypyrrole is ca. 0.25, whereas the N/C ratio of poly(N-vinylpyrrolidone) is somewhat lower at 0.166. (originating from the FeCl3 oxidant used to polymerize the pyrrole). From the XPS spectra the Cl/N atomic ratios were In XPS studies of conducting polymers the surface carbon signal is generally somewhat more intense than expected on estimated to be 0.21 and 0.27 at polypyrrole mass loadings of 1.0 and 8.9%, respectively.Taking into account the likelihood the basis of elemental microanalyses. This is most likely due to the relatively high surface energies of these materials.14 of surface degradation and concomitant loss of dopant species, these values are in reasonable agreement with the normally Thus, precise agreement between N/C ratios calculated from XPS data and structural formulae is not necessarily expected. accepted doping range for polypyrrole (0.25–0.33).There is no evidence for any Fe signals in the spectral region from 708 to Nevertheless, if the polypyrrole were present as an overlayer on the latex surface, the XPS N/C ratios of the two coated 720 eV, which suggests that these two latexes are significantly Table 1 Summary of the particle size, conducting polymer loading and electrical conductivity of the polypyrrole-coated polystyrene (PS) latexes examined in this study nitrogen PS latex content of polypyrrole polypyrrole sample diametera/mm latexb (mass%) loadingb (mass%) layer thicknessc/nm sd/S cm-1 1 1.8 1.71 8.9 21 1 2 1.6 1.25 6.5 13 3 3 1.8 0.94 4.6 10 0.8 4 1.8 0.34 1.0 2 <10-6 aAs measured by DCP (confirmed by electron microscopy).bFrom CHN elemental microanalyses. Polypyrrole loadings were obtained by calculating reduced nitrogen contents relative to that of polypyrrole ‘bulk powder’ (16.5% N) prepared in the absence of latex.cCalculated assuming a uniform polypyrrole overlayer as described in ref. 2. dFour-point probe measurements on compressed pellets at room temperature. J. Mater. Chem., 1997, 7(8), 1349–1355 1351less contaminated with iron salt(s) than the polypyrrole-coated polystyrene latex examined by Chehimi and co-workers.9 High-resolution C 1s XPS spectra of the same three latexes are shown in Fig. 3. A ‘shake-up’ satellite at ca. 291.5–292.0 eV is clearly visible in the spectra of both the uncoated latex and also the coated latex which contains 1.0% polypyrrole (sample 4). This feature has been previously assigned to a p–p* transition for the aromatic rings of the polystyrene component.9 Its presence suggests that the overlayer is either very thin (i.e.less than the XPS sampling depth) and/or is rather ‘patchy’. This observation is also consistent with the reduced N/C ratio for this sample (see above). No ‘shake-up’ satellite is visible in the C 1s spectrum of the latex containing 8.9% polypyrrole (sample 1), which suggests that the overlayer in this latter sample is suciently thick and/or more uniform to obscure the underlying polystyrene latex.Similar observations were reported by Chehimi and co-workers, who examined a polystyrene latex coated with a similarly thick overlayer of polypyrrole.9 Time-of-flight secondary ion mass spectrometry Fig. 4 depicts three ‘negative ion’ TOF-SIMS spectra in the low mass range. The first spectrum is of the uncoated, poly(Nvinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter.This spectrum is identical to that obtained for the pristine poly(N-vinylpyrrolidone) stabilizer, which is consistent with this component being located at the latex surface. There is a mass peak at 26 u which is assigned to CN- fragments from the pyrrolidone unit of the stabilizer. The second spectrum is that of a polypyrrole chloride ‘bulk powder’ synthesized by conventional precipitation polymerization in the absence of any latex.A peak at 26 u is again prominent: this has been previously assigned to the CN- anion and is therefore charac- Fig. 4 Negative-ion TOF-SIMS spectra of: (a) an uncoated, poly(Nvinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter; (b) polypyrrole chloride bulk powder prepared by conventional precipitation polymerization in the absence of any latex; and (c) a polypyrrolecoated, poly(N-vinylpyrrolidone)-stabilized polystyrene latex of 1.8 mm diameter (8.9% polypyrrole mass loading; sample 1 in Table 1) teristic of polypyrrole.14 It is noteworthy that, in this spectrum, the 26/25 peak ratio is greater than unity (the 25 u peak is assigned to the C2H- anion).In addition, there are two peaks at 35 and 37 which are attributable to 35Cl and 37Cl. The third spectrum is of a polypyrrole-coated, poly(N-vinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter (8.9% polypyrrole loading by mass; sample 1). Again, the 26/25 peak ratio is greater than unity and there are two strong peaks due to the isotopic chloride anions.Thus these TOF-SIMS spectra are consistent with the XPS data and further support the hypothesis that chloride-doped polypyrrole is formed as an overlayer on the surface of the polystyrene latex particles. Raman spectroscopy The Raman results are summarized in Fig. 5. The spectrum of an uncoated polystyrene latex is shown in Fig. 5(d). There are several strong signals which are characteristic of polystyrene at approximately 1002 cm-1 (n1 ring-breathing mode), 1603 cm-1 (n9b ring stretch), and 622 cm-1 (n6b ring deformation). This spectrum is in excellent agreement with the Fig. 3 High-resolution XPS spectra of the C 1s region: (a) an uncoated, poly(N-vinylpyrrolidone)-stabilized polystyrene latex of 1.8 mm diam- Raman spectra of polystyrene reported by previous work- eter; (b) and (c) polypyrrole-coated, poly(N-vinylpyrrolidone)-stabilized ers.15,16 A polypyrrole-coated polystyrene latex is shown in polystyrene latexes of 1.8 mm diameter with polypyrrole mass loadings Fig. 5(b). The conducting polymer loading on this latex is only of 1.0% and 8.9% respectively (see samples 4 and 1 in Table 1). Note 1.0% by mass (N.B.essentially identical spectra were also that the p–p* shake-up satellite at ca. 291.5–292.0 eV due to the obtained at polypyrrole loadings of 3.0 and 8.9%), yet this aromatic rings in the underlying polystyrene latex is absent in spectrum Raman spectrum is identical to that of pure polypyrrole.17 (c). This is consistent with a thicker, more uniform polypyrrole overlayer. Surprisingly, no signals attributable to polystyrene are 1352 J.Mater. Chem., 1997, 7(8), 1349–1355to that of the uncoated polystyrene latex [Figure 5(d)]. Thus, in contrast to the coated latex, the two components in the heterogeneous admixture are essentially additive, as expected. We conclude that the ‘core–shell’ particle morphology of these polypyrrole-coated polystyrene latexes is responsible for the complete attenuation of the signal from the polystyrene core, even at a polypyrrole overlayer thickness of only 2 nm.UV–VIS reflectance spectroscopy During the course of the Raman control experiments described above, it was noted that the polypyrrole-coated polystyrene latexes were distinctly more coloured (i.e. appeared a darker shade of grey) than the corresponding heterogeneous admixtures of dried polypyrrole bulk powder and polystyrene latex with the same conducting polymer loadings.An attempt to quantify this observation was made using diuse reflectance UV–VIS spectroscopy. The results are depicted in Fig. 6 for a polypyrrole-coated polystyrene latex with a polypyrrole mass loading of 4.6% (sample 3) and the equivalent heterogeneous admixture (polypyrrole mass loading 5.0%).It is clear that the coated latex has a significantly lower reflectance (i.e. higher absorbance), which is again consistent with the ‘core–shell’ Fig. 5 Raman spectra of: (a) a heterogeneous mixture comprising 3% particle morphology proposed for this material. polypyrrole chloride bulk powder and 97% poly(N-vinylpyrrolidone)- stabilized polystyrene latex; (b) a polypyrrole-coated, poly(N-vinylpyr- Scanning force microscopy rolidone)-stabilized polystyrene latex with a polypyrrole mass loading of 1.0% (sample 4 in Table 1); (c) the dierence spectrum obtained by Our earlier studies1,2 of polypyrrole-coated polystyrene par- subtracting spectrum (b) from spectrum (a); (d) an uncoated poly(N- ticles using scanning electron microscopy suggested that the vinylpyrrolidone)-stabilized polystyrene latex.Note the strong poly- polypyrrole overlayer was remarkably smooth and uniform at styrene bands in spectrum (a); these features are not visible in spec- polypyrrole loadings lower than approximately 10% by mass. trum (b). However, these morphological studies were somewhat restricted by the relatively low SEM resolution.It is well known that observed, even though this relatively strong Raman scatterer scanning force microscopy (SFM) has a much higher resolution has its strongest signal (1002 cm-1) occurring in a region of than SEM so it was decided to use this technique to study the near-baseline Raman intensity for polypyrrole. Why are there nanomorphology of both the uncoated and coated polystyrene no Raman features attributable to polystyrene in a composite latexes.Fig. 7(a) shows an image of an isolated, uncoated which contains 99% polystyrene by mass? This observation is 1.8 mm polystyrene latex. These particles have a relatively believed to be directly related to the ‘core–shell’ particle smooth, featureless morphology, with relatively few imperfecmorphology of the coated latexes.The polypyrrole overlayer tions. In contrast, an image of a polypyrrole-coated polystyrene must either completely absorb the Raman excitation light latex particle (1.0% polypyrrole loading by mass) is shown in and/or attenuate the Raman signal arising from the underlying Fig. 7(b). The nanomorphology of the polypyrrole overlayer is polystyrene latex.In this context it is worth emphasising that clearly somewhat rougher than that of the uncoated latex the excitation wavelength of 1064 nm is very near the strong, particles and appears to be composed of individual nanosized broad optical absorption band due to polypyrrole (lmax ca. features of 10–20 nm. It is noteworthy that these features were 950–1000 nm).We note that similar ‘skin-eect’ observations not observed in our SEM studies,2 presumably due to this have been reported by other workers18 in Raman studies of the surface chemistry of graphite or carbon fibres and, more importantly, by Hearn et al. for the Raman spectra of polypyrrole- coated polyester fibres.19 However, it is remarkable that such ecient attenuation is observed in the present work since, at a conducting polymer mass loading of 1.0%, the thickness of the polypyrrole overlayer on the polystyrene latex particles is extremely thin (ca. 2 nm; see sample 4 in Table 1). In contrast, the conducting polymer overlayers on the polypyrrole-coated polyester fibres studied by Hearn et al. were much thicker (>100 nm).19 In order to verify the unusual observations described above, control experiments were carried out using a heterogeneous admixture with a mass composition of 3% polypyrrole chloride ‘bulk powder’ and 97% uncoated polystyrene latex.The Raman spectrum of this admixture is shown in Fig. 5(a). There is no possibility of a ‘core–shell’ morphology for such an admixture and, as expected, the signals associated with the polystyrene latex are readily observed, even though this control Fig. 6 UV–VIS reflectance spectra (obtained using a diuse reflectance sample has a slightly lower polystyrene content than the core– accessory) for (a) a polypyrrole-coated polystyrene latex (polypyrrole shell latex [Figure 5(b)]. Some polypyrrole bands are also loading 4.6% by mass; sample 3 in Table 1) and (b) a heterogeneous apparent in Fig. 5(a) but these are much less intense than those admixture comprising 5% polypyrrole bulk powder and 95% poly- observed in Fig. 5(b). Finally, spectrum (b) was subtracted from styrene latex (1.8 mm diameter). The observed lower reflectance for the spectrum (a) to obtain a dierence spectrum [Fig. 5(c)]. former spectrum indicates increased light absorption by the core– shell particles.Although somewhat noisier, spectrum (c) is essentially identical J. Mater. Chem., 1997, 7(8), 1349–1355 1353Scanning electron microscopy and IR spectroscopy studies At this point in our investigation all the experimental evidence suggested a ‘core–shell’ morphology for the polypyrrole-coated polystyrene particles. We were aware that the Lehigh group had successfully used solvent extraction to examine the particle morphology of poly(methyl methacrylate)/polystyrene ‘core– shell’ latexes.22 Since there is some literature evidence that polypyrrole is lightly cross-linked,23 extraction of the uncrosslinked polystyrene ‘core’ with a suitable organic solvent was attempted.If successful, the polypyrrole ‘shells’ should remain as insoluble residues.Accordingly, a dried polypyrrole-coated 1.6 mm polystyrene latex (6.5% polypyrrole loading by mass; sample 2 in Table 1) was treated with THF (see Experimental section). After isolation and drying, the mass loss was found to be consistent with quantitative extraction of the polystyrene component. Analysis of the black residues using FTIR spectroscopy confirmed that this material was essentially just polypyrrole (see Fig. 8), with very little evidence for the original polystyrene component (compare the relatively weak intensity of the band at 702 cm-1 with the corresponding very strong band observed in the IR spectrum of the polypyrrole-coated polystyrene latex).Examination of these polypyrrole residues by SEM revealed a ‘broken egg-shell’ morphology (see Fig. 9), with the ‘egg-shell’ diameter corresponding to that of the original coated particles. Thus, this solvent extraction experiment confirms beyond all reasonable doubt that these composite particles do indeed possess a ‘core–shell’ morphology. There are two possible explanations for the success of the extraction experiment: (1) the THF could permeate the continuous polypyrrole overlayer, causing the polystyrene core to swell and eventually leading to rupturing of the polypyrrole overlayer; (2) the polypyrrole overlayer is not completely continuous and the THF diuses into the latex core via imperfections or defects in the polypyrrole overlayer.Finally, we note that the ‘broken egg-shell’ polypyrrole morphology is highly unusual and may be useful in catalysis applications.Thus, smaller polystyrene latex particles could possibly serve as a colloidal ‘template’ for the synthesis of high surface area polypyrroles with unusual morphologies. Conclusions Several micrometre-sized polypyrrole-coated polystyrene latexes have been characterized in terms of their surface Fig. 7 SFM images of (a) an uncoated 1.8 mm diameter poly(Nvinylpyrrolidone)- stabilized polystyrene latex; (b) a polypyrrole-coated polystyrene latex with a polypyrrole mass loading of 1.0% (sample 4) and (c) a polypyrrole-coated polystyrene latex with a polypyrrole mass loading of 8.9% (sample 1) latter technique’s poorer resolution.On the other hand, similar nanomorphologies for polypyrrole overlayers on textile fibres (STM) were reported using scanning tunnelling microscopy (STM) in an earlier study.20 An SFM image of a polypyrrolecoated polystyrene latex at a higher polypyrrole loading (8.9% by mass) is shown in Fig. 7(c). This thicker polypyrrole overlayer has a distinctly ‘globular’ morphology, with features of the order of 50 nm. These globular features were also observed in our earlier SEM studies2 and are very similar to the morphology of relatively thick polypyrrole coatings on quartz fibres.21 At least ten polypyrrole-coated latex particles were Fig. 8 FTIR spectra of (a) a polypyrrole-coated polystyrene latex (mass analyzed by SFM. In each case, the polypyrrole overlayers loading 6.5%; sample 2 in Table 1) and (b) the polypyrrole broken appear to be reasonably continuous, with little or no evidence egg-shells residues remaining after solvent extraction of the polystyrene of bare patches.This is consistent with both the Raman component using THF (a non-solvent for the polypyrrole component). spectroscopy observations discussed above and also with a The polystyrene bands are much weaker in the latter spectrum, thus confirming quantitative extraction of this component.‘core–shell’ particle morphology. 1354 J. Mater. Chem., 1997, 7(8), 1349–1355Defence Research Agency are both thanked for partially funding the purchase of the FTIR spectrometer. Dr. E. Then (Science Museum, UK) is thanked for his assistance in con- firming our original UV–VIS reflectance spectroscopy studies. References 1 S. F. Lascelles and S. P. Armes, Adv.Mater., 1995, 7, 864. 2 S. F. Lascelles and S. P. Armes, J.Mater. Chem., preceding paper. 3 (a) A. R. Goodall, M. C. Wilkinson and J. Hearn, J. Polym. Sci., Polym. Chem. Ed., 1977, 15, 2193; (b) C. W. A. Bromley, Colloids Surf., 1986, 17, 1. 4 (a) A. J. Paine, W. Luymes and J. McNulty, Macromolecules, 1990, 23, 3104; (b); C. K. Ober, K. P. Lok and M. L. Hair, J. Polym. Sci., Polym. L ett., 1985, 23, 103. 5 H. Ge, P. R. Teasdale, G. G. Wallace, J. Chromatogr., 1991, 544, 305. 6 (a) H. Kawaguchi, Microspheres for Diagnosis and Bioseparation in PolymerMaterials for Bioanalysis and Bioseparation, ed. T. Tsuruta et al., CRC Press, London, 1993; (b) P. J. Tarcha, D. Misun, M. Wong and J. J. Donovan, Polymer L atexes: Preparation, Characterisation and Applications, ed. E. S. Daniels, E. D.Sudol and M. S. El-Aassar, ACS. Symp. Ser. no. 492, 1992, 22, 347; (c) M. R. Pope, S. P. Armes and P. J. Tarcha, Bioconjugate Chem. 1996, 7, 436. 7 D. B. Cairns, S. P. Armes, M. M. Chehimi, M. Delamar and S. Y. Luk, Macromolecules, to be submitted. 8 (a) A. E. Wiersma and L. M. A. vd Steeg, Eur. Pat., 589 529; (b) A. E. Wiersma, L. M. A. vd Steeg and T. J. M. Jongeling, Synth. Met., 1995, 71, 2269. 9 C. Perruchot, M. M. Chehimi, M. Delamar, S. F. Lascelles and S. P. Armes, L angmuir, 1996, 12, 3245. 10 J. C. Vickerman, A. Brown and N. M. Reed, Secondary Ion Mass Fig. 9 Scanning electron micrographs of (a) a polypyrrole-coated poly- Spectrometry5Principles and Applications, Clarendon Press, styrene latex (mass loading 6.5%; sample 2) and (b) the resulting Oxford, 1989.polypyrrole residues after solvent extraction of the underlying poly- 11 B. Drake, C. B. Prater, A. L. Weisenhorn, S. A. C. Gould, T. R. styrene core particles using THF (a non-solvent for the polypyrrole Albrecht, C. F. Quate, D. S. Cannell, H. G. Hansma, and P. K. component). The broken egg-shells morphology clearly evident in the Hansma, Science, 1989, 243, 1586. latter micrograph confirms the core–shell particle morphology of the 12 Y.Deslandes, D. F. Mitchell and A. J. Paine, L angmuir, 1993, original polypyrrole-coated latex particles. 9, 1468. 13 (a) M. M. Chehimi, S. F. Lascelles and S. P. Armes, composition and nanomorphology using a wide range of Chromatographia, 1995, 41, 671; (b) M. M. Chehimi, M. L. Abel, Z. Sahraoui, K. Fraoua, S. F.Lascelles and S. P. Armes, Int. J. Adhes. techniques. XPS and TOF-SIMS studies were consistent with Adhes., 1997, 17, 1. a relatively uniform conducting polymer overlayer containing 14 (a) M. L. Abel, S. R. Leadley, A. M. Brown, J. Petitjean, M. M. chloride dopant anions. XPS provided some evidence for the Chehimi and J. F. Watts, Synth. Met., 1994, 66, 85; (b) S. Y. Luk, underlying polystyrene latex at lower polypyrrole loadings. W.Lineton, M. Keane, C. DeArmitt and S. P. Armes, J. Chem. Raman studies were also consistent with a core–shell particle Soc., Faraday T rans., 1995, 91, 905. morphology, since only vibrational bands due to polypyrrole 15 R. S. Venkatachalam, F. J. Boerio, P. G. Roth and W. H. Tsai, J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 2447.were observed. Remarkably, no evidence was found for the 16 (a) C. H. Jones and I. J. Wesley, Spectrochim. Acta, Part A, 1991, underlying polystyrene component, even for polypyrrole over- 47, 1293; (b) R. A. Nyquist, C. L. Putzig, M. A. Leugers, R. D. layer thicknesses as low as 2 nm. This is attributed to unusually McLachlan and B. Thill, Appl. Spectrosc., 1992, 46, 981. ecient attenuation of the polystyrene bands by the polypyr- 17 C. M. Jenden, R. G. Davidson and T. G. Turner, Polymer, 1993, role overlayer. The enhanced absorbance (reduced reflectance) 34, 1649. of the polypyrrole-coated polystyrene latex particles relative 18 (a) F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126; (b) M. A. Tadayyoni and N. R. Dando, Appl. Spectrosc., 1991, 45, to their corresponding heterogeneous admixtures was con- 1613. firmed by UV–VIS reflectance spectroscopy. At low polypyr- 19 M. J. Hearn, I. W. Fletcher, S. P. Church and S. P. Armes, Polymer, role loadings (1.0 mass%) the nanomorphology of the 1993, 34, 262. conducting polymer overlayer was relatively smooth, but a 20 S. P. Armes, M. Aldissi, M. Hawley, J. G. Beery and S. Gottesfeld, rougher, more globular nanomorphology was observed using L angmuir, 1991, 7, 1447. SFM at higher loadings (8.9 mass%). Solvent extraction of 21 S. P. Armes, S. Gottesfeld, J. G. Beery, F. Garzon, M. Mombourquette, M. Hawley and H. H. Kuhn, J.Mater. Chem., the underlying polystyrene latex core was quantitative and 1991, 1, 525. revealed a ‘broken egg-shell’ morphology for the polypyrrole 22 S. Shen, M. S. El-Aasser, V. L. Dimonie, J. W. Vanderho and residues, thus providing irrefutable evidence for a ‘core–shell’ E. D. Sudol, J. Polym. Chem., Part A: Polym. Chem., 1991, 29, 857. particle morphology. 23 F. P. Bradner, J. S. Shapiro, H. J. Bowley, D. L. Gerrard and W. F. Maddams, Polymer, 1989, 30, 914. This work has been carried out with the support of the Defence Research Agency, Fort Halstead, UK. DSM Research and the Paper 7/00236J; Received 10th January, 1997 J. Mater. Chem., 1997, 7(8), 1349–1355 1355

ISSN:0959-9428    

DOI:10.1039/a700236j

出版商:RSC    

年代:1997

数据来源: RSC