摘要:

J. MATER. CHEM., 1992,2(l), 115-1 18 Hall-effect Observation in the New Organic Semiconductor Bis(1,2,5=thiadiarolo)=p=quinobis(1,3=dithiole)(BTQBT) Kenichi Imaeda,*" Yoshiro Yamashita," Yongfang Li,"t Takehiko Mori," Hiroo Inokuchi" and Mizuka Sanob a Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Division of Natural Sciences, International Christian University, Mitaka, Tokyo 7 87, Japan Single crystals of bis(l,2,5-thiadiazolo)-p-quinobis(l,&dithiole) (BTQBT) were grown either by recrystallization from a nitrobenzene solution or by sublimation in nitrogen atmosphere. The electrical resistivities were 1.2 x lo3C! cm and 2.7 x lo5R cm at room temperature for a crystal grown by recrystallization and that obtained by sublimation, respectively.These values are remarkably low for a single-component organic crystal. In addition, the BTQBT crystals have a small anisotropy in resistivity (pl/p1,=2), which is ascribed to strong intermolecular interactions inherent in the crystal structure. They show a Hall effect which is an unusual observation in organic semiconductors. The sign of carriers was determined to be positive and the Hall mobility was found to be ca. 4 cm2s-' V-' at room temperature. Keywords: Organic semiconductor; BTQBT; Electrical conductivity; Hall effect The studies on organic conductors composed of electron donors and electron acceptors progressed rapidly after the discovery of TTF-TCNQ.' Many organic metals and organic superconductors have been obtained through foresighted mol- ecular design and superior crystal-growth techniques.2 Recently, superconductivity was found above 20 K in alkali- metal-doped fullerenes (T,=28 K for RbxC60).3 There are two approaches to the development of new organic conductors.One is to prepare multicomponent systems such as charge- transfer (CT) complexes. The other is to create single-compo- nent systems. Organic molecular crystals are generally in- sulators owing to the wide gap between valence and conduction bands. The way to achieve electrical conduction in organic crystals with n-electron systems is by enhancement of inter- molecular interactions through intermolecular overlap of n-orbitals, which reduces the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and enhances the number of charge carriers.One of the strategies for enhancement is the introduction of chalcogen atoms such as sulfur, selenium and tellurium with large van der Waals radii. Actually, tetrathiotetracene (TTT),4 tetraseleno- tetracene (TST),4 tetrakis(alky1thio)tetrathiafulvalene (TTC, -TTF),' tetrakis(methyltel1uro)tetrathiafulvalene (TTeC -TTF)6 and hexamethylenetetratellurafulvalene (HMTTeF)7 show markedly low resistivities (104-105 R cm) at room temperature. We have synthesized the heterocyclic compounds contain- ing 1,2,5-dithiazole rings in the hope of increasing inter- heteroatomic interactions in the solid state. We found the intermolecular S-..N contacts of 3.03 A in bis(1,2,5-thiadiazo1o)tetracyanoquinodimethane (BTDA-TCNQ)8 and 3.05 A in 4,7-dimethyl-4,7-dihydro[l,2,5]thiadiazolo [3,4-b] pyrazine.' We have reported a novel crystal structure of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT)." As shown in Fig.1, BTQBT molecules are stacked to form columns along the c axis with a uniform spacing of 3.46A. It is noteworthy that we have observed short intercolumnar S-.-S contacts of 3.26A. In this paper, we present the characteristic transport properties of BTQBT crystals derived from strong two-dimensional intermolecular interactions. t Permanent address: Institute of Chemistry, Academia Sinica, Beijing, China Formula 1 Experimental The synthesis and purification of BTQBT were reported in a previous paper." Dark-red single crystals were grown by the following two methods: (i) recrystallization from a nitroben- zene solution; (ii) sublimation from BTQBT powder in a sealed glass tube with 50 Torrl N2 gas heated at 370 "C.Both methods yielded needle-shaped crystals; typical dimensions were 1-2 mm (length) x 0.1 mm (width) x 0.1 (thickness) mm and 3-8 mm x 0.3 mm x 0.03 mm for crystals grown by recrys- tallization (named rec-crystal hereafter) and those grown by sublimation (named sub-crystal), respectively. Electrical resis- tivity measurements were performed by the two-probe method in an inert atmosphere with 20Torr He gas to avoid the surface effect of oxygen etc., and electrical contacts were made with gold paint. The resistivity measurement under pressure up to 10 kbar was carried out by using a BeCu clamp cell at room temperature in the pressure medium liquid (Idemitsu Daphne Oil #7373)." The pressure was monitored with a manganin sensor.The Hall effect measurement was carried out at room temperature using a conventional four-probe 11 Torr x133.322 Pa. Fig. 1 Crystal structure of a BTQBT crystal grown by recrys-tallization method under a magnetic field up to 1.3 T. The magnetic field was calibrated with an InAs Hall sensor (F. W. Bell BHT921). A constant d.c. voltage of 1OV was applied across the ends of the crystal. The current and the Hall voltage were detected with an electrometer (Keithley 617) and a digital multimeter (Takeda Riken TR6856), respectively.Results and Discussion Electrical Resistivity Fig. 2 shows the temperature dependence of resistivity along the long axis (stacking axis) of a BTQBT rec-crystal and a BTQBT sub-crystal. Both crystals are found to behave as simple semiconductors. The resistivity at room temperature bRT)is 1.2 x lo30cm and the activation energy (E,) is 0.21 eV for the rec-crystal, while PRT =2.7 x lo5 0cm and E, =0.24 eV J. MATER. CHEM., 1992, VOL. 2 for the sub-crystal. The resistivity values of both crystals are remarkably low for single-component organic crystals. The rec-crystal has a resistivity lower by a factor of ca. lo2 than the sub-crystal. The rec-crystal may have included a very small amount of nitrobenzene which was used as a solvent for crystal growth and can act as an electron acceptor, though no difference in lattice parameters between rec- and sub- crystals was observed.A wide sub-crystal allowed us to measure the anisotropy in resistivity. The anisotropy is found to be px:py:pz= 1:2 :100, where p,(= p,) denotes the resistivity measured in the direction parallel to the long axis x of a needle-like crystal, p,, that perpendicular to the x axis within a broad face of the crystal, and pz that perpendicular to the broad face. Since the shortest S...S contacts extend along the b axis as shown in Fig. 1, the y direction seems to correspond to the b axis. The ratio of the resistivity along the intercolumn direction to that along the intracolumn direction (pl/pII) is very small (ca.2). This result supports the existence of strong two-dimensional intermolecular interactions which are inherent in the crystal structure. The effect of pressure on the resistivity of a rec-crystal is shown in Fig. 3. The resistivity decreases monotonically with increasing pressure and becomes ca. 1/5 of that at the atmos- pheric pressure. It has been reported that the resistivities of organic molecular crystals and poorly conductive CT com- plexes drop steeply under high pressure. For example, the resistivities of isoviolanthrone and fluoranil-perylene decrease to ca. 1/100 by application of pressure up to ca. 10 kbar.l2*I3 Such a large decrease in resistivity is assumed to result from the strong pressure-induced increase of intermolecular over- lapping which is initially very small at ambient pressure in conventional molecular crystals.In contrast, the BTQBT crystal shows a very small change in resistivity over the same range of pressure. This behaviour resembles the behaviour of highly conductive CT cornplexe~'~ and organic metal^,'^.'^ in which the decrease in resistivity by pressure is very small. This can be qualitatively understood as resulting from small relative increase of intermolecular overlapping which is initially already large in highly conductive organics. We have also measured the temperature dependence of resistivity at P =9.5 kbar and found that the activation energy slightly decreased from E, (P=0 kbar) =0.21 eV to E, (P=9.5 kbar) = 0.18 eV.Although the resistivity is not simply related to intermolecular overlap, the small pressure effect on the resis- 0 0 0 0 00 t 1 I I I I21 J I I345678910 103~1~ Plkbar Fig. 2 Temperature dependence of the resistivity of (a)a BTQBT rec- Fig. 3 Pressure dependence of the resistivity normalized at ambient crystal and (b) a BTQBT sub-crystal pressure of a BTQBT rec-crystal J. MATER. CHEM., 1992, VOL. 2 tivity observed in BTQBT also supports the existence of strong intermolecular interactions through large n-orbital overlap between and within the columns of BTQBT molecules. Hall Effect The detection of a Hall voltage for organic molecular crystals was extremely difficult because of their very high resistivity.Heilmeier et ~1.'~ succeeded in and Heilmeier and Harri~on'~ the measurement of a Hall mobility for metal-free phthalo- cyanine and copper phthalocyanine single crystals. Up to now, these are the only convincing observations. The low electrical resistivity of a BTQBT crystal enabled us to measure a Hall mobility. Table 1 summarizes the results obtained from a Hall effect measurement for a BTQBT rec-crystal and the inset illustrates our experimental geometry. The dimensions of the crystal used in this measurement were 1.7(1)mm x O.O9(w) mm x 0.075(t)mm. The Hall voltage (V,) observed as a difference between the output with the magnetic field on (H, = 1.3T) and that with the magnetic field off (H,= 0) was +0.25 mV. When the direction of a magnetic field was reversed, the Hall voltage became -0.25 mV. The sign of the Hall voltage shows that the sign of carriers is positive. The thermoelectric power measurement of a rec-crystal gave S = + 500 pV K-' at room temperature.This positive sign is in agreement with the sign determined by the Hall effect measure- ment. The Hall coefficient (RH) was determined to be 4 x lo3 cm3 C-' from the equation of RH = (VHt)/(IxHz).The Hall mobility (pH),which is calculated by division of the Hall coefficient by the resistivity (pH=RH/p), was found to be 4cm2 s-' V-'. We obtained almost the same value of pHfor a sub-crystal, though the observed Hall. voltage was somewhat unstable. It is widely accepted that the drift mobilities of most organic molecular crystals are ca.1 cm2 s-' V-' at room tempera- ture.'* The value of 4cm2 s-' V-' for BTQBT crystals is considered to be meaningfully larger than that for other molecular crystals. Yamashita and Kurosawa" have devel- oped a theory that the conduction mechanism can be explained by the following two models depending on the magnitude of mobility: (a)a band model for p >> 1 cm2 s -' V-' and (b)a hopping model for p<< 1 cm2 s-' V-'. According to our extended Huckel molecular orbital calculation for a BTQBT crystal," the intermolecular transfer integral (t) between the HOMOS along the stacking direction is t= 0.123 eV. The band width (W) which is given by 4x the transfer integral is Wx0.5eV. This value is extraordinarily Table 1 Hall-effect measurement for a BTQBT rec-crystal IHz large for organic molecular crystals and supports qualitatively the observed high value of the mobility according to the wide- band model (W>k,T).The mobility p can be approximately expressed in the form p = ez/m*,where z is the relaxation time and m* is the effective mass. The condition z>h/k,T leads to:" p> 20(m/m*)cm2s-' V-' at 290 K. Since the transfer integral is related to m* by m* = h2/2td2(d being the distance between molecules), we obtain m* = 3m with t= 0.123eV and dx3 A for BTQBT. This leads to a rough estimation of p>6.7 cm2 s-' V-', which is of the same order as the observed px4 cm2 s-' V-'. Taking into account the results on crystal structure, electrical resistivity, Hall mobility and band calculation, a band picture may be plausible for the conduction mechanism of BTQBT crystals.So far, we have not addressed the problem of the intrinsic or extrinsic nature of the observed resistivity of BTQBT crystal. The most important finding of this work is the observation of the high Hall mobility, which reflects large intermolecular overlapping and this result does not depend on whether the electrical conduction is intrinsic or not. In the future we plan to clarify the nature of the conducting property for BTQBT, measuring e.g. the temperature dependence of the Hall mobility. In summary, the transport properties of BTQBT crystals were investigated in terms of electrical resistivity and Hall effect, The strong two-dimensional intermolecular interactions originating from the unique crystal structure gave a small anisotropy in resistivity (p,/pll ~2)and a small change in resistivity with pressure.Most significantly, a Hall effect could be observed in BTQBT crystals and the Hall mobility was found to be ca. 4cm2 s-' V-'. These results, with a band structure calculation, suggest that the electrical conduction in BTQBT crystals can be explained in terms of a band model. We are grateful to Professor Y. Maruyama of Institute for Molecular Science for his helpful advice and discussion on Hall mobility measurements. References I L. B. Coleman, M. J. Cohen, D. J. Sandman, F. G. Yamagishi, A. F. Garito and A.J. Heeger, Solid State Commun., 1973, 12, 1 125. 2 Proc. Int. Con. Sci. Technol. Synth. Met. (ICSM '90), Synth. Met., 1991, 41-43. 3 M. J. Rosseinsky, A. P. Ramirez, S. H. Glarum, D. W. Murphy, R. C. Haddon, A. F. Hebard, T. T. M. Palstra, A. R. Kortan, S. M. Zahurak and A. V. Makhija, Phys. Rev. Lett., 1991,66,2830. 4 I. F. Shchegolev and E. B. Yagubskii, Extended Linear Chain Compounds, ed. J. S. Miller, Plenum, New York, 1982, vol. 2, ch. 9, pp. 385-392. 5 K. Imaeda, T. Enoki, Z. Shi, P. Wu, N. Okada, H. Yamochi, G. Saito and H. Inokuchi, Bull. Chem. SOC. Jpn., 1987,60, 3163. 6 H. Inokuchi, K. Imaeda, T. Enoki, T. Mod, Y. Maruyama, G. Saito, N. Okada, H. Yamochi, K. Seki, Y. Higuchi and N. Yasuoka, Nature (London), 1987,329, 39. 7 S.Matsuzaki, H. Okumura, H. Takenouchi, T. Kyouda and M. t(((7 10 Y. Yamashita, S. Tanaka, K. Imaeda and H. Inokuchi, Chem.~~ ~~ magnetic field (H,) 1.3 T Lett., 1991, 1213. applied voltage ( Vx) 10 v I1 H. Fujiwara, H. Kadomatsu and K. Tohma, Rev. Sci. Instrum., current (I,) 3.6 pA 1980, 51, 1345. Hall voltage (V,) 0.25 mV 12 H. Inokuchi, Bull. Chem. SOC. Jpn., 1955,28, 570. sign of carriers positive 13 M. Schwarz, H. W. Davies and B. J. Dobriansky, J. Chem. Phys., Hall coefficient (RH) 4 x lo3 cm3 C-' 1964,40, 3257. Hall mobility (pH) 4 cm2 s-l V-' 14 T. Mori, K. Imaeda, R. Kato, A. Kobayashi, H. Kobayashi and H. Inokuchi, J. Phys. SOC.Jpn., 1987, 56, 3129. I Sano, Synth. Met., 1991, 39, 385. 8 C. Kabuto, T. Suzuki, Y. Yamashita and T. Mukai, Chem. Lett., 1986, 1433. 9 Y. Yamashita, J. Eguchi, T. Suzuki, C. Kabuto, T. Miyashi and S. Tanaka, Angew. Chem., Int. Ed. Engl., 1990, 29, 643. 118 J. MATER. CHEM., 1992, VOL. 2 15 K. Imaeda, T. Enoki, T. Mori, H. Inokuchi, M. Sasaki, K. I9 J. Yamashita and T. Kurosawa, J. Phys. Soc. Jpn., 1960,15,802. Nakasuji and I. Murata, Bull. Chem. SOC.Jpn., 1989, 62, 372. 20 T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito and 16 G. H. Heilmeier, G. Warfield and S. E. Harrison, Phys. Rev. H. Inokuchi, Bull. Chem. Soc. Jpn., 1984, 57, 627. Lett., 1962, 8, 309. 21 H. Meier, Organic Semiconductors, Verlag Chemie, Weinheim, 17 G. H. Heilmeier and S. E. Harrison, Phys. Rev., 1963, 132, 2010. 1974. 18 L. B. Schein and D. W. Brown, Mol. Cryst. Liq. Cryst., 1982, 87, 1. Paper 11043485;Received 20th August, I991

ISSN:0959-9428    

DOI:10.1039/JM9920200115

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992, 2(1), 119-124 Synthesis and Transition Temperatures of some Novel, Chiral Nematic, laterally Attached, Side-chain, Siloxane Polymers Russell A. Lewthwaite, George W. Gray and Kenneth J. Toyne* School of Chemistry, The University, Hull, HU6 7RX, UK The synthesis of a chiral nematic, laterally attached polymer, from an enantiotropic chiral nematic side-chain precursor is described, and ways of achieving selective esterifications of 2,4-dihydroxybenzoates are discussed. This compound is the first example of its kind to exhibit the chiral nematic phase in both the monomer and the derived polymer; the results show that the polymer has a lower chiral nematic to isotropic clearing point than the monomer. Keywords: Chiral nematic polymer; Laterally attached polysiloxane ; Liquid crystal Side-chain liquid-crystalline polymers are a relatively recent discovery, the first examples being synthesised by Finkelmann et d.,'who produced methacrylate polymers which were of the terminal type.The polymerisation process for these mono- meric side-chains has the effect of increasing the order of the mesophase, and so a nematic monomer usually gives a smectic homopolymer.2 In order to obtain nematic homopolymers in this way, it was necessary to synthesise a non-mesogenic monomer, so that upon polymerisation the increase in order of the system would generate a nematic polymer; these require- ments were demonstrated by Finkelmann and Rehage in the early 1980~.~,~Until their work, chiral nematic polymers had only been prepared by the use of copolymers where one of the monomers contained a chiral unit to impart chirality to the resulting The preparation of laterally attached side-chain liquid- crystalline polymers6 overcame the problem of the increasing order which results from polymerisation of terminal mono- mers.Unlike their terminal analogues, lateral polymers have so far been found to be exclusively nematic, with no smectic tendencies. The introduction of such laterally attached mol- ecules into a known smectic terminal polymer gives a nematic polymer with a lateral side-chain concentration of only 16% within the terminal polymer system, and also the polymers show glass transitions instead of melting points when the lateral group is above a concentration of only YO.^ These consequences of lateral attachment should make the pro- duction of chiral nematic (cholesteric) laterally attached, side- chain polymers relatively easy, but this has not proved to be the case.Several attempts have been made at their synthesis, but these have given amorphous polymers from either amorphous8 or monotropic chiral (S, *, N*) side-chain precur- sor~.~However, a 'side-on' cholesteric polymer has been made using an esterified cholesterol moiety as a lateral side chain." Our initial work to produce chiral nematic, laterally attached, siloxane polymers centred on side-chains with similar struc- tures, e.g. I, to those of existing nematogenic polymers. 'O(CH2)&H =CH2 I The side-chain precursor, I, had a chiral nematic to isotropic transition at -2.4 "Cand the siloxane polymer obtained from I was an amorphous glass (&= -12.7 "C).The mesophase thermal stability of the side-chain was then increased by the replacement of the terminal pentyl group by an octyloxy group, and also by extension of the spacer group by one carbon atom (to six carbons). These changes had the effect of increasing the monotropic chiral nematic to isotropic tran- sition by 37.1 "C to 34.7 "C, but once again the polymer was amorphous, although the Tg value had been raised to 4.6 "C. It was clear that the thermal stability of the chiral nematic mesophase had to be increased still further. The structural changes which were made to give the required enhancement of the chiral nematic stability were the incorporation of a further ester function rather than direct coupling of the two aromatic rings (to give non-mesogenic 15a), and then the inclusion of a further aromatic ring to give compound 15b, which has an enantiotropic chiral nematic phase.CH3 C H3CH2tHCH20 +32q-..2+' O(CH2)aCH =CHp 15 a, x = OC8H17, b, x =G m 8 H 1 7 This type of system incorporates the point of attachment of the spacer group more towards the centre of the molecules, as for the non-chiral esters reported by Keller et al.," but the system we have prepared allows the attachment of different outer aromatic groups. We now report a calamitic side-chain precursor (15b) which has increased mesophase thermal stab- ility and consequently shows an enantiotropic chiral nematic mesophase, which also exists in the resulting siloxane homo- polymer.Experimental The synthesis of compounds 15 (a and b) proved to be troublesome, with the principal difficulty arising because selectivity was needed for the reaction of the hydroxy groups in the starting material, compound 9. The hydroxy groups in this molecule would usually react selectively because the 2-hydroxy function is sterically hindered by the neighbouring methyl ester group. However, the production of an ester at the 4-hydroxy group as a first step would be of limited use as the methyl ester has to be hydrolysed at a later stage, and this would lead to hydrolysis of the ester prepared at the 4-position.A benzyl ester, rather than the methyl ester, was prepared so that the benzyl group could subsequently be selectively removed by hydrogenolysis; the esterification step at the 4-hydroxy group occurred in good yield, but the hydrogenolysis of the benzyl group gave an unreactive acid (because of H-bonding with the 2-hydroxy group), and only a 32% yield was obtained when a 0.5 mol excess of the phenol was used. Protection of the 4-substituted 2-hydroxybenzoic acid with a methoxycarbonate was tried, based on procedures previously reported for similar systems but these were of limited US^.'^,'^ Therefore, the approach shown in Schemes 1 and 2 was tried. Both the esterification steps (to give compounds 12 and 14) proceeded in good yield, with the second esterification step occurring selectively at the 4-hyd- roxy function, as expected, The only problem arising in this route was the penultimate step (to give compounds 15).The usual method employed for the etherification of a phenol, using the alkyl bromide and potassium carbonate in refluxing butanone, failed for compound 15b and left the unreacted starting material; we believe this unreactivity was due to the insolubility of the intermediate anion. Production of the phenoxide anion using sodium hydride was an improvement but gave an impure product, and the only reagent to give an isolatable, pure product of 15b was diethyl azodicarboxylate which, even so, was limited in its success in the case of 15a.The poly(hydrogenmethy1)siloxanebackbone used for the polymerisations was obtained from Wacher Chemie (m= 46+ 3 by 'H NMR). Instrumentation Infrared spectra were produced on a Perkin-Elmer 783 spec-trometer, 'H NMR spectra were produced on a JEOL JNM-HO oCOzCH3 1 Y3 CH3CHZtHCHzOe c 0 Z c H 3' I 4I 7O -OH 5 HO eOCHz Ph 6I C8Hl 70 eOCHzPh 7 I c8H17000H 8 Scheme 1 J. MATER. CHEM., 1992, VOL. 2 HO-&OzCH3 -PhCHz0 I 9 10 I OCHZPh OCHzPh 12 OH / 13 OHCH3\ CH3CH2!HCHz0 -@Oz-&Oze 14 15 Scheme 2 GX270 FT NMR using CDC13 as solvent and TMS as reference, and mass spectra were obtained using a Finnigan 1020 Automated GC/MS. Optical rotations were measured using a ETL-NPL Automatic Polarimeter Control Unit Type 143A. Differential scanning calorimetry was carried out using a Perkin-Elmer DSC 7, with TAC 7/PC instrument cooler and controlled cooling accessory, and optical microscopy was performed using an Olympus BH-2 polarising microscope in conjunction with a Mettler FP52 hot stage and Mettler FP5 temperature control unit.Compounds were purified, where specified, by column chromatography using silica gel (60-120 mesh), or by flash chromatography using silica gel (200-400 mesh). Synthesis (S)-Methyl 4-(2-Methylbutoxy)benzoate (2) (S)-(+)-1-Bromo-2-methylbutane (27.2 g, 0.18 mol) was added to a mixture of methyl 4-hydroxybenzoate (22.8 g, 0.15 mol), anhydrous potassium carbonate (103.5 g, 0.75 mol), dry but- J.MATER. CHEM., 1992, VOL. 2 anone (350cm3) and potassium iodide (0.1 g). The mixture was stirred, and heated under reflux with the exclusion of moisture until TLC analysis showed the reaction to be complete. The mixture was cooled, poured into water (300cm3), washed with diethyl ether (2x200cm3) and the combined organic extracts were then washed with 5% sodium hydroxide (200 cm3), water (200 cm3) and dried (MgS04). The solvent was removed, and the residue was purified by flash chromatography (dichloromethane) to leave a straw-coloured oil. Yield, 24.2g (72%); m/z 222(M+), 191, 152, 121(100%) vmax/cm- 3080, 2970-2880, 1720, 1610, 1585, 1520, 1470, 1255, 1170, 11 10, 1015, 850; 6-0.95 (6 H, d, t, 2 x CH3), 1.25 (1 H, my CH), 1.55 (1 H, m, CH), 1.85 (1 H, m, CH), 3.75 (2 H, octet, CH,O), 3.85 (3 H, s, OCH3), 6.85 (2 H, d, arom), 7.95 (2 H, d, arom).(S)-4-(2-Methylbutyloxy)benzoicAcid (3) Compound 2 (24.2 g, 0.1 1 mol), sodium hydroxide pellets (19.8 g, 0.495 mol), methanol (450 cm3) and water (45 cm3) were heated under reflux for 2 h. The mixture was cooled, diluted with water (400 cm3) and acidified with concentrated hydrochloric acid (to pH 2) to give a white precipitate, which was filtered off, washed with water and dried (in vacuo). The crude material was recrystallised [petrol (b.p. 40-60 "C) and a small amount of toluene] to leave white crystals. Yield, 16.9g, (87%); transitions ( "C), K1-96.7*K2*112.54; m/z 208(M+), 191, 138(100%), 121, 70; v,,,/cm-' 3200-2100, 2980-2880, 1680, 16 10, 1580, 1520, 1470,1260,1035,960-920, 855; 6-0.98 (6 H, d, t, 2 x CH3), 1.28 (1 H, m, CH), 1.58 (1 H, m, CH), 1.90 (1 H, m, CH), 3.83 (2 H, octet, CH,O), 6.93 (2 H, d, arom), 8.03 (2 H, d, arom), the carboxylic acid proton was not detected.4-Hydroxy-4'-octyloxybiphenyl(5) 4,4'-Dihydroxybiphenyl (1 5.0 g, 8 1 mmol), 1-bromooctane (15.6 g, 81 mmol) and ethanol (150 cm3) were heated under reflux for 1 h; a solution of potassium hydroxide (4.87 g, 87 mmol) in water (15 cm3) was added over 1 h and the resulting mixture was heated under reflux for 3 h. The solvent was removed and the residue was purified by flash chromatog- raphy (dichloromethane) and recrystallised (methanol) to leave white powdery crystals. Yield, 7.07 g (32%); m.p.146-148 "C; m/z 298(M+), 282, 255, 240, 226, 212, 199, 186(100%); v,,,/ cm-' 3600-3100, 2970, 2960, 1610, 1580, 1505, 1450, 1380, 1250, 1180, 820; 6-0.85 (3 H, t, CH3), 1.30-1.55 (10 H, m, 5 x CH2), 1.80 (2 H, m, CH,), 4.00 (2 H, t, CH20), 4.65 (1 H, s, OH), 6.90 (4 H, 2 x d, arom), 7.43 (4 H, 2 x d, arom). 1-Benzyloxy-4-octyloxybenzene(7) The reaction was carried out using the procedure described for the preparation of compound 2: 4-benzyloxyphenol (6) (6.93 g, 35 mmol), 1-bromooctane (8.1 1 g, 42 mmol), anhy- drous potassium carbonate (24.2 g, 0.18 mol), dry butanone (150 cm3), 18-crown-6 (0.1 g) and potassium iodide (0.1 8).The crude residue was recrystallised (ethanol) to give white plate- lets.Yield, 9.94 g, (91%); m.p. 64-66 "C; m/z 312(M+), 109, 9 I( 100%); v,,,/cm -3070,2970-2880, 15 15, 1460, 1245, 1230, 1035, 1025, 830, 740, 700; 6-0.88 (3 H, t, CH,), 1.33 (10 H, m, 5 x CH,), 1.73 (2 H, m, CH,), 3.88 (2 H, t, CH,O), 5.00 (2 H, s, CH,O), 6.85 (4 H, m, arom), 7.38 (5 H, m, arom). 4-Octyloxyphenol (8) Compound 7 (9.9 g, 32 mmol), 5% palladium-on-carbon cata- lyst (0.6g) and ethyl ethanoate (200cm3) were stirred under a positive pressure of hydrogen until TLC analysis revealed the completion of the reaction. The mixture was then filtered (Hyflo-Supercel) and the solvent was removed. The residue was purified by flash chromatography (dichloromethane) and recrystallised [petrol (b.p. 40-60 "C)] to give white crystals. Yield, 7.0 g (99%); m.p.57-61 "C; m/z 222(M+), 178, 167, 149, 136, 123, 1 10( 100%); vmax/cm- ' 3600-3200, 3050, 2970-2860, 1520, 1460, 1380, 1250, 1050, 830; 6-0.85 (3 H, t, CH3), 1.27- 1.55 (10 H, m, 5 xCH,), 1.75 (2H, m, CH,O), 3.87 (2 H, t, CH,O), 4.38 (1 H, s, OH), 6.73 (4 H, m, arom). Methyl 2,4-Dibenzyloxybenzoate (10) The reaction was carried out using the procedure described for the preparation of compound 2: methyl 2,4-dihydroxyben- zoate (9) (67.2 g, 0.40 mol), benzyl bromide (164.2 g, 0.96 mol), anhydrous potassium carbonate (276.0 g, 2.0 mol) and dry butanone (1 dm3). The product was used without further purification. Crude yield 100%; m.p. 49.5-51.5 "C; m/z 348(M+), 334, 316, 257, 181, 167, 91(100%); vmax/cm-' 1720, 1610, 1580, 1450, 1270, 1190, 1120, 1020, 735, 700; 6-3.83 (3 H, s, OCH3), 5.03 (2 H, s, CH20), 5.13 (2 H, s, CH,O), 6.55 (2 H, m, arom), 7.33 (10 H, m, 2 x Ph), 7.85 (I H, d, arom).2,4-Dibenzyloxybenzoic Acid (11) The reaction was carried out using the procedure described for the preparation of compound 3: compound 10 (149.2 g, 0.40 mol), sodium hydroxide pellets (33.0 g, 0.83 mol), meth- anol (750cm3) and water (75cm3). The white powder was used without further purification. Crude yield 126.9 g (95%); m.p. 112-115°C; m/z 334(Mf), 316, 290, 243, 181, 152, 91(100%); v,,,/cm-' 3700-2000,3050,3040,1730, 1705,1610, 1580, 1505, 1460, 1280, 1040, 740, 700; 6-5.08 (2 H, S, CHZO), 5.20 (2 H, s, CH,O), 6.68 (2 H, m, arom), 7.38 (10 H, m, 2 x Ph), 8.13 (1 H, d, arom), 10.68 (1 H, s, OH).4-Octyloxyphenyl 2,4-DibenzyEoxybenzoate (12a) Compound 8 (5.55 g, 25 mmol), compound 11 (8.35 g, 25 mmol), N,N'-dicyclohexylcarbodiimide(6.18 g, 30 mmol), 4-(N-pyrrolidino)pyridine(0.93 g, 6.3 mmol) and dry dichloro- methane (100 cm3) were stirred overnight with the exclusion of moisture. The solvent was removed and the residue was purified by flash chromatography (dichloromethane), and recrystallised [petrol (b.p. 40-60 "C) and a small amount of toluene] to give white crystals. Yield 12.5 g (93%); m.p. 83-86 "C; m/z 448, 317, 181, 166, 149, 131, 119, 109, 91(100%); v,,,/cm-' 3080,3040,2970-2770,1725, 1610,1585, 1510, 1470, 1250, 1200, 1140, 1030, 875, 830, 735, 715, 700; 6- 0.88 (3 H, t, CH3), 1.33 (10 H, m, 5 x CH,), 1.78 (2 H, m, CH,), 3.93 (2 H, t, CH2O), 5.10 (2 H, S, CHZO), 5.18 (2 H, S, CHZO), 6.63 (2 H, m, arom), 6.88 (2 H, d, arom), 7.05 (2 H, d, arom), 7.40 (10 H, m, 2 x Ph), 8.05 (1 H, d, arom).4'-Octyloxybiphenyl-4-yl 2,4-Dibenzyloxybenzoate(12b) The reaction was carried out using the procedure described for the preparation of compound 12a: compound 5 (7.05g, 25 mmol), compound 11 (8.35 g, 25 mmol), N,N'-dicyclohexyl- carbodiimide (6.1 8 g, 30 mmol), 4-(N-pyrrolidino)pyridine (0.93 g, 6.3 mmol) and dry dichloromethane (100 cm3). The residue was purified by flash chromatography (dichlorome- thane), and recrystallised (toluene) to give fine white crystals. Yield, 11.4 g (74%); m.p. 118-122 "C; m/z 420, 317, 227, 91(100%); v,,,/crn-' 3080,2980-2860,1745,1610,1580,1505, 1460, 1250, 1215, 1180, 1030, 880, 830, 805, 740, 700; 6-0.88 (3 H, t, CH3), 1.33 (10 H, m, 5 x CH2), 1.78 (2 H, myCH,), 3.98 (2 H, t, CH20), 5.10 (2 H, s, CH,O), 5.18 (2 H, s, CH20), 6.65 (2H, m, arom), 6.95 (2H, d, arom), 7.20 (2H, d, arom), 7.45 (14 H, m, 2 x Ph, arom), 8.08 (1 H, d, arom).4-Octyloxyphenyl 2,4-Dihydroxybenzoate (13a) The reaction was carried out using the procedure described for the preparation of compound 8: compound 12a (12.5 g, 23 mmol), 5% palladium-on-carbon catalyst (1.5 g) and ethyl ethanoate (1 50 cm3). The residue was recrystallised [petrol (b.p. 40-60 "C)and a small amount of ethanol] to give white crystals. Yield, 7.15 g (84%); m.p.133-135 "C; m/z 358(M+), 336, 322, 307, 292, 277, 245, 222, 137(100%), 110; vmax/cm-l 3440, 3300, 2940-2860, 1660, 1630, 1595, 1510, 1470, 1350, 1250, 1190, 1145, 1080, 1050, 870, 815; 6-0.90 (3 H, t, CH3), 1.30 (10 H, m, 5 xCH2), 1.78 (2 H, m, CH2), 3.98 (2 H, t, CH20), 6.45 (2 H, m, arom), 6.93 (2 H, d, arom), 7.08 (2 H, d, arom), 7.93 (1 H, d, arom), 10.23 (2 H, s, OH). 4-Octyloxybiphenyl-4-yl 2,4-Dihydroxybenzoate (13b) The reaction was carried out using the procedure described for the preparation of compound 8:compound 12b (1 1.4 g, 19 mmol), 5% palladium-on-carbon catalyst (1.5 g), ethyl ethanoate (75 cm3) and tetrahydrofuran (75 cm3). The residue was purified by flash chromatography (dichloromethane), and recrystallised (toluene) to give grey-white crystals.Yield, 6.20 g (77%); m.p. 167-168 "C; m/z 434(M+), 298, 282, 268, 254, 240, 228, 199, 186(100%), 137; v,,,/cm-' 3420, 3320, 2970-2860, 1665, 1635, 1610, 1595, 1505, 1470, 1350, 1270, 1205, 1145, 860, 840, 810; 6-0.90 (3 H, t, CH3), 1.30 (10 H, m, 5 x CH,), 1.80 (2 H, m, CH,), 4.00 (2 H, t, CH,O), 6.48 (2 H, m, arom), 6.98 (2 H, m, arom), 7.23 (2 H, d, arom), 7.50 (2 H, d, arom), 7.60 (2 H, d, arom), 8.05 (1 H, m, arom), 10.70 (2, s, OH). (S)-4-OctyloxyphenyE 2-Hydroxy-4-[4-(2-methylbutoxy) benzoyEoxy]benzoate (14a) The reaction was carried out using the procedure described for the preparation of compound 12a:compound 13a (7.15 g, 20 mmol), compound 3 (3.46 g, 16.6 mmol), N,N'-dicyclohex- ylcarbodiimide (4.1 1 g, 20 mmol), 4-(N-pyrrolidino)pyridine (0.62 g, 4.2 mmol), dry dichloromethane (50 cm3) and dry tetrahydrofuran (50 cm3).The residue was purified by flash chromatography (dichloromethane), and recrystallised [petrol (b.p. 40-60 "C)] to give fine white crystals. Yield, 8.1 1 g (89%); transitions ( "C) K*71.5.N*- 122.5.1; m/z 548(M+), 533, 355, 327, 222, 191(100%), 137, 121, 110; v,,,/cm-' 3700-3300, 2980-2870, 1735, 1685, 1610, 1515, 1470, 1350, 1250, 1195, 1165, 1255, 890, 850, 820; 6-0.90-1.03 (9 H, m, 3 x CH3), 1.40-1.80 (15 H, m, 7 x CH,, CH), 3.88 (2 H, octet, CH,O), 3.98 (2 H, t, CH,O), 6.88-7.00 (6 H, m, arom), 7.13 (2 H, d, arom), 8.15 (3 H, m, arom), 10.70 (1 H, s, OH). (S)-4'-OctyloxybiphenyE-4-yl 2-Hydroxy-4-[4-(2-methylbutoxy) benzoyloxylbenzoate (14b) The reaction was carried out using the procedure described for the preparation of compound 12a: compound 13b (6.20 g, 15 mmol), compound 3 (2.57 g, 12 mmol), N,N'-dicyclohexyl- carbodiimide (3.06 g, 15 mmol), 4-(N-pyrrolidino)pyridine (0.46 g, 3.0 mmol), dry tetrahydrofuran (75 cm3).The residue was purified by gravity chromatography (dichloromethane), and recrystallised [petrol (b.p. 40-60 "C)and a small amount of toluene] to give white crystals. Yield, 5.40 g (70%); tran- sitions ( "C) K1 65.2 K2 86.5 N* 240.6 I; m/z 624(M +), 594, 580, 567, 310, 298(100%), 186, 121; v,,,/cm-' 3600-3250, 2970-2860, 1745, 1685, 1615, 1505, 1480, 1350, 1250, 1205, 1170, 1070, 895, 845, 810; 6-0.88 (3 H, t, CH3), 0.98 (3 H, t, CH3), 1.05 (3 H, d, CH3), 1.33-1.80 (15 H, m, 7 xCH2, CH), 3.85 (2 H, octet, CH,O), 4.00 (2 H, t, CH,O), 6.85 (2 H, m, arom), 6.93 (4 H, 2 x d, arom), 7.23 (2 H, d, arom), 7.50 (2 H, d, arom), 7.63 (2 H, d, arom), 8.13 (3 H, 2 x d, arom), 10.68 (1 H, s, OH).J. MATER. CHEM., 1992, VOL. 2 (S)-4-OctyEoxyphenyl4-[4-(2-Methylbutoxy)benzoyloxy]-2-(pent-4-eny1oxy)benzoate(15a) Diethyl azodicarboxylate (0.39 g, 2.2 mol) in dry tetrahydro- furan (10 cm3) was added to a stirred mixture of 14a (1.20 g, 2.2 mmol), triphenylphosphine (0.58 g, 2.2 mmol) and dry tetrahydrofuran (20 cm3) under dry nitrogen. After 1 h, pent- 4-en-1-01 (0.19 g, 2.2 mmol) in tetrahydrofuran (5 cm3) was added dropwise and the resulting mixture was stirred until completion of reaction (TLC).The solvent was removed from the mixture and the residue was purified by flash chroma- tography (dichloromethane) and recrystallised [petrol (b.p.40-60 "C)] to afford the product as white crystals. Yield, 0.24 g (19%); m.p. 68-70 "C;[alD= -2.1" at 21 "Cin CHCl,; m/z 425, 395(100%), 222, 205, 191, 136, 121; vmaX/cm-' 3080, 2970-2860, 1750, 1710, 1645, 1605, 1585, 1505, 1470, 1255- 1235, 1190, 1030, 985, 915, 870, 850, 810; 6-1.00 (9 H, m, 3 xCH,), 1.28-1.53 (13 H, m, 6xCH2, CH), 1.73 (2 H, m, CH2), 1.93 (2 H, m, CH2),2.28 (2 H, m, CH2), 3.85 (2 H, octet, CH,O), 3.95 (2 H, t, CHZO), 4.07 (2 H, t, CHZO), 5.00 (2 H, m, =CH2), 5.85 (1 H, m, CH=), 6.90-7.13 (8 H, m, arom), 8.10 (3 H, m, arom). (S)-4'-Octyloxybiphenyl-4-yl 4-[4-(2-Methylbutoxy)benzoyloxy]-2-( pent-4-enyloxy)benzoate (15b) The reaction was carried out using the procedure described for the preparation of compound 15a:diethyl azodicarboxylate (1.74 g, 10 mmol), compound 14b (6.20 g, 10 mmol), tri-phenylphosphine (3.14 g, 10 mmol), dry tetrahydrofuran (20 cm3), pent-4-en-1-01 (0.86 g, 10 mmol).The solvent was removed from the mixture and the residue was purified by flash chromatography (dichloromethane) and recrystallised (ethyl acetate, 3 x) to afford the product as white crystals. Yield 3.77 g (55%); transitions ( "C) K*91.9*N*- 142.201; [a],,= -25.3" at 21 "Cin CHCl,; m/z 692(M+), 395,298, 205, 19 1( 1 OOYO),12 1 ;v,,,/cm -'3090,2970-2860, 1 800, 1780, 1655, 1630, 1595, 1550, 1520, 1475, 1300, 1250, 1210, 1085, 1050, 895, 885, 855, 815; 6-0.90 (3 H, t, CH3), 1.00-1.10 (6 H, d, t, 2 xCH3), 1.33-1.60 (13 H, m, 6xCH2, CH), 1.83 (2 H, m, CH2), 1.98 (2 H, m, CH2),2.33 (2 H, m, CH2),3.88 (2 H, octet, CHZO), 4.00 (2 H, t, CHZO), 4.10 (2 H, t, CH2O), 5.00 (2 H, m, =CH2), 5.83 (1 H, m, CH=), 6.95 (6 H, m, arom), 7.25 (2 H, d, arom), 7.50 (2 H, d, arom), 7.60 (2 H, d, arom), 8.13 (3 H, m, arom).Siloxane Polymer (16a) Compound 15a (0.23 g, 0.37 pmol), poly(hydrogenmethy1) siloxane (20 pg, 0.34 pmol) and dry dichloromethane (20 cm3) were stirred and heated under reflux, with the exclusion of moisture. The catalyst, platinum divinyltetramethyldisiloxane complex dm3, 3-3.5% platinum in xylene, from Petrarch) was added every morning and evening until the reaction mixture showed the absence of an Si-H absorption band (2160 cm-') in the infrared spectrum; reactions were typically complete after 4 days.The solvent was removed and the polymer was dissolved in dry dichloromethane and repeat- edly precipitated with methanol until the alkene precursor was removed (monitored by TLC). The polymer was then filtered (membrane filter, 0.5 pm) and dried at 130 "C in uucuo for 5 h. Transitions ( "C)g.6.1 *N**24.8*1. Siloxane Polymer (16b) The reaction was carried out using the procedure described for the preparation of compound 16a: compound 15b (0.30g, 0.43 pmol), poly(hydrogenmethy1)siloxane (20 pg, 0.41 pmol), dry dichloromethane (20 cm3). Transitions ( "C) g 26.7 N* -123.5 I.The parent compounds (17a and 17b) were synthesised in J. MATER. CHEM., 1992, VOL. 2 a similar way, except that methyl 4-hydroxybenzoate was used instead of methyl 2,4-dihydroxybenzoate and, of course, the final etherification step (using pent-4-en-1-01) was not needed; their transition temperatures are shown in Table 1. Discussion It can be seen from Table 1 that the inclusion of a lateral substituent in the parent molecules 17a and 17b has a marked effect on their mesophase thermal stabilities. The small hydroxy group has the effect of depressing both the clearing points of the chiral nematic mesophases by 45-5OoC, and also the melting points of the respective molecules. This was also shown by Taniguchi et a1.,14 CH, X I1 X = H K 68 Sc' 120 SA172 I X=OH K28w 86S~141I where the clearing temperature of their compound (structure 11) was lowered by 31 "C and the melting point by 40 "C, also the S,* to SA transition was depressed by 34 "C.This is a well established phenomenon for small lateral substituents, and even the inclusion of a fluoro substituent can cause a compound showing solely smectic polymorphism to exhibit a nematic mesophase, and can also dramatically reduce its clearing temperature. These very marked depressions are caused because the broadening of the molecule perturbs the alignment and associations of the molecules within the smectic mesophase more than it does in the nematic mesophase so that the smectic tendencies of the compound are reduced, and a nematic mesophase can be formed.The introduction of alkene side-chains (15a, 15b) causes a further reduction in clearing temperatures compared with the small hydroxy group. Work with lateral substituents of varying lengths has been reported by Weissflog and Dem~s'~~'' and it is noted in this work that the major depressions in clearing points come from substitution by methyl, ethyl and propyl groups. Increasing the chain length of these lateral substituents Table 1 Transition temperatures for polymers 16a and 16b, their precursors 14a, 15a, 14b, 15b and their related mesogenic core systems 17a, 17b compound no. CH,CH,$HCH*O ~ c 0 2 ~' -X=H 17a X=OH 14a X =O(CH,),CH =CH, 15a siloxane polymer 16a CH, transition temperatures/ "C x c02~0c8H K -95.0*N** 171.8 I K.71.5-N**122.5-1 K -70.0-1 g -6.1(0.16)".N* *24.8(0.41)b.I X X=H 17b K * 144.0* S, * -156.3* N* -285.6.1 X=OH 14b K, -65.2*K, e86.5*N*-240.6.1 X=O(CH,),CH=CH, 15b K -91.9*N*-142.2-1 siloxane polymer 16b g.26.7(0.12)"*N** 123.5(0.45)".1 The values in parentheses are heat capacity changes ("/Jg-' K-') and enthalpy changes (*/J g-').causes the mesophase (or monotropic) clearing temperatures to level off at a chain length of ca. 16 carbon atoms and no odd-even effect is detected. It should be noted that any smectic tendencies of the parent compound are eliminated to give a nematic phase when any lateral alkyl group (even methyl) is incorporated within the system.The same argu- ments can be applied to the effects of these alkyl groups, as for those given above for the fluoro and hydroxy substituents for their ability to reduce the ordering of the mesophase. Long lateral groups can be used to advantage; for example, their effect causes an increase in the dielectric anisotropies of cyano compounds because they can interfere with the anti- parallel correlation of the systems and so give smaller interactions of the cyano groups and the aromatic rings.'* On polymerisation, the side-chain 15b gave polymer 16b, and this change again gave a depression in the clearing point of the material of 18.7 "C. This result is in accordance with the effect of a lateral group appended on the left-hand ring of a nematic homop~lyrner,'~ but disagrees with results by Keller et al." where the clearing temperature is higher in the polymer when the lateral group is appended to the middle ring (although this side-chain has two identical aromatic esters on either side of the middle ring); both the above cases involve siloxane homopolymers.The difference between the two poly- mers, Ma and 16b is the presence of an extra aromatic ring in the latter's side-chain, and this difference is reflected in the clearing temperatures of the respective polymers. The four- ring side-chain clears 98.7 "C higher than its three ring ana- logue, an increase of this magnitude is as expected for the inclusion of a further aromatic ring. The pitch lengths of the chiral nematic phases have not yet been measured, although they are expected to be short (in the ultraviolet region), owing to the short spacer length,2*3 and the twisting power of the 2-methylbutyl chiral group.Leube and Finkelmannl' have attached an undec- 10-enyl spacer at the 6-position of a cholestanol derivative to give a chiral nematic side-chain with a clearing point of 40 "C, and the derived siloxane polymer has a chiral nematic to isotropic transition of 47 "C. Their results using a 'side-on' side-chain show that polymerisation does not lead to a smectic polymer and only gives a small rise in the clearing temperature of the polymer relative to that of the side-chain. Our results, demon- strating a reduction of the clearing point, are the first examples of such an effect in chiral systems on producing a laterally attached polymer 16b, from its side-chain precursor 15b.Compound 15b has been logically synthesised from smaller units, and its structure will be more capable of controlled modification to achieve specific physical properties than steroid-based systems such as those reported by Leube and Finkelmann. Further work in this area to be reported later is involved with altering the point of attachment of the spacer group, and the effect of this change on mesophase thermal stability, 17 and also with investigating the use of longer spacer groups, with the expectation that these changes will result in selectively reflecting polymer^.^,^,^^,^^ Conclusions In this paper we have presented a synthetic pathway which leads to laterally attached, side-chain polysiloxanes and, with simple adaptions this could give polymers with different chiral groups, spacer groups and terminal alkoxy groups.The poly- mer 16a has a small, 18.7 "C,chiral nematic range, and this is increased to 96.8 "C for compound 16b when the phenyl moiety is replaced by a biphenyl; this change shows how the system is sensitive to changes within the core region of the lateral side-chain. The attachment of the mesogenic cores 17a I24 J. MATER. CHEM., 1992, VOL. 2 and 17b (via compounds 15a and 15b) within the polymers 16a and 16b respectively, has had the resulting effect of reducing the clearing temperatures of the materials by 147.0 and 162.1 "C.This demonstrates how a high mesophase thermal stability is required for the mesogenic core, if the 5 6 7 8 Ya. S. Freizon, S. G. Kostromin, N. I. Boiko, U. V. Shibaev and N. A. Plate, A.C.S. Polym. Preprint, 1983, 24, 279. F. Hessel and H. Finkelmann, Polym. Bull., 1985, 14, 375. G. W. Gray, J. S. Hill and D. Lacey, Angew. Chem., 1989, 1146. F. Hessel, R. P. Herd and H. Finkelmann, Makromol. Chem., 1987, 188, 1597. resulting polymer is required to exhibit mesomorphism, and it may lead the way to the use of five-ring structures, as the clearing temperatures of the polymers will only be ca. 250 "C. 9 10 11 G. W. Gray, J. S. Hill and D. Lacey, Mol. Cryst. Liq. Cryst., 1991, 197, 43. H. Leube and H. Finkelmann, Polym. Bull., 1988, 20, 53.P. Keller, F. Hardouin, M. Mauzac and M. F. Achard, Mol. Cryst. Liq. Cryst., 1988, 155, 171. The authors would like to thank AKZO Corporate Research, Arnhem, The Netherlands for financial support for R. A. L., and Dr D. F. Ewing, Mrs B. Worthington, Mr R. Knight and Mr A. D. Roberts for spectroscopic measurements. 12 13 14 15 E. Fischer and H. 0. L. Fischer, Ber. Deut. Chem., 1913, 46, 1138. E. Fischer, Ber. Deut. Chem., 1909, 42, 215. U. Taniguchi, M. Ozaki, K. Nakao and K. Yoshino, Mol. Cryst. Liq. Cryst., 1989, 167, 191. G. W. Gray, M. Hird and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1990, 195, 221. References H. Finkelmann, H. Ringsdorf and J. H. Wendorff, Makromol. 16 17 18 W. Weissflog and D. Demus, Cryst. Res. Tech., 1983, 18, 21. W. Weissflog and D. Demus, Cryst. Res. Tech., 1984, 19, 55. S. Takenaka, H. Morita, S. Kusabayashi, Y. Masuda, M. Iwano Chem., 1978, 179, 273. and T. Ikemoto, Chem. Lett., 1988, 1559. H. Finkelmann and G. Rehage, Makromol. Chem. Rapid Com- mun., 1982, 3, 859. 19 G. W. Gray, J. S. Hill and D. Lacey, Mol. Cryst. Liq. Cryst. Lett., 1990, 7, 47. H. Finkelmann and G. Rehage, A.C.S. Polym. Preprint, 1983, 24, 277. 20 21 H. Finkelmann and H. J. Kock, Disp. Technol., 1985, 1, 81. P. J. Shannon, Macromolecules, 1984, 17, 1873. H. Finkelmann, J. Koldehoff and H. Ringsdorf, Angew. Chem. Int. Ed. Engl., 1978, 12, 935. Paper 11045275; Received 30th August, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200119

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1), 125-130 Preparation and Characterisation of Polyaniline Colloids using a Monodisperse Poly(ethy1ene oxide)-based Steric Stabiliser Peter Tadros,a Steven P. Armes*a and Shen Yung Lukb a School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton BN7 9QJ, UK Courtaulds Research, P.O.Box 777, 107 Lockhurst Lane, Coventry, UK The synthesis of polyaniline colloids using a new polymeric steric stabiliser of narrow molecular weight distribution based on poly(ethy1ene oxide) is reported. This model stabiliser contains one tertiary amine group per polymer chain which is believed to participate in the in situ aniline polymerisation, leading to the chemical grafting of the stabiliser onto the surface of the polyaniline particles.These polyaniline colloids were characterised by a wide range of techniques including electron microscopy, FTIR and visible absorption spectroscopy, microanalysis, d.c. conductivity and thermogravimetric analysis. The colloids possess a polydisperse non-spherical morphology and remarkably high solid-state electrical conductivity (0.1-1.0 S cm-'), despite the presence of the electrically insulating grafted stabiliser. The mass ratio of grafted stabiliser : polyaniline was determined indirectly by 'H NMR spectroscopy analysis of the post-reaction super- natant solutions. In addition, flocculation experiments were carried out to assess the aggregation stability of these dispersions in the presence of added electrolyte. Keywords: Polyaniline ; Colloid; Steric stabiliser ; Po/y(ethy/eneoxide) Polyaniline is generally recognised to be the only completely air-stable conducting polymer.' It can be synthesized either ~hemically~-~or electrochemically5-7 as a bulk powder or film.Although soluble in various solvents such as THF, DMF, or NMP in the undoped state, the doped (conducting) form is insoluble in most solvents (except concentrated acids8) and Fig. 1 Chemical structure of the poly(ethy1ene oxide)-based steric is therefore an intractable and unprocessable material. Various stabiliser (PEO-1)workers have attempted to improve the processability of polyaniline, usually by modifying the polymer chain with alkyl, alkoxy, or sulfonic group^.^-" Recently, Armes and Aldissi reported the preparation anionic polymerisation of ethylene oxide using an N-phenyldi- of sterically stabilised colloidal polyaniline particles via ethanolamine initiator and NaOH as a ~atalyst.'~' Molecular-dispersion polymerisation.12 The steric stabiliser was a weight distribution data were obtained by gel-permeation random (statistical) chromatography (GPC) in THF eluent using monodisperse poly(2-vinylpyridine-co-4-aminostyrene) copolymer.Since then we have reported the use of other sta- polystyrene homopolymer calibration standards and a refrac- poly(viny1 al~ohol),'~ tive index detector. Similar results were obtained by Dr. S.bilisers based on poly(4-~inylpyridine),'~ pol y(N-vin y limidazole)' and pol y(N-vin ylpyrrolidone). Holding at RAPRA using LiBr/DMF eluent and poly(ethy1ene All of these random copolymer stabilisers were of rather oxide) standards.The number-average molecular weight of broad molecular weight distribution (M,/M, z2.5-6.5) and PEO-1 was determined by 'H NMR spectroscopy in D20 contained 1-40 mol% pendant primary amine groups. Our (360 MHz Bruker instrument) by comparing the peak integral visible absorption spectroscopy studies showed that, for each due to the aromatic protons of the tertiary aniline group stabiliser type, these amine groups were oxidised prior to (6~6.7-7.4) with the peak integral due to the methylene the in situ aniline polymerisation, which suggested that the protons of the poly(ethy1ene oxide) chains (6z3.5-3.9). An copolymer stabilisers are chemically grafted to the polyaniline FTIR spectrum of a film of the stabiliser cast from methanol particles. solution onto a KBr disc was also recorded using a Perkin- In 1989, Barry and Kuhn of the Milliken Research Corpor- Elmer 1710 instrument.ation filed a patent application describing the preparation of polyaniline colloids using a novel low-molecular-weight poly Preparation of Polyaniline Colloids (ethylene oxide)-based steric ~tabi1iser.l~" This stabiliser had a narrow molecular weight distribution and contained only (NH&S208 (1.49 g) [or KI03 (0.46 g)] and PEO-1 (1.0-3.0 g) one tertiary amine graft site per polymer chain. The present were dissolved together in 50 cm3 1.2mol dm- hydrochloric paper is focused on the preparation and complete characteris- acid and this solution was then aged at 20 "C for 60 min with ation of these poly(ethy1ene oxide)-based polyaniline colloids continuous stirring.Aniline monomer (0.50 cm3) was then using a wide range of experimental techniques. injected uia a syringe and the reaction mixture was stirred for at least 16 h. The resulting dark green dispersions were centrifuged at 50 000 rpm for 18 h using a Beckman L2-65B Experimental ultracentrifuge. The supernatants were decanted and the sedi- Characterisation of the Steric Stabiliser ments redispersed in 1.2 mol dmP3 HCl. The colloids were then filtered under gravity to remove undispersed sediment. The poly(ethy1ene oxide)-based steric stabiliser (PEO-1; see Some of these experiments were repeated with the reaction Fig.1) was kindly donated by Dr. H. Kuhn of the Milliken mixture being pre-cooled to 0 "C for at least 30 min prior to Research Corporation. It was synthesized by the solvent-free addition of the aniline monomer. Visible Absorption Spectroscopy Experiments on Aged Stabiliser Solutions (NH4)2S208 (1.49g) [or KI03 (0.46g)l and PEO-1 (3.0g) were dissolved together in 50 cm3 1.2 mol dm-3 hydrochloric acid and this solution was then aged at 20 "C for 60 min with continuous stirring. During this time period the visible absorp- tion spectra of these ageing oxidant/stabiliser solutions were recorded at regular intervals over the range 400-600 nm in a 10mm quartz cell using a Perkin-Elmer PU 8720 UV-VIS absorption spectrometer.Characterisation of Polyaniline Colloids Visible absorption spectra of the diluted polyaniline disper- sions were recorded in aqueous acid using the above instru- ment. Infrared spectra of the dried polyaniline colloids were recorded using a Nicolet 740 FTIR spectrometer in conjunc- tion with a Spectra-Tech IR Plan microscope. Transmission electron microscopy (TEM) studies were made on dilute dispersions dried down on carbon-coated copper grids (ex. Bio-Rad) using a JEOL-IOOC instrument. Scanning electron microscopy studies were made using an ASID4D ultra-high resolution scanning attachment to the above TEM instrument. The latter samples were not sputter- coated with a gold overlayer since the polyaniline colloids were sufficiently conductive to prevent significant charging problems.CHN and S elemental microanalyses were determined using a Perkin-Elmer 2400 instrument. Chlorine analyses were determined by the Schoniger oxygen combustion method. Conductivity measurements were made at 20 "C using conven- tional four-point probe techniques. Weight fractions of the as-prepared polyaniline dispersions and their respective super- natants (after centrifugation) were determined by drying to constant weight in a 60 "C vacuum oven. The relative thermal stabilities of the dried polyaniline colloids were assessed using a Perkin-Elmer TGA-7 instrument. The stabiliser/conducting polymer composition of the poly- (ethylene oxide)-stabilised polyaniline colloids (ca. 20 mg) dis- solved in 0.5 cm3 conc.D2S04(this solvent dissolves both the stabiliser and the normally insoluble polyaniline component) was investigated directly by 'H NMR spectroscopy. Since the chemical structure of both components is known, the mass ratio of stabiliser to conducting polymer could, in principle, be calculated from the ratio of the aromatic peak integral due to the polyaniline protons at 6 z7.5-8.5 to the aliphatic peak integral due to the methylene protons of the stabiliser (6 ~4.2-5.2). In a control experiment, known masses (ca.6 mg) of polyaniline and poly(ethy1ene oxide) homopolymers were co-dissolved in 0.5 cm3 conc. D2S04. The theoretical mass composition of this sample was calculated from the NMR peak integral ratio and compared to the actual mass compo- sition.The clear post-centrifugation supernatants were assayed for ungrafted PEO-1 stabiliser by the following procedure. A known mass of toluene-4-sulfonic acid, sodium salt (53& 1 mg) was dissolved in 0.50 cm3 1.2 mol dm-3 HCl containing a known concentration of PEO-1 and dm3 of D20 was then added by micropipette. The 'H NMR spectra of these samples were recorded using the 360 MHz Bruker instrument. The peak integrals of both the aromatic protons (6~7.1-7.5) and the methyl protons (6 z2.1-2.2) of the toluene-4-sulfonic acid were ratioed to the methylene protons due to the PEO- 1 stabiliser (6~3.2-3.5) and these values were used to con- struct a calibration curve. The above procedure was repeated using 0.50 cm3 of the supernatant solutions of unknown PEO- 1 concentration (instead of the PEO-1 stock solution) and J.MATER. CHEM., 1992, VOL. 2 thus the concentration of non-adsorbed PEO-1 present in the supernatant solution was determined from the calibration curve. Flocculation experiments were carried out as follows: vary- ing quantities of MgS04 were added to six sample bottles each containing 10 cm3 of a 0.10% m/v polyaniline dispersion in 1.2 mol dmP3 HC1 so as to obtain final MgS04 concen- trations in the range 0.0-1.0 mol dm-3. These bottles were then immersed in a waterbath and equilibrated at 47 "C for 5 h. Results and Discussion The degree of polymerisation (n+m)of PEO- 1 was determined to be 125f6 by 'H NMR spectroscopy, which compares reasonably well with the manufacturer's nominal degree of polymerisation of 100 for the batch sample.This result cor- responds to a number-average molecular weight for PEO- 1 of 5600 & 300 g mol- '. Analysis by I3C NMR spectroscopy gave a degree of polymerisation of ca. 100 but in view of the poorer signal-to-noise ratio in the spectrum this latter value was considered to be less reliable. In principle the number- average molecular weight of PEO-1 can also be determined from its nitrogen content, but in practice the latter was below the detection limit for our CHN elemental microanalysis instrument. Gel permeation chromatography studies on PEO-1 indicated a very narrow molecular weight distribution (M,/M,~l.09).Both the 'H and 13C NMR spectra of PEO- 1 were consistent with the chemical structure depicted in Fig. 1. The FTIR spectrum of the stabiliser contained no bands attributable to N-H vibrations. This indicates that the aromatic amine group in each stabiliser molecule is diethoxylated as expected, since the stabiliser was synthesized using N-phenyldiethanolamine (rather than aniline) as an initiator. Thus, these aromatic amine groups are not terminally attached to a single poly(ethy1ene oxide) chain (n,m>0). The results shown in Table 1 confirm that stable colloidal dispersions of polyaniline can be prepared under certain conditions using the poly(ethy1ene oxide)-based steric stabil- iser PEO- 1. This stabiliser has several novel features compared to the random or graft copolymer stabilisers reported pre- vio~sly.'~-'~~'~~~~It is a relatively low molecular weight stabiliser of rather narrow molecular weight distribution.In addition, it has only one graft site per stabiliser chain and, unlike our previous stabilisers, this graft site is a tertiary (rather than primary) aromatic amine. However, a somewhat higher initial stabiliser concentration is needed for the forma- tion of a stable colloidal dispersion (40-60 g dmP3 rather than 7.5-10 gdrnp3). This is presumably a consequence of the lower number of graft sites per stabiliser molecule [one site per chain for PEO-1 compared to 9-20 sites per chain Table 1Reaction conditions for the preparation of sterically stabilised polyaniline colloids experi-ment no.oxidant stabiliser concentration/ g dm-, polymerisationtemperature/ K colloid formation? 1 KIO, 20 293 no 2 KIO, 40 293 no 3 KIO, 60 293 no 4 KIO, 20 273 no 5 KIO, 40 273 no 6 KIO, 60 273 no 7 8 9 10 (NH4)2S208 (NH4)2S20* (NH4)2S208 (NH4)2S208 20 40 60 40 293 293 293 273 no Yes Yes Yes J. MATER. CHEM., 1992, VOL. 2 for poly(2-vinylpyridine)-, poly(N-vinylimidazo1e)- and poly- (N-vinylpyrro1idone)-basedcopolymer stabili~ers.'~,'~,'~ The gravimetric determination of the digerence in solids content between the post-reaction solution and the post-reaction supernatant confirmed that the aniline polymerisation pro- ceeded to high yield (>70%) under the reaction conditions employed. The PEO-1 stabiliser is compatible (does not form insoluble complexes) with both (NH4)2S20s, the preferred oxidant for the polymerisation of aniline, and the KI03 oxidant.However, only the former oxidant results in the formation of stable dispersions of polyaniline particles. Use of the latter oxidant leads instead to macroscopic precipitation of the conducting polymer even at 0 "C,at which a slower rate of polymerisation might be expected. We have no satisfactory explanation for this surprising observation at the present time. The visible absorption spectroscopy studies on the ageing oxidant/stabiliser solutions confirm that the tertiary amine sites of PEO-1 are oxidised to radical-cation species under the colloid synthesis conditions prior to the addition of aniline monomer.Similar observations were reported for the polydis- perse random copolymer stabilisers containing primary amine group^.'^-'^ In the present study the absorption peak maxi- mum of the oxidised species is blue-shifted from 520-540 to 495 nm. This blue shift is presumably related to the different substituents on the amino graft sites. In control experiments a similar peak at 495 nm was observed for the oxidation of the small molecule model compound N-phenyldiethanolamine (for which n+m=2, see Fig. 1) under identical reaction con- ditions. No such peak was observed for an oxidant/poly(ethyl- ene oxide) homopolymer solution of similar molecular weight to PEO-1. We infer from the above experiments that the oxidised PEO-1 stabiliser inevitably participates in the in situ aniline polymerisation during the colloid synthesis, leading to the chemical grafting of the stabiliser onto the surface of the polyaniline particles.The time dependence of the absorption peak at 495 nm is shown for both the (NH4)2Sz08 and the KI03 oxidants in Fig. 2. Clearly, the former oxidant is more efficient for short ageing times (<60 min). However, after the 1 h ageing period 1.o 0.8 CI.-cf -L v 0.6 v)a3d II x 4; 0.4 0) C (0e tn n a 0.2 0 0 10 20 30 40 50 60 ageing time/min Fig. 2 Increase in absorbance at 495 nm of oxidised PEO-1 stabiliser solutions with ageing time for (a)KIO, and (b)(NH4)2S206oxidants used for the colloid synthesis, the 495 nm peak absorbances for the two oxidants are almost identical.Thus, the results of these experiments cannot account for the observation that the use of the KI03 oxidant leads to the macroscopic precipi- tation of polyaniline rather than the formation of stable colloidal dispersions. A typical transmission electron micrograph of a diluted polyaniline dispersion (expt. 9) is shown in Fig. 3. Unlike the colloidal polyaniline reported by Vincent and Waterson,18 the 'particle morphology is clearly non-spherical and is more similar to the 'rice-grain' morphology we have previously observed for polyaniline particles obtained using polydisperse statistical copolymer stabi1isers.'*-l6 A scanning electron micrograph of a polyaniline colloid film (expt.9) is shown in Fig. 4. Although particle coalescence has almost certainly not occurred during solvent evaporation (owing to the high Tgof the conducting polymer component) it is nevertheless extremely difficult to distinguish between the individual poly- aniline particles within the film. This observation is in contra- distinction to our scanning electron microscopy studies of films fabricated from colloidal polypyrrole dispersions which have clearly revealed the presence of the original individual spherical polypyrrole particles within the film matrix.20 The FTIR spectra of two polyaniline colloids are shown in Fig. 5. Spectrum B is of expt. 8 and is identical to that of bulk polyaniline in the range 750-1650 cm-',' with no bands due to the PEO-1 stabiliser being observed.However, spec- trum A of expt. 9 contains six additional bands at 1467, 1342, 1281, 1110, 963 and 843 cm-', which are characteristic of the PEO-1 stabiliser. Turning our attention to the 2500-3700 cm-' region we observe a band at ca. 2900 cm- due to the C-H stretch of the methylene component of the PEO- 1 stabiliser in both spectrum A and spectrum B. Thus this observation confirms the presence of the PEO-1 stabiliser in both the colloidal polyaniline samples. Furthermore, the inten- sity of this feature is noticeably weaker in spectrum B, which is consistent with our observations in the 750-1650 cm-' spectrum region and suggests that the PEO-1 content of expt.8 is signiJicantlylower than that of expt. 9. Our initial attempts to obtain the PEO-l/polyaniline mass ratio of the polyaniline colloids directly by 'H NMR spec- troscopy in conc. D2S04 were unsuccessful. The polyaniline colloid dissolved completely in this solvent as expected and the theoretical PEO- l/polyaniline mass ratio was calculated by comparing the peak integral of the aromatic protons of the conducting polymer component to that of the aliphatic protons of the stabiliser. However, in our control experiments with known masses of bulk polyaniline and ungrafted PEO-1 stabiliser codissolved in conc. D2S04 the observed peak integrals were inconsistent with the known molar ratio of the two components.In addition, some chemical degradation of the PEO-1 stabiliser was observed. Thus we decided that this seemingly simple method was unreliable for the determination of the stabiliser/conducting polymer mass ratio in these sys- tems. Fortunately, we were more successful with our indirect method, which involved an NMR assay of the post-reaction supernatant solution for ungrafted stabiliser using toluene-4- sulfonic acid as a quantitative reference compound. A cali-bration curve was constructed using stock solutions contain- ing known concentrations of both PEO-1 and toluene-4- sulfonic acid (see Fig. 6). The concentration of ungrafted PEO-1 in each post-reaction supernatant was determined from this calibration curve. For expt.8 and 9 the post-reaction concentrations of PEO-1 in the supernatant solution were 37.0 and 48.4 g dm-3, respectively. Assuming an overall yield of polyaniline of ca. 0.57+0.03 g,2 and knowing the initial stabiliser concentration in each case (see Table l), this corre-sponded to stabiliser :polyaniline mass ratios of 20/80 and J. MATER. CHEM., 1992, VOL. 2 Fig. 3 Transmission electron micrograph of a poly(ethy1ene oxide)-stabilised polyaniline colloid (expt. 9) Fig. 4 Scanning electron micrograph of a poly(ethy1ene oxide)- stabilised polyaniline colloid (expt. 9) 50/50 w/w, respectively. These results are entirely consistent with our FTIR spectra which suggested a significantly lower PEO-1 content in the polyaniline colloid obtained from expt. 8 relative to that of expt.9. Thus we conclude that the stabiliser/ polyaniline relative mass composition of the colloid appears to depend markedly on the initial stabiliser concentration. Complete microanalysis data were obtained only for expt. 9. For this sample C=48.52%, H=6.27%, N= 5.97%, C1= 5.74% and S =2.92% (0=30.58% by difference). There are two main conclusions we can draw from these data. First, the polyaniline component is doped with a mixture of chloride and sulfate anions. Secondly, since the pure PEO-1 stabiliser contains no detectable nitrogen, we may estimate the stabiliser :polyaniline mass ratio from the reduced nitrogen content of the polyaniline colloid relative to the nitrogen content of bulk polyaniline powder (10.62Y0).~Using the above data we calculate the stabiliser :polyaniline mass ratio in the colloid resulting from expt.9 to be 44: 56, which is in reasonable agreement (within experimental error) with our NMR results above. Our thermogravimetric analysis results indicate that the polyaniline colloids were slightly less air-stable than bulk polyaniline powder (prepared under identical reaction con- ditions in the absence of stabiliser). This is presumably related to the increased surface area of the former mate~ial.'~-'~ The colloidal stability towards particle aggregation of one of the polyaniline dispersions (expt. 9) was assessed in the presence of various concentrations of added MgS04 electro- lyte at 47 "C (see Fig. 7).Unsurprisingly, the polyaniline dispersion was less stable in higher concentrations of electro- lyte. The resulting particle flocculation was weak and revers- ible upon dilution of the system, which suggests that the PEO-1 stabiliser is simply 'salted out' and thus the adsorbed stabiliser layer becomes sufficiently reduced in thickness to allow the ever-present van der Waals attractive forces between the particles to dominate over the short-range steric repulsion forces.'l The chemical grafting of the PEO-1 stabiliser onto the surface of the polyaniline particles prevents stabiliser desorption occurring during flocculation and upon dilution of the system these adsorbed stabiliser layers become resol- vated, resulting in the redispersion of the polyaniline particles if the system is gently agitated.Thus, both the initial particle flocculation and its subsequent reversibility is indirect evidence for an external layer of adsorbed PEO-1 steric stabiliser surrounding each polyaniline particle. J. MATER. CHEM., 1992, VOL. 2 3.5 3.0 B I 3700 3500 3300 3100 2900 2700 2500 wavenumber/cm-' 1650 1500 1350 1200 1050 900 750 waven u m ber/cm -' Fig. 5 FTIR spectra of poly(ethy1ene oxide)-stabilised polyaniline colloid films in the wavelength range 750-1650 cm-' and 2500- 3700 cm-': (A) expt. 9 and (B) expt. 8 The long-term colloidal stability of the polyaniline disper- sions was not particularly good, with irreversible flocculation of ca. 1.0% w/v dispersions in 1.2 mol dm-3 HCl occurring if left to stand at room temperature over a period of several weeks even in the absence of added electrolyte.This stability is similar to that observed for our poly(viny1 alcohol)-stabilised polyaniline colloid^,'^ but is distinctly poorer than our poly- (vinyl pyridine)-stabilised polyaniline colloids.'2v'3 These lat- ter systems can remain as dispersed, stable colloids for more than 12 months at room temperature. We believe that the relatively poor colloid stability of the polyaniline dispersions in the present study is probably due to either desorption or chemical degradation of the grafted stabiliser. Studies are underway to investigate this further. The electrical conductivities of thin films fabricated from these polyaniline dispersions were remarkably high (0.1-0.5 0 0 10 20 30 40 50 60 70 stabiliser concentration/g dmP3 Fig.6 Calibration graph for the determination of ungrafted stabiliser concentration remaining in the post-reaction supernatant by 'H NMR spectroscopy: peak integral ratios of the CH2 stabiliser protons to (a)the methyl protons and (b)the aromatic protons of the toluene- Csulfonic acid reference compound. The dashed lines indicate the determination of the supernatant concentration of ungrafted stabiliser for experiment 8 1.0 S cm-'), despite the presence of the outer layer of insulat- ing steric stabiliser. Long-term colloid stability problems notwithstanding, we believe that these novel poly(ethy1ene oxide)-stabilised polyaniline dispersions represent a signifi-cantly more processable form, of polyaniline, with conductivit- ies only slightly lower than bulk polyaniline powder or film.Since this is our first opportunity to study stabilisers of narrow molecular weight distribution we are currently focus- ing on investigating the effect of varying the molecular weight of the PEO-1 steric stabiliser on the chemical and physical properties of the resulting polyaniline colloids. Conclusions We have confirmed that sterically stabilised polyaniline col- loids may be prepared in good yield using a poly(ethy1ene oxide)-based stabiliser. Unlike the copolymer stabilisers we have previously described, this new stabiliser is of low molecu- lar weight and has a narrow molecular weight distribution.It contains only one tertiary amine graft site per polymer chain (as opposed to multiple primary amine graft sites) and so a relatively high stabiliser concentration is required for the formation of stable polyaniline dispersions. Our visible absorption spectroscopy studies of ageing oxidant/stabiliser solutions suggest that the stabiliser is chemically grafted to the polyaniline particles. The polyaniline particles have a polydisperse non-spherical morphology similar to that observed for colloidal polyaniline stabilised with random copolymers of broad molecular weight distribution. Unfortunately the long-term stability of these dispersions towards particle flocculation/aggregation is not particularly high relative to some of our other sterically stabilised polyaniline colloids.The presence of the PEO-1 stabiliser has been qualitatively confirmed by FTIR spec- troscopy and quantitatively determined by a novel indirect method based on 'H NMR spectroscopy. Our flocculation experiments are consistent with an outer layer of adsorbed steric stabiliser surrounding each polyaniline particle in the dispersed phase. In view of the latter observation it is not easy to explain the relatively high solid-state d.c. conductivity Fig. 7 Colloid stability of poly(ethy1ene oxide)-stabilised polyanilinc concentration from left to right is 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3 of compressed films or pellets fabricated from such dispersions (0.1-1 .O S cm -I).Compared to bulk polyaniline powders, these colloidal dispersions signijicantly improve the pro-cessability of the normally intractable conducting polymer component. The authors gratefully acknowledge the generous financial contribution from ICI Agrochemicals which enabled this work to be carried out. We wish to thank the following people for their kind assistance in the course of this work: Dr. J. Edwards of ICI Chemicals and Polymers (microanalyses), Dr. S. Hold-ing (Rapra G.P.C. facility), D. Lacey (photography) and Dr. J. Thorpe (electron microscopy). SERC is also thanked for a capital equipment start-up grant which was used to purchase the ultracentrifuge and the TGA-7. References See, for example, Proc. ICSM '88, Synth. Met., 1989, 27-29, and references therein.(a) S. P. Armes and J. F. Miller, Synth. Met., 1988, 22, 385; (b) J. F. Miller, BSc Thesis, University of Bristol, 1987. A. Pron, F. Genoud, C. Menardo and M. Nechtschein, Synth. Met., 1988, 24, 193. J-C. Chiang and A. MacDiarmid, Synth. Met., 1986, 13, 193. P. M. McManus, S. C. Yang and R. J. Cushman, J. Chem. SOC., Chem. Commun., 1986, 1556. J. MATER. CHEM., 1992, VOL. 2 dispersions at 47 "C in the presence of MgSO, electrolyte. Electrolyte 6 J. C. Lacroix, K. K. Kanazawa and A. Diaz, J. Electrochem. SOC., 1989, 136, 1308. 7 A. Watanabe, K. Mori, Y. Iwaski, S. Murakawi and Y. Naka- mura, J. Polym. Sci., Polym. Chem. Ed., 1989, 27, 4431. 8 Y. Cao, A. Andreatta, A. J. Heeger and P. Smith, Polymer, 1989, 30, 2305. 9 S. Ni, L. Wang, F. Wang and L. Shen, Polymer Commun., 1989, 30, 123. 10 D. MacInnes and B. L. Funt, Synth. Met., 1988, 25, 235. 11 J. Kallitsis, E. Koumanakos, E. Dalas, S. Sakkopoulos and P. G. Koutsoukos, J. Chem. SOC., Chem. Commun., 1989, 1146. 12 S. P. Armes and M. Aldissi, J. Chem. SOC., Chem. Commun., 1989, 88. 13 S. P. Armes, M. Aldissi, S. F. Agnew and S. Gottesfeld, Langmuir, 1990, 6, 1745. 14 S. P. Armes, M. Aldissi, S. F. Agnew and S. Gottesfeld, Mol. Cryst. Liq. Cryst., 1990, 190, 63. 15 R. F. C. Bay, S. P. Armes, C. Pickett and K. Ryder, Polymer, 1991, 32, 2456. 16 C. DeArmitt and S. P. Armes, J. Colloid Interface Sci., in the press. 17 (a)C. Barry and H. H. Kuhn, US.Pat. Appl., 07/448459, 1989; (b) H. H. Kuhn, Am. DyestuflRep., 1966, 55, 100. 18 B. Vincent and J. Waterson, J. Chem. SOC., Chem. Commun., 1990, 683. 19 E. C. Cooper and B. Vincent, J. Phys. D, 1989, 22, 1580. 20 S. P. Armes, M. Aldissi, M. Hawley, J. G. Beery and S. Gottesfeld, Langmuir, 1991, 7, 1447. 21 K. E. J. Barrett, Dispersion Polymerisation in Organic Media, Wiley, London, 1975. Paper 1/04688H; Received 10th September, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200125

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1) 131-137 Gas-sensing Properties of Semiconducting Films of Crown-ether-su bstituted Phthalocyanines Philippe Roisin," John D. Wright*,* Roeland J. M. Nolte,bOtto E. Sielckenb and Stephen C. Thorpe" a Centre for Materials Research, University Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH, UK Department of Organic Chemistry, University of Nijmegen, 6525 ED Nijmegen, The Netherlands " Health and Safety Executive, Broad Lane, Sheffield S3 7HQ, UK The effects of nitrogen dioxide and ammonia on the semiconductivity properties of solution-deposited thin films of the tetra-substituted 15-crown-5, 18-crown-6 and 21-crown-7 metal-free and copper phthalocyanines are reported. The 15-crown-5 films showed good reversible conductivity changes at room temperature in NO, concentrations up to 5ppm. The sensor characteristics worsen as the size of the crown-ether ring increases and this is consistent with the effect of greater separation of adjacent adsorbed species as molecular size increases.Lateral repulsions, which are believed to play a dominant role in controlling the response and reversal processes, are reduced by this increase. Treatment of the films with aqueous KCI solution led to dramatic changes in gas-sensing properties and, for the 15-crown-5 derivative, in film morphology. After KCI treatment the film conductivity decreased very rapidly and reversibly on exposure to NO,, even at room temperature, and the 15-crown-5 film changed from a polycrystalline needle structure to an extremely smooth structure.These effects are tentatively ascribed to pronounced changes in the molecular assembly induced by the interaction of the potassium ions with the crown-ether moieties, and to consequent changes in the porosity of the film to small gas molecules. Above 3ppm NO, the response begins to saturate, suggesting depletive chemisorption on an n-type material. However, similar responses are also observed for the electron-donor gas ammonia, indicating the presence of both donor and acceptor impurities in the materials. The gas-sensing properties of the KCI treated films at room temperature are the best of any organic semiconductor film yet reported. Keywords: Phthalocyanine ; Crown-ether ; Nitrogen dioxide ; Semiconductivity ; Gas sensor The semiconductivity of phthalocyanine films is strongly increased in the presence of adsorbed electron-acceptor gases.This effect is due to charge transfer between the electron- donating phthalocyanine molecules and the electron-accepting gas, which has been shown to reduce the activation energy for charge-carrier generation, and it has been studied widely in recent years with the objective of producing a new gener- ation of gas sensors.' Although the sensitivity of devices based on sublimed films of conventional metal-phthalocyanine com-plexes is excellent, offering detection of NO2 down to ppb levels, for example, the response rate and reversibility of such devices is generally lower than the optimum for a practical sensing device.The latter properties appear to be influenced by both the chemical and physical structure of the films,' and although there are numerous reports in the literature of attempts to optimise these structures, very little success has been achieved in producing materials which respond and reverse rapidly at room temperature. However, recently a series of phthalocyanines with four crown-ether rings attached has been synthesized2 (Fig. l), and it seemed likely that these materials might show interesting gas-sensor properties since the crown-ether groups can act both as spacers, providing control over the lateral separation of neighbouring phthalo- cyanine molecules, and as functional groups capable of pro- moting better molecular assembly within the films.The variable size of the non-conjugated and relatively low-polaris- ability crown-ether groups permits controlled variation of the separation between adsorption sites for electron-acceptor mol- ecules on the surface of films of the materials, as well as systematic variation in the polarisability of the material immediately surrounding an adsorption site. Such variations are expected to have a significant effect on gas adsorption and hence on gas-sensing properties of the films, as will be a:M=2H, n=O b:M=2H, n=l c:M=2H,n=2 tooyd:M=Cu,n=O e:M=Cu,n=l w f: M=Cu, n=2 Fig. 1 Crown-ether-substituted phthalocyanines discussed later. Treatment of these materials with alkali-metal ions has been shown to influence the way in which the molecules assemble to form solid films.2 Such ions would therefore provide the opportunity both to improve the struc- tural ordering of the films and to block the empty spaces in the centres of the crown-ether moieties which would otherwise provide potential channels for diffusion of small gas molecules into the bulk of the film.We now report the first studies of gas-sensing properties of the films of such materials, which confirm the strong effect of alkali-metal ions and show that these films have unique properties of very fast response and reversal to NO2 at room temperature. Experimental Materials 4,5,4,5’,4,5”,4”,5”’-Tetrakis(1,4,7,10,13-pentaoxatridecamethyl-ene)phthalocyanine (la), 4,5,4‘,5’,4,5”,4”,5”’-Tetrakis( 1,4,7, 10,13,16-hexaoxahexadecamethylene)phthalocyanine (lb), and 4,5,4,5’,4,5”,4”, 5”’-Tetrakis (1,4,7,10,13,16,19-heptaoxanona-decamethylene) phthalocyanine (lc) and their copper complexes (Id-f) were prepared from the corresponding benzo crown ethers by conversion to the dicyanobenzo crown ethers (by bromination and treatment with CuCN) and cyclisation.2 Purification was effected by column chromatography on neutral alumina with chloroform-methanol (10 :1 v/v).Films were deposited on alumina substrates fitted with interdigitated platinum electrodes, with integral platinum heaters, by spreading a concentrated solution of the material in chloroform over the substrate with a Pasteur pipette and allowing the solvent to evaporate slowly in a closed chamber which also contained a small amount of chloroform in a beaker.With practice it was found possible to obtain uniform films in this way, although little control of film thickness was possible. For some measurements these films were subsequently treated with 1 drop of an aqueous solution containing 1 g dm-3 KCl, again leaving the films in a closed chamber until the solvent had evaporated, typically within 2 h at room temperature. Although the materials are not water-soluble, it was observed that the drop of KCl solution became coloured after some time, particularly for the 15-crown-5 derivative, suggesting substantial interactions between the ions and the molecule and possibly surface reconstruction by a dissolution mechanism.Methods The films were examined using a Cambridge Stereoscan 250 Scanning Electron Microscope operated at either 6.8-7 or 20 keV. Charging effects were avoided by earthing the inter- digitated electrodes of the substrate, and it was not necessary to metallise the films. Electrical conductivity measurements were made in various gas atmospheres using a computer-controlled test rig. Two mass-flow controllers were used, one controlling the flow of clean dry air produced via a Signal AS80 air purifier and the second controlling the flow from a cylinder containing a standard gas mixture (8ppm for NO2 and 100ppm for ammonia) supplied by British Oxygen Company Special Gases Division. The total volume flow was maintained constant by computer control, with the gas concentration determined by the mixing ratio of the two gas streams.For studies of reversal of the gas effects in clean air, a solenoid valve provided a flow of clean air only over the sample. The d.c. conductivities of the phthalocyanine films were measured using a battery- powered voltage source and an electrometer interfaced to the microcomputer to permit current readings as a function of time. Since many of the films had high resistance at room temperature, the currents to be measured were very small, typically lO-’-lO- A, so the sample chamber was mounted in a Keithley Model 6104 shielded test enclosure to reduce electrical noise. Two main types of experiment were carried out in the present work.In the first, the electrical conductivity was first measured for 2min with the sample in clean air, then for 5 cycles of 2 rnin in an N02-air mixture followed by reversal in clean air for 2 min, with concentrations of 1, J. MATER. CHEM., 1992, VOL. 2 2, 3, 4 and 5 ppm in successive exposure cycles. The second type of experiment explored the reproducibility of the response to a single NOz concentration (3 ppm) in a similar sequence of 5 exposure and reversal cycles. Similar cycles of measure- ments were also made for some samples using 10-50 ppm of ammonia, or 5 successive cycles with 20 ppm ammonia. Results and Discussion Metal-free Crown-ether-substituted Phthalocyanines Fig. 2-4 show the responses of the 15-crown-5, 18-crown-6 and 21-crown-7 phthalocyanine films to 1-5ppm NO2 and 5 cycles of 3 ppm NO2.Repeat measurements on separately prepared films of the same materials gave results showing similar response and reversal curve shapes, although the size of the response varied by up to one order of magnitude for different films. These variations are most likely to result from differences in film thickness, which is difficult to control with the solution film deposition method used, and are consistent with some contribution arising from NO2 diffusing into the bulk of the films. Comparison of the data for the three compounds shows that the response speed (as judged by the extent to which the conductivity increase has achieved an equilibrium value after 2 min) and the reversibility follow the order 15-crown-5 >18-crown-6>21-crown-7. The response of the 15-crown-5 compound in particular is rapid and nearly completely reversible in these conditions even at room tem- perature. This feature is unique among the materials thus far reported in the literature, with the possible exception of lead phthalocyanine films subjected to extreme heat treatment at 360 0C.394 Electron microscope photographs of the films (Fig.5) showed pronounced differences in morphology, with the 15- crown-5 film having a polycrystalline needle structure, the 18- 31 0 2 4 6 8 10 12 14 16 18 20 22 time/min 0 2 4 6 8 10 12 14 16 18 20 22 time/min Fig. 2 Semiconductivity changes at room temperature in a film of 15-crown-5 metal-free phthalocyanine exposed to clean air for 2 min, followed by exposure to (a) 1, 2, 3, 4 and 5 pprn NO, in dry air for 2 min, and (b) five separate periods of 2 min of 3 ppm NO, in dry air, with reversal in clean air for 2 min between each exposure J.MATER. CHEM., 1992, VOL. 2 0 2 4 6 8 10 12 14 16 18 20 22 tirne/min 0 2 4 6 8 10 12 14 16 18 20 22 tirne/rnin Fig. 3 Semiconductivity changes at room temperature in a film of 18-crown-6 metal-free phthalocyanine exposed to NO2 concentration cycles as for Fig. 2 "I 0 2 4 6 8 10 12 14 16 18 20 22 tirne/rnin I/Il,/ 0 2 4 6 8 10 12 14 16 18 20 22 ti rne/rnin Fig. 4 Semiconductivity changes at room temperature in a film of 21-crown-7 metal-free phthalocyanine exposed to NO2 concentration cycles as for Fig.2 crown-6 film being almost completely uniform and featureless and the 21-crown-7 film apparently amorphous but cratered. Although the 15-crown-5 film morphology provides a larger surface area, which may contribute to the larger response, these differences in morphology do not account completely for the observed trends in NOz response. The differences do, however, correlate with the molecular sizes in a logical way. The four crown-ether moieties in all cases act as spacers between neighbouring molecules, with the 21 -crown-7 provid- ing the largest intermolecular lateral separation and 15-crown- 5 the smallest. We have shown previously' that the response kinetics of phthalocyanine films in gas-sensing applications are strongly influenced by the lateral electrostatic repulsions between negatively charged electron-acceptor molecules adsorbed on the surface.In particular, the response to gases such as NO2 appears to be determined by the rate of displacement of adsorbed oxygen, which is accelerated by lateral repulsions with traces of strongly bound NOz. If the lateral spacing between molecules is increased, these repulsion effects will also be reduced, leading to slower response and poorer reversibility. This model accounts for the differences between the responses shown in Fig. 2-4. The rapid response of the 15-crown-5 compound compared with typical unsubstituted phthalocyanines at similar tempera- tures suggests that the surface charge-transfer interactions which lead to weak chemisorption of electron-acceptor gases on these materials are weaker for the crown-ether substituted compounds. The energy needed to transfer an electron from the phthalocyanine film to the adsorbed electron-acceptor gas molecule is determined by the ionisation potential of the phthalocyanine (which is unlikely to be strongly influenced by the crown substituents), the electron affinity of the acceptor gas and the polarisation energies of the resulting charged species.It is the latter quantities which play the most import- ant role in the crown-ether substituted materials. The four crown-ether groups represent a considerably less polarisable environment than the n-electron systems of the phthalocyan- ine ring.Thus the energetics of the surface charge-transfer process are less favourable and the gases are more weakly bound, though still sufficiently strongly interacting to facilitate the charge-carrier generation process. There appears to be a delicate balance between the effects of increasing molecular size in reducing the lateral repulsions between adsorbed species (which is an unfavourable thing to do from the point of view of fast response and reversal) and reducing the polarisation energy (which has a favourable influence). This leaves considerable scope for further synthetic chemistry attaching a wider variety of spacer groups onto the phthalo- c yanines. Copper Complexes of Crown-ether-substituted Phthalocyanines Results of a broadly similar nature were obtained for the copper complexes of the 18-crown-6 and 21-crown-7 com- pounds, as shown in Fig.6 and 7, although careful examin- ation of Fig. 4 and 7 shows significant differences between the metal-free and copper complexes of the 21 -crown-7 derivative. Thus, the responses to increasing NO2concentrations increase monotonically for the metal-free derivative, whereas for the copper complex they peak at 2 ppm and thereafter decrease. Also, for the repeated exposures to 3 ppm NOz for the metal- free compound the increases are similar for all five exposures, and the reversal becomes slightly better in the later cycles, whereas for the copper complex both the increases and the reversals decrease in magnitude on successive cycles.These differences can be explained in terms of the well-known tendency of copper(I1) to prefer tetragonal six-fold co-ordination in a Jahn-Teller distorted environment. This enhances binding of NO2 molecules near the centre of the large molecule, whereas for the metal-free compound the gas must adsorb on the n-electron cloud around the periphery of the phthalocyanine ring. NOz bound near the centre of the molecule exerts the lowest possible repulsion on neighbouring gas molecules, thus reducing the response and reversal rates as observed. The behaviour of all these materials is therefore explicable in terms of theories already developed to account for the behaviour of unsubstituted phthalocyanines.While it is encouraging that no drastically new models are required to account for the properties of these quite strongly different materials, further studies by different techniques (for example, J. MATER. CHEM., 1992, VOL. 2 16 I 0 2 4 6 8 10 12 14 16 18 20 22 time/rnin 0 2 4 6 8 10 12 14 16 18 20 22 time/min Fig. 6 Semiconductivity changes at room temperature in a film -of 18-crown-6 copper phthalocyanine exposed to NO, concentration cycles as for Fig. 2 10 48 0 -I III'IIIII 0 2 4 6 8 10 12 14 16 18 20 22 time/rnin 13 Q l2 z 11 s =" 10 J0 9 " 0 2 4 6 8 10 12 14 16 18 20 22 tirne/rnin Fig.7 Semiconductivity changes at room temperature in a film of 21-crown-7 copper phthalocyanine exposed to NO, concentration cycles as for Fig. 2 direct measurement of the strengths of surface interactions6) and on a wider range of materials with different central metal ions and different large spacer groups are needed to confirm the generality of the interpretations proposed here.J. MATER. CHEM., 1992, VOL. 2 0 2 4 6 8 10 12 14 16 18 20 22 time/min 6 I(b) 3 g3i-l2 0 2 4 6 8 10 12 14 16 18 20 22 time/min Fig. 8 Semiconductivity changes at room temperature in a film of KCl treated 15-crown-5 metal-free phthalocyanine exposed to NO, concentration cycles as for Fig. 2 3 a " 0 2 4 6 8 10 12 14 16 18 20 22 ti me/mi n 31 1 0 2 4 6 8 10 12 14 16 18 20 22 time/min Fig. 9 Semiconductivity changes at room temperature in a film of KCl treated 18-crown-6 metal-free phthalocyanine exposed to NO, concentration cycles as for Fig.2 Effects of KCl Treatment Fig. 8-10 show the effects of NOz on the electrical conduc- tivity of the same films used for the experiments of Fig. 2-4, after treatment with KCl solution. The change in response characteristics is remarkable. The initial conductivities of the 135 21 I a mI s! 21 2 3 0 2 4 6 8 10 12 14 16 18 20 22 ti me/min 0 2 4 6 8 10 12 14 16 18 20 22 time/min Fig. 10 Semiconductivity changes at room temperature in a film of KC1 treated 21-crown-7 metal-free phthalocyanine exposed to NOz concentration cycles as for Fig. 2 KCl treated films are ca.an order of magnitude larger than before treatment, and the effect of NOz is to cause an immediate rapid decrease in the conductivity. This decrease is equally rapidly and completely reversed in clean air. Fig. 11 shows the scanning electron microscope images of the surfaces of the KCl treated 15-crown-5 and 18-crown-6 films. The surface structure of the 15-crown-5 film has become dramati- cally more uniform, with complete loss of the needle crystallite morphology, while the 18-crown-6 film shows signs of a slightly rougher texture than before treatment, which on higher magnification is seen to be a consequence of the deposition of small crystallites onto the film surface. It seems likely that these crystallites are of KC1, but further work is needed to prove this and to show whether or not the effects on electrical properties can be obtained without the deposition of such crystallites if the KCl treatment is optimised.These results suggest that the KCl treatment increases the n-type contribution to the film conductivity, which is then reduced by depletive chemisorption of NO2. Alkali-metal salts are well known to complex with crown ethers, and the effect is related to the relative sizes of the alkali-metal ion and the central cavity in the crown ether.7 The cavity radii (determined from CPK models') are 0.86- 0.92 for 15-crown-5, 1.34-1.43 A for 18-crown-6 and 1.7- 2.1 A for 21-crown-7, while the potassium ion radius is 1.38 A. Thus the ion will fit into the 18-crown-6 and 21-crown-7 cavities, but not into the 15-crown-5 cavity.The effects of alkali-metal salts on the molecular assembly of crowned phthalocyanines have been discussed,2 and cations which are too large to fit into the cavities serve to bridge crown-ether moieties of two different phthalocyanine molecules and thus promote structural ordering. In the case of cations which will fit into the cavities, it is believed that the counter-anions strongly prefer locations close to the cations on electrostatic grounds, and therefore intercalate between adjacent crowned phthalocyanine molecules containing up to four potassium 1Fig. 11 SEM photographs of KCI treated films: (a) 15-crown-5, ((b)and (c) 18-crown-6 ions each, forming equally strongly ordered structures.These tight structures are in marked contrast to the relatively porous structures of the parent crowned phthalocyanines, and may severely limit the ingress of oxygen into the films. This may be the origin of the loss of p-type conductivity in the films. Although it might be expected that this effect would lead to a lower initial conductivity in the films, the tighter structure will also greatly enhance the electron transport properties of the material, and this would account for the observed higher conductivity in clean air. Furthermore, the improvement in J. MATER. CHEM., 1992, VOL. 2 1.2 1 .1 z z I 1.0 2 2! J 0.9 v w -0.8 0.7 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 log (“O*l/PPm) I , I 1 I ,0.8 I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 log (“O,l/ppm) 0.2 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 log (“O,l/ppm) Fig.12 Calibration curves in 1-5 ppm NO2 for KCl treated metal- free phthalocyanine sensors at room temperature: (a) 15-crown-5, (b) 18-crown-6 and (c)21-crown-7 surface uniformity of the film would minimise the number of strong adsorption sites, grain boundaries and pathways for slow diffusion of gases into the bulk of the film, which are all contributors to slow components in the response of the semiconductivity to adsorbed gases. A mechanism involving depletive chemisorption also pre- dicts that in the presence of higher concentrations of NOz the observed decrease in conductivity should become smaller and eventually change sign as p-type doping dominates.Fig. 12 shows calibration curves for the KC1 treated sensors, J. MATER. CHEM., 1992, VOL. 2 10 \ f 4 0 2 4 6 8 10 12 14 16 18 20 22 time/min 91 I 0 2 4 6 8 10 12 14 16 18 20 22 time/min Fig. 13 Semiconductivity changes at room temperature in a film of KCl treated 15-crown-5 metal-free phthalocyanine exposed to clean air for 2 min, followed by exposures to (a) 10, 20, 30, 40 and 50 ppm NH, in dry air for 2min, and (b) five separate 2min periods of 20 ppm NH, in dry air, with reversal in clean air for 2 min between each exposure from which it is clear that the response of the 15-crown-5 compound is already showing clear signs of such a reversal above NO, concentrations of ca.3 ppm, while the trend is progressively weaker for the other two compounds. It is not yet known whether this trend is associated with differences in molecular assembly or differences in impurity content for the different materials. As a test of the proposed depletive chemisorption mechan- ism for the above effects, experiments were carried out for a fresh film of the 15-crown-5 compound treated with KC1 and then exposed to ammonia in various concentrations. As shown in Fig. 13, ammonia (a donor gas) also produces a negative response, suggesting depletive chemisorption on a p-type material. The result suggests that the KCl treated films may have both donor and acceptor impurity levels contributing to the conductivity, so that both acceptor and donor gases can lead to depletive chemisorption.As this was a fresh film, no comparison can be made of the magnitudes of the effects of NO2 and NH3. The results described herein prove that crown-ether phthalocyanines proffer a rich and varied research field. In particular, the dramatic results obtained for the materials reported here clearly demonstrate their potential for gas- sensing devices, because the rapidity and reversibility of the effects are superior to those of other materials of the phthalo- cyanine class so far proposed for gas-sensor applications. References J. D. Wright, Prog. Surf. Sci., 1989, 31, 1. 0.E. Sielcken, M. M. van Tilborg, M. F. M. Roks, R. Hendriks, W. Drenth and R. J. M. Nolte, J. Amer. Chem. Soc., 1987, 109, 426 1. T. A. Jones, B. Bott and S. C. Thorpe, Sensors and Actuators, 1989, 17, 467. Y. Sadaoka, T. A. Jones and W. Gopel, Sensors and Actuators, 1990, B1, 148. P. B. M. Archer, A. V. Chadwick, J. J. Miasik, M. Tamizi and J. D. Wright, Sensors and Actuators, 1989, 16, 379. R. C. Weaver and J. D. Wright, Sensors and Actuators, 1991, B4, 505. P. D. Beer, in Chemical Sensors, ed. T. E. Edmonds, Blackie, London, 1988, p. 22. N. K. Dalley, in Synthetic Multidentate Macrocyclic Compounds, ed. R. M. Izatt and J. J. Christensen, Academic Press, New York, 1978, p. 207. Paper 1/04771J; Received 16th September, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200131

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992, 2(1), 139-140 Structure of the Cubic Intercalate MgxTiS, Philip Lightfoot, Franciszek Krok, Jan L. Nowinski and Peter G. Bruce Centre for Electrochemical and Materials Sciences, Department of Chemistry, University of St Andrews, St Andrews, Fife KY76 9ST, UK The structures of the intercalates Mg,TiS,-c(O <x <0.25),prepared by chemical means, have been refined using high-resolution X-ray powder diffraction. The solid-solution adopts a structure derived from that of a spinel, with space group Fd3m. Magnesium exclusively occupies the octahedral 16c sites within the cubic close-packed sulfide array rather than the tetrahedral 8a sites, as would occur in a normal spinel. A linear increase of the cubic lattice parameter, a, and occupancy of the 16c site versus x is shown.Keywords: intercalation ; Titanium disulfide ; Powder diffraction ; X-Ray diffraction ; Rietveld analysis The intercalation of divalent cations into solid hosts has Results and Discussion received relatively little attention to date. This is in part due All the samples showed an almost single-phase (cubic, to the lack of suitable chemical routes to the intercalation of aE9.8 A) pattern. A small peak at 28~53" is probably due such species. The intercalation of magnesium, in particular, to the rhombohedra1 TiS, polymorph. Refinements were is of interest because of the possibility of developing a carried out in the standard manner, initially refining scale magnesium battery which utilises a suitable host as a cathode. and background parameters, followed by lattice parameters, Recently,' we communicated the successful intercalation of peak shape and finally structural parameters, Starting param- magnesium into the cubic and layered polymorphs of TiS2 eters for the refinements were those of a cubic spinel, space using the organometallic reagents di-n-butyl magnesium group Fd3rn.The Ti was placed on the octahedral B-sites [(C,H,),Mg] and the magnesium phenoxide, magnesium 2,6- (16d) and S on the X-sites (32e) of the cubic-close-packed di-tert-butylphenoxide [Mg(O-2,6-Bu1C,H3),]. In the case of array. The latter site has a variable positional parameter, x,the cubic polymorph, Mg,TiS,-c, a continuous range of solid which was set at 0.25 in the initial refinement.The tetrahedral solution of x =0-0.25 was established, but the location of the A-sites were left vacant and the Mg was located on theMg2+ ions within the TiS,-c host and the structure of the octahedral 16c sites, which are empty in spinel. Refined solid solution in general were not determined. In this paper parameters included the Mg occupancy and an overall tem- we report the structure of Mg,TiS,-c obtained from high- perature factor. resolution X-ray powder diffraction. Final parameters obtained from the Rietveld refinements are presented in Table 1. All the samples studied refined very Experimental satisfactorily on the basis of the cubic spinel model, a typical refined profile being shown in Fig. 1. Lattice parameters show The solid solutions were prepared as described previously,' a smooth linear expansion with increasing x, with a corre- by reaction of the powdered TiSz-c with solutions in n-sponding increase in the occupancy of the 16c site by mag- heptane of the organometallic reagents mentioned in the nesium (Fig.2). The values of magnesium content determined introduction. The compositions of the intercalated materials on the basis of the structure refinement are, in general, in were characterised by atomic absorption analysis using a Pye- very good agreement with those obtained by chemical analysis. Unicam SP9. Bond lengths around the two metal sites are given in Table 2. X-Ray powder diffraction data were collected on a STOE The Mg-S distance is in all cases significantly shorter than STADI/P high-resolution system, employing Ge-monochrom- that found in rocksalt-like MgS (2.60 A), as expected for a atised Cu-Ko!, radiation, and a small linear position-sensitive partially filled site.detector covering 6" in 28. Data sets covered the range 5 <28/ The spinel lattice has additional possible sites for intercal- degrees <110, in 0.02"steps. Samples were sealed in 0.5 mm ation of guest species at the positions 8a 0.125, 0.125, 0.125, capillaries, and an appropriate cylindrical absorption correc- 8b 0.375, 0.375, 0.375 and 48f x, 0.125, 0.125. All of these sites tion was applied to the data. Data were analysed by the have tetrahedral co-ordination to the surrounding sulfur Rietveld method using the GSAS program,, with peak shapes sublattice.In order to test the uniqueness of the present described by a pseudo-Voigt function. Scattering factors were model, these sites were included in our refinements. In all taken from International Tables3 Table 1 Refined parameters for Mg,TiS2 0 9.7389( 1) 0.2488( 1) - 0.0060(6) 8.9 1.43 0.035 0.068 9.7545( 1) 9.7576(1) 0.2509( 1) 0.2510(1) 0.074(4) 0.049(5) 0.0 1 18(4) 0.0 195( 5) 8.9 10.2 1.04 I .38 0.08 9.7751(2) 0.2512( 1) 0.096(4) 0.0 120(4) 9.1 1.09 0.15 0.10 9.80 1q3) 9.7888(2) 0.2520(1)0.25 16( 1) 0.I44(6)0.1 17(5) 0.0 103(6)0.0044(4) 9.0 9.0 1.02 0.98 0.25 9.8559(4) 0.25 19(2) 0.224(7) 0.025( 1) 10.1 1.56 a x refers to composition based on chemical analysis. J. MATER. CHEM., 1992, VOL.2 2.0 (? 1.5 z \B 1.0 3 '0.5 0.0 I 1 I I I I I I I I 3.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2O/lOO degrees Fig. 1 Final observed (plus marks), calculated (solid line) and differ- ence (below) profiles for the Rietveld refinement of Mg,, ,,TiS,-c -(a)9.84 -9.72 ! II I 0.0 0.1 0.2 0.3 X 0.2-A 0 0 w Y r" 0.1 -8 rn 8 0.0 0.0 '0.1 0.2 0.3 X Fig. 2 Cubic lattice parameter, a, (above) and refined Mg 16c site oc- cupancy, (below) as determined by Rietveld refinement, versus x for the solid solution Mg,TiS,-c. x is the value determined analytically Table 2 Bond lengths for Mg,TiS, bond length/A X Ti-S Mg-S 0 2.446(2) - 0.035 0.06% 2.430( 1) 2.430(1) 2.44%(1) 2.449( 1) 0.08 0.10 2.432(1) 2.432(1) 2.455( 1) 2.463( 1) 0.15 0.25 2.431(1) 2.456( 2) 2.470(2) 2.478(3) cases, however, the Mg occupancy refined to zero within three e.s.d.s.For the final refinements, therefore, the occupancies of these sites were set at zero. Magnesium exclusively occupies the octahedral 16c sites, as was found to be the case for Li intercalation into the cubic titanium di~ulfide,~ rather than the tetrahedral 8a sites, as would be expected for a normal spinel. Little is known about magnesium thiospinels and, in par- ticular the compound MgTi2S4 appears to be unknown. Magnesium prefers the tetrahedral site in the oxide spinels MgA12045 and MgTi204,6 and in the thiospinels of the larger trivalent cations MgMy'S4 (MI1'= Sc, Yb, Lu).~However, MgIn2S4' and Mg,.8,1n2,113S49 somewhat surprisingly appear to adopt the inverse spinel structure, with Mg and In sharing the octahedral site.We thank SERC for financial support. P.G.B. thanks the Royal Society for the award of a Pickering Research Fel- lows hip. References P. G. Bruce, F. Krok, J. Nowinski, V. C. Gibson and K. Tavakolli, J. Mater. Chem., 1991, 1, 705. A. C. Larson and R. B. Von Dreele, Los Alamos National Laboratory Rep., LA-UR-86-748, 1987. International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, Vol. IV. Y. Saidi, I. Abrahams and P. G. Bruce, Mater. Res. Bull., 1990, 25, 533. G. E. Bacon, Acta Crystallogr., 1952, 5, 684. A. Lecerf, Thesis, Bordeaux University, 1962. M. Patrie, J. Flahaut and L. Domange, C.R. Acud. Sci., 1964, 258, 2585. H. Hahn and W. Klingler, 2. Anorg. Chem., 1950, 263, 177. B. Eisenmann, M. Jakowski and H. Schaefer, Mater. Res. Bull., 1984, 19, 77. Paper 1/05000A; Received 30th September, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200139

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1) 141-142 MATERIALS CHEMISTRY COMMUNICATIONS New Preparation Method for Highly Siliceous Zeolite Films Tsuneji Sano,* Yoshimichi Kiyozumi, Kazuyuki Maeda, Makoto Toba, Shu-ichi Niwa and Fuji0 Mizukami National Chemical Laboratory for Industry, Higashi, Tsukuba, lbaraki 305, Japan Highly siliceous zeolite films of silicalite and ZSM-5 were prepared by a new preparation method using cellulose moulding. Keywords: Zeolite ; Film ; Cellulose moulding The molecular sieving properties of zeolite have been utilized in various chemical and physical processes such as hetero- geneous catalysis and gas separation. Recently, great interest has been focused on zeolite membranes or films, owing to their uniform pore sizes and resistance to high temperatures.The most common form of the zeolite membrane has been made by embedding zeolite crystals in polymer or ceramic matrices, or by the in situ crystallization of zeolites on porous substrate^.'-^ More recently, we have found that a pure ZSM- 5 zeolite film ca. 30-100 pm in thickness, which is made up of zeolite crystals themselves, can be obtained on a Teflon slab.' However, the film obtained is fragile and the control of the shape of zeolite film is difficult. In this communication, we describe a new preparation method of zeolite film using a cellulose moulding such as a filter paper. The hydrothermal synthesis of silicalite and ZSM-5 zeolite films was performed as follows. Aluminium nitrate and col- loidal silica (Cataloid SI-30 from Shokubai Kasei Co.; 30.4 wt.% SO2, 0.38 wt.% Na,O, 69.22 wt.% water) were added to a stirred mixture of tetrapropylammonium bromide (TPABr) and sodium hydroxide in solution, to give a hydrogel with a composition of 0.1 TPABr-0.05 Na,O-(0-0.01) Al2O3-SiO2-80H20.The hydrogel was then transferred to a 50 cm3 stainless-steel autoclave with a Teflon sleeve. A piece of filter paper was immersed in the solution and placed vertically along the axis of the autoclave. The autoclave was placed in an air-heated oven at 170 "C for increasing time periods. After the completion of crystallization under autogen- ous pressure without stirring, the autoclave was cooled down, and the sample was washed and dried at 120 "C for 24 h. For comparison, the zeolite film was also prepared using a Teflon slab.' At first, silicalite was hydrothermally synthesized in the presence of cellulose moulding, such as a filter paper.Fig. 1 shows the X-ray diffraction diagrams of the silicalite-cellulose composites prepared at varying crystallization times. The incident beam angle of the X-ray was I", indicating that the X-ray diffraction data seem to come from the surface of the composite. The intensities of the reflection peaks correspond- ing to silicalite increased with an increase in the crystallization time. After crystallization for 24 h, the peaks corresponding to the cellulose were not observed in the X-ray diffraction diagram of the composite. The shape of the silicalite-cellulose composite as shown by optical photography was the same as that of the filter paper used, indicating that the shape of zeolite film is easily controlled.The thermogravimetric (TG) measurement of the composite is presented in Fig. 2. Two distinct steps were observed in the TG curve of the composite. Taking into account that only one step was observed in the 70 60 50 cn8 40 00 7 : 30 20 10 0-5 10 20 30 40 50 28ldegrees Fig. 1 X-ray diffraction diagrams of silicalite-cellulose composites prepared at various crystallization times: (a)0, (b)3, (c) 12, (d) 24, (e) 48 h TG curve of the pure silicalite film prepared with the Teflon slab, it seems to be obvious to ascribe the first step to the decomposition of the filter paper and the second one to the destructive desorption of TPA cations occluded in the zeolite framework.The composite was calcined at 500 "Cfor 20 h in order to decompose and burn the cellulose. However, the composite was not disintegrated by the calcination process. This suggests that the zeolite crystals are bonded together strongly in the form of a continuous film. Fig. 3 shows a scanning electron micrograph of the composite after crystalliz- ation for 48h. The surface of the filter paper consists of the densely packed zeolite crystals 5-30 pm in size. The thickness of the film was ca. 500 pm and was ca. 2 times that of the filter paper. The structural strength of the zeolite film was strongly improved, compared with the zeolite films obtained on the Teflon slab.For the composite prepared at the short crystallization time, several zeolite crystals were observed in the cellulose fibre structure. J. MATER. CHEM., 1992, VOL. 2 -50 -0 The pore size distribution of the silicalite film calcined at 500°C for 20 h was measured by a mercury porosimeter -10 -s v) -20v v) (Micromeritics AutoPore 9200). The micropore volume other than zeolitic was 0.08 cm3 g-' for 30-600 A in diameter and was considerably smaller than that of highly compressed silica or alumina pellets, indicating very little microporosity. In the case of ZSM-5 zeolite film, the zeolite synthesis using the filter paper needed a longer crystallization period com- -E 'E -303 pared to the synthesis using the Teflon slab.From these results, it is found that highly siliceous zeolite films with high structural strength can be prepared using cellulose moulding and that the shape of zeolite film can be -40 - controlled easily. Although the permeability properties of the zeolite films could not be determined, it should be possible to apply this kind of zeolite film to many fields such as gas separation, water treatment and hydrocarbon processing. The -50 = I I I exact role of the cellulose in the synthesis of the zeolite films 100 200 300 V"C 400 500 could not be clarified because of the limited data. We consider that the OH groups contained in the cellulose provide the unique sites for the crystallization of zeolite. Fig. 2 Thermogravimetric curves of silicalite films prepared with (a) filter paper and (b)Teflon slab at crystallization time of 48 h References D. L. Wernick and E. J. Osterhuber, J. Membrane Sci., 1985, 22, 137. T. Bein, K. Brown and C. J. Brinker, Stud. Surf. Sci. Catal., 1989, 49, 887. A. S. Michaels, Chemtech, 1989, 162. Jpn. Kokai Tokkyo Koho, 59-213615, 1984; 63-291809, 1988. US.Pat., 4800187, 1989. A. Danner and K. K. Unger, Chem. Ing. Tech., 1990,62,487. K. Suzuki, Y. Kiyozumi, T. Sekine, K. Obata, Y. Shindo and S. Shin, Chem. Exp., 1990, 5, 793. T. Sano, Y. Kiyozumi, M. Kawamura, F. Mizukami, H. Takaya, T. Mouri, W. Inaoka, Y. Toida, M. Watanabe and K. Toyoda, Zeolites, in the press. Communication 1/05232B; Received 15th October, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200141

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1), 143-144 BOOK REVIEWS Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. By C. J. Brinker and G. W. Scherer. Academic Press, New York-London, 1990. Pp. 912. Price $139.00. ISBN 0 12 134970 5. This is an excellent book aimed at postgraduate students, current practitioners and newcomers to the field of sol-gel science. It is the first text book of its kind and in many ways the style of the book is reminiscent of the great treatise on silica chemistry written by the late R. K. Iler. In both books, novice and expert alike are provided with an up-to-date picture of the known scientific information relating to the subject matter under discussion. In this book, the term sol-gel is defined broadly as the preparation of ceramic materials by the formation of a sol, gelation of the sol and removal of the solvent.The focus of the book is on materials prepared from non-aqueous routes, although ceramics prepared via aqueous routes and from organically modified materials (ormosils or ceramers) are also considered. Most information is available for silicate systems but, wherever possible, comparison is made with non-silicate systems. A brief introduction including basic terminology, brief historical background and background texts, sets the scene for the remainder of the book which looks at each step of the sol-gel process in turn. Chapters are provided on (1) hydrolysis and condensation mechanisms involved in the formation of particles a few nanometres in diameter; (2) the stabilisation of monodisperse particles; (3) theories of gelation including a comparison of predicted and observed changes near the gel point; (4) the ageing of gels, including structural changes which occur whilst the material is bathed in a liquid phase which can have far-reaching consequences for the success of the drying and sintering stages of the process; (5) the theory and practice of drying, including the problems of structural deformation and shrinkage which can lead to cracking of samples; and (6) the theory and practice of sintering, including proposed mechanisms for densification.The materials as-formed prior to the sintering process have very high surface areas, typically 300-1000 m2 g-' and have functional groups both on the surfaces of the material and within the bulk phase.The majority of groups are accessible to further reaction and a chapter is included which details some possible chemical modifications which may be made to the bulk ceramic prior to sintering. A further chapter provides a comparison of gel-derived and conventional ceramics where it is clear that major differences are observed for materials prepared at low temperatures only, the advantages of the sol- gel route in the preparation of ceramics largely disappearing when materials are treated at high temperatures. Sol-gel materials are expensive to produce in comparison with more conventional ceramic preparation methods and in order for their use to be exploited they have to show distinct functional/applications advantages over similar materials pre- pared by more conventional routes.A wide range of current and potential applications, including thin films are discussed in the remaining two chapters of the book and although the coverage given is largely cursory in nature, references are provided to enable readers to make a more comprehensive study of the technological possibilities in this area, The authors are to be complimented on their literary style as all chapters are clearly set out and easy to read. The nature of the book is, however, one of compilation rather than a critical review of the information available. It is to be hoped that any subsequent editions, in addition to containing sub- stantially more information on mechanisms in the formation of, and structure development in, non-silicate systems, mixed systems and organically modified materials would include a more rigorous appraisal of available chemical information, particularly with respect to proposed mechanistic pathways and structural models.In this research area, much of the work which has been done has been based upon alchemy rather than pure science. This volume goes some way towards redressing the balance, putting this research field on a more rigorous scientific footing and should encourage soundly based experimentation. This volume is the definitive text in this area of science and technology and represents a much needed addition to the scientific literature.C.C.Perry Received 22nd October, 199 1 Cement Chemistry. By H. F. W. Taylor, Academic Press, London, 1990. Pp. 475. Price US $110. ISBN 0-12-683900-X. At least 1 ton of concrete is produced annually per head of world population and perhaps double that amount in the more developed countries. In spite of this, the chemistry of cements is ignored in many undergraduate chemistry courses in the UK, whilst research on cement and concrete is spread thinly among Chemistry, Materials Science, Civil Engineering and Building Science departments in higher education insti- tutes, research associations and industry. Cement chemistry has, however, flourished at the University of Aberdeen under the dynamic leadership of Professor H. F. W. Taylor and this book is the culmination of his many years at the forefront of the subject.Unlike the multi-author two volumes The Chemistry of Cements edited by Taylor in 1964, the present work is a single- author text and is all the better for this. It is concerned with hydraulic cements, defined by Taylor as those that set and harden as a result of chemical reactions with water, and if mixed with water in appropriate proportions continue to harden even if stored under water after they have set. Portland cement is of course the most important example. The book starts by considering this material and its constituent phases, including a detailed account of their polymorphism and crystal structure. The phase equilibria of binary and ternary systems within the CaO-A1,O,-Fe2O3-SiO2 quaternary system are discussed and also the role of minor elements.There follows an important chapter on the chemistry of Portland cement manufacture, including consideration of the energy changes involved. The properties of Portland clinker and cement are reviewed in terms of the techniques used to study them. The next four chapters give a detailed coverage of the processes that occur when Portland cement or its constituent phases are mixed with water and the characteristics of the hardened material. The complex natures of the C-S-H gel phase formed from the anhydrous silicate phases and the more crystalline AFm and AFt phases formed from the aluminate, ferrite and sulfate phases are given an incisive and up-to-date discussion, which will be carefully scrutinised by research workers.The next two chapters cover the chemistry of other types of cement, notably an excellent introduction to composite cements (based on the incorporation of blastfurnace slag, pulverized fuel ash, natural pozzolanas or microsilica) and calcium aluminate cements. The increasing use of admixtures is reflected in a detailed consideration of the role of retarders, accelerators, air-entraining agents, water reducers and super- plasticizers. Finally concrete chemistry is discussed, with emphasis on aspects relating to the durability of concrete, including the destructive alkali-silica reaction. The book contains an authoritative in-depth treatment of a comprehensive range of topics that perhaps only Taylor could provide.The author’s distinctive style, so admired by readers of his papers, is reflected in the logical sequence and the precise manner in which topics are summarised. The text is closely argued, so that some sections will warrant careful re-reading, yet it is at all times lucid and concise. There is a wealth of detailed information that will be particularly valu- able to the research worker and to those lecturing on the subject. The book is not, however, a catalogue of published research, but provides a critical account of the state of current knowledge in the major areas of cement chemistry. The coverage of the literature is very wide. About 1300 references are listed with a high proportion referring to papers published during the 1980s. For a first edition it is remarkably free from errors.It is a pleasure to be able to conclude a review with such a whole-hearted endorsement of a book. Just as Taylor’s previous two-volume text has been widely read and referred to in numerous publications, so this book will become the standard text for the next generation. Chemists with active interests in cements will want to purchase their own copy and will lend it to colleagues with reluctance, for fear of not having it to hand when they want to check some detail of the subject. J. H. Sharp Received 23rd October, 1991 Novel Materials in Heterogeneous Catalysis. ACS Sym-posium Series 437. Ed. R. T. K. Baker and L.L. Murell, ACS, Washington. 1990. ISBN 0-8412-1863-3. Pp. 366. Price US $89.95. This book was developed from a symposium on ‘New Cata- lytic Materials and Techniques’ held at the American Chemical Society meeting at Miami Beach, September 1989. The thirty chapters that it contains, most originating from the USA and presumably the papers from the symposium, are grouped into seven sections or subject areas. Nine chapters constitute the first section which considers zeolitic materials. Following an interesting, but somewhat irrelevant, article on the crystallization of lead iodide in space the emphasis is on ALP0 and SAP0 materials. Powerful physical techniques: MASNMR, ESR, XPS, Mossbauer spec- J. MATER. CHEM., 1992, VOL. 2 troscopy, combined TPD-TGA and neutron scattering are applied to structural problems.Regretably, only four of the papers report catalytic results and only one of these can be said to be detailed. To the reviewer, the highlight was a short paper on high-alumina-content silica-aluminas (up to 85% A1203) with high pore volume and controllable pore size which exhibited cracking activity for gas oil that increased with high-temperature steaming. Layered structures are represented by five articles and clusters by four; all the former report catalytic findings, whereas the latter do not proceed beyond studies of gas adsorption. Two papers consider membrane reactors, where incorporation of either a ceramic or metallic membrane, permselective to hydrogen, lifts thermodynamic restrictions on the yield in dehydrogenation reactions.We are reminded that the ceramic membrane was developed for uranium isotope separation in the 1940s: less a novel material, more a delayed reaction, now facilitated by advances in sol-gel techniques. Metal oxide catalysts are swiftly dealt with in two papers; one on the preparation and physical characterization of Nb205 on A1203, the other a study of methane coupling on PbO-MgO-Al203. The number of ACS divisions supporting the symposium suggests that catalysis in fuel production was to be a major topic at the original meeting. The papers in this section fall somewhat short of this, but it is satisfying to find that all five present catalytic results. Articles on carbon monoxide hydro- genation, photocatalytic oxidation of hydrocarbons on metal- loporphyrines and oligomerization of isobutene on improved phosphoric acid catalysts have little in common.Two papers, linked by their relevance to direct coal liquefaction, complete the section and emphasise the importance of catalyst precur- sors and activation conditions in determining activity. The volume concludes with three accounts of new techniques applicable to solid surfaces: surface tension measurements by laser interferometry, Raman spectroscopy showing the import- ance of substrate structure on supported vanadium oxide layers and SEM ‘and FEM applied to gold clusters on metal substrates. Overall, it is a readable volume, but falls considerably short of a definitive work. That only just over half of the contri- butions report catalytic results is disappointing and is not to be expected from the title. Presentation is good except for the usual range of technology exhibited in camera-ready material. Unfortunately it is expensive at $89.95 and will find its way into institutional libraries rather than private collections. R. Rudham Received 23rd October, 199 1

ISSN:0959-9428    

DOI:10.1039/JM9920200143

出版商:RSC    

年代:1992

数据来源: RSC