摘要:

J. MATER. CHEM., 1991, 1(6), 939-941 Dielectric Relaxation Peaks in the Low-frequency/High-temperature Range of the Loss Permittivity Curve studied by the Thermostimulated Depolariration Current (TSDC) Technique in 2,3,7,8-Tetramethoxychalcogenanthrenes-TCNQ Charge-transfer Complexes Ricardo Diaz Calleja,” Enrique Sanchez Martinez” and Gunter Klarb ” E. T.S.I.I.,Universidad Politecnica de Valencia, Camino de Vera sln, E-46071 Valencia, Spain lnstitut fur Anorganische und Ange wandte Chemie der Universitat Hamburg, Martin-Luther King Platz 6, D2000 Hamburg 13, Germany The dielectric behaviour of the 1: 1 charge-transfer complexes of the 2,3,7,8-tetramethoxychalcogenanthrenes (5,10-dichalcogena-cycl~diveratrylenes) Vn,EE’ (E=E’ =S, Se; E =S, E’= Se) with 7,7,8,8-tetracyano-pquinodi-methane (TCNQ) is studied by the thermostimulated depolarization current (TSDC) technique as an alternative experimental method to the more conventional a.c.measurements. In the low-frequency side of the dielectric loss curves (E” vs. frequency plots) the relaxation peaks of the compounds are hidden by a continuous increase in dielectric loss. By a transformation of complex permittivity E* =E’-id’ into complex polarizability a* =a’-id’ loss peaks can be observed in the a” vs. frequency plots (E. Sanchez Martinez, R. Diaz Calleja, P. Berges, J. Kudning and G. Klar, Synth. Met., 1989, 30, 67). The existence of these relaxations is now proven by the TSDC met hod. Keywords: Dielectric relaxation; Charge transfer; Polarizability; Thermostimulated depolarization current Dielectric loss curves contain one or several relaxation peaks, many of which are dipolar in origin.These peaks are related to small motions in the molecule concerning these groups. Free charges can also produce peaks, for example peaks appearing at higher temperatures than the peak associated with the glass-rubber transition in amorphous polymers. In heterogeneous samples consisting of different phases and having different dielectric constants and conductivities, charges accumulated near the interface when the sample is heated and subjected to an electric field can be neutralized in a TSDC experiment resulting in peaks. This phenomenon, called the Maxwell-Wagner effect can be expected in semicrys- talline polymers, the amorphous part of which has higher conductivity than the crystalline part, in systems in which an electret metallized on only one side is shorted together with an air gap, or in a two-layer system.In order to prevent these effects we have used two-sided silver metallized samples and direct-contact electrodes. In our powdered samples, on the other hand Maxwell-Wagner-Sillars (M WS) relaxation is usually described by a single relaxation time, i.e. a Debye-like relaxation. However, an analysis of the dielectric and loss curves carried out previously1T2 indicates that this relaxation follows a Cole-Cole equation and is governed mainly by a distribution of relaxation times. Owing to the specific charac- ter of our samples a contribution of an MWS phenomenon cannot be totally excluded, but we think that the observed peak is of primarily dipolar origin.In many cases a continuous increase of the loss permittivity has been observed in the low-frequency side of the spectrum, commonly attributed to d.c. conductivity behaviour. For example, this effect has been found in the spectra of the 1 :1 charge transfer (CT) complexes of the 2,3,7,8-tetramethoxy- chalcogenanthrenes (5,1O-dichalcogena-cyclo-diveratrylenes), Vn,EE’ (E =E’=S, Se; E =S, E’ =Se), with 7,7,8,8-tetracyano- quinodimethane (TCNQ), in which the relaxation peak is partially (Vn2S2 .TCNQ) or totally (Vn,SSe*TCNQ and Vn,SE2 -TCNQ) hidden by this increase of conductivity.’ I I H3C CH3 VnZEE’ TCNQ A transformation of the complex permittivity E* =E’ -id’ into the complex polarizability a* =a’ -ia” according to a*--(E * -1)/(E* +2) (1) can be used;3 loss peaks have been observed in the a” us.frequency curves, previously not seen in the a’ us. frequency curve^.^,^ Eqn. (1) acts as a normalization procedure for the polarizability a*. E* and a* representations are equivalent, and from a macroscopic point of view the use of E* or a* is only relevant to demonstrate the presence of a relaxation peak. On the other hand, thermostimulated depolarization cur- rents (TSDCS)~.~ have been widely used as a complementary technique of the more conventional dielectric a.c. measure- ments in the audiofrequency range in order to detect and analyse dipolar relaxation.TSDC gives a better resolution of the relaxation peaks, owing to the fact that the equivalent frequency is lower than in a.c. dielectric measurements. We therefore applied the TSDC technique to the CT complexes Vn,EE’-TCNQ in order to interpret the results’ of the a.c. measurements in terms of this method. According to TSDC a neat peak should appear in the low-temperature range of the TSDC spectrum of each compound, which corresponds to that produced by the transformation in the a” us. frequency curve. Experimental The dielectric a.c. measurements are described elsewhere. ’ The thermostimulated depolarization currents were deter- mined by use of a SOLOMAT TSC/RMA spectrometer. The technique employed was as f01lows.~ The sample in disc form was metallized with colloidal silver and a d.c.electric field was applied at a high temperature. The sample was then cooled to low temperature with the field maintained. Next it was short-circuited and reheated at a linear rate, measuring the discharge current with an electrometer as a function of temperature. Measurements were made between 153 and 343 K after polarization under the conditions given in Table 1. Depolarizations were done at a rate of 7 K min-’. Results The TSDC thermograms of the CT complexes are given in Fig. 1. In each case a clean depolarization peak is present. The parts of the spectra at higher temperatures, showing a continuous increase of the conductivity, are not reproduced.Activation energies E, are usually calculated from the initial slope of the intensity of current5 according to d(ln j,)/d(l/T)= -rnE,/R (2) where j, is the reduced current (intensity of current/area), R is the gas constant, and m is a parameter related to the broadness of the peak. Thus, for rn= 1 we have a Debye single peak. But, in general, the values of rn are temperature depend- ent and can be obtained from dielectric loss measurements 20.0 10.0 0.0 I I 1 I I 160 180 200 220 N 160 180 200 220 T/K J. MATER. CHEM., 1991, VOL. 1 or alternatively from polarization loss. We adopt for E” (or a”) an equation E” sech(rnx) (3) where is the loss at the maximum, x is ln(o,,,/o)= (E,/R)(l/T-l/Tmax),and E, is the activation energy of dielec- tric a.c.data. As proposed by Fuoss and Kirkwood7 the parameter rn can be estimated from a cosh-’ (E~,~/E’’)us. x plot. The parameter rn is approximately equivalent to the param- eter 1 -h in the equation proposed by Cole and Cole for the permittivity E*=E~+(E~-E,)/[~+(ioz)’-”] (4) where E~ and E, are the unrelaxed and relaxed permittivities, respectively. It has been demonstrated’ that the 1-h parameter in eqn. (4)is the same as the corresponding one in a*=a, +(ao-am)/[I+(ioz)’-”’] (5) where a* is calculated from E* by means of eqn. (1). The corresponding values of m and 1 -h for the three compounds are given in Tables 1 and 2, respectively. For Vn2S2*TCNQrn was calculated from the E” curve. In the two other cases, where no maximum was present in the E” curves, m had to be evaluated from the a” curves.The values of j, were directly taken from the thermograms in Fig. 1. The activation energies E, were then calculated using eqn. (2). The values of E, obtained (Table 1) are close to those calculated from a.c. measurements. ’ In fact, small discrepancies between the two sets of values are normally found and may be due to the different techniques employed. Discussion Activation energies are valuable for estimating the tempera- ture at which the TSDC peak corresponding to the a.c. peak must appear. For this purpose a time (frequency) temperature transformation has to be made. In fact, TSDCs are obtained as a time response and a.c.audiofrequency measurements in the frequency dominion. Frequency-time conversion requires Fourier transformation. However, exact formulae are not obtainable; approximate ones were proposed several years ago by Schwarzl and Struik’ and Van Turnhout6 according to 1.475RT2j,(T) &’I( r)= EOEE, where R is the gas constant, T is the temperature, j, is the current density, c0 is the vacuum permittivity, E is the electric field, and E, the activation energy. Also, f=-0.113E, SRT~ (7) Fig. 1 Thermostimulated depolarization currents for the CT com- where E,, R and T have been formerly defined, s is the inverse plexes Vn,EE.TCNQ. (a) E=E’=S; (b) E=S, E=Se; (c) E=E= of the heating rate, and fis the frequency. Eqn. (7) gives the equivalent frequency at the maximum and E acts as a Se compound ~~ ~~ Vn,S, .TCNQ Vn,SSe *TCNQ Vn ,Se, -TCNQ go Table 1 TSDC data for the CT complexes Vn,EE.TCNQ“ T*/K E/V mm-’ m EJkJ mol-’ j710-3 HZ 323 500 0.53 57.4 2.52 313 100 0.57 57.0 2.32 353 50 0.43 55.9 3.06 Tp=polarization temperature; E =electric field; E, =activation energy; m and fcalculated by means of eqn.(2) and (7). J. MATER. CHEM., 1991, VOL. 1 94 1 Table 2 Data evaluated by comparison of the a.c. and TSDC measurements for the CT complexes Vn,EE'.TCNQ compound a0 a, EO Vn,S, -TCNQ 0.836 0.384 7.77 Vn,SSe * TCNQ 0.952 0.400 60.5 Vn,Se, -TCNQ 0.890 0.390 25.3 I I T/K Fig. 2 Dielectric loss permittivities for the CT complexes Vn,EE'*TCNQ, calculated by means of eqn.(6). 0,E = E'= S; A, E=S, E'=Se; 0,E=E'=Se normalization factor in order to reduce the current to a common basis in terms of E"(T). The equivalent frequencies f are given in the last column of Table 1 and the calculated E"(T)curves are shown in Fig. 2. Values of meet in an acceptable range and are of the same order of magnitude as the value directly measured for Vn2S2 -TCNQ by means of a dielectric bridge. A conclusive test of the equivalence of the two sets of relaxations is to apply an Arrhenius equation in order to check the position along the temperature axis in connection with the respective frequencies. This is easy in the case of Vn2S2 *TCNQ owing to the previously mentioned fact that for this compound a relaxation peak at ca.200 Hz at low temperatures (275 K) is seen in the E" us. frequency curve. Accordingly, gives a temperature of 184 K for the estimated frequency of 2.52 x Hz close to the experimental value of 190 K for the peak temperature in the TSDC spectrum. For the other two compounds, as there are no peaks visible in the &"-f diagrams, frequencies have to be translated from E* to a* according to &o+2 (l-h)-lL=f,0a (9) E, I-h LlHz flHz 2.87 0.63 3 x lo3 I xi03 3.00 0.68 4 x 10, 9.7 2.92 0.59 140 7.7 0.5 0.6 0.7 0.8 Uf Fig. 3 Cole-Cole plots in terms of the polarizability for the CT complexes Vn,EE'-TCNQ. (a) E=E'=S; (b) E=S, E'=Se; (c) E= E' = Se frequencies of the maximum loss in the permittivity and in the polarizability representations, respectively, and 1 -h is the Cole-Cole exponent in eqn.(4) and (5). f, can be calculated according to eqn. (9) from f, taken from the al-f curves. The values of ao, a,, cO,E,, 1 -h, andf, are given in Table 2. Again, by application of eqn. (8) for Vn,SSe*TCNQ and Vn2Se2 .TCNQ temperatures of 204 and 20 1 K, respectively, are obtained, in good agreement with the observed ones of 197 and 195 K (Fig. 1). Occasionally, discrepancies between the two sets of values can arise due to the long succession of calculation steps. Although it is not the purpose of this paper to interpret the dielectric relaxation behaviour in terms of some specific molecular motion in these CT complexes (we lack information about conformational energies of these materials) we ascribe tentatively and partially the relaxation in these molecules (Vn2EE') to the butterfly flapping motion, as observed in folded molecule^,^ in which the dipolar methoxy groups -OCH3 in addition to the E and E' chalcogenides are dielectrically active.However, an MWS contribution to this peak is not totally excluded. We thank the Volkswagenstiftung and the Fonds der Chem- ischen Industrie for financial support. References 1 E. Sanchez Martinez, R. Diaz Calleja, W. Gunsser, P. Berges and G. Klar, Synth. Met., 1989, 67, 30. 2 W. Hinrichs, P. Berges, G. Klar, E. Sanchez Martinez and W. Gunsser, Synth. Met., 1987, 20,357. 3 G. Schufmann and W. Gunsser, Z. Naturforsch., Teil A, 1977, 32,1059. 4 E. Sanchez Martinez, R. Diaz Calleja, P. Berges, J. Kudning and G. Klar, Synth. Met., 1989, 32, 79. 5 C. Bucci, R. Fieschi and G. Ghidi, Phys. Rev., 1966, 148, 816. 6 J. Van Turnhout, Thermally Stimulated Discharge of Polymer Electrets, Elsevier, Amsterdam, 1975. 7 R. M. Fuoss and J. G. Kirkwood, J. Am. Chem. SOC., 1941, 63, c0 and E, can be calculated from a. and a,: 385. 8 F. R. Schwarzl and L. C. E. Struik, Adu. Molec. Relax. Proc.,2ao+1. 2a,+l 1967, 1, 201.&O=-, 1-ao &,== 1 -a, 9 Y. Koga, H. Takahashi and K. Higasi, Bull. Chem. SOC. Jpn., 1973,46, 3359. a. and a, can be easily estimated from the a"-@' plots (Cole- Cole diagrams; Fig. 3). Moreover,f, andf, in eqn. (9) are the Paper 1/01600H; Received 5th April, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100939

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 943-946 Formation of Q8M10{[(CH3),Si0],,Si80,,} from QM,{[( CH,)3SiO],Si},Olivine and Dioptase Graham J. Bratton, Brian R. Currell, John R. Parsonage and Gurvinder K. Wall* School of Biological and Chemical Sciences, Thames Polytechnic, Woolwich, London SE18 6PF, UK The preparation of three-dimensional silicate organosiloxane hybrids by the controlled polymerisation of monosilicic acids derived from silicate minerals such as olivine has been studied. A two-stage reaction process, for the formation of ([(CH,),SiO],,Si,O,,} [Q8Ml0; Q =SiO,,,, M=(CH,).$iO,/,)] has been outlined, which includes silicic acid polymerisation and polyorganosiloxane (QM polymer) redistributions. The structure of Q8M,, has been characterised and shown to be a major product of the partial trimethylsilylation of the minerals olivine or dioptase.Keywords: Silicate organosiloxane hybrid; Olivine ; Dioptase A trimethylsilylation reaction can be used to convert silicic acids which have been produced in solution, to their corre- sponding trimethylsilyl derivatives. '-' This reaction may be formally summarised as: -[Si(0-M ')2],0-+2q HCl +-[Si(OH),],O -+2qM + (1) -[Si(OH),],O-+2q(CH3)3SiOH-+-(Si[OSi(CH,),]>,O-+2qH20 (2) removed at 50 "C (1 2 mmHg) followed by 20 "C (1 mmHg) to yield a white viscous gum. The capillary GLC analysis (chloroform solvent) of this showed the presence of QM301 ,2CH2CH3, QM4, Q2M501 /2CH2CH3, Q2M6, Q4M8, Q3M8, QsMs, Q6M8, Q8Mlo isomers and M12isomers.The isolation of the Q8Mlo isomers was carried out by gel-permeation chromatography (Biobead S-XL gel). The GLC analysis of the fraction containing the Q8M1,) polymers indicated two isomers in a 1 : 1 ratio (found C, 27.4; H, Various trimethylsil ylation methods have been propo~ed~-~ 7.1 %;{(CH3)3[SiO]loSi8011) requires C, 27.9; H, 7.0%. Found and they have successfully eliminated unwanted silicic acid condensation reactions that occur in ~olution.~ This paper describes the preparation of three-dimensional polyorganosiloxanes [e.g. Q8Mlo; Q=SiO,,,, M =(CH3), SiO1,2] from a monosilicic acid (SO:-anion) derived from olivine. This has been achieved by modifying the trimethylsilyl- ation conditions4 so that silicic acid polymerisation [reaction (I)] is encouraged.The polymerisation proceeds in a con- trolled fashion, thereby avoiding gel f~rmation.~ The cubic Q8Mlo species has been shown to be the favoured species from these silicic acid condensations since under similar trimethylsilylation conditions dioptase (Si60ii -anion) and QM4 (trimethylsilylated ion SO:-anion) also give good yields of Q8M The polyorganosiloxanes produced have been characterised by chromatography, spectroscopy and elemental analysis. Experiment a1 Preparation of Q8Mlo In a typical reaction hydrochloric acid (p = 1.18 g cm3, 50 cm3), ethanol (1 56 cm3) and hexamethyldisiloxane (7.2 g) were added to olivine (10 g) in a slurry with water (40 cm3); the reactants were stirred under reflux at 72 "C for 6 h.The reaction mixture was extracted with chloroform (6 x 50 cm3); these chloroform extracts were combined and centrifuged to remove any interfacial product, and any excess of volatile material removed at 50 "C (12 mmHgJ.) to yield the polyorganosiloxanes. The polyorganosiloxanes were stirred under reflux at 72 "C for a further 3 h with hydrochloric acid (p = 1.18 g cm3, 50 cm3), water (40 cm3) and ethanol (1 56 cm3). The reaction mixture was again extracted with chloroform (6 x 50 cm3); the extracts were combined and excess of volatile material t 1 mmHgzl33.3 Pa. m.w. 1300;{(CH3)3[Si0]10Si8011},requires 1290). This procedure was repeated with dioptase (10 g) and QM4 (5 g) as the 'mineral' source. In the case of QM4 no additional hexamethyldisiloxane was added.Analytical Procedures A Varian Vista GLC coupled to a 402 data station was used. Liquid samples, neat (0.1 mm3) and solid samples as chloro- form solutions (10-20% w/v, 0.3-0.7 mm3) were injected onto a 25m capillary column of SE30 bonded phase onto fused quartz with the injection splitter set at 99: 1. The oven temperature was programmed to rise from 60°C (held for I .O min) to 300 "C (held for 10.0 min) at 70 "C min -'. For the gel-permeation chromatography (GPC) a Waters 50 1 high-performance liquid chromatograph in a gel-per-meation mode, equipped with a differential refractive-index detector and a column bank containing 2 x 10 nm, 2 x 50 nm, 2 x lo2 nm and 1 x lo3 nm microstyrogel columns was used.Toluene was used as the eluting solvent with a flow rate maintained at 1.5 cm3 min-'. 'H NMR (ASIS) spectra were recorded using a Bruker WH400 spectrometer, the polyorganosiloxane materials were analysed in deuterated benzene using internal tetramethyl- silane as reference. 29Si NMR spectra were recorded using a JEOL FX-90 instrument with a multinuclear probe covering the range 2.8- 36.2 MHz. An INEPT pulse sequence was used, and the samples were run in deuterochloroform with tetramethylsilane as the internal reference. Results and Discussion Formation and Characterisation of QsMlo from Olivine The structure of silicate minerals can be elucidated by tri- methylsilylation and the subsequent analysis of the poly- organosiloxanes produced.For example, olivine (an J. MATER. CHEM., 1991, VOL. 1 Table 1 GPC analysis of trimethylsilylation products from olivine and dioptase and redistribution products from QM4 react ants product analysis (from GPC) name wt*k mm wt. QM4 QzM, ME MIO Mi2 m.w. C(%) H(%) Ia olivine 10.0 47.2 13.2 2.3 0.4 - - 430 31.6 8.4 6.9 Ib olivine 10.0 7.2 0.1 0.1 m.w. 1300 1270 27.4 7.1 10.1 IIa dioptase 10.0 47.2 0.2 0.1 m.w. 1050 1030 29.3 7.8 9.4 IIb dioptase 10.0 7.2 0.4 0.2 m.w. 1120 1080 28.0 7.3 4.4 I11 QM4 10.0 - 0.8 0.6 m.w. 1063 920 30.3 8.0 m.w. is the average molecular weight for QM4-M,,. SO2: olivine, 32.2% (75 pm); dioptase, 38.1% (75 pm); QM4, 15.6%. N.B. Very little or no residual mineral was obtained for olivine (0.2 g in both cases).Some high-molecular-weight material (insoluble) was obtained for olivine (0.2g in both cases).mm is hexamethyl disiloxane, molecular weight 162.38. orthosilicate containing discrete Si0:- tetrahedra) will give 90% yields of{[(CH,),SiO],Si} (QM,) as shown in reaction (Ia), Table 1. Modification of the Lentz trimethylsilylation technique by replacing propan-2-01 as the phase-transfer agent with ethanol and decreasing the amount of hexamethyldisilox-ane used has enabled the preparation of a range of polyorgano- siloxanes (m.w. 860-2 100) with the major component being ([(CH3)3Si0]1oSig011}, (QsMlo, m.w. 1290) see reaction (Ib), -95 -110 -125 Table 1. The two QgMlo isomers are the major components of the polymeric product; however, a range of other polymers was also obtained (Fig.1 and 2). The molecular weight of the QgMlO isomers, from the GPC analysis, was shown to be 1'4 12 10 81300 (calculated m.w. 1290). Molecular models (Fig. 3) of the two QgMlo isomers show that steric hindrance between bulky trimethylsilyl groups is alleviated in the cage str~ct~re,~~~~ this explains why only trace amounts of the expected Q8M8 polymer closed cage are obtained. Garzo et al." have previously identified QgM1O as a minor component in silicic acid redistributions. Condensation Reaction The products obtained during trimethylsilylation depend on the competition of the reactions shown below: 3 SiOH +HOSi f -+ 3 SiOSi f +H20 (3) 9SiOH + HOSi(CH,), -,3 SiOSi(CH& + H20 (4) T -I~In the presence of an excess of trimethylsilanol reaction (4) 0.4 0:3 012 0.1 0.0predominates and low-molecular-weight products are pro- 4PPm) 99 Fig.2 400 MHz 'H NMR and 29Si NMR spectra of Q8M,, M,, isomers duced; but when there is only a limited amount of trimethyl- 2 silanol present reaction (3) predominates and higher-molecular I weight products are produced. Silicic acid condensation proceeds in a stepwise fashion as in the case of soluble silicates. QM Polymer Redistribution Reactions If the amount of hexamethyldisiloxane is limited or if QM4 Fig. 1 Capillary GL chromatogram of Q8Mlo and MI2isomers is added to an acidic medium polymerisation-depolymeris- J.MATER. CHEM., 1991, VOL. 1 Fig. 3 Two isomers of QsMlo ation reactions can proceed by the following pathways: QM4+ HCl+QM,OH +(CH,),SiCl (5) 2QM3OH+QZM6 +H2O (4) (CH3),SiC1+H20+(CH3),SiOH +HCl (7) 2(CH3),SiOH+(CH3),SiOSi(CH3), +H20 (8) If an excess of trimethylsilanol is present, as in reaction (Ia) (Table I), the re-capping process will predominate and thus low-molecular-weight products are produced. The polymeris- ation processes which occur from QM4 to yield Q8Mlo may include at least three interconnected routes which lead to the +QM2 +QM2 +QM20 -0-0 -o-o-o -o-o-o-o QM4 Q2b Q3M8 Q4M10 +ah420-0 c---0 II I o-o-o o-o-o Q5M10 Q4M10 +QM2j +OM2/ o-o-o 0 II I o-o-o o-o-o-o Q6M12 -M21 0-0-0 +Mz O'o'O 111-1 I o-o-o o.o/o 06M10 Q6M12 +QM21 o-o-o-o 0'O.o P-F0Ill I I-o-o-o 0.p 0-0' Q7M12 Q5M6 +QM21 Q6MlO \ o-o-o-o Ill o-o-o-o %MI4 +QM2j o-o-o '-0 o-o-o, 4Ill I I I I0 o-o-o formation of Q8Mlo (Fig.4). These routes go for example via Q2M6, Q3M8, Q4M10 and low-molecular-weight QxM12 (x>8) polymers. Most of these species are observed in the capillary GLC of the Q8Mlo polymer as minor components (Fig. 1). Depolymerisation occurs to a smaller extent than the polymerisation reactions, since the overall trend is to higher- molecular-weight products. Condensation Products The two three-dimensional structures of Q8MlO and QxM12 (x>8) isomers are the favoured limiting species of silicic acid polymerisation and QM polymer redistributions during the modified trimethylsilylation reactions of mineral silicates.The parent silicate anion must be small, i.e. up to Si60ii-, and the trimethylsilylation conditions are modified as shown in Table 1. If dioptase is trimethylsilylated under Lentz4 con- ditions [reaction (24, Table 11 instead of the expected Q6MI2 product a large yield of Q6M1O is produced [Fig. 5(a)].This has been reported previously by various worker^.'^.'^ Two of the three Q6M1O isomers have been positively identified by comparison and a much smaller GLC peak, although not positively identified was assigned to the third isomer of Q6M10* 12 3 'i 5 I 1212 i ;i,& Q5M10 Fig. 4 Stepwise formation of QsMlo J.MATER. CHEM., 1991, VOL. 1 We acknowledge the support of International Paints plc. and the NMR spectra run at the University of Warwick and City of London Polytechnic and G.J.B. thanks SERC for support. m D-Q II III I and I1 are the most likely Q6M1O isomers to be formed from dioptase as they are easily obtainable by rearrangement of the Q6M12 polymer; these two isomers are responsible for the largest peaks seen in the capillary GLC chromatogram. The remaining isomer (111) would be formed by the combi- nation of two Q3M6 species and not from Q6MI2 rearrange- ment and is therefore present in small quantities. The capillary GLC chromatogram shows the presence of a variety of other products from QsMlo to QM4 and a series of M12 isomers.The redistributions become more apparent if dioptase is trimethylsilylated under modified conditions [see Table 1 reac- tion (Ib) and Fig. 5(b)]the yield of Q6M10 decreases drastically whilst the yields Of Q8Mlo and QxM12 (x> 8) isomers increase. Redistribution of QM4 If QM4 is refluxed under modified Lentz conditions4 as shown in reaction (111) (Table 1) a significant quantity of high- molecular-weight products are obtained [Fig. 5(c)]. The iso-mers of Q8Mlo and Q,M12 (x>8) appear to be quite stable, as further redistribution reactions appear not to occur as shown by reaction (Ib) in Table 1. Even the trimethylsilylation of olivine under modified Lentz conditions shows a high yield of Q8Mlo and M12 isomers; the average molecular weight of the product remains constant at m.w.1300 even after 15 h reaction time with the favoured formation of Q8MIo. References 1 B. R. Currell and J. R. Parsonage, J. Macromol. Sci. Chem., 1981, 16, 141. 2 H. P. Calhoun and C. R. Masson, Rev. Silicon Germanium, Tin Lead Compounds, 1981,5 (4), 153. 3 A. D. Wilson and S. Crisp, Organolithic Macromolecular Mater- ials, Applied Science, London, 1977. 4 C. W. Lentz, Inorg. Chem., 1964, 3, 574. 5 J. Gotz and C. R. Masson, J. Chem. SOC. A, 1970, 2683. 6 F. D. Tamas, A. K. Sarker and D. M. Roy, Hydraulic Cement Pastes: Their Structure and Properties, Cement and Concrete Association, London, 1976, 55. 7 P. K. Iler, The Chemistry of Silica: Solubility, Polymerisation, Colloid and Surface Properties and Biochemistry, Wiley, New York, 1979. 8 B. R. Currell, H. G. Midgley, J. R. Parsonage and E. A. Vidgeon, Analyst (London), 1982, 107, 117. 9 R. K. Harris, NMR and the Periodic Table, Academic Press, London, 1978. 10 L. S. Dent Glasser, E. E. Lachowski, R. K. Harris and J. Jones, J. Mol. Struct., 1979, 51, 239. 11 G. Garzo, A. Wargha and K. Ujszazi, J. Chem. SOC., Dalton Trans., 1984, 1857. 12 H. P. Calhoun and C. R. Masson, J. Chem. SOC., Dalton Trans., 1978, 1342. 13 L. S. Dent Glasser, E. E. Lachowski, R. K. Harris and J. Jones, J. Mol. Struct., 1979, 51, 239. Paper 1/O 16856; Received I 1 th April, 199 1

ISSN:0959-9428    

DOI:10.1039/JM9910100943

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 947-954 Synthesis and Conformational Study of Cuticle Collagen Models and Application as a Bioadhesive Hiroyuki Yamamoto* and Tomoyuki Takimoto Institute of High Polymer Research, Faculty of Textile Science and Technology, Shinshu University, Ueda 386, Japan The synthesis of models of marine cuticle collagens, conformational aspects of native and model proteins and bonding strengths on substrates are investigated. Based on the amino acid composition of acid-soluble cuticle collagen of the Polychaete Nereis japonica, two random-sequence copolypeptides and two repeating-sequence polypeptides have been synthesized using N-carboxyanhydride and active-ester methods. Native marine cuticle collagen exhibited a thermally induced conformational transition at a melting temperature (T,) of 28 "C.Sequential (Ala-Gly-Glu-Hyp-Gly-Gly),, and (Hyp-Gly-Gly-Glu-Ala-Gly),, exhibited T, at 33 and 28 "C, respectively, while no critical T, was observed in the cases of the random copolypeptides A and 6,suggesting that the amino acid sequence plays an important role in the marine cuticle collagen. Adhesive formula?ions consisting of copoly(Ly~'~ Tyr'), native and synthetic model collagens added by oxidase, were found to have the highest tensile shear strength of 5.1 N mm-' on iron, while the value for cop~ly(Lys'~ Tyr') itself was 2.6 N mrnp2. Mixed adhesive formulations consisting of copoly(Ly~'~ Tyr') and collagens (native and synthetic) showed tensile shear strengths of 3.2-3.9 N mm-2 on iron.The effect of oxidase to increase the bonding strength was observed in general but the bonding strengths of the adhesive formulation consisting of both native and synthetic cuticle collagens were weaker on alumina, skin and bone than those on iron. The results of this study have been compared to those for bonding strengths of commercial gelatin from cattle bone. Keywords: Bioadhesive; Collagen ; Amino acid; Polypeptide For over 2000 years man has derived collagen and gelatin from fish and animal bone or skin. These derivatives were found to be useful adhesives between woods, and wood and stone. In recent years there has been increased development in the application of collagen as a photographic, edible, and medical coating material.' Research is now taking place to find a new collagen or gelatin that will serve a specialized purpose as a biomaterial or bioadhesive.Collagen, the most abundant structural protein, has a gross amino acid compo- sition strictly different from other proteins and has been shown to have a unique triple-helical structure.2 We have investigated the polymer chemistry of some model polypeptides from marine proteins, secreted from marine invertebrates such as mussels and barnacle^.^-^ In this on- going investigation, we used the cuticle collagen of the marine worm Nereis japonica, which is readily available in the sea around Japan.8-" This paper describes first the synthesis of cuticle collagen models and the conformational study of the model polypeptides, and second the evaluation of the bonding strengths of some formulations as bioadhesives using synthetic marine cuticle model collagens.Experimental Materials We isolated native cuticle collagen from Nereis japonica of Polychaete in Annelida (purchased locally at Niigata). Purified cuticle collagen was obtained by the acid extraction method by dissolving in 0.1 mol dm-3 acetic acid3 as outlined in Scheme 1. The results of the amino acid composition were almost the same as in the earlier articles reported by Kimura (Gly 35.2%, 4-Hyp 17.0% and 3-Hyp 0.59%, Glu 12.7%, Ala 9.5%, Arg 7.6%, Ser 5.9%, Thr 4.2%, Pro 2.3%, Leu 1.9%, Asp 1.0%, Ile 0.89%, Val 0.77%, Phe 0.23%, Met 0.08%, Tyr 0.06%, Lys 0.05%, and His 0.02%).8710 All amino acids except Gly are L-configurational.N-tert-Butyloxycarbonyl-0-benzylhydroxyproline[Boc-Hyp(Bzl)], Boc-Ser(Bzl), Boc-Thr(Bzl), y-methyl glutamate [Glu(OMe)], dicyclohexylcarbodiimide (DCCI) and 1-hydroxybenzotria-zole (HOBt) were purchased from the Protein Research Foun- dation and, when necessary, were converted to Hyp(Bzl), Ser(Bzl), and Thr(Bz1) hydrochlorides, respectively. The func- tional groups of the side chains were protected by y-benzyl ester (OBzl) for Glu, and tosyl (Tos) for Arg. p-Nitrophenol, benzyloxycarbonyl (Z) chloride, dicyclohexylamine (DCHA), boron tribromide (BBr,), trifluoromethanesulphonic acid (TFMSA) and gelatin (cattle bone, lot 076-00185) were pur- chased from Wako Pure Chemical Industries. o-Nitroben- zenesulphenyl (Nps) chloride was purchased from the Tokyo Chemical Industry.The 4mol dm-3 solution of hydrogen chloride was purchased from Kokusan Chemical Works. Dichloroacetic acid (DCA) and dimethylformamide (DMF) were purchased from Wako. Tyrosinase (4900 units mg- ' solid) was purchased from Sigma. Methods Thin-layer chromatography (TLC) was carried out using precoated silica gel plates (Merck Kieselgel G Type60). The following solvent systems were used: A, chloroform-meth- anol-acetic acid (95 :5 :3, v/v); B, n-butanol-pyridine-water (4: 1 : 1, v/v); and C, n-butanol-water-acetic acid (4: 5: 1, v/v; upper layer). Spots were located on the plates by spraying with a 1% solution of ninhydrin in water-saturated butanol and heating (amine hydrohalides), or by heating (Boc deriva- tives) or by spraying with a hydrobromic acid solution and heating (Z derivatives), followed by a ninhydrin spray and heating.Optical rotation was measured with a Jasco DIP-4 polarimeter at 589 nm and at 25 "C. Elemental analyses were carried out with a Yanagimoto CHN recorder MT-3. The intrinsic viscosities were measured in DCA at 25 "C, using an Ubbelohde viscometer. The molecular weights and degrees of polymerization (DP) were estimated from the following empirical equations: [q] =3.2 x lop4 for poly[Tyr(Z)]; frozen cuticles (30 g) wash (in distilled water, overnight) solution (in 0.1 mol dm4 AcOH, overnight) centrifugation (1OOOOg ,30 min) supernatant precipitate + 20% NaCl 2.5% NaCl centriiugation(5OOOg , 10 min) supernatant precipitate solution (in 0.1 mol dm" AcOH,overnight) lyophilized purified cuticle collagen (150 mg) Scheme 1 Purification scheme for acid-soluble cuticle collagen [q] =2.78 x 10-'M,0.87for poly [Glu(OBzl)], and log(DP) = 1.47 log [q] +2.99 for poIy[Lys(Z)] (concentration in 10 g dm-3).'2-'4Circular dichroism (CD) measurements were performed with a JASCO J-40A recording spectropolarimeter equipped with a Haake constant-temperature circulating water bath and the data are expressed in terms of mean residue ellipticity [el (cm' degree dmol-').The conformational stability of the native protein and model polypeptides was studied at a heating rate of 0.1 "C min-' by measuring the amplitude of the dichroic bands at 197nm.The concentration range was 0.072-0.24 mg cm-3 and thermostatted cells with cell lengths of 0.2-1.O mm were used. Amino acid analysis was carried out with a Hitachi amino acid analyser 835. The bond strengths of three types of the adhesive model proteins on various substrates were measured according to the Japanese Industrial Standard (JIS). The tensile shear strength was measured using iron and alumina test pieces (12.7 x 30-38 mm) according to the JIS K6849 method. How- ever, to measure the bonding strengths on biomaterials some modifications of the JIS were done, since the JIS only describes the detailed procedure for measuring a variety of bonding J. MATER. CHEM., 1991, VOL. 1 strengths on metals, plastics and woods.The tensile shear strength was measured with a tensile testing machine (Imada SV-50) using test pieces: iron (Fe, JIS SlOC), alumina (A1203, JIS H4000), cattle bone and porcine skin. Each testing piece was polished, degreased with trichloroethane, and then treated with an adhesive (polypeptide). Two pieces of the same substrate were stuck together (12.7 x 10-20 mm) and allowed to stand for 3 days at 23 "C and 60% relative humidity (JIS); the skin needed 6 days under the same drying conditions. The adhered pieces were examined on the testing machine described above at a rate of 10 mm min- '. The bond strengths were taken as the average values of three measurements each. Commercial gelatin, sequential A and B and copoly(Lys'o Tyr) were dissolved in water (lo%, w/v), while native marine cuticle collagen and random A and B were dissolved in 2-chloro- ethanol (5%, w/v) because of low solubility in water.The mixed adhesive formulations were prepared by mixing com- mercial gelatin or native and synthetic cuticle collagens with copoly(Lys'o Tyr) at the ratio of 1 : 1 (v/v). Synthesis Cuticle model collagens were synthesized via two different procedures according to the amino acid N-carboxyanhydride (NCA) method and the hexapeptide p-nitrophenyl active-ester meth~d,~both followed by polycondensation. Random Copolypeptides Cuticle collagen model polypeptides were synthesized accord- ing to the NCA method. Eight amino acid NCAs were synthesized from amino acids and protected amino acids by three different procedures as described in our previous article.' The three and eight NCAs were copolymerized to prepare copolypeptides A and B (Table 1) in dioxane with triethyl- amine (TEA) as an initiator.The resulting protected copoly- peptides were obtained in 88Yo yield (protected copolypeptide A) and in 73% yield (protected copolypeptide B). The molecu- lar weights were estimated by viscometry in DCA to be [q]kzA =0.137 (protected copolypeptide A) and 0.166 (pro-tected copolypeptide B). The protecting groups in the side chains of the above protected polypeptides A and B were cleaved simultaneously by treating with TFMSA (10 equiv. mol)." Two kinds of cuticle collagen model polypeptides were obtained in 71 (copolypeptide A) and 70% (copoly-peptide B) yield. The model copolypeptides A and B were hydrolysed for 70 h at 110 "C using 6 mol dm-3 hydrochloric Table 1 Preparation of cuticle collagen model compounds by copolymerization NCA amino acid analysisb amino acid NCA method' g mol% found (molo/o) copolypeptide A GlY A 1.28 54.2 60.3 HY P(BZ1) Glu(OBz1) B B 1.51 1.20 26.2 19.6 22.6 17.1 GlY A copolypeptide B 0.83 37.3 40.8 Ala A 0.26 10.1 13.1 Pro B 0.07 2.4 2.9 Glu(OBz1) Ser(Bz1) Thr( Bzl) Arg(Tos) HYP(BZ1) B B B C B 0.77 0.31 0.23 0.63 0.98 13.4 6.3 4.4 8.1 18.0 15.9 5.5 3.8 5.4 12.6 "A, PCl,; B, COCl,; C, SOCl,. bValues for the hydrolysates of the deprotected copolypeptides A and B.J. MATER. CHEM., 1991, VOL. 1 acid and analysed with a Hitachi amino acid analyser 835 (Table 1). Sequential polypeptide The syntheses of sequential model polypeptides are outlined in Schemes 2 and 3; all the reactions were monitored by TLC. Z-Gly (compound 1 in Scheme 2) and Z-Ala (1 3)were prepared as described by Greenstein and Winitz.I6 Z-Gly-p-nitrophenyl Ala GlY Glu ester (ONp) (2) was prepared as described by Bodanszky and du Vigneaud.I7 Nps-Glu(0Me) DCHA (10) were prepared as described in our previous paper.I8 Poly(Ala-Gly-Glu-Hyp-Gly-Gly) (19). Z-Gly-Gly-OMe (4)was prepared from Z-Gly-ONp (2) and Gly-OMe (3) as an oily product; R, 0.59(A), 0.71 (B). Compound 4 was saponified to Z-Gly-Gly (5);m.p. 178 "C; Rf 0.20(A), 0.53 (B) (Found: C, H ~OMeOH Bzl Boc 8 ONP OMe Bzl' OH 73 HCI-H -0Me Nps-$OH HCbH 9 ONP OMe' Bzl Z 14 OMe Nps OMe' Bzl 11 ONP Z 15 OH HC1-H 12 ONP OMe Bzl Z 16 ONP OMe Bzl HBr*H 17 ONP OMe Bzl ( 18 1" Scheme 2 Preparation of sequential poly(A1a-Gly-Glu-Hyp-Gly-Gly).Z, Benzyloxycarbonyl; ONp, p-nitrophenyl ester; OMe, methyl ester; Boc, tert-butyloxycarbonyl; Nps, o-nitrophenylsulphenyl; OBzl, benzyl ester; Bzl, benzyl ether GlY Glu Ala 32 Scheme 3 Preparation of sequential poly(Hyp-Gly-Gly-Glu-Ala-Gly).Abbreviations as for Scheme 2 950 54.1; H, 5.2; N, 10.5%.C12H1405N2 requires C, 54.1; H, 5.3; N, 10.5%). Compound 5 was coupled with p-nitrophenol using DCCI to give Z-Gly-Gly-ONp (6);19 m.p.158 "C; Rf 0.55 (A), 1.0 (B) (Found: C, 56.0; H, 4.5; N, 10.5%. Cl8Hl7O7N3 requires C, 55.8; H, 4.4; N, 10.90/,). Compound 6 was converted to HBr-Gly-Gly-ONp (7); m.p. 184-186 "C; R, 0 (A), 0.13 (B) (Found: C, 34.4; H, 4.1; N, 11.4%. Cl0Hl2O5N3-H20requires C, 34.1; H, 4.0; N, 11.9%). Boc- Hyp(Bz1) was coupled with the compound 7 using DCCI to give Boc-Hyp(Bz1)-Gly-Gly-ONp(8) as an amorphous foam; Rf 0.66 (A), 0.88 (B) (Found: C, 58.5; H, 5.9; N, 9.9%. C27H3209N4requires C, 58.3; H, 5.8; N, 10.1%). Compound 8 (8.7 g, 16 mmol) was treated with 4mol dm-3 HCI gas to give tripeptide HC1 Hyp(Bz1)-Gly-Gly-ONp (9); m.p. 66-71 "C; [a];'= -5.6" (c=3, methanol)?; Rf 0 (A), 0.51 (B) (Found: C, 51.8; H, 5.5; N, 10.5%.C22H2507N4C1 H20 requires C, 51.7; H, 5.3; N, 11.0%). Nps-Glu(0Me) (10) was coupled with Hyp(Bz1)-Gly-Gly- ONp using DCCI to give Nps-Glu(0Me)-Hyp(Bz1)-Gly-Gly-ONp (1 1); m.p. 61-65 "C; [a];' = -46.0" (c= 1, DMF); R, 0.87 (A), 0.87 (B) (Found: C, 54.7; H, 5.2; N, 10.6%. C34H36012N6S requires C, 54.2; H, 4.8; N, 11.2%). Compound 11 (7.5 g, 10 mmol) was converted to HCl *Glu(OMe)-Hyp(OBz1)-Gly- Gly-ONp (12); m.p. 67-69 "C; [a];'=6.0" (c= 1, methanol); R, 0 (A), 0.23 (B) (Found: C, 51.3; H, 5.4; N, 10.2%. C28H34010N5C1 H20 requires C, 51.4; H, 5.6; N, 10.7%). Z-Ala-Gly-OMe (14) was prepared from Z-Ala (1 3) and Gly-OMe2' and was saponified to Z-Ala-Gly (15); m.p. 103- 108 "C; [a];'= -18.0" (c= 1, methanol); R, 0.24 (A), 0.57 (B) (Found: C, 55.4; H, 5.6; N, 10.1%. C13H1605N2 requires C, 55.7; H, 5.8; N, 10.0%).Z-Ala-Gly-Glu(0Me)-Hyp(Bz1)-Gly-Gly-ONp(16) was pre- pared from compound 15 and Glu(0Me)-Hyp(Bz1)-Gly-Gly-ONp using DCCI; m.p. 68 "C; [a]i5 =3.3" (c=5, DMF); Rf 0.68 (A), 0.72 (B) (Found: C, 57.5; H, 5.8; N, 10.9%. C41H47014N7requires C, 57.1; H, 5.5; N, 11 A%). Compound 16 (6.3 g, 7.3 mmol) was treated with 3.1 mol dm-3 HBr to give HBr .Ala-Gly-Glu(0Me)-Hyp(Bz1)-Gly-Gly-ONp(1 7); m.p. 135 "C; [a];'= -14.0' (c= 1, methanol); R, 0 (A), 0.74 (B) (Found: C, 43.3; H, 5.8; N, 10.6%. C33H42012N7Br.6H20 requires C, 43.2; H, 5.9; N, 10.7%). Compound 17 was polymerized in dry DMF to givepoly(A1a-Gly-Glu(0Me)-Hyp(Bz1)-Gly-Gly)(18) using (a) TEA (0.36 cm3; 2.6 mmol) or (b)TEA (0.36 cm3) and HOBt (35 mg 0.26 mmol) with stirring for 10 days, yielding (a) 1.0 g (67%) and (b) 1.1 g (69%) (Found: C, 43.9; H, 7.0; N, 11.3%.C2,H3,08N6*8H20 requires C, 44.3; H, 7.2; N, 11.5%). The protected polyhexapeptide 18 with [q];5ci =0.15 was treated with 1 mol dm-3 boron tribormide to give the final product poly(A1a-Gly-Glu-Hyp-Gly-Gly)(1 9), yielding 0.70 g (95%) (Found: C, 41.4; H, 6.8; N, 14.6%. C19H2809N6*4H20 requires C, 41.0; H, 6.5; N, 15.1%; amino acid analysis A1al .00G1y3 .05Hyp1.2gGlU1 .13). PoZy(Hyp-Gly-Gly-Glu-Ala-Gly) (32). Gly-ONp (20) was coupled with compound 13 to give Z-Ala-Gly-ONp (21); m.p. 155- 162 "C; [a]h5= -9.0" (c= 1, DMF); R, 0.59 (A), 0.54 (B), 0.59 (C) (Found: C, 56.6; H, 4.5; N, 10.8%.C19H1907N3 requires C, 56.9; H, 4.8; N, 10.5%). Compound 21 was treated with 3.1 mol dm-3 HBr to give HBr-Ala-Gly-ONp (22) as an amorphous foam; [cx]h5 = 10.0" (c= 1, methanol); R, 0.36 (B), 0.16 (C) (Found: C, 36.4; H, 4.4; N, 11.2%. CllHl4O5N3Br.H20 requires 36.1; H, 4.4; N, 11.5%). Nps- Glu(OBz1) (23), which has been prepared in a similar way to compound 10, was coupled with compound 22 using DCCI to give Nps-Glu(OBz1)-Ala-Gly-ONp (24) as an amorphous t [alDvalues are recorded in units of 10-' degree cmz g-'. J. MATER. CHEM., 1991, VOL. 1 foam; yield 67%, m.p. 90-93 "C; [a];'= -11.0" (c= 1, DMF); R, 0.59 (A), 0.89 (B) (Found: C, 55.0; H, 4.8; N, 10.6%. C29H29010N5Srequires C, 54.5; H, 4.6; N, 11.0%).Compound 24 was treated with 4 rnol dmP3 HCl gas to give tripeptide HCl -Glu(OBz1)-Ala-Gly-ONp (25) as an amorphous foam; yield 82%, m.p.52-54 "C; [a];' = -12.0" (c= 1, ethanol); R, 0.58 (B), 0.62 (C) (Found: C, 52.9; H, 5.6; N, 10.4%. C23H2708N4Clrequires C, 52.8; H, 5.2; N, 10.7%). HBr-Gly- Gly-OMe (26) was prepared from compound 4; m.p. 150- 155 "C; R, 0.60 (B), 0.46 (C) (Found: C, 26.7; H, 4.7; N, 12.3%. C5HI1O3N2Brrequires C, 26.4; H, 4.9; N, 12.3%). Compound 26 was coupled with Boc-Hyp(Bz1) using DCCI to give Boc-Hyp(Bz1)-Gly-Gly-OMe (27) as an oily mass; yield 90%, R, 0.63 (A), 0.84 (B) (Found: C, 58.8; H, 7.0; N, 9.2%. C22H3107N3requires C, 58.8; , 7.0; N, 9.4%). Compound 27 was saponified with 1 mol dm-3 NaOH to Boc-Hyp(Bz1)- Gly-Gly (28) as an oily mass; yield 77%, R, 0.22 (A), 0.57 (B), 0.64 (C) (Found: C, 58.0; H, 6.6; N, 9.8%.C21H2907N3 requires C, 57.9; H, 6.7; N, 9.7%). Boc-Hyp(Bz)-Gly-Gly-G1u(OBzl)-Ala-Gly-ONp(29) was prepared from compound 28 and compound 25 using DCCI; yield 90%, m.p. 113 "C; [a]h5= -16.0" (c=1, DMF); R, 0.10 (A), 0.70 (B), 0.66 (C) (Found: C, 57.7; H, 6.4; N, 10.5%. CUHs3Ol4N7*H20 requires C, 57.3; H, 6.0; N, 10.6%). Compound 29 (2.2 g, 2.4 mmol) was treated with 4 mol dmP3 HCl gas to give HC1-Hyp(Bz1)-Gly-Gly-Glu(OBz1)-Ala-Gly-ONp (30); yield, 1.62 g (79%); m.p. 77 "C; = -28.0" (c= 1, methanol); R, 0.70 (B), 0.58 (C) (Found: C, 55.5; H, 5.8; N, 11.5%. C39H46012C1 requires C, 55.7; H, 5.5; N, 11.7%). Compound 30 was polymerized in DMF to yield poly [Hyp(Bzl)-Gly-Gly-Glu(OBz1)-Ala-Gly](31) using TEA; yield, 1.1 g (goo/,) [Found: C, 59.0; H, 5.8; N, 12.3%.(C33H4009N6)n requires C, 59.6; H, 6.1; N, 12.7%]. The protected polyhex- apeptide 31 with [t7];:A=0.16 was treated with TFMSA to give the final poduct poly(Hyp-Gly-Gly-Glu-Ala-Gly)(32), yielding 0.62g (85%) [Found: C, 41.3; H, 7.1; N, 14.0%. (C19H2809N6)nrequires C, 41.4; H, 6.8; N, 14.6%; amino acid A1al .00G1y2 .72HYP0 .87G1U0 ,991. Results Native Cuticle Collagen The first isolation and characterization of cuticle collagen from a marine worm, Nereis japonica, has been reported by Kimura,' Kimura and Tanzer,'?'' and Murray and Tanzer." The Nereis cuticle collagen is extremely hydroxyproline-rich and proline-poor, and its molecular weight is several times greater than that of the vertebrate tropocollagen, which is ca.300000. The marine cuticle collagens were found to exhibit a number of unusual properties such as thermal stability and a diversity of molecular structure during evolution. The anomalous properties of cuticle collagen in annelids may be due to the chemical composition.' In this article, native cuticle collagen from the marine worm Nereis japonica has been prepared according to the acid- soluble purification method described by Kimura.8 The cuticle collagen thus obtained was reported to have a high molecular weight of 1 700000;8 it has little solubility in water but is sol- uble in dilute acetic acid or 2-chloroethanol (up to 5% w/v).Native cuticle collagens from marine worms were reported to cause thermal denaturation by optical rotation measure- ments similar to ordinary collagen^.^,^ The physicochemical properties have also been investigated measuring viscosity, flow birefringence, and sedimentation velocity and equilib- rium.21 CD spectroscopy was used to investigate the thermal properties of the native cuticle collagen and synthetic cuticle collagen models. Fig. 1 shows the results of CD spectra of J. MATER. CHEM., 1991, VOL. 1 200 210 220 230 240 Alnm Fig. 1 CD spectra of native cuticle collagen in 0.01 mol dm-3 acetic acid: 0,at 15 "C; 0,at 45 "C native cuticle collagen in 0.01 mol dm-3 acetic acid at two different temperatures. Native cuticle collagen exhibited a weak positive dichroic band at ca.215-220 nm and a strong negative dichroic band at 196 nm. Ordinary collagens exhibit a thermally induced conformational transition from a super- coiled triple-helical structure at lower temperatures to a single random-coil structure at higher temperatures in solution, and the conformational transition temperature is recognized as the melting temperature (T,). Native and collagenase digested cuticle collagens exhibited similar thermally induced confor- mational transition as depicted in Fig. 2. This thermal behav- iour may give a clue for verifying the synthesized cuticle model proteins. Synthetic Cuticle Collagen Models Two kinds of random copolypeptide model compound con- taining three and eight amino acids listed in Table 1 have been synthesized.The molecular weights of both random copolypeptide models were ca. 9000 and the average DPs were ca. 100. Both of the random copolypeptides were insol- uble in water or dilute acetic acid, but slightly soluble in 951 organic solvents such as trimethylphosphate (TMP) and 2-chloroethanol. Because of these solubility characteristics, the CD spectra of the two random copolypeptides were measured in a mixture of TMP-(0.005 mol dmP3) acetic acid (1 :2, v/v) solvents. The CD spectral shapes of two random copolypeptides resemble each other and also resemble the CD spectrum of native cuticle collagen in 0.01 mol dm-3 acetic acid at 45 "C in Fig. 1. The thermal stability of the two random copolypeptides was also examined, but no critical T, was observed exhibiting a linear ellipticity-temperature relationship (not shown).Sequential polyhexapeptides, poly(A1a-Gly-Glu-Hyp-Gly-Gly) and poly(Hyp-Gly-Gly-Glu-Ala-Gly),were synthesized following the routes in Schemes2 and 3, after several unsuc- cessful approaches were tried, as describd later in the Dis- cussion. The polyhexapeptides were estimated to have molecular weights 10 000 (DP= 130) and 9400 (DP = 120). Since the polyhexapeptides are soluble in water, the CD spectra of the polyhexapeptides were measured in 0.01 mol dm-3 acetic acid, using the Kimura method for native cuticle collagen measurement.8 The CD spectra of two polyhexapep- tides exhibited positive dichroic bands at 214 nm and negative dichroic bands at 197 nm, respectively, as depicted in Fig.3. The sequential polypeptides exhibited thermally induced con- formational changes with T, at 33 and 28 "C as shown in Fig. 4, which were not observed in the cases of random copolypeptides A and B. We have been investigating a series of marine adhesive model protein systems. In our previous article we reported the results of an insolubilization mechanism using a series of copoly(Lys" Tyr ') (x = 1-10) and an oxidase tyrosinase.' Here, copoly(Lys'o Tyr') was chosen as a model adhesive protein, which is one of the simplest marine adhesive model com- pounds, and has been reported to exhibit a tensile shear strength of 2.6 N mm-' on iron. In the present article we prepared some adhesive formulations consisting of copoly(Lyslo Tyr') and commercial gelatin (cattle bone), native cuticle collagen or synthetic cuticle collagen model proteins. Table 2 shows the results of the tensile shear strength on various substrates for the adhesive formulation containing native cuticle collagen and synthetic collagen model polypep- tides as fillers.Because of its low solubility in water, the native cuticle collagen was made 5% solution in 2-chloroethanol. Since tyrosinase does not act in 2-chloroethanol, Table 2 does not include the bonding results of a copoly(Lys Tyr)-native cuticle collagen-tyrosinase formulation. Commercial cattle 200 210 220 230 240 I/nm Fig. 3 CD spectra of cuticle collagen model sequential polypeptides Fig.2 Melting profiles of native and digested cuticle collagen in in 0.01 mol dm-3 acetic acid: sequential (Ala-Gly-Glu-Hyp-Gly- 0.01 mol dm-3 acetic acid: 0, native; 0, after collagenase digestion Gly),,, 0 at 15 "C and 0 at 45 "C; sequential (Hyp-Gly-Gly-Glu- at 23 "C Ala-Gly),,, A at 15 "C and A at 45 "C J. MATER. CHEM., 1991, VOL. 1 Table 2 Bonding strength of cuticle collagen model proteins and their adhesive formulations sample/surface gelatin' native cuticle collagend random Ad,e random Bd." sequential A" sequential Be (LYs1O, Tyr)nf(Lys", Tyr),-gelatin (Lys", Tyr),-gelatin-enzymeg (Lys", Tyr),-native cuticle collagend (Lys", Tyr),-native cuticle collagen-enzymed (Lys", Tyr),-sequential A (Lys", Tyr),-sequential A-enzyme (Lys", Tyr),-sequential B (Lys", Tyr),-sequential B-enzyme tensile shear strength/N mm-2 iron alumiiia skin" boneb 3.48 1.29 0.57 0.95 1.19 0.61 0.54 0.37 0.02 0.46 0.0 0.42 1.10 1.42 0.29 0.03 1.36 1.15 0.36 0.13 2.55 2.26 0.16 1.07 3.24 1.67 0.69 1.18 3.68 1.71 0.19 1.89 3.23 0.77 0.43 1.18 inactive 3.95 1.88 0.43 0.43 4.56 2.03 0.50 1.01 3.94 2.18 0.50 0.63 5.1 I ~ ~ ~~~~~ 2.27 0.3 I 1.28 ~ a Porcine skin; cattle bone; 'commercial gelatin from cattle bone; 2-chloroethanol solution; random A and B, random copolypeptides containing three (A) and eight (B) amino acids; sequential A, poly(A1a-Gly-Glu-Hyp-Gly-Gly)and sequential B, poly(Hyp-Gly-Gly-Glu-Ala-Gly); degree of polymerization =360; enzyme, tyrosinase.\ \ 0' b \ \ \ \ '0-I3 I I I 0 10 20 30 40 50 60 T/"C Fig. 4 Melting profiles of sequential polypeptides in 0.01 mol dm-3 acetic acid: 0, sequential (Ala-Gly-Glu-Hyp-Gly-Gly),,with a T, at 33 "C; A, sequential (Hyp-Gly-Gly-Glu-Ala-Gly),, with a T, at 28 "C; 0,digested native cuticle collagen taken from Fig. 2 bone gelatin from Wako exhibited a bonding strength of 3.5 N mm-2 on iron, which is a weaker bonding strength than reported earlier. Native cuticle collagen (1.2 N mm -2, and synthetic sequential collagen model proteins (ca. 1.4 N mrnp2) exhibited weak adhesibility. The highest bonding strength, 5.1 Nmm-2, was found in the case of a copoly(Lyslo Tyr ')-sequential B-tyrosinase formulation on iron.The bonding strengths of random copolypeptides A and B examined in the same way were very weak on metals (ca. 0.02NmmP2) and no further examination as to adhesive functions had been done. In general the bonding strength of adhesive formulation of poly(Lys'o Tyr')-sequential cuticle collagen models with tyrosinase were higher than the same adhesive formulations without the enzyme (except skin). The bonding strengths of commercial gelatin and native cuticle collagen from marine worm are ca. 0.59 N mm-2 on skin and are 0.39-1 N mm-2 on bone. When tyrosinase was added to copoly(Lys lo Tyr')-gelatin, an increased bonding strength of 1.9 N mm-2 on bone was obtained.The bonding strengths of copoly(Lys lo Tyr')-sequential model collagens were 0.43-0.50 N mm-* on skin and 0.43-0.63 N mm-2 on bone. When tyrosinase was added, their bonding strengths increased three-fold up to 1.3 N mm -'. The adhesive formu- lations composed of copoly(Lys'o Tyr') with tyrosinase exhib- ited greater bonding strengths on solid surfaces such as iron, alumina and bone than those without the enzyme. Conversely, a decreased enzyme effect was observed on soft skin surfaces. Discussion The synthesis and conformation of cuticle collagen from marine worm Nereis japonica has been investigated from the standpoint of polymer chemistry. Two samples of random copolypeptides were synthesized by the usual NCA method. These cuticle collagen models having major amino acid com- positions but random amino acid sequences, however, exhib- ited only a thermal perturbation of the conformation and no adhesibility on metals, and are not suitable as cuticle collagen model compounds.At present no primary structure of the marine cuticle collagens has been discovered. From the amino acid compo- sitions and several secondary and tertiary structural character- istics of ordinary collagens, a variety of repetitive sequences such as Gly-Ala-Glu-Hyp-Gly-Gly,Ala-Glu-Gly-Hyp-Gly-Gly, Glu-Ala-Gly-Gly-Hyp-Gly,Ala-Gly-Glu-Gly-Gly-Hyp, can be considered. At first we tried to synthesize a sequential polyhexapeptide (Ala-Gly-Glu-Hyp-Gly-Gly), (Scheme 2) via the following strategy.An attempt to couple a formulation of Z-Ala-Gly-Glu(0Me)-Hyp(Bz1)-Gly-Gly-ONp,Z-Ala-Gly-Glu(0Me) with Hyp(Bz1)-Gly-Gly-ONp hydrochloride was tried. However, the selective removal of the two protecting groups of 2 and tert-butyl ester from tripeptide Z-Ala-Gly- Glu(0Me)-OBu' to Z-Ala-Gly-Glu(0Me) by 3 mol dm-3 hydrogen chloride-dioxane, trifluoroacetic acid-water (4 : 1, J. MATER. CHEM., 1991, VOL. 1 v/v)22,23 and 11 mol dm3 hydrochloric acid-tetrahydrofuran (1O:7, v/v)~~was unsuccessful, causing a partial cleavage of the Z protecting group, although amino acid derivatives such as Z-amino acid-OBu' were selectively converted to Z-amino acids as previ~usly.~~-~~ This discrepancy can be understood from the fact that a prolonged reaction time was necessary in the case of the longer tripeptide compounds.Other unsuc- cessful strategies, not described in detail here, were also tried. Two sequential polyhexapeptides were synthesized according to Schemes 2 and 3. Sequential poly(A1a-Gly-Glu-Hyp-Gly-Gly) with DP = 130 and poly(Hyp-Gly-Gly-Glu-Ala-Gly)with DP= 120 exhibited a thermally induced change of the CD spectra with melting temperatures at 33 and at 28 "C, respect- ively, in 0.01 mol dm-3 acetic acid. This and the results of random copolypeptides suggest that the amino acid sequence is important in cuticle collagen molecules. Apart from the thermal tendency, the absolute amplitudes of the dichroic bands arising from the sequential polyhexapeptides were small and the spectral shapes of native and synthetic sequential cuticle collagens differed somewhat from each other, whereas the T, of the native (28 "C) and the synthetic (33 and 28 "C) were close.One of the reasons why the ellipticities of the sequential polyhexapeptides are small may be attributable to their low molecular weights, because ellipticity depends on the backbone chain length,25 and in fact the digested cuticle collagen exhibited much smaller ellipticities, having almost the same T, as the native one, as depicted in Fig. 1 and 4. Experimental investigations of the conformational stability of a variety of synthetic collagen model compounds have been In general, the melting temperature of collagens is higher when the collagens involve more Hyp.29 In spite of the fairly high Hyp content in the native cuticle collagens from Nereis japonica than ordinary collagens, its T, is lower than that for ordinary collagens. This is because the Hyp residue in the triplet sequence Glu-X-Y (X and Y, imino acids) localizes in the X po~ition.~'In the present synthesis of the sequential polyhexapeptides we chose the Hyp residue in the Y ordinary collagen model position for sequential A and the X position for sequential B of the triplet sequences.The results in Fig. 4 exhibited that the difference of the melting tempera- tures between the sequential A and B was ca. 5 "C and the sequential B is a better marine cuticle collagen on the basis of its T,. In the progressive study of this series of marine cuticle collagens, other polyhexapeptides and helix-forming sequences such as Gly-Ala-Hyp-Gly-Glu-Arg and Gly-Hyp- Ala-Gly-Glu-Arg will perhaps be better models on the basis of the melting phenomenon, since the sequences of Gly-Pro- Hyp, Gly-Pro-Ala and Gly-Ala-Hyp are known to form stable triple helices.31 Most of the collagen model polypeptides hitherto synthe- sized contained imino acids 33 and 67 mol% due to the triplet sequence. The present polyhexapeptides contain 17 mol% Hyp residues, while the native cuticle collagen contains ca. 20 mol% imino acids. Unfortunately, since the molecular weights of our polyhexapeptide cuticle collagen models were very low compared with the native sample, we were unable to investigate the molecular reasons for the conformational transition; for example, a transition from a triple-helical conformation to a single random-coil conformation.In the cases where we compared the results of the thermal confor- mational transition between the sequential collagen models, the amino acid sequence (though not yet determined) of the native cuticle collagens is necessary to discuss the super helical conformation and its transition. Finally, we evaluated the bonding strengths of the adhesive formulation of native and synthetic cuticle collagens in Table 2 comparing it with our past bioadhesive result^.^ The tensile shear strengths of the native and synthetic cuticle collagens were not so high (ca. 1.36 N mm-2 on iron) among our synthetic adhesive model proteins (over 100 types), in which the highest sample poly(L-lysine) hydrobromide, exhibited 12.1 N mm-2 on iron.The tensile shear strengths of native and synthesized sequential cuticle collagens on alumina (ca. 1.42 N mm-2) are better, while the highest sample, poly(Lys2 Tyr'), exhibited 2.65 N mm-2. The tensile shear strengths (ca. 0.54 N mm-2) of native and synthetic cuticle collagens on skin and bone have comparable bonding strengths to those of silk gland protein and silk fibroin, whose tensile strengths on bones (bovine and porcine) were in the 0.49-0.69 N mm-2 range.32 The collagen or gelatin adhesive formulations composed of copoly(Lys'' Tyr') with tyrosinase exhibited greater bonding strengths on solid surfaces such as iron than those without the enzyme.Conversely, a decreased enzyme effect was observed on the soft skin surface. As to the source of the differences in adhesion between iron and skin, the greater amount of mechanical energy which can be stored in skin, relative to iron, may give very different results from the same adhesive formulation. Such different bonding charac- teristics on skin and bone may give some practical applications as bioadhesive formulations. A surface chemical study using the contact angle and the surface tension to evaluate inter- actions between the adhesive proteins and the substrates is the next target of this and will be reported soon. In addition to the synthetic barnacle arthropodin model pro- tein~,~~the present native and two synthetic sequential cuticle collagens seem to offer promise as a bioadhesive formulation in a wet environment. In our ongoing study several sequential collagen model proteins with essential amino acid sequences and compositions will be synthesized in order to increase our knowledge regard- ing marine cuticle collagen in connection with adhesive pro- teins secreted from marine invertebrates.The authors wish to express their special thanks to Professor Shigeru Kimura of the Tokyo University of Fisheries for his helpful discussion and for the information on how to identify and purchase the marine worm. This was partly supported by a Grant-in-Aid for Scientific Research (No. 024555012) from the Ministry of Education, Science and Culture.References J. H. Waite, Int. J. Adhesion Adhesives, 1987, 7, 9. V. Christine, US. Pat. (Appl.), 034078, 1987. H. Yamamoto, J. Chem. SOC., Perkin Trans. I, 1987, 613. H. Yamamoto, J. Adhesion Sci. Technol., 1987, 1, 177. H. Yamamoto, A. Nagai, T. Okada and A. Nishida, Marine Chem., 1989, 26, 331. 6 A. Nagai and H. Yamamoto, Bull. Chem. SOC. Jpn., 1989, 62, 2410. 7 H. Yamamoto, S. Kuno, A. Nagai, A. Nishida, S. Yamauchi and K. Ikeda, Int. J. Biol. Macromol., 1990, 12, 305. 8 S. Kimura, Bull. Jpn. SOC. Sci. Fisheries, 1971, 37, 419. 9 S. Kimura and M. L. Tanzer, J. Biol. Chem., 1977, 252, 8018. 10 S. Kimura and M. L. Tanzer, Biochemistry, 1977, 16, 2554. 11 L. W. Murray and M. L. Tanzer, in Biology of Invertebrate and Lower Vertebrate Collagens, ed.A. Bairati and R. Garrone, Plenum Press, New York, 1985, p. 243. 12 J-P. Vollmer and G. Spach, Biopolymers, 1967, 5, 337. 13 P. Doty, J. H. Bradbury and A.M. Holtzer, J. Am. Chem. SOC., 1956, 78, 947. 14 M. Hatano and M. Yoneyama, J. Am. Chem. SOC., 1970, 92, 1392. 15 H. Yajima, N. Fujii, H. Ogawa and H. Kawatani, J. Chem. SOC. Chem. Commun., 1974, 107. 16 J. P. Greenstein and M. Winitz, in Chemistry of the Amino Acids, Wiley, New York, 1964, vol. 2, p. 891, 926. 17 M. Bodanszky and V. du Vigneaud, J. Am. Chem. SOC., 1959, 81, 5688. 954 J. MATER. CHEM., 1991, VOL. 1 18 H. Yamamoto and T. Hayakawa, Biopolymers, 1979, 18, 3067. 26 F. R. Brown 111, J. P. Carver and E. R. Blout, J. Mol. Biol., 1969, 19 M. Bodanszky, J.T. Sheehan, M.A. Ondetti and S. Lande, J. Am. Chem. SOC., 1963,85,991. 27 39, 307. B. R. Shaw and J. M. Schurr, Biopolymers, 1975, 14, 1951. 20 L. Zervas, D. Borovas and E. Gazis, J. Am. Chem. SOC., 1963, 28 H. P. Germann and E. Heidemann, Biopolymers, 1988, 27, 157. 85, 3660. 29 K. Inouye, S. Sakakibara and D. J. Prockop, Biochim. Biophys. 21 22 L. Murray, J. H. Waite, M. L. Tanzer and P. V. Hauschka, in Methods in Enzymology, ed. L. Cumingham and D. Frederikson, Academic Press, New York, vol. 82, 1982, p. 65. H. Gregory, J. S. Morley, J. M. Smith and M. J. Smithers, 30 31 32 Acta, 1976, 420, 133. T. V. Burjanadze, Biopolymers, 1982, 21, 1489. R. Doltz and E. Heidemann, Biopolymers, 1986, 25, 1069. H. Yamamoto, T. Okada, A. Nagai and A. Nishida, Nippon 23 J. Chem. SOC. (C), 1968, 715. E. Schnabel, H. Klostermayer and H. Berdt, Liebigs Ann. Chem., 33 Kagaku Kaishi, 1988, 1771. Surface Reactive Peptides and Polymers, ed. C. S. Sikes and A. P. 24 1971, 749, 90. G. Losse, D. Zeidler and T. Grieshaber, Liebigs Ann. Chem., 1968,715, 196. 34 Wheeler, American Chemical Society, Washington, DC, 1989. H. Yamamoto and A. Nagai, Marine Chem., 1991, in the press. 25 E. Heidemann and W. Roth, Adv. Polymer Sci., 1982, 43, 143. Paper 1/01812D; Received 18th April, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100947

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 955-963 Growth of ‘124’ and ‘247’ Phases studied by High-resolution Transmission Electron Microscopy in HoBa,Cu,O,- Ceramics prepared under Normal Oxygen Pressure Yong Van and Marie-Genevieve Blanchin* Departement de Physique des Materiaux, (UA CNRS no 7 72),Universite Claude Bernard, 69622 Villeurbanne Cedex, France High-resolution transmission electron microscopy (HREM) has been used to study the structure of superconductor HoBa,Cu,O,-, (‘123’) ceramics prepared under normal oxygen pressure. HREM images reveal that lattice disorder as well as high densities of stacking faults occur locally in the perfect ‘123’ structure. HREM micrographs obtained from thin areas of those faulted regions show a good correspondence to projections of the HoBa,Cu,O, (‘124’) and Ho,B~,CU,O,~ (‘247’) structures proposed from neutron diffraction studies.The b/2 shift between double Cu-0 chains occurring in both structures is confirmed by comparison between experimental and simulated HREM images corresponding to different projections of these structures. This demonstrates that owing to departure from nominal composition, stacking sequences occur locally in the specimens, which can give rise to growth of ‘124’ and ‘247’ phases normally obtained under higher oxygen pressure. Keywords: Superconductor; ‘123’ phase ; ’I24’ phase ; ‘247’ phase ; High-resolution transmission electron microscopy Since superconductivity around liquid-nitrogen temperature was discovered,’ the R,Ba,Cu, (with rare-earth- metal elements R =Y, Ho etc.)family proved to be one of the most interesting groups of high- T, superconductors, from the points of view of fundamental research and applications.In the Y2Ba4Cu6 +” family, three members were disco-vered.’ The first member (n=0) is YBa,Cu,O, --x,well known as the ‘123’ phase, and is prepared under normal oxygen pressure. Two other members, i.e. for n= 1, Y2Ba4C~7015+x (the ‘247’ phase) and for n=2, YBa2Cu408 (the ‘124’ phase), are normally obtained under higher oxygen The ‘123’ phase has an oxygen-deficient perovskite structure. Two kinds of oxygen vacancies are ordered in the YBa2Cu307 structure: the first forms a two-dimensional arrangement in the Y planes whereas the other forms vacancy chains alternat- ing with Cu-0 chains parallel to the b axis in the Cu(1) planes.The YBa2Cu408 structure differs from YBa2Cu307 in that single linear Cu-0 chains parallel to the b axis are replaced by double Cu-0 chains with edge-sharing, square- planar oxygen coordination; this induces interesting properties (a strong dependence of T, on hydrostatic pressure, thermal expansion and compressibility anomalies and anisotropy in the a-b The Y,B~,CU,O~~+~ compounds have a structure that consists of alternating ‘123’ and ‘124‘ units and it was reported that T, could be varied between 14 and 70 K by changing the stoichiometry in the range+0.092 x 2 -0.72.6-9 It follows that combined investigation of ‘123’, ‘124’ and ‘247’ phases allows the study of the influence of the single and double Cu-0 chains on high-T, supercon- ductivity.Like the YB~,CU,O,-~ compounds, the high-T, HoBa2Cu307--x superconductors have an orthorhombic structure corresponding to an oxygen-deficient perovskite structure (depicted in Fig. 3, later) with lattice parameters a = 0.3819 nm, b=0.3893 nm, c= 1.1653 nm at room tempera-ture,” but they exhibit quite high critical current densities.” They can contain high densities of stacking faults correspond- ing to the occurrence of double Cu-0 chains, whose structure and mechanism of formation have been detailedI2 with respect to similar features for the Y-Ba-Cu-0 ~ystem.’~-’~In the present paper, we focus our attention on local growth of ‘124’ Fig. 1 Medium-magnification micrograph from a thin area of a HoBa,Cu,O, -x specimen.Regions P and P’ correspond to the perfect ‘123’ structure whereas high densities of stacking faults are observed Fig. 2 Enlargement of an HREM image along the a or b axis from in regions D, D‘ and D” region D” in Fig. 1 and ‘247’ phases in HoBa2Cu307 -x ceramic superconductors prepared under normal oxygen pressure. The structure of these phases has been investigated by high-resolution trans- mission electron microscopy (HREM). The results are dis- cussed through comparison of HREM experimental images with simulated images computed from the structural models proposed in the literature. Experimental High-T, HoBa2Cu307 -x ceramic samples were produced by ALCATEL-ALSTHOM Recherche Laboratories, Marcoussis, France.The samples were made from powder synthesized by a solid-state reaction at 950 “C under atmospheric pressure for 48 h. Cylinders (4~4.5mm, 1=40 mm) were shaped by isostatic pressing and were sintered, i.e. the cylinders were heated for ca. 13 h at 920 “C and maintained at this tempera- ture for 3 h under pure oxygen at normal pressure. The specimens were then slowly cooled to 450 “C over 10 h, then maintained for 12 h at 450 “C for oxidation and slowly cooled to room temperature over 12 h under the same atmosphere. According to the data available in the literature,20 such a procedure will give a value for xin HoBa,Cu307 --x of ca. 6.95. Susceptibility measurements gave T,z86 K. The specimens t = 0.388nm projected potential t= 1.164 nm C 1 00 0 cu t = 1.940 nm 1-2.716 nm J.MATER. CHEM., 1991, VOL. I for electron microscopy were prepared by the standard tech- nique for ceramic oxides. HREM observations were performed using a JEOL 200CX-UHP2S microscope at 200 kV with a top entry stage. The pictures were obtained within times short enough to avoid deterioration of the oxygen content.I3 The objective lens spherical aberration constant was determined as C,=O.89 mm. The simulation of the HREM images was performed on a microcomputer using a program based on multislice theory.13 Results Stacking Faults in the ‘123’ Phase The specimens were processed having the nominal compo- sition HoBa,Cu,O, -x. X-Ray diffraction experiments showed that the crystals were chiefly isostructural with HoBa,Cu307 (space group Pmmm).Nevertheless local departures from that structure were revealed by HREM images at higher spatial resolution.Fig. 1 is a medium-magnification electron micro- graph from one of the samples. Regions P and P’ correspond to the perfect ‘123’ phase, whereas high densities of planar defects are observed in regions D, D’ and D”. Fig. 2 is a high-magnification HREM image from the region D” of Fig. 1, seen along the [100) or the [OlO] direction. This Af= -45 nm Af = -50 nm Af = -55 nm.. Fig. 3(a) J. MATER. CHEM., 1991, VOL. 1 (b) Af=-45 nm Af=-50 nrn Af = -55 nrn t-0.388 nrn projected potential t= 1.164 nrn 1 00 t = 1.940 nrn0C" 0 0Ho t=2.716nrn Fig.3 Series of computer simulated images of the '123' structure for increasing crystal thickness, t, depending on defocus of the objective lens A$ (a) [loo] projection, (b) [OlO] projection image was taken close to the Scherzer defocus ~ondition'~ and represents the projected potential of the structure within a good approximation in such a thin foil. The large dark dots correspond to columns of Ho and Ba atoms, and Cu columns are seen as weaker dark spots between the Ba and Ho columns. However, the HREM images along the [loo] or [OlO] direction of the ordered '123' structure cannot be discriminated without difficulty owing to the limited resolution of the 200 kV electron microscope used for the observations.As shown by the images simulated for thin crystals observed close to the Scherzer defocus condition (Fig. 3), the occupied oxygen columns ([loo] projection), or the oxygen vacancy columns ([OIO] projection) located in the Cu(1) planes (between the Ba planes) are always seen as the brightest dots for both projections. Despite the lack of resolution with respect to imaging of the oxygen sublattice, such a row of very bright dots can be considered as the fingerprint of the Cu(1) planes in the corresponding HREM images. In the high- magnification HREM micrograph of Fig. 2, the planar faults readily appear as double rows of bright dots between the Ba planes. Correspondingly, the planar defects may be thought to consist of a pair of Cu(1) planes, and thus the additional material is expected to have the composition CuO.Detailed contrast studies of such extrinsic stacking faults in the HoBa- cuo'2 and the YBaCu0'3-'9 systems have been published together with the corresponding structural models. Clearly some faults exhibit a mirror symmetry (marked M in Fig. 2) Fig. (a) High-magnification HREM image along the a or axis across the regions P and D in Fig. 2. (b)Optical diffraction pattern from the area '124' in the negative of (a). (c) Optical diffraction pattern from the area '247' in the negative of (a) J. MATER. CHEM., 1991, VOL. 1 Y Ba2Cu408 (Ammm) Fig. 5 (a)Enlargement of the HREM image of the region ‘124‘ in Fig. 4(a). The projection of the ‘124‘ unit cell is outlined, (b) Structure of YBa,Cu,O,, showing the double CuO, chains connecting planes of CuO, pyramids (from ref.2) whereas other faults exhibit a glide symmetry (G in Fig. 2). A gradual change from mirror to glide character can occur due to interaction with permutation t~inning’~?’~.’~ (marked PT in Fig, 2). On the other hand, a change from single CuO plane to double CuO layers can also be observed (circled in Fig. 2). Local Growth of the ‘124’ Phase It should be noted that the density of the CuO double layers can be quite high in some regions of the crystals, as seen in Af= -40 nm Af = -45 nm Af = -50 nm Af = -55 nm Af I-60 nm Fig. 2. The intervals between the stacking faults are then only a few unit cells of ‘123’ phase. The new phase can be thus expected to grow from the ordered arrangement of the faults, as shown by the HREM image of Fig.4(a), obtained across the regions P and D of Fig. 1. This micrograph clearly reveals that region P corresponds to the perfect ‘123’ phase whereas region D corresponds to ‘124’ and ‘247’ phases. The ‘124‘ phase, which has been extensively studiedin the YBaCuO has the orthorhombic structure Ammm at room temperature [depicted in Fig. S(b)].It differs from the ‘123’ phase by the intercalation of a double CuO layer, i.e. double t= 1.54 nm t= 2.32 nm f = 3.09 nm Fig. qa) J. MATER. CHEM., 1991, VOL. 1 t= 1.54 nrn t= 2.32 nrn t = 3.09 nrn Af= -40 nm Af= -50 nm Af= -55 nm Af= -60 nm Fig. 6 Series of computer-simulated images of the ‘124‘ structure for increasing crystal thickness, t, depending on defocus of the objective lens AJ (a)[IOO] projection, (b)[OlO] projection Cu-0 chains, between the Ba planes [Fig.5(b)]. of this structure, close to the Scherzer defocus condition and In HoBa,Cu,08 the Y atoms are readily replaced by the Ho for increasing crystal thickness, are reproduced in Fig. 6. In atoms. The simulated images observed along the a and b axes this case the HREM images along the a and b axes can be c =5 Y2Ba4Cu& 5 (Arnmrn ) Fig. 7(a) Enlargement of the HREM image of region ‘247’ in Fig. qa);inset is the simulated image. (b)Structure of Ho,Ba,Cu,O1, consisting of alternating ‘123’ and ‘124‘ units (from ref. 2) discriminated easily from the b/2 shift within the double Cu-0 chains.21 Fig.5(a) is an enlargement of the region '124' in Fig. 4(4.Part of the micrograph of Fig. 5(a), as delineated, exhibits a good correspondence to the projection of HoBa,Cu,O, along the b axis [Fig. 6(b)]:the metal columns are seen as dark dots and the double CuO layers are imaged as rows of very bright dots exhibiting a mirror symmetry. According to Fig. 6(b),the experimental image matches best (a 1 Ba--c Hod I Ba4 . , ,. c . . ., . , Ba -.,.-....,-, , .. Af= -45 nm HO-Ba-Ba-Ho-Ba-., .. . Ba ----c Ho 4 Ba-c projected potential Af= -55 nm Af= -65nm J. MATER. CHEM., 1991, VOL. 1 with the simulated images along [010] direction for crystal thickness in the range 1.54-2.32 nm and at defocus values between -45 and -55 nm.Fig. 5(a) clearly reveals that the perfect stacking sequence of double CuO layers, leading to local growth of the '124' phase, does not extend over very large volumes of crystal, but is limited by the occurrence of structural defects, i.e. double CuO layers exhibiting a glide character [labelled G in t= 1.54 nm t = 3.08 nm t ~4.61nm Fig. 8(a) J. MATER. CHEM., 1991, VOL. 1 96 1 (b1 t= 1.55 nm t= 3.10 nm t = 4.65 nm Af=-&I In-l projected potential Af= -55 nm Af= -65nm Fig. 8 Series of computer simulated images of the '247' structure for increasing crystal thickness, t, depending on defocus of the objective lens Af (a)[loo] projection, (b)[OlO] projection Fig.5(a)] or terminating in the crystal [circled in Fig. 5(u)]. Local Growth of the '247' Phase Moreover some local lattice disorder is superimposed with It can be seen from Fig. 4(a)that the '247' phase can be found that large density of extended defects, this being apparent besides regions corresponding to the '123' and '124' phases. showed that from the corresponding variations in the dot intensity of the Previous studies of the YBaCuO ~ystem~-~*~~ experimental HREM images. the '247' structure consists of alternating units of '123' and J. MATER. CHEM., 1991, VOL. 1 -1 t= 1.'09nm t=2.18 nm t = 3.27 nm ............... Ho ............ Ba Af= -55 nm Ba'Ba Ho'Ho Ba Ba projected potential Af= -75 nrn I Af = -95 nm Af= -1 15 nrn Fig. 9 (a) High-magnification HREM image of a '247' region observed along the [l 101 direction; inset is the simulated image.Seen along [OOI] direction, the shift in the lattice fringes (marked by dashed white lines) can be observed which shows a glide character of the CuO double layers in this projection. (b)Computer-simulated images of the '247' structure in the [l lo] projection for increasing crystal thickness, t, depending on defocus of the objective lens Af J. MATER. CHEM., 1991, VOL. 1 ‘124‘ as depicted in Fig. 7(b).The unit cell is A-centred (space group Ammm), the neighbouring ‘1 23’ blocks being translated by b/2 with respect to each other. A doubling of the stacking sequence along [OOI] is thus necessary, leading to a large c parameter [Fig.7(b)]. Fig. 4 reproduces optical diffraction patterns from areas of the negative of Fig. 4(a) corresponding to ‘124‘ [Fig. qb)] and ‘247’ [Fig. 4(c)]. The superstructure spots along the c axis are clearly viewed in Fig. qc), which corresponds to the long period arising from ‘123’ and ‘124‘ alternating units in the ‘247’ phase. This is consistent with the results given for the YBaCuO system by Beeli et aL21 Fig. 7(a) is a high-magnification micrograph of the region ‘247’ in Fig. 4(a), in which alternation of ‘123’ and ‘124’ units can be clearly seen (white arrows); the double CuO layers exhibit a glide character as in the projection of the ‘247’ structure along the a axis [Fig. 7(b)]. Simulated images of the ‘247’ phase observed close to Scherzer defocus condition along the a and b axes are reproduced in Fig.8. The double CuO layers exhibit a mirror symmetry in the simulated images along the b axis, whereas they show a glide symmetry along the a axis as observed in the experimental image of Fig. 7(a). Here again the extension of the ‘247’ phase appears to be limited by stacking faults. The existence of the ‘247’ phase in our specimens was confirmed by HREM observations along other directions. Fig. 9(a) is an HREM image from a region observed along the [l 101direction, in which the alternation of ‘123’ and ‘124‘ units is found again. The series of simulated images of the ‘247’ phase viewed along the [llO] direction at different defocus values and for increasing crystal thickness are shown in Fig.9(b). A good match with the experimental image is found for defocus values between -95 nm and -105 nm in the thickness range 1.09-3.27 nm. Accordingly variation in the contrast of the experimental image is observed owing to the change in the foil thickness. Although no atomic structure image can be achieved along this direction owing to the limited resolution of the 200 kV electron microscope used for the observation, the shift in the lattice fringe imaged in Fig. 9(a) clearly reveals that the double CuO layers exhibit a glide character. This confirms the existence of a b/2 shift within the double Cu-0 chains of the ‘247’ phase present in our specimens, in agreement with the results obtained for the YBaCuO system for neutron diffraction6-’ and electron microscopy.2o Discussion HREM observations showed that our specimens consisted of a matrix having the ‘123’ structure in which ‘124‘ and ‘247’ structures could be found locally.The present specimens were prepared by standard ceramic processing under normal oxy- gen pressure to have the nominal composition HoBa2Cu307-x. Obviously excess of CuO could occur locally, and could be accommodated by intercalation of double CuO layers. The ordering of such double layers, which can alternate with simple layers, gives rise to the stacking sequence of the ‘124‘ and ‘247’ phases. The ‘124‘ phase was first observed2 in partially decomposed powders and then as a component of multiphase thin films.Later it was shown that polycrystalline ‘124‘ can be synthesized under normal pressure using a mineralizer and very fine powders from the decomposition of nitrates.2 On the other hand the existence of the ‘247’ phase was only reported in multicomponent samples with nominal composition YBa2Cu30, -solidified under high pressures of oxygen (2900 bar) or in small single crystals grown with a high-pressure travelling solvent method at oxygen pressure of 130 bars2 The present study reveals that small amounts of ‘247’ phase can be formed locally in polycrystalline ceramic samples with nominal composition HoBa2Cu307 -synthe-sized under normal oxygen pressure. Conclusion The present study confirms that carefully controlled reactions and thermal treatments are crucial to obtain chemically and structurally homogeneous polycrystalline samples.The key role played by the ordering of double Cu04 chains in trans- formation from ‘123’ phase into ‘124‘ and ‘247’ phases is confirmed. Many other stacking sequences of ‘123’ or ‘124‘ units could be obtained in the family of homologous RBa2CU6+ compounds. Control of complex chemical reactions under moderate if not normal oxygen pressure should be investigated to generate such a series of new superconductor compounds. We thank Dr. P. Dubots and Mr. A. Wicker (Alcatel-Alsthom Recherche, France), who provided the samples, for stimulating discussions on the subject. Dr. B. Poumellec (Laboratoire des Composes Non-Stoechiometriques, Universite Paris-Sud, ORSAY, France) is gratefully acknowledged for his interest in the present study.References 1 M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Hung, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett., 1987, 58, 908. 2 E. Kaldis and J. Karpinski, Eur. J. Solid State lnorg. Chem., 1990, 27, 143. 3 J. Karpinski, E. Kaldis, E. Jilek, S. Rusiecki and B. Bucher, Nature (London), 1988, 336, 660. 4 P. Marsh, R. M. Fleming, M. L. Mandich, A.M. De Santolo, J. Kwo, M. Hong and L. J. Martinez-miranda, Nature (London), 1988, 334, 141. 5 P. Fischer, J. Karpinski, E. Kaldis, E. Jilek and S. Rusiecki, Solid State Commun., 1989, 69, 531. 6 J. Karpinski, C. Beeli, E. Kaldis, A. Wisard and E.Jilek, Physica C, 1988, 153-155, 830. 7 J. Karpinski and E. Kaldis, Nature (London), 1988,331, 242. 8 P. Bordet, C. Chaillout, J. Chenavas, J. L. Hodeau, M. Marezio, J. Karpinski and E. Kaldis, Nature (London), 1989, 334, 596. 9 J. Karpinski, S. Rusicki, B. Bucher, E. Kaldis and E. Jilek, Physica C, 1989, 161, 618. 10 P. Fischer, K. Kakurai, M. Steiner, K. N. Clausen, B. Lebech, F. Hulliger, H. R. Ott, P. Bruesch and P. Unternahrer, Physica C, 1988, 152, 145. 11 M. Wacennovsky, H. W. Weber, 0.B. Hyun, D. K. Finnemore and K. Ereiter, Physica C, 1989, 160, 55. 12 Y. Yan, M.G. Blanchin and A. Wicker, Physica C, 1991, 175, 651. 13 Y. Yan and M. G. Blanchin, Philos. Mag. A, 1990, 61, 513. 14 Y.Matsui, E. Takayama-Muromachi and K. Kato, Jpn. J. Appl. Phys., 1988, 27, L 350. 15 Y. Matsui, E. Takayama-Muromachi and A. Ono, Jpn. J. Appl. Phys., 1988, 26, L 777. 16 B. Domenges, M. Hervieu, C. Michel and B. Raveau, Europhys. Lett., 1987, 4, 21 1. 17 H. W. Zandbergen, R. Gronsky, K. Wang and G. Thomas, Nature (London), 1988,331, 596. 18 G. Van Tendeloo and S. Amelinckx, Phys. Status Solidi (a), 1987, K1,103. 19 G. Van Tendeloo, D. Broddin, H. W. Zandbergen and S. Amelinckx, Physica C, 1990, 167, 627. 20 B. Poumellec, Physica C, 1990, 166, 289. 21 C. Beeli, H-U. Nissen, Y. Kawamata and P. Stadelmann, Z. Phys. B, 1988, 73, 313. Paper 1/01966J; Received 26th April, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100955

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991,1(6), 965-970 Time-resolved Powder Neutron Diffraction Study of Thermal Reactions in Clay Minerals David R. Collins,*" Andrew N. Fitch" and C. Richard A. Catlowb a Department of Chemistry, University of Keele, Staffordshire ST5 5BG, UK Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21, Albemarle Street, London WlX 4BS, UK Thermally induced reactions in three contrasting clay samples have been studied, in situ, using time-resolved powder neutron diffraction. The dehydroxylation of kaolinite was observed in the range 440-600 "C, and the initial stages of mullite formation at 950-1000 "C. A multi-component clay material composed of disordered kaolinite and illite, with excess quartz, known as Etruria Marl, dehydroxylated in the range 400-600 "C and began to recrystallize to mullite at 1000 "C.The quartz-cristobalite phase transition in this material was observed at 1150 "C. An Mg-vermiculite was shown to dehydrate in stages, giving stable hydrates with d-spacings of 14.4, 11.7, 10.3 and 9.4 A. The vermiculite dehydroxylated at 400-850 "C and began to recrystallize to enstatite at 1000 "C. Keywords: Kaolinite; Vermiculite ; Etruria Marl ; Time-resolved powder neutron diffraction Time-resolved powder neutron diffraction (TRPND) is ide- ally suited to studying the dehydration and dehydroxylation of clay minerals as well as the formation of high-temperature anhydrous crystalline phases. The technique allows us to monitor, in situ, the structural changes of a crystalline material under non-equilibrium conditions, by the rapid acquisition of a series of complete diffraction patterns.In addition, the technique also monitors the hydrogen atom content of a material, irrespective of crystallinity, through the time evolution of the incoherent background intensity. In this work the structural changes are thermally induced, although pressure induced changes have also been studied using TRPND.' The thermal reactions of layer silicates have been studied by other in situ techniques, such as thermal analysis and spectroscopy (see the recent review by Brindley and Lemai- tre2). However, these methods do not yield information pertaining directly to structural changes. Previous diffraction experiments have not been performed in situ, and have involved heat treatment and quenching, and then subsequent analy~is.~Although such experiments have provided valuable information regarding the identification and stability ranges of phases, they cannot give a detailed record of thermal evolution and may indeed fail to detect certain short-lived phases.In contrast, TRPND has the potential to give a complete record of structural changes, allowing in principle, the identification of all crystalline phases. The potential of possible applications of TRPND in solid- state chemistry are numerous (see Pannetier4). The viability of studying thermally induced dehydration reactions in inor- ganic crystalline materials by TRPND has been demonstrated previously on Inverse Weberites.' Indeed we note that the latter study was performed on the same diffractometer DlB, as used in the present study.We describe experiments on three different clay samples, namely a pure kaolinite, a pure Mg-vermiculite and a material known as Etruria Marl. The latter is of particular industrial importance as it is used as a raw material for the manufacture of bricks. We have collected data for all three samples between room temperature and ca. 1150 "C observing various stages of water loss and the formation of high-temperature anhy- drous phases. Experimental Starting Materials and Sample Preparation Kaolinite A well crystalline kaolinite known as the soft variety of Georgia kaolinite was used [Clay Minerals Society Source Clay (KGa-1) Washington County, Georgia, USA].6 No sample preparation was needed as the kaolinite was already of suitable powder form.Etruria Marl The Etruria Marl (from Staffordshire, UK) is a multi-phased material containing disordered kaolinite and illite with quartz and minor iron oxide phases. The material was already of suitable crystal size and no sample preparation was necessary. Mg-Vermiculite Obtained from Llano, Texas, USA, this vermiculite is of particularly high crystallinity and ordering and has been used in previous X-ray diffraction ~tudies.~.~ It occurs in macro- scopic flakes ca. 10 mm x 10 mm x 3 mm, with a pronounced cleavage parallel to the layers. An electric herb mill (SEB mini hachoir) was used to reduce the particle size of the material to a suitable powder form.Such a procedure was deemed preferable to grinding the material with a pestle and mortar as the latter is believed to damage the crystal ~tructure.~ A more uniform crystal size was achieved by sieving the powder. In order to ensure complete saturation with Mg interlayer cations, the sample was refluxed with solutions of MgC1, at 70 "C for 7 days. During this period the solutions were changed daily. After the final reflux the excess chloride ions were removed by washing with distilled water, using silver nitrate as a test to ensure their complete removal. The sample was partially dried in a desiccator over silica gel and sub- sequently placed in a second desiccator over a water bath to ensure complete hydration.Neutron Diffraction Experiments All the data were collected on the two-axis diffractometer D1 B, at the Institut Laue Langevin, Grenoble, France. The instrument is equipped with an 3He/Xe position-sensitive J. MATER. CHEM., 1991, VOL. 1 detector containing 400 cells spanning an 80" range in 28. Individual diffraction patterns were collected at intervals of 150s at a wavelength of 2.52 A. At the beginning of the experiment an A1203 standard was run to correct for the zero-point error of the diffractometer. To collect data over as wide a temperature range as possible we used a furnace equipped with a.niobium heating element which allowed the samples to be heated under vacuum up to 1150 "C.The samples were held in a 10 mm x 50 mm open- ended niobium sample can. However, because of the strong scattering of neutrons by niobium, data collected using the niobium furnace and sample can contained large contami- nation peaks. Owing to a programming error the kaolinite sample was held at 610 "C for ca. 3 '/z h before continuing to heat up to 1150 "C. All the samples were held at this tempera- ture for 2 h before cooling. During this time we continued to collect data until the temperature had cooled to ca. 200 "C, at which point we were able to open the furnace and change the sample. Experimental details of the angular ranges, temperatures and heating rates over which the data were collected are given in Table 1.The temperatures correspond to those of the thermocouple placed in the samples and are believed to be accurate within a few degrees. However, there exists the possibility of temperature gradients within the sample. Interpretation of Data The diffraction patterns for each of the different samples are represented conveniently as a 3D plot, where intensity is expressed as a function of 28 and time or temperature. These data are analysed subsequently by fitting an appropriate peak- shape function to the Bragg reflections and a second-order polynomial to the incoherent background, using the computer program ABFFIT." Thus we are able to monitor accurately the position, FWHM and intensity of the Bragg reflections together with the incoherent background intensity as a func- tion of time and temperature. In practice the intensity of the incoherent background is determined by measuring the average intensity over a small angular range (e.g.So), where no reflection occurs. From this, the time evolution of the hydrogen atom content can be monitored. However, the observed incoherent background intensity contains contributions not only from the hydrogen atoms, but also those due to instrumental background, air scattering, and scattering by amorphous material in the sample. An approximate correction, to obtain the incoherent scattering intensity due solely to hydrogen, can be made by subtracting off the incoherent background intensity measured for the fully dehydrated material. This approach ignores effects caused by changes in the amorphous content of the sample, and the temperature dependence of the incoherent background arising from the sample and furnace.Table 1 Summary of experimental details" heating rate/ temperature samples 28 range/" "C min-' range/ "C kaolinite 5-85 3.75b 20-1 150 Etruria marl 5-85 3.75 30-1 150 Mg-vermiculite 5-85 1.8 40-250 4.0 250-1 150 Wavelength =2.5 18 A;collection time for individual diffraction pat- terns = 150 s. Owing to a programming error the temperature of the kaolinite sample was held constant at 630 "C for ca. 200 min. The thermal evolution of a crystalline phase can be moni- tored by the intensity of one or more of its Bragg reflections. This neglects the effects on the thermal evolution of the Debye-Waller factors. However, the approximation is believed to be reasonable for low-angle isolated reflections over a limited temperature range.5 The amount of each phase in the sample at a given temperature can then be estimated by measuring the intensity of one or more Bragg reflections and then normalizing to the intensities from pure phase reference sample^.^ It is very difficult to determine reliably the relative amount of each phase in a sample from medium-resolution powder diffraction data, especially when non-crystalline phases are present. Instead we have limited our analysis to expressing the time/temperature evolution of each phase in each sample rather than attempting to apportion the total time dependence of the diffraction pattern to precise contributions from individ- ual phases.Using a suitable Bragg reflection we are able to calculate the ratio Illmax,for each phase, where I is the intensity at a given temperature and I,,, is the maximum intensity of that reflection. Finally we note that the observed peak positions, FWHM and intensities are dependent on the time/temperature resol- ution of the experiment. Results and Discussion Kaolinite The 3D plot of the kaolinite data is shown in Fig. 1 and the thermal evolution profile in Fig. 2. At 25 "C (during furnace evacuation and prior to heating) there is a small decrease in the incoherent background which we believe represents the loss of a small amount of grain-boundary water.Indeed since there is no corresponding reduction in the kaolinite intensity, this loss of hydrogen cannot originate from within the kaolin- ite crystal structure. The reduction in the intensities of the kaolinite 001 and 002 reflections and the incoherent back- ground show the disappearance of kaolinite, due to dehydrox- ylation, begins at 440 "C and is complete at 600 "C. The variation of the d-spacings of the 001 and 002 reflections with temperature between 25-600 "C is linear, giving a thermal expansion coefficient along c* of 1.60 x K-I. Following dehydroxylation but before the onset of mullite recrystalliz- ation, we observe an amorphous rneta-kaolin region. Although the majority of the hydrogen has been removed during dehydroxylation there still appears to be small amounts present in the amorphous rneta-kaolin region as suggested previously by IR experiments."-13 The removal of hydrogen does not appear to be complete until ca.900 "C.The formation of mullite begins at 950 "C. We note that this is a somewhat lower temperature than previous reports of mullite forma- tion.14-16 However, it is possible that the early stages of formation of the Bragg peaks attributed to mullite, are in fact the poorly understood spinel phase, which is rep~rted~,~.'~-'' to be stable in the range 900-1000 "C. Since the structural details of the latter phase have not been fully determined, it is not possible to distinguish between the two phases using these data.We note, however, that the intensities of the peaks referred to as the mullite 110 and 210 reflections show local maxima at 990 "C; and we suggest that this effect may be due to the presence of the spinel phase which is replaced by mullite at temperatures above 990 "C. There is, however, no further evidence for this suggestion and, of course the increase with temperature of the Debye-Waller factors of the 1I0 and 210 reflections may significantly affect their observed inten- si ties. J. MATER. CHEM., 1991, VOL. 1 12003 I 001 time/min 002 Fig. 1 3D plot of diffraction patterns for kaolinite. Inset shows the time-temperature profile 0.6OD8l "1i I I0.0 A t A A A A A A A A A A AA AA A AA &A AA A Fig.2 Thermal evolution profile for kaolinite. Incoherent background (-), kaolinite 001 (0)and mullite 110 (A)intensities Etruria Marl The multi-component 3D diffraction pattern of the Etruria Marl (see Fig. 3) is more complex than that of kaolinite. The 001 kaolinite reflections in the Etruria Marl are poorly defined, probably as a consequence of poorer crystallinity. However, the variation in d-spacing of these reflections as a function of temperature is similar to those observed in the pure kaolinite sample. The absence of any illite peaks, particularly the 10 8, 001 reflection, suggests that this sample of Etruria Marl contains little illite, or the illite which it does contain is of exceptionally poor crystallinity.The presence of quartz is, however, demonstrated by its strong 101 reflection. The thermal evolution profile for Etruria Marl is shown in Fig. 4. At 25 "C we observe a sharp decrease in the incoherent background, which we attribute to the loss of grain-boundary water. We note that the intensities of both the 002 kaolinite and quartz 101 reflections increase with reduction of the incoherent background intensity. It is possible that the enhancement of the kaolinite and quartz reflections is a direct consequence of the removal of the grain-boundary water. The reduction in hydrogen-atom content will lead to a reduction in the number of neutrons scattered incoherently, thereby allowing more neutrons to interact coherently with the afore- mentioned crystalline phases. After the grain-boundary loss the incoherent background remains constant up to 400 "C, at which point dehydroxylation begins.The simultaneous reduction in the intensity of the kaolinite 002 reflection and the incoherent background is accompanied by a further increase in the quartz 101 intensity. The latter continues to increase until the dehydroxylation process is complete at 600 "C. Although the increase in intensity of the quartz 101 reflection is probably a consequence of the reduction of the kaolinite and incoherent background intensities, it may poss- ibly represent the a-B phase change. The position of the quartz 101 reflection as a function of temperature indicates normal thermal expansion up to 570"C, whereupon the d- J.MATER. CHEM., 1991, VOL. 1 Fig. 3 3D plot of diffraction patterns for Etruria Marl. C =cristobalite, K =kaolinite, M =mullite and Q =quartz. Inset shows the time- temperature profile spacing abruptly ceases to increase and remains constant. We suggest the point of this abrupt change in behaviour corre- 1.o sponds to the ct-fl quartz phase transition which has pre- viously been observed'' at 573 "C. Between 600 and 1000 "C we observe an amorphous region, which we believe represents 0.8 rneta-kaolin. As with the pure kaolinite sample, small amounts of hydrogen remain trapped until temperatures in excess of 800 "C. The appearance of the mullite at 1000 "C is slightly later than that from the pure kaolinite sample and is consistent 0.6 with the previous observations that mullite formation from a x-E disordered kaolinite is retarded compared with that of a well 2 At 1150 "C we observed the quartz- ordered ka~linite.'~,~'-~~ cristobalite phase transition.The intensity of the quartz 1010.4 reflection decreases as that of the cristobalite 111 reflection increases. 0.2 Mg-VermiculiteA0 0.0 0 A 3D plot of the Mg-vermiculite data is shown in Fig. 5. The 1 I variation in the position of the 001 vermiculite reflection as 0 400 800 1200 a function of temperature, determined by fitting a Gaussian TI"C to the aforementioned reflection, is shown in Fig. 6. The Mg- Fig. 4 Thermal evolution profile for Etruria Marl. Incoherent back- vermiculite initially exhibits a basal spacing of 14.4 A, which mullite 110 (A),quartz 101 (.) andground (-),(.),kaolinite 001 probably represents a two-sheet hydrate.24 This steadily cristobalite 111 (0)intensities decreases to 14.0 8, at 80"C, whereupon the basal spacing J. MATER.CHEM., 1591, VOL. 1 0 200 Fig. 5 3D plot of diffraction patterns for Mg-vermiculite. Inset shows the time-temperature profile 15 13 5 cn .-0 0 P-0 811 LL 9 0 200 400 600 800 1000 TI"C Fig. 6 Position of 001 reflection of Mg-vermiculite, as a function of temperature determined by fitting one Gaussian collapses rapidly to 11.7 A, giving a one-sheet hydrate. We then observe a small decrease in d-spacing to 11.5 8, at 170 "C.Following this we observe a further abrupt collapse of the layers giving a reflection with a d-spacing of 10.3 8, at 200 "C. Since such a d-spacing is of an intermediate value between 400 600 800 timejmin those typical of one- and zero-sheet hydrates, its interpretation needs further consideration as it is difficult to form a physical picture of hydrates containing a non-integral number of interlayer water sheets. The 10.3 8, d-spacing may correspond to the 002 reflection of an ordered arrangement of alternating zero- and one-sheet hydrates. This would yield a similar basal spacing as that observed in the 20.6 8, phase reported in a previous study of Mg-~ermiculite.~~ Alternatively it may corre- spond to the average position of a 001 d-spacing arising from a randomly interstratified distribution of one- and zero-sheet hydrates.In order to distinguish between these two possibil- ities, analysis of higher-order 001 reflections is needed. Since these higher-order reflections are extremely weak and poorly defined, owing to the high incoherent background intensity, such an analysis was not possible. However, as the 10.3 8, reflection is very broad (see Fig. 7) we suggest the interstratifi- cation is probably random. Following this we observe a steady decrease in the d-spacing to 10.0 8, at 700 "C,whereupon it falls more rapidly to 9.3 A, yielding a fully collapsed structure at 800 "C. Shortly after, dehydroxylation destroys the 001 reflection. Analysis of the FWHM of the fitted Gaussian as a function of temperature (Fig.7) reveals very significant broadening during the periods of rapid layer collapse at 80 "C and 170- 200 "C. The broadening probably arises through extensive local variation in hydration states within the sample. After the second layer collapse, the FWHM of the fitted Gaussian is much larger than that fitted to the two- and one-sheet hydrates. We suggest that this broadening, which corresponds to a d-spacing of 10.3 A, probably indicates a disordered 970 3.0 8 2.6 8 2.2 m a m8 0 8 2 1.8 1.4 1.o 8 'IL 8. m '8 0.6 0 200 400 600 800 I000 T/"C Fig. 7 FWHM of 001 reflection of Mg-vermiculite as a function of temperature determined by fitting one Gaussian interstratification of one- and zero-sheet hydrates.Subsequent analysis of the data during the second layer collapse at 160-200 "C,by fitting two Gaussian functions to the 001 reflections, revealed peaks with d-spacings of 11.4 and 10.4 A. These are characteristic of a one-sheet hydrate and a randomly inter- stratified mixture of one- and zero-sheet hydrates, respectively. The first-layer collapse at 80 "C was too rapid to allow the identification of different hydration states. The thermal evolution profile (see Fig. 8) shows dehydration to occur between 25 and 200 "C. There then follows a period up to 400 "C where the incoherent background intensity and the intensity of the vermiculite 001 reflection remain constant.Above 400 "C the incoherent background begins to fall indi- cating the onset of dehydroxylation. Dehydroxylation and 1.o 0.8 0.6 2-2 0.4 0.2 0.0 0 400 800 1200 T/"C Fig. 8 Thermal evolution profile for Mg-vermiculite. Incoherent back- ground (-), and enstatite 420 (V)intensities (.)Mg-vermiculite 001 J. MATER. CHEM., 1991, VOL. 1 hence the removal of the Mg-vermiculite is complete by 850°C. At ca. 1000°C we observe the abrupt growth of the 420 enstatite reflection. From these data it appears that the final stages of dehydroxylation and .the onset of enstatite formation are not simultaneous as has been suggested pre- viously.2 However, we acknowledge that as a result of residual hydrogen, the relatively high incoherent background may obscure the initial stages of the formation of the 420 enstatite reflection.Conclusion This work has demonstrated the use of TRPND to study dehydration, dehydroxylation and high-temperature phase changes in three contrasting clays. We observe structural changes and changes in the hydrogen-atom content during dehydration and dehydroxylation. In addition the formation of mullite is observed from the kaolinite and Etruria Marl samples, whereas enstatite recrystallizes from the Mg-ver- miculite. We thank the I.L.L. for the use of the neutron-beam facilities and SERC for financial support during the experiment. We are indebted to Dr. P. G. Slade for providing the sample of vermiculite. D.R.C. gratefully acknowledges receipt of a stud- entship from Steetley Brick and Tile Ltd.References 1 Z.L.L. Annu. Rep., 1985, 59. 2 G. W. Brindley and J. Lemaitre, in Chemistry of Clays and Clay Minerals, ed. A.C.D. Newman, Mineralogical SOC., London, 1987, p. 319. 3 F. Onike, G. D. Martin and A. C. Dunham, Mater. Sci. Forum, 1986, 7, 73. 4 J. Pannetier, Chem. Scr., 1986, 26A, 113. 5 Y. Laligant, G. Ferey and J. Pannetier, Chem. Scr., 1988, 28, 101. 6 A. A. Hassanipak and E. Eslinger, Clays Clay Miner., 1985, 2, 99. 7 H. Shiroza and S. W. Bailey, Am. Miner., 1966, 51, 1124. 8 P. G. Slade, P. A. Stone and E. W. Radoslovich, Clays Clay Miner., 1985, 33, 51. 8 P. G. Slade, P. A. Stone and E. W. Radoslovich, Clays Clay Miner., 1985, 33, 51. 9 P. G. Slade, personal communication, 1987.10 A. Antoniadis, J. Berruyer and A. Filhol, Acta Crystallogr., Sect. A, 1990,46,692. 11 V. Stubican, Miner. Mag., 1959, 32, 38. 12 V. Stubican and R. Roy, J. Phys. Chem. Zthaca, 1959,65, 1348. 13 R. Pampach, Polka Akad. Nauk, Prace Mineralogiczne, 1966, 6, 53. 14 M. Bulens and B. Delmon, Clays Clay Miner., 1977, 25, 271. 15 M. Bulens and B. Delmon, Bull. SOC. Chim. Belg., 1977, 86, 405. 16 J. Lemaitre, M. Bulens and B. Delmon, in Proc. Znt. Clay Conf., Mexico City, Applied Publishing, Wilmette, Illinois, 1975, 539. 17 G. W. Brindley, Prog. Ceram. Sci., 1963, 3, 3. 18 G. W. Brindley and M. Nakahira, J. Am. Ceram. SOC., 1959, 42, 311. 19 H. Yamadu and S. Kimura, J. Ceram. Assoc. Jpn., (YogkoKyokai Shi), 1962, 70, 65. 20 A. F. Wright and M. S. Lehmann, J. Solid State Chem., 1981, 36, 371. 21 H. D. Glass, Am. Miner., 1594, 39, 193. 22 H. M. Richardson, in X-Ray Zdentijcation and Crystal Structures of Clay Minerals, ed. G.W. Brindley, Mineralogical SOC., London, 1951, p. 76. 23 K. Tsuzuki, J. Earth Sci., 1961, 9, 305. 24 G. F. Walker, Clays Clay Miner., 1959, 4, 101. Paper 1/02056K; Received 1st May, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100965

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 971-976 97 1 Electrochemical Salt Formation in Bis(phthalocyaninato)ytterbium(iii)-Stearic Acid Langmuir-Blodgett Films Michael Petty," David R. Lovett,*" John Millerb and Jack Silverb "Department of Physics, University of Essex, Colchester, UK bDepartment of Chemistry and Biological Chemistry, University of Essex, Colchester, UK The negative deviation of areas per molecule from calculated values and low collapse rates of Langmuir monolayers of bis(phthalocyaninato)ytterbium(m)-stearic acid mixtures is discussed in terms of phase miscibility. Postdeposition stearic acid salt formation in Langmuir-Blodgett films is investigated using X-ray diffraction and infrared absorption studies. It is found that the salt formation occurs upon reduction of the bis(phthalocyaninat0) ytterbium(n1).The redox potentials of pure and mixed films of bis(phthalocyaninato)ytterbium(III) are also investigated. A wetting and/or charge-injection mechanism was observed for mixed films. Keywords: Electrochemical salt formation; Bis(phthalocyaninato)ytterbium(m); Stearic acid; Langmuir-Blodgett film ; Electrochromism In a recent paper,' we showed that bis(phtha1ocyaninato)ytter-bium(~~~)([Yb(pc)(pc.)],where pc is the phthalocyanato 2 -anion and pc. is the phthalocyanato 1 -monoradical anion) can be deposited onto a variety of substrates by the Langmuir- Blodgett (LB) technique. The family of compounds to which this material belongs is known to be electrochromic when in evaporated film formzp4 and this was also found to be the case with LB films."' Unfortunately, monolayers of [Yb(pc)(pc.)] were not very stable with respect to collapse, and deposited films exhibited artifacts of this collapse.An attempt was made to improve this situation by mixing [Yb(pc)(pc.)] with stearic acid.5 The greatest success was achieved with a mixture of 1:5 [Yb(pc)(pc-)]:stearic acid deposited from a mol dm-3 CdC1, subphase. This mono- layer collapsed at a rate of just 1.1% h-' and deposited Y-type with unity transfer ratios. In the course of the work some effects were observed for the mixed materials, on a pure- water subphase, which have since been explored further. Mixed monolayers of stearic acid and [Yb(pc)(pc.)] were noted to collapse at a lower rate than monolayers of the parent compounds.The surface pressure uersus area per molecule (A,) isotherms of these mixtures also displayed a negative deviation from calculated values. These phenomena are described and explained in this paper. X-Ray diffraction analysis of the LB films of these materials suggested that the stearic acid was converted into its potassium salt upon oxidation and/or reduction of the [Yb(pc)(pc.)] in a KCl electrolyte. This work has been corroborated with infrared (IR) measurements and is discussed here. In addition, the redox potentials of the [Yb(pc)(pc.)] in both pure and mixed forms have been investigated. Experimental The two-compartment trough and dipping mechanism employed have been described elsewhere.' Deposition was performed at a speed of 4-10 mm min -from a pure-water subphase (resistivity >18 MR cm; temperature =21 & 1 "C).Films were deposited onto clean, hydrophilic indium-tin-oxide (ITO) coated and gold-coated glass slides. [Yb(pc)(pc.)], synthesized in the Department of Chemistry and Biological Chemistry at the University of Essex,' was mixed in the molar ratios 1:0, 1:1, 1:5, 1:9, 1:99 and 0:l [Yb(pcXpc.)] :stearic acid. These mixtures were spread onto the subphase as solutions in chloroform. Surface pressure us. area per molecule isotherms were recorded by compressing the monolayers to 35 mN m-'. Monolayer collapse-rate tests and deposition were carried out at a surface pressure of 30 mN m- '. Constant pressure was maintained using an analogue electronic feedback system.Low-angle X-ray diffraction analysis was performed on films deposited onto IT0 slides using a Raymax RX3D diffractometer. IR spectra were recorded using a Perkin-Elmer 225 infrared spectrophotometer. In order to increase the effective thickness of the films, a variation of the method of Francis and Ellison6 was used to prepare the LB samples for study. Films deposited onto gold-coated substrates were mounted opposite a parallel gold reflector to form a waveguide. Two concave mirrors directed IR light into the waveguide arrangement. The emerg- ent light was collected by a further two mirrors and focused onto the entrance slit of the spectrophotometer. For electrochemical analysis the IT0 electrodes coated with LB films were placed in a custom-built W-shaped cuvette containing a 5% KC1 solution.The transparent IT0 layer formed the working electrode; the counter and reference electrodes being a 1 cm2 platinum gauze and a saturated calomel electrode (SCE) respectively. An E.G. and G. Prince- ton Applied Research model 264A Polarographic Analyzer/ Stripping Voltmeter was used to control the three-electrode cell. Cyclic voltammograms (CVs) were recorded (scan rate 20 mV s-l) on an XY-recorder. All potentials were recorded with respect to the SCE at room temperature. Results and Discussion Monolayer Behaviour Surface pressure uersus area per molecule isotherms for mono- layers of the mixtures on a pure water subphase have been reported previously.' The isotherms of the pure [Yb(pc)(pc-)] showed large hysteresis, but this was progressively reduced on the increase of the stearic acid content in the mixture.The average area per molecule (A,) for the various mixtures, measured in the usual fashion, plotted against mole fraction of [Yb(pc)(pc.)] is shown in Fig.l(a). Also shown in Fig.l(a) J. MATER. CHEM., 1991, Vol. 1 (b'30-L J ., c Fig. 1 A graph of (a)area per molecule and (b)monolayer collapse rate versus mole fraction of [Yb(pc) (pc.)] in monolayers spread on a pure water subphase (x =experimental data, 0 =calculated data based on the area per molecule of [Yb(pc)(pc.)] and stearic acid being 70 and 20 A', respectively) are the calculated average A, based upon an A, of 20 and 70A2 for the individual molecules of stearic acid and [Yb(pc)(pc.)], respectively, and assuming the two phases to be irnmis~ible.~The negative deviation of the experimental curve from the theoretical plot indicates that the [Yb(pc)(pc.)] and stearic acid are indeed miscible.The stability of the com-pressed monolayers was measured at a constant surface pressure of 30 mN m-' and Fig.l(b) shows the percentage hourly fall in A, averaged over 2 h following compression for each of the mixtures. By this test it was found that the pure [Yb(pc)(pc.)] monolayers were only slightly less stable than those of stearic acid. However, the monolayers of the mixed materials exhibited collapse rates much less than those of the parent compounds. It is suggested that, arising from the miscibility, the [Yb(pc)(pc.)] molecules are aligning on the water surface with interspatial filling by the smaller stearic acid molecules, which prevent slippage and, hence, collapse.X-Ray Diffraction Analysis The d-spacings obtained from the X-ray diffraction spectra for the LB films of the 1 : 5 and 1 :9 mixtures are given in Table 1. None of these films had previously been oxidized or reduced in an electrochemical cell. The first-order maximum of the 1 :9 film is shifted slightly with respect to the second- and third-order maxima because of refractive effects.8 For this reason, whenever possible, peaks other than the first- order were used to calculate d-spacings.The maxima in these spectra are believed to be due to the fatty acid present in the mixed films. The results for the 1 : 5 and 1 :9 films gave an average d-spacing for a stearic acid bilayer of 41.3 f0.4 A. Owing to the difficulties encountered in depositing stearic acid m~ltilayers,~ consistent data for the bilayer spacing in stearic acid LB films are scarce. Reported values range from 40.2f0.3 3 50.9f0.3 3 diffraction maxima. 39.7 k0.2 A'' to 45.8 A," which encompasses the average compounds has already been ~bserved.'~-'~ In these cases an explanation was proposed in which the film underwent a reorganization into small crystallites soon after deposition. These crystallites distributed the diffraction intensity over a wide angular range making individual maxima weak and unobservable in the case of thin films.Such a recrystallization may also occur in LB films of pure [Yb(pc)(pc.)]. Striking changes in the X-ray spectra occurred for films which had been oxidized and reduced in an electrochemical cell. The average d-spacings of 50.65 f0.05 and 50.77 f0.09 I$ for the 1 :5 and 1 :9 films, respectively, (see Table 1) are consistent with values obtained from LB films of stearic acid salts.g.10,15-17 It would therefore appear that upon oxidation and/or reduction of the [Yb(pc)(pc.)] in these films the stearic acid in the film is converted to a salt, in this case potassium stearate because of the KC1 electrolyte employed. In order to accommodate the metal ions the film would have to change dimensions and this is confirmed by the X-ray data.However, no sign of stress in these cycled films was observed under the polarizing optical microscope. Similar d-spacing changes in the spectra of films of a 1 :99 [Yb(pc)(pc.)] tricosanoic acid mixture did not occur when the films were placed in an electrochemical cell and the redox potentials of the [Yb(pc)(pc.)] a~plied.~ Infrared Absorption Analysis The IR spectrum of a 47-layer LB film of cadmium stearate (subphase: 10-4moldm-3 Cd2+, pH 7) obtained by the reflection technique is shown in column (a)of Table 2. In this case, the acid bands of the stearic acid spectrum [see column (f) of Table21 have been replaced by the two carboxylate bands at 1540 and 1430 cm-'.These are respectively due to the asymmetric and symmetric CO stretching of the COT carboxylate group. The IR spectrum of a 51-layer 1 :5 mixture LB film obtained by the reflection technique is shown in column (b)of Table 2. The presence of stearic acid in the sample is best confirmed by the carbonyl stretch band and the CH2 progression bands, whilst the 1260, 1105-1020 and 815 cm-' bands indicate the presence of [Yb(pc)(pc.)]. This film was then placed in a two- J. MATER. CHEM., 1991, Vol. 1 Table 2 Minima wavenumbers (/cm-') in IR transmission spectra of deposited films (a) (b) (4 (4 (4 0 remarks 2960 m 2950 m 2960 m 2960 m 2950 w 2910 s 2910 s 2910 s 2910 s 2910 s 2920 s 2870 m 2860 w 2870 w 2850 vw 2840 s 2840 s 2850 s 2840 m 2840 s 2850 s 2660 vw 2320 m 2330 vw 1700 s 1700 s 1708 m 1710 s 1635 w 1620 w 1595 w 1580 w 1550 w 1540 m 1520 m 1505 w 1470 w 1470 w 1470 m 1472 w 1470 m CH2 6 1460 w 1460 m CH, 6 1430 s 1450 w 1430 m 1450 w 1430 w 1450 m 1432 m 1445 w OH 6, C-0 C0.i Vsymm v 1410 w 1410 w 1405 m 1405 m 1395 m 1395 w band progression.1375 w 1378 w (series of peaks) 1355 w 1360 m 1365 m 1348 w 1340 w 1330 m 1325 m 1320 vw 1320 w 1325 m 1310 w 1310 m 1315 m 1310 w 1310 w 1300 w 1295 m 1295 m 1295 m 1295 m 1295 m OH 6, C-0 v 1280 m 1280 w 1284 w 1275 m 1275 w 1266 m OH 6, C-0 v 1260 m 1260 s 1260 s 1260 s 1255 w 1240 m 1240 m 1240 m 1240 w 1235 w 1220 m 1220 m 1220 m 1224 m 1220 w 1212 w 1200 m 1200 m 1200 m 1196 w 1200 w 1185 m 1185 m 1185 m 1185 w 1170 vw 1165 w ll50vw 1140 vw 1100 m 1105 s 1050 w 1040 w 1020 w 1100s 1080 w 1050 w i:broad $m (broad peak) 1020 1020 peak) 1112m 1115w 1100 w 1070 w 935 w 940 w 890 m OH 6 820 w 815 m 810 m 810 s 785 vw 760 w 725 m 725 m 725 m CH2 doublet in solid 715 m 685 w phase with n L4.640 vw (a) Cadmium stearate, 47 layer LB film. (b)51 layer LB film of 1:5 mixture (c) 51 layer LB film of 1:5 mixture after oxidation. (d) 51 layer LB film of 1 :5 mixture after reduction. (e) Tricosanoic acid, 42 layer LB film. (f) Stearic acid mull.The labelling s (strong), m (medium), w (weak) and vw (very weak) indicate the band intensity on an arbitrary scale. electrode cell containing a 5% KC1 solution and the trum of the cadmium stearate film, column (a) of Table21. [Yb(pc)(pc.)] oxidized by applying +0.9 V. The film was Thus, there is no IR evidence for free acid in the mixed film returned to the neutral state by applying OV to obtain the after the reduction of the [Yb(pc)(pc.)]. spectrum shown in column (c) of Table 2. This spectrum is If salt formation is to occur in the mixed films, then a essentially the same as that of the virgin film. It is important simple application of the theory of electrostatics indicates that to note that the spectrum of the film after oxidation still this cannot happen when the [Yb(pc)(pc.)] is oxidized.Oxi- contains the carbonyl stretch at 1700cm-Upon re-insertion dation requires the film electrode to be positive, hence repel- of the film in the electrochemical cell the [Yb(pc)(pc*)] was ling any positive metal ions. Reducing the [Yb(pc)(pc.)] reduced by applying -0.9 V. After return to the neutral state requires the film electrode to be negative, therefore attracting by applying 0 V, the spectrum in column (d) of Table 2 was positive metal ions into the film. The spectra summarized in measured. The most striking feature of this spectrum is the Table2 do indeed show that conversion of the free acid to complete absence of the carbonyl stretch band [c. the spec- the salt only occurs when positive metal ions are attracted into the film.The COY asymmetric stretch band expected between 1500 and 1600 cm- 'was not observed but the sudden disappearance of the 1700 cm -' band and the appearance of the 1450 cm-' band (CO, symmetric stretch) confirms stear- ate formation. The symmetric stretch was the stronger of the two COT bands in the cadmium stearate film spectrum and would therefore be the easier to detect. The positions of these bands are dependent upon the nature of the positive coun- terion." For comparison, Table 2, column (e),shows the IR spectrum of a 42-layer LB film of tricosanoic acid. This spectrum is similar to that of stearic acid [column (f)] except that the band progression has shifted and the OH bending and CO stretching bands are now found at 1432 and 1266 cm-', respectively.Subsequent to the recording of this spectrum the reduction potential of the [Yb(pc)(pc.)] was applied to this film in a 5% CdCl, electrolyte (note that a CdC12 electrolyte was used in this case only). The IR spectrum of the film after this process overlaid exactly the spectrum of the virgin film, within experimental errors; thus indicating that the salt was not formed. The disappearance of the IR absorption bands of the free acid and the appearance of those of the CO, group upon reduction of the [Yb(pc)(pc.)] is conclusive evidence for potass- ium-ion inclusion into the mixed layer films. These data are further confirmed by the X-ray diffraction spectra, and it is reasonable to conclude that metal ions (only K+ is present) have complexed with the stearic acid in the film.A simple model of the reduction of the [Yb(pc)(pc.)] in a 1 :5 [Yb(pc)(pc.)]:stearic acid film would predict that only the cations required to balance the charges on these molecules would be attracted into the film. In this case 1/5 of the stearic acid protons might be expected to be substituted. The com- plete lack of the 1700 cm-' band in the spectrum of the film that had been reduced is therefore somewhat surprising as this suggests that all the protons have been replaced. A more realistic scenario is that during reduction K+ cations form a diffuse region within the film in order to balance the charge on the electrode. This enables the K+ ions to exchange with the protons of the stearic acid.When the [Yb(pc)(pc.)] is returned to the neutral state all the uncomplexed cations flow out leaving, in this case, potassium stearate as the new main phase. Electrochemical Analysis A typical CV of a 1 :0 film is shown in Fig. 2; here, a positive current is taken to represent reduction. (We have already given a full description of the UV-VIS absorption spectra of the 1 :0 and mixed material films in the various redox state^.^) The cycle was started with the application of +1.20 V, i.e. the film became oxidized. At the current peak RG the red oxidation product of the application of the initial potential was reduced back to the green neutral state. The reduction peak GB is due to the conversion of the neutral material to its blue reduced form.The steeply rising current A indicates the onset of the reduction of the tin oxide electrode to tin. Upon reversal of the voltage sweep the current dropped rapidly to B which represents the re-oxidation of the tin to tin oxide. The oxidation peak BG shows the conversion of the reduced [Yb(pc)(pc.)] to the green neutral state. Oxidation of the material to the red state is shown by the peak GR. These data, along with those for other [Yb(pc)(pc.)]:stearic acid ratios, are presented in Table 3. Nernstian behaviour is ruled out by the shape of the peaks which are broader than the theoretical 90 mV full-width-at- half-rnaxirn~m'~(peak widths are up to 500 mV FWHM). In addition, non-reversible behaviour is indicated for both the J.MATER. CHEM., 1991, Vol. 1 35-A 30-25-20-15-.4 10-L 5-L 3 0--5 --1 0--15--20-GR 1111111111111 -25 c 1.2 0.8 0.4 6.0 -0.4 -0.8 -1.2 potential vs. SCE/V Fig. 2 The cyclic voltammogram of an LB film of pure [Yb(pc)(pc.)] oxidation and reduction processes by the non-unity value of the ratios of the red-green:green-red (0.2) and blue-green :green-blue (0.31) peak currents.20 This irreversibility is further confirmed by the large differences between the red- green (140 mV) and blue-green (240 mV) current summits. For reversible processes with the number of electrons involved in the redox reaction equal to unity the theoretical difference would be expected to be 59.5 mV at 25 "C.The first two reduction scans of a virgin 1 :0 film are shown in Fig. 3. Note that the peak potential of the first reduction peak (-0.63 V) is more cathodic than the subsequent peaks. When the oxidation process was the first to be analysed for a virgin film the converse was found to be true; the first oxidation peak being always more anodic than subsequent oxidation cycles. This is in common with the findings for evaporated films of other bi~phthalocyanines.~*~~~~~It was found that the films could be made to revert partially to the virgin state by scanning the potential to the other redox system, for instance to the red-green transition after several cycles at the blue-green system. Upon cycling around the (in this case) blue-green system again the first reduction peak was more cathodic than subsequent peaks.Again the converse of this was also true. The CV of an LB film of the 1 :9 mixture is shown in Fig.4. The most striking feature of this voltammogram is the reduction peak at -0.53 V which occurred for the first cycle only. A second, smaller reduction peak is also visible in the first scan at ca. -0.7 V. In subsequent cycles only the second peak at ca. -0.7 V was recorded in this region. These CVs are typical of those taken of mixed material films. The redox potentials of the 1 :5 and 1 :9 films show striking differences when compared to those of the pure [Yb(pc)(pc.)] film. The green-blue transition shifts in potential by at least -0.2 V, from ca.-0.5 V to ca. -0.7 V or lower. This is accompanied by a new reduction peak appearing at the same potential as the green-blue process in the pure films. No J. MATER. CHEM., 1991, Vol. 1 Table 3 Peak potentials of pure and mixed material films mixing“ ratio RG~IV IP‘jV GBd/V BG‘/V GRJ~ I :og +0.48 -0.53 -0.25 +0.65 1:5 +0.46 -0.55 -0.80 -0.23 +0.59 I :9 +0.46 -0.53 -0.73 -0.23 +0.59 I :9 +0.46 -0.50 -0.75 -0.22 +0.58 I :9 +0.48 -0.53 -0.75 -0.23 +0.59 “[Yb(pc)(pc.)]:stearic acid. bRed-green transition. ‘Ion penetration peak. dGreen-blue transition. ‘Blue-green transition. ’Green-red transition. 8Pure [Yb(pc)(pc.)] LB film. 50 - 45- 40- 35- 30- 25.25- i! 20- 15- 1& 5- 0- -502 I 0’0 I I -02 I I -04 I I -06 I l-08 l l -10 LB film *Ol OOi:;-5 -104 I,,,,,, IIIIII 1.2 0.8 0.4 O!O -0.4 -0.8 -1.2 potential vs.SCEjV Fig. 4 The cyclic voltammogram of a 1 :9 LB film change in colour of the films was observed coincident with this new reduction peak. It was found that films of the 1 :5 and 1 :9 mixtures could be returned to their virgin states by drying. Upon re-immersing the slides in electrolyte and measuring their CVs the new peak appeared on the first cycle. This process could be repeated apparently indefinitely. Storing films in electrolyte did not produce the additional reduction peak upon re-measuring their CVs. Thus the new peak in the CVs of mixed material films would appear to be associated with a wetting mechanism; injecting ions and/or water into the largely hydro- phobic film.This explanation of ion penetration is reminiscent of that proposed for similar peaks produced by another phthalocyanine compound,22 though this material contained no fatty acid. Why the redox potential of the [Yb(pc)(pc.)J should also be displaced is not yet clear. It has previously been observed that in sublimed films of pure phthalocyanine materials the ion penetration peak is associated with an electrochemically induced phase change, which allows inclusion of the counter ion^.^^ This phase is believed to be similar to that formed by annealing a sublimed film.23,24 We note that in the case of the 1 :0 film such a peak is not observed. This probably indicates that when pure [Yb(pc)(pc.)] is deposited by the LB technique it is already in the correct phase for ion incorporation.The effect of covering an IT0 electrode with a fatty-acid film was investigated. A 49-layer Y-type LB film of tricosanoic acid was deposited onto an IT0 electrode and its CV recorded. The total absence of the redox currents seen for the plain potential reached -1.45 or +2.53 V. This effect has been reported for single monolayers of alkyl mercaptans (C12-C18) deposited onto a gold ele~trode.~’ In this case the monolayer was capable of suppressing gold oxidation by up to five orders of magnitude. Thus, the hydrophobic alkyl chains act as an insulator to the flow of solvated ions. Conclusion The A, and collapse rate data for the mixed material mono- layers given previously’ have been explained here.[Yb(pc)(pc.)] has been shown to be miscible with stearic acid. This accounts for both the negative deviation of the A, of the mixtures from theory and the increase in stability of the mixed monolayers with respect to the parent compounds. Although X-ray diffraction peaks attributable to the [Yb(pc)(pc.)J were not observed in the mixed material films, virgin films did produce the diffraction spectrum of stearic acid. When the [Yb(pc)(pc.)] in these films was oxidized and reduced the fatty-acid spectrum changed to that consistent with a fatty-acid salt. IR analysis showed that the salt was formed upon reduction of the [Yb(pc)(pc.)], attracting, in this case, potassium cations into the films. It has long been known that salts of certain metal ions can be difficult if not impossible to deposit.Blodgett” showed that copper or aluminium ions in the subphase at concentrations of lo-’ mol dm-3 com- pletely prevented film deposition. The postdeposition salt- formation technique described above may prove a viable method of producing LB films of compounds that would otherwise not deposit. The redox potentials of LB films of [Yb(pc)(pc.)] have been measured using cyclic voltammetry. These studies have shown that for mixed material films there exists a wetting or charge- injection mechanism associated with the presence of the fatty acid. The results for a pure fatty acid film indicate that the 976 J.MATER. CHEM., 1991, Vol. 1 fatty acid is non-conducting and electrolyte impermeable over the range of redox potentials of the [Yb(pc)(pc.)]. Earlier work,’ showed that films consisting of 1 :99 mixtures of [Yb(pc)(pc.)]-tricosanoic acid were not electrochromic. It was postulated that electrochromism could occur only if 3 4 5 G.C.S. Collins and D.J. Schiffrin, J. Electroanal. Chem., 1982, 139, 335. C.S. Frampton, J.M. O’Connor, J. Peterson and J. Silver, Dis-plays, 1988, 9, 174. M. Petty, D.R. Lovett, J.M. O’Connor and J. Silver, Thin Solid Films, 1989, 179, 387. regions of [Yb(pc)(pc.)] within the film were sufficiently close together so that conduction from one region to the next was possible and that this was not the case in the 1 :99 films.We are now able to expand on this postulate. It has now been shown using cyclic voltammetry that a pure tricosanoic acid film, which approximates to the 1 :99 mixture, indeed, does 6 7 8 9 S.A. Francis and A.H. Ellison, J. Opt. SOC. Am., 1959, 49, 131. M. Petty, Ph. D. Thesis, University of Essex, 1990. N.B. McKeown, M.J. Cook, A.J. Thompson, K.J. Harrison, M.F. Daniel, R.M. Richardson and S.J. Roser, Thin Solid Films, 1988, 159, 469. G.L. Clark, R.R. Sterrat and P.W. Leppla, J. Am. Chem. SOC., 1935, 57, 330. not conduct and also effectively blocks the electrolyte from 10 G.L. Clark, R.R. Sterrat and P.W. Leppla, J. Am. Chem. SOC., the IT0 electrode. This is further confirmed by the IR analysis of tricosanoic acid films, as these studies show that electro- chemical salt formation does not occur within a pure fatty- acid film.Hence, it is not sufficient for the [Yb(pc)(pc-)] to be present merely in the film; this material must also form a conducting matrix, as the co-matrix of fatty acid is both non- 11 12 13 14 15 1936, 58, 2199. K.B. Blodgett, J. Am. Chem. SOC.,1935, 57, 1007. R.H. Tredgold, A.J. Vickers, A. Hoorfar, P. Hodge and E. Khoshdel, J. Phys. D: Appl. Phys., 1985, 18, 1139. R.H. Tredgold, Rep. Prog. Phys., 1987, 50, 1609. A.J. Vickers, Ph. D. Thesis, University of Lancaster, 1984. K. Mizushima, T. Nakayama and M. Azuma, Jpn. J. Appl. Phys., conducting and water repellent. 16 1987, 26, 772. M. Pomerantz and A. Segrnuller, Thin Solid Films, 1980, 68, 33. 17 M. Prakash, J.B. Peng, J.B. Ketterson and P. Dutta, Thin’Solid Thanks are due to the British Technology Group for continued support on electrochromics to J.S., to SERC for providing a studentship to M.P., to the Royal Signals and Radar Establish- 18 19 Films, 1987, 146, L15. L.J. Bellamy, The Infrared Spectra of Complex Molecules, Chap-man and Hall, London, 3rd edn., 1986. F. Castaneda and V. Plinchon, J. Electronanal. Chem., 1987,236, ment, Malvern, for the loan of equipment, and to Z. Ali Adib at the University of Lancaster for assistance with X-ray analysis. 20 21 163. D.T. Sawyer and J.L. Robert:, Experimental Electrochemistry for Chemists, John Wiley, New York, 1974. F. Castaneda. V. Plinchon, C. Clarisse and M.T. Riou, J. Electroanal. Chem., 1987, 233, 77. 22 J. Silver, P. Lukes, S.D. Howe and B. Howlin, J. Muter. Chem., 1991, 1, 29. References 23 P.J. Lukes, Ph. D. Thesis, University of Essex, 1989. 1 M. Petty, D.R. Lovett, P. Townsend, J.M. O’Connor and J. Silver, J. Phys. D: Appl. Phys., 1989, 22, 1604. 24 25 J. Silver and M. Ahrnet, unpublished results. H.O. Finklea, S. Avery and M. Lynch, Langmuir, 1987, 3, 409. 2 P.N. Moskalev and I.S. Kirin, Opt. Spectrosc., 1970, 29, 220. Paper 1/02212A;Received 10th May, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100971

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 977-988 Synthesis and Characterization of Isomeric Biphenyl-containing Poly(ary1 ether-bisketone)s. Part 1.-Polymers derived from 4,4'-(p-FluorobenzoyI)biphenyl and Bisphenols Atul Bhatnagar, Rajarathnam S. Mani, Barry R. Weeks and Dillip K. Mohanty* Department of Chemistry and Center for Applications in Polymer Science, Central Michigan University, Mt. Pleasant, MI, 48859, USA A series of amorphous and semicrystalline poly(ary1 ether-bisketone)s have been synthesized from bisphenols and 4,4'-bis(p-fluorobenzoyl)biphenyl via nucleophilic aromatic substitution reactions. Model compound studies were carried out with a variety of substituted phenols, 4,4'-bis( p-fluorobenzoyl)biphenyl and 4,4'-bis( p-chloroben- zoy1)biphenyl. The bishalide monomers were synthesized by the reaction of biphenyl-4,4'-dicarboxylic acid with thionyl chloride followed by Friedel-Crafts acylation with the appropriate aryl halide.Potassium carbonate mediated reaction of these monomers in dimethylacetamide or diphenyl sulphone gave high-molecular-weight polymers in excellent yield. Polymers with semicrystalline morphologies were synthesized from soluble high- molecular-weight amorphous precursors with removable bulky substituents. Unlike the corresponding mono-ketone analogues, the amorphous poly(ary1 ether-bisketone)s exhibited poor solubility in a wide variety of solvents, indicative of improved solvent resistance. The glass-transition and melting temperatures of the polymers are among the highest known for poly(ary1 ether-ketone)s.In addition, the polymers exhibit excellent thermal stability and afford tough films by compression moulding. Keywords: Nucleophilic aromatic substitution ; Amorphous material; Glass-transition temperature Poly(ary1 ether-ketone)s belong to a class of materials known as engineering thermoplastics. 1*2 During the past decade ICI has commercialized an aromatic poly(ary1 ether-ket~ne),~~' PEEK (l),which has contributed to the enhanced scientific interest in this class of materials. Poly(ary1 ether-ketone)s exhibit many desirable characteristics including exceptional thermo-oxidative and dimensional stability, resistance against radioactive irradiation and excellent mechanical properties. The introduction of crystallinity into a poly(ary1 ether-ketone) backbone results in improving the solvent resistance and modulus.PEEK exhibits a high degree of crystallinity and a melting point (T,) of 335 "C. On the other hand, PEEK suffers from poor creep behaviour above its relatively low glass- transition temperature (T,) of 145 oC.6 Therefore, attempts have been made to either increase the glass-transition tempera- tures of semicrystalline poly(ary1 ether-ketone)s in general or to introduce cross-link sites into the PEEK The amorphous poly(ary1 ether-ketone)s, characterized by back- bones containing sp3 or sp3d2 hybridized atoms (e.g. 2 and 3, respectively) in addition to the ether and carbonyl linkages, are useful for a variety of applications, such as fabrication of foils, films or membranes.Such materials exhibit lower glass- transition temperatures than the analogous amorphous poly(ary1 ether-sulphone)s, 4 and 5.lO-l' In order to achieve higher glass-transition temperatures and/or higher melting points, two carbonyl groups (in contrast to PEEK which contains one such group) have been introduced into the polymer repeat unit structures (e.g. 6a and 6b).12-14 Polymer 6a was synthesized by the reaction of isopropylidenebiphenyl- diol (bisphenol-A) with either 7a or 7b via nucleophilic aro- matic substitution reactions. Electrophilic aromatic substitution reactions have been used also for the synthesis of poly(ary1 ether-ketone)s. For example polymer 8a, prepared via this procedure, exhibits high Tg (170 "C) and T, (381 "C)." Furthermore, according to a recent patent claim, polymer 8b containing two keto substituents and one biphenyl linkage in the polymer repeat unit could also be synthesized by following the same procedure.I6 A wide variety of aromatic polymers containing a biphenyl linkage in the basic repeat unit of the polymer have been synthesized.For example, conventional biphenyl functional poly(ary1 ether-sulphone), synthesized by the reaction of biphenol with 4,4'-dichlorodiphenyI sulphone, exhibits excel- lent thermo-oxidative stability and high toughness2 These desirable properties have been attributed to the presence of the biphenyl linkage. l7 Furthermore, we have recently reported the synthesis and characterization of a series of poly(ary1 ether-bissu1phone)s containing a biphenyl moiety in the repeat unit structures.These polymers exhibit some of the highest known Tgs for poly(ary1 ethers)." In order to achieve some of these improved characteristics with poly(ary1 ether-ketone)s, we have synthesized a number of monomers, 9, suitable for the synthesis of poly(ary1 ether-bisketone)s. These monomers, which contain a biphenyl linkage and two keto groups were prepared from isomeric biphenyldicarbox- ylic acids. In this paper, which is the first of a series, we report the synthesis of bisketo functional monomers (derived from biphenyl-4,4'-dicarboxylic acid), related model compound studies and polymers based upon isopropylidenebiphenyldiol (bisphenol-A), hydroquinone, tert-butyl hydroquinone, biphe- nyl-4,4-diol and 4,4'-dihydroxydiphenyl sulphone (bisphenol- S).The polymers have been characterized by spectroscopic, thermal and thermomechanical means. Experimental Materials Dimethylacetamide (DMAc) (Aldrich) was dried over calcium hydride and then distilled at reduced pressure. Chlorobenzene (Fisher) and aniline (Aldrich) were purified following the same procedure. Diphenyl sulphone (DPS) (Aldrich) was recrys- tallized from acetone. 4,4'-Isopropylidenebiphenyldiol (bisphenol-A), kindly sup- plied by Dow Chemical, was purified by recrystallization from toluene and dried at reduced pressure at 80°C for 24 h. Hydroquinone, biphenol and 4,4'-dihydroxydiphenyl sul-phone (bisphenol-S) (Aldrich) were recrystallized from acetone.J. MATER. CHEM., 1991, VOL. I 1 0I:: II I2 x=c, Y=C; 3 x=s02 Y=C ; 4 x=c, Y=S02 I I 6 7 8a 8b X=CI, F 9 tert-Butylhydroquinone (Aldrich) was crystallized from hex- ane-ether. Molecular sieves (3 A) (Fisher), were dried over- night in an oven at 100 "C. Thionyl chloride was stirred over triphenyl phosphite and distilled at normal pressure prior to use. Anhydrous potassium carbonate (Fisher) was dried over- night in an oven at 100 "C. The required diacid, biphenyl- 4,4'-dicarboxylic acid was purchased from Spectrum. All other reagents were used as received. Biphenyl-4,4'-dicarbonyl DichEoride (10) A 1000 cm3, three-Decked, round-bottomed flask fitted with a condenser, a nitrogen inlet and a mechanical stirrer was charged with 60.5 g (0.25 mol) of biphenyl-4,4'-dicarboxylic acid, 600 cm3 of thionyl chloride (5.0 mol, excess) and 10 cm3 of dimethylformamide (DMF).A nitrogen blanket was main- tained during the course of the reaction. The reaction mixture was heated under reflux for a period of 4h. Upon the formation of the acid chloride, excess thionyl chloride was removed by distillation. The crude product was then crys- tallized (dichloromethane) to afford 10 as a white crystalline solid: m.p. 184-186 "C; v/cm-' 1788, 1723, 1598, 1206, 885, 845; mass spectrum (rule) (relative intensity) 278 (1 5), 243 (loo), 152 (39), 76 (10). 4,4-Bis(p-chlorobenzoyl)biphenyl(11) A 1000 cm', three-necked, round-bottomed flask fitted with a condenser, a nitrogen inlet and a mechanical stirrer was charged with 54.45g (0.25mol) of 10 and 500cm3 of dry chlorobenzene.The reaction vessel was cooled in an ice bath and anhydrous aluminium chloride (52.8 g, 0.40 mol) was added in small increments to the reaction mixture. After the initial exotherm, the ice bath was removed and the mixture was refluxed for 10 h. The deep-orange reaction mixture was then poured into strongly acidified cold water with vigorous stirring. The precipitated solid was collected by filtration at reduced pressure and washed with copious quantities of water J. MATER. CHEM., 1991, VOL. 1 followed by saturated aqueous sodium hydrogencarbonate solution. The crude product was crystallized five times (boiling 1,2-dichlorobenzene) and extracted with dichloromethane to remove residual 1,2-dichlorobenzene to afford 11 as white needles (63% yield): m.p.308.9 "C (DSC); v/cm-' 1645, 1604, 1587, 1290, 856; mass spectrum (m/e) (relative intensity) 432 (66), 43 1 (33), 430 (92), 3 19 (loo), 141 (22), 139 (65), 11 1 (28). (Found: C, 72.52; H, 3.58; C1, 16.40. Calc. for C26H1602C12: C, 72.56; H, 3.72; C1, 16.28%.) 4,4-Bis(p-jluorobenzoyl)biphenyl (12) Compound 12 was prepared by the procedure described for 11 by using 60.0 g (0.216 mol) of 10, 55 g (0.58 mol) of fluorobenzene and 58.1 g (0.44 mol) of anhydrous aluminium chloride. The reaction mixture was refluxed for 18 h. The product was isolated in high yield and purified by recrystalliz- ation (chloroform) to afford white flakes (67% yield): m.p.274.5 "C (DSC); v/cm-' 1641, 1601, 850; mass spectrum (m/e) (relative intensity) 398 (loo), 303 (77), 123 (68), 95 (26); 'H NMR (CDCI,) 6 7.45(m), 7.9(m); I9F NMR (CDC1,) 6 106.1; 13C NMR (CDCI,) 6 115.56 (d, JC-F=21 Hz), 127.3, 130.6, 132.63 (d, JCpF=9 Hz), 133.73 (d, JC-F=4 Hz), 136.9, 143.8, 165.42 (d, JCpF=251 Hz), 194.7. (Found: C, 77.98; H, 3.97; F, 9.53. Calc. for C26H1602F2: C, 78.38; H, 4.02; F, 9.65Yo.) Model Compound Synthesis (13a-d) Model compounds 13a-d were synthesized according to the following general procedure." (Analytical data for the com- pounds are summarized in Table 1.) A three-necked, 100 cm', round-bottomed flask fitted with a nitrogen inlet, a ther-mometer, and a Dean-Stark trap fitted with a condenser was charged with 0.005 mol of 11 or 12,0.01 mol of the desired phenol (Table l), 7.0 g (excess) of anhydrous potassium car- bonate, 35 cm3 of DMAc and 20 cm3 of toluene.The reaction mixture was heated at solvent reflux at 145 "C and water, the byproduct of the reaction, was removed by azeotropic distil- lation. The reaction mixture was heated to 160 "C for 8-22 h and then cooled to room temperature. It was filtered and the filtrate was distilled under reduced pressure to remove all solvents. If possible, the residue was dissolved in appropriate solvent and the solution was washed repeatedly with water, dried over anhydrous magnesium sulphate and the solvent was removed by rotary evaporation at reduced pressure.The crude product was purified by crystallization. General Procedure for Polymer Synthesis A typical synthesis of poly(ary1 ether-bisketone) was conduc- ted in a 500 cm', four-necked, round-bottomed flask equipped with a nitrogen inlet, a thermometer, an overhead stirrer, and a Dean-Stark trap. A detailed synthetic procedure used to prepare 14a is provided. The flask was charged with 5.7g (0.025 mol) of bisphenol-A and 10.7501 g (0.025 mol) of 12 and carefully washed in with 125 cm3 of DMAc. Anhydrous potassium carbonate, 10 g (excess) was added followed by 45 cm3 of toluene. The reaction mixture was then heated until toluene began to reflux at 140°C. Water (byproduct of the reaction) was continuously removed via the Dean-Stark trap.The reflux temperature was maintained for 4-6 h until the accumulation of water was no longer evident in the Dean- Stark trap. The reaction mixture became a light yellow at the initial stage of the reaction, owing to the formation of the phenoxide, and slowly deepened to brown with time. The reaction temperature was gradually raised to 165 "C by remov- ing toluene from the Dean-Stark trap. The reaction mixture was heated at that temperature for a period of 18 h. An additional amount (20cm3) of DMAc was added to reduce the high viscosity of the reaction mixture and the heating at 165 "C was continued for an additional 6 h. In reactions involving less reactive phenoxides or phenoxide of lower solubility, 45 g of DPS was added in place of DMAc.These reactions were carried out at 230 "C for a period of 3-4 h. The viscous reaction mixture was allowed to cool to room temperature, diluted with 100cm3 of DMAc and filtered to remove inorganic salts. The filtrate was acidified with several drops of glacial acetic acid to neutralize the phenoxide end groups and the polymer was precipitated with a 10-fold volume of methanol. The polymer was then dried at reduced pressure at 50 "C for 8-10 h. It was redissolved in the appro- priate solvent, the solution was filtered, acidified with glacial acetic acid, and the polymer coagulated in methanol. The fibrous solid was dried as before. For the attempted one-step synthesis of semicrystalline, hydroquinone functional poly(ary1 ether-bisketone), DPS, instead of DMAc, was used as the reaction medium.The reaction was carried out at elevated temperature (see Results and Discussion) and the polymer was coagulated by pouring the reaction mixture while still hot (160 "C) into a 10-fold volume of acetone. The coagulated polymer was then extracted with acetone, water and acetone, in that order, using a Sohxlet apparatus. The polymer was then dried as before. trans De-tert-butylation of tert-Butylhydroquinone Functional Poly(aryl ether-bisketone) (14c) A 100cm3, round-bottomed flask equipped with a magnetic stirrer and a glass stopper was charged with 0.874g (0.0015 mol) of 14c, 12 cm3 of trifluoromethanesulphonic acid and 30 cm3 of toluene.Note that the latter two reagents were taken in excess. The reaction mixture was stirred vigorously for a period of 18 h. The resulting product, polymer 14e, was isolated by pouring the reaction mixture into a large excess of acetone. The coagulated polymer was isolated by filtration at reduced pressure and thoroughly washed with copious quantity of water to remove residual acid. It was then washed with methanol and was dried under reduced pressure at 50 "C for a period of 8 h. Heterogeneous Hydrolysis of Biphenyl Functional Poly(ary1 ether-bisketimine) (14d) Polymer 14d (1.388 g, 0.002 mol) and a stoichiometric amount of hydrochloric acid (0.004 mol, 37 cm3 of 0.108 mol dmP3 aqueous solution) were added to a glass container inside a Parr High Pressure Reactor. The reactor was assembled and the temperature of the reaction vessel was increased to 300 "C.The hydrolysis was carried out with vigorous stirring. At the end of the hydrolysis period (1.5 and 24 h), the vessel was cooled and the polymeric product was collected by filtration. The polymer was extracted with hot water, washed with methanol and dried under reduced pressure at 50 "C for 8 h. 4,4'-Bis(p-fluoro-a-phenyliminobenzyl)biphenyl(15) A 100 cm3 round-bottomed flask fitted with a condenser was charged with 0.398 g (0.001 mol) of 12, 0.023 g (0.0025 mol) of freshly distilled aniline, 30cm3 of dry chlorobenzene and 20 g of 3 A molecular sieves. The reaction mixture was heated under solvent reflux for 24 h.It was then cooled and filtered and the sieves were washed with dry dichloromethane. The solvent was removed from the filtrate by evaporation at reduced pressure. The crude product was isolated in high yield and was purified by recrystallization (dichloromethane) to afford 15 (yield 52%) as a yellow crystalline solid: m.p. 245- 247 "C; v/cm- ' 1630, 1592, 1494; mass spectrum (m/e) (relative intensity) 549 (40), 548 (loo), 547 (29), 473 (32), 381 (23), 198 (43), 77 (52); 'H NMR (CDCl,) 6 6.7(m), 6.95(m), 7.15(m), 7.45-8.0(m); 19F NMR (CDCl,) 6 110.2, 112.0; 13C NMR (CDCI,) 6 115.17 (d, J=21.6 Hz), 115.23 (d, J=21.6 Hz), W Table 1 Analytical data for model compounds 13a-d elemental analysis compound phenol bishalide yield (YO) m.p.1 "C vlcm -6, m/e (rel.intensity) calc. found 13a 4-tert-but ylphenol 11 or 12 ca. 96 233-235 1647, 1593 31.53,34.51, 658(29), C, 83.95 C, 83.97 1496, 1307 117.19,119.83, 644(37), H, 6.43 H, 6.46 1290 127.01,127.05, 643(IOO), 130.65,131.88, 4 17(24), 132.51,137.73, 3 14(56), 14330,147.84, 253(42) 153.28,162.28, 194.98 13b 4-phen ylphenol 11 or 12 ca. 98 338-340 1643, 1597 699(22), C, 85.96 C, 86.08 1487, 1301 698(50), H, 4.87 H, 5.00 1289 453(20), 349(19), 273(loo), 152(21)1% phenol 12 ca. 98 268-270 1645, 1599 546(42), C, 83.52 C, 83.40 1494, 1307 453(1 I), H, 4.76 H, 4.96 1288. 1263 377(2l), 197(loo), 140(15) 13d 4-h ydroxybenzophenone 12 ca. 96 >360 1645, 1592 755(22), C, 82.74 C, 82.56 1501, 1311 677(1I), H, 4.57 H, 4.62 1265 576(loo), 499(3I), 301(87) J.MATER. CHEM., 1991, VOL. 1 98 1 (1) soc12 (2) DMF reflux. 4 h 10 11; X=CI 12; X=F Scheme 1 120.38, 120.9, 123.3, 123.36, 126.48, 126.64, 126.83, 126.98, ation with chlorobenzene and fluorobenzene, respectively 128.62, 129.79, 129.85, 130.09, 130.16, 131.34, 131.39, 131.48, 131.59, 131.90, 131.94, 135.20, 135.78, 135.81, 135.86, 138.77, 138.92, 140.13, 140.35, 142.35, 142.53, 150.96, 162.44 (d, J= 249 Hz), 164.3 (d, J=251 Hz), 166.52, 166.67. (Found: C, 83.10; H, 4.38; F, 6.94; N, 5.13. Calc. for C38H26N2F2: C, 83.21; H, 4.74; F, 6.93; N, 5.1OYo.) Characterization 'H and 13C NMR spectra were recorded using a General Electric QE-300 instrument. 19F NMR was recorded using an IBM NR 80 instrument and fluorotrichloromethane as an internal standard.IR spectra were obtained with a Nicolet DxB FT-IR spectrophotometer. Glass-transition tempera-tures, taken as the midpoint of the change in slope of the baseline, were measured either with a DuPont DSC 2100 or a Perkin-Elmer DSC-7 at a heating rate of 10 "C min-'. Thermogravimetric analysis (TG) of the polymer samples was conducted with a heating rate of 10 "C min-' in nitrogen. Intrinsic viscosity measurements for the amorphous polymers were determined by using a Cannon-Ubbelohde dilution viscometer and solutions in either N-methylpyrrolidone (NMP) or trichloromethane (CHC13) (25 "C). Dynamic mech- anical behaviour was assessed with a Polymer Laboratories dynamic mechanical thermal analyser (DMTA), bending with a heating rate of 4 "C min-' (1 Hz).Results and Discussion Monomer Synthesis The carbonyl group is known to activate a fluorine atom in para orientation, towards nucleophilic aromatic substitution reactions. For example, 4,4'-difluorobenzophenone is used for the synthesis of PEEK and other poly(ary1 ether-ket~ne)s.~*~ On the other hand, under similar reaction conditions, 4,4'- dichlorobenzophenone is thought to be an unsuitable mono- mer for poly(ary1 ether) ~ynthesis.~ This has been attributed to the low reactivity of a chlorine substituent owing to its larger size and lower electronegativity compared to a fluorine atom. It has been shown that the bischlorides 7a or 7b with two keto substituents, are suitable for polymer synthesis with bisphenol-A and other bisphenols.2.'2 However, this list of bisphenols did not include more acidic compounds such as hydroquinone or 4,4'-dihydroxybenzophenone. Therefore, both bischloro- (1 1) and bisfluoro- (12)substituted compounds were synthesized for the present investigation in order to study the feasibility of using 11 as a suitable, yet less expensive monomer for polymer preparations.The bishalides 11 and 12 were prepared starting from biphenyl-4,4-dicarboxylic acid, followed by acid chloride formation and Friedel-Crafts acyl-(Scheme 1). Compound 11 was insoluble in all common low- boiling solvents. It could only be purified by recrystallization from boiling 1,2-dichlorobenzene. In order to obtain monomer grade material, the crude product was recrystallized at least five times.The purity of 11, which was of critical importance, was ascertained by DSC after each recrystallization. The DSC thermogram of 11 after the fifth crystallization is shown in Fig. 1. On the other hand, purification of 12 was relatively simpler. It could be recrystallized from a large volume of trichloromethane to afford monomer grade material. The DSC thermogram of high purity 12 is also shown in Fig. 1. The insolubility of 11, and the highly cumbersome nature of its purification, allowed for the synthesis of the compound in small quantity for model compound studies only. The large- scale preparation of 11 required for polymer synthesis was not undertaken.Model Compound Studies Model compound studies were carried out by treating phenol or substituted phenols with 11 or 12 in DMAc in the presence of excess potassium carbonate (Scheme 2). An analysis of the data in Table 1 indicates that the reaction of p-tert-butylphe- no1 or 4-phenylphenol with either 11 or 12 results in the desired product in essentially quantitative yield. During the reaction with 4-phenylphenol, the resulting product precipi- tates from the reaction mixture. These observations suggest the following. First, it is possible to use the bischloride, 11, or the bisfluoride, 12, to synthesize high-molecular-weight amorphous polymer with bisphenol-A. Secondly, the polymer resulting from 4,4'-dihydroxybiphenyl and either of the bishal- ides would be semicrystalline in nature and a high-boiling I 1 I I 1 I I 1 I 150 200 250 300 350 T/"C Fig.1 DSC thermograms of (a)4,4-bis(p-chlorobenzoyl)biphenyl,and (b)4,4'-bis(p-fluorobenzoy1)biphenylmonomers J. MATER. CHEM., 1991, VOL. 1 DMAc. K2CO3 toluene, 165 "C,8 -10h -H20.-KX 13 ad X Z I 13a IH 13c Scheme 2 solvent such as DPS would be required for the synthesis of the polymer. The reaction of phenol with 12 was also quanti- tative and the resulting product precipitated from the reaction mixture at 165 "C. However, the replacement of the chloro groups of 11 by phenoxide derived from phenol was not satisfactory. Only a 50% yield of the desired compound could be realized. This difference in reactivity of the phenoxide anion as compared with the anions derived from p-tert- butylphenol and 4-phenylphenol towards chlorine displace- ment, may be attributed to the relative nucleophilicities of the anions under consideration.l9 The required set of data needed to support this contention is not available. However, it is possible to correlate the order of basicity of the phenoxide anions with their relative nucleophilicity. That this can be done, is due to the relatively small size of the oxygen anionic centre and also due to the fact that the attacking atom is the same (oxygen) in all three cases under consideration.20 A comparison of the relative acidity con-stants for phenol, 4-phenylphenol and p-tert-butylphenol reveals the following order: 4-phenylphenol -p-tert-butylphenol <phenol.21 It therefore follows that the order of the conjugate base strength and nucleophilicity is Ph-0-<4-Ph-Ph-O--p-(CH3)3C-Ph-O-.Thus, it is not surprising that the anion derived from phenol cannot replace the less reactive chlorine atoms of 11, whereas the more reactive nucleophiles can. This conclusion was further supported by carrying out the reaction of 4-hydroxybenzo- phenone (a stronger acid than phenol)21 with 11 and 12. The reaction was quantitative with the more reactive bisfluoride (12) only (Table 1). In addition to elemental analysis, the structures of the model compounds were verified by mass spectrometry. 13C NMR analysis lent further support for the structure determi- nation of 13a.The observed I3C absorbances (Table 1) are in close agreement with the calculated values.22 The model compound reaction corresponding to the polymer derived from 4,4'-dihydroxydiphenyl sulphone and 12 was not carried out because the required phenol, 4-hydroxyphenyl sulphone, was not readily available. Synthesis and DSC Analysis of Amorphous Poly(ary1 ether- bis ke tone)s Polymerization of the bishalide, 12, with either bisphenol-A or bisphenol-S could be readily carried out in presence of excess potassium carbonate in a DMAc-toluene (2 :1) solvent mixture (Scheme 3). In both reactions, a polymerization tem- perature of 165 "C was sufficient to synthesize high-molecular- weight poly(ary1 ether-bisketone)s.However, for the polymer- ization reaction of bisphenol-S with 12, it took significantly longer time (18 h) to attain a sufficient rise in viscosity compared to the 8 h necessary for the reaction of bisphenol- A with the same bishalide. Since the sulphone moiety is strongly electron withdrawing, the resulting bisphenoxide from bisphenol-S can be regarded as a weak nucleophile in contrast to the bisphenoxide from bisphenol-A. Similar obser- vations have been made for the reactions of the phenoxide from bisphenol-S and a variety of activated halides2.l7 The solubility behaviour of polymers derived from bisphenol-A, 14a, and from bisphenol-S, 14b, was remarkably different from that of the monoketone counterparts, 2 and 3, respectively.For example, poly(ary1 ether-bisketone), 14a, is insoluble in tetrahydrofuran and chlorinated hydrocarbons except for trichloromethane. It is only soluble in dipolar aprotic solvents such as DMAc (Table 2). On the other hand, the corresponding monoketone analogue, poly(ary1 ether-ketone) 2, is soluble in all aforementioned solvents at room temperature. Similarly, the bisphenol-S functional poly(ary1 ether-bisketone), 14b, is insoluble in all common solvents except for N-methylpyrroli- done (NMP) and hot DMAc. Once again, this solubility behaviour is in sharp contrast to that of the monoketone analogue 3 which is soluble in the solvents listed in Table 2 at room temperature. The reason for the difference in solubility behaviour may be attributed to the presence of the biphenyl and two ketone moieties in the repeat unit structures of polymers 14a and 14b.The intrinsic viscosity value for 14a in trichloromethane at 25 "C (q=60 cm3 g- I) and for 14b in NMP at 25 "C (q=60 cm3 g-') (Table 3), suggest a high to moderate molecular weight for these polymers. The molecular structure of the polymers was confirmed by both 13C NMR and FTIR. The 13C NMR spectrum of polymer 14a is shown in Fig. 2. The observed peak positions were in agreement with calculated chemical shifts.22 Further- more, owing to the high molecular weight of the polymers, additional absorbances due to the terminal units were not observed. The FTIR spectra (KBR) afthe polymers established the presence of the ether and keto linkages (absorbances at 1250 and 1655 cm- ', respectively) in the repeat-unit structures.The glass-transition temperatures of amorphous poly(ary1 J. MATER. CHEM., 1991, VOL. I HO-Ar-OH + K2C03, DMAC or DPS toluene, 10-12 h, A -H20, -KFI Y 14C 14d Scheme 3 Table 2 Solubility" behaviour of poly(ary1 ether-bisketone) 14a-f and poly(ary1 ether-bisketimine) 14d solvents polymers methylene chloride trichloromethane tetrahydrofuran dimet h y lacetamide N-methylpyrolidone 14a S 14b S* 14c S 14d S 14e 1 14f i 10% mjv. s =soluble at r.t.; s* =soluble when heated; i =insoluble. Table 3 Intrinsic viscosity data and glass-transition temperatures of amorphous poly(ary1 ether-bisketone) and poly(ary1 ether-bisketimine) 14d po1ymer 14a 14b 14C 14d intrinsic viscosity values glass-transition temperatures (/ "C) (DSC) in chloroform at 25 "C/cm3 g-heating rate 10 "C/min 60 60" 154 99 Solvent =NMP.ether-bisketone)s 14a and 14b were determined by DSC. As expected, polymer 14b exhibited a higher Tg (219 "C) in the second heating than 14a (q=186 "C),which contains a rela- tively less polar isopropylidene linkage (Table 3). This is due to the presence of a sulphone moiety in the polymer repeat unit. The DSC scan from the first heating was not conclusive. Furthermore, these <s observed for 14a and 14b are signifi- cantly higher than the reported values for the analogous poly(ary1 ether-ketone)s, containing only one keto group in the repeat unit structures.2 For example, bisphenol-A func- tional polymer 2 exhibits a Tgat 150 "C, which is ca.36 "C lower than the Tg of 14a and the q of 14b, bisphenol-S functional poly(ary1 ether-bisketone), is ca. 20 "Chigher than that of 3, which contains only one keto group in the repeat unit structure. Finally, these high qvalues for poly(ary1 ether- 186 219 206 21 1 bisketone)s can be attributed to the presence of the biphenyl linkage and the two keto functionalities in the repeat unit structures of the polymers. Synthesis and DSC Analysis of Semicrystalline Poly(ary1 ether- bisketone)s From the solubility behaviour of 13c, and from the reported insolubility of PEEK, 1,3,23in all common solvents at room temperature, except for strong protic acids, the poly(ary1 ether-bisketone) 14e, derived from hydroquinone and suitable bishalide, 12, was expected to be semicrystalline in nature with a high melting point.Therefore, a direct, one-step syn- thesis of 14e from hydroquinone and 12 was attempted in DPS, in the presence of excess potassium carbonate. The 1'"'"''1'''~1"''I 200 150 100 50 0 6 (PPm) Fig. 2 13C NMR spectrum of poly(ary1 ether-bisketone) in CDC1, based on 14a reaction was conducted at 250 "C for 5 h and at 320 "C for an additional 3 h, following a reported procedure for PEEK synthe~is.~During the synthesis, a noticeable rise in solution viscosity was not observed even after holding the reaction mixture at 320 "C.In addition, the reaction mixture was heterogeneous in nature during the entire course of the attempted synthesis. An increase in the reaction temperature to 330°C did not change the heterogeneous nature of the reaction mixture significantly. This is in sharp contrast to what is observed during PEEK synthesis by the same pro- cedure. In the latter case, an initial heterogeneous reaction mixture becomes fairly homogeneous at 320°C as the tem- perature approaches the melting point of PEEK (335 "C). These observations suggested that the resulting oligomers probably have higher melting temperatures than PEEK and continue to remain insoluble even at 320 "C. Furthermore, the precipitated oligomeric material was highly powdery in nature, indicative of the low molecular weight.As expected, it was insoluble in all common solvents except for strong protic acids such as concentrated sulphuric acid and methane- sulphonic acid. The low molecular weight of the polymer was verified by intrinsic viscosity measurements in concentrated sulphuric acid at 25 "C (q=28 cm3 g-'). Sulphonation of the polymer backbone during such measurements resulting in higher than actual qinh value has been The DSC thermogram of the oligomer showed a melting endotherm at 425 "C (80 J g-') in the first heating. The sample was quench cooled and reheated; a Tg at 281 "C, a crystallization exotherm at 351 "C (-3.6 J g-') and a melting endotherm at 404 "C (41.5J g-') were observed.These observations suggest that the direct one-step synthesis of 14e would not be possible unless a solvent with a higher boiling point than DPS is utilized with a boiling point approaching close to 400 "C. This led us to consider a modified methodology for the synthesis of high-molecular-weight poly(ary1 ether-bisketone) 14d. The procedure made use of the previously reported strategy of introducing a bulky substituent to synthesize an amorphous high-molecular-weight precursor followed by sub- sequent removal of the substituent to afford the desired high- molecular-weight semicrystalline polymer. For example, the synthesis of amorphous, tert-butylhydroquinone functional poly(ary1 ether-ketone) of high molecular weight by the reaction of tert-butylhydroquinone and 4,4'-difluorobenzo- phenone has been rep~rted.'~'~~ This was followed by the partial or complete removal of the bulky tert-butyl groups by J.MATER. CHEM., 1991, VOL. 1 treatment with anhydrous aluminium chloride25 or trifluor- omethanesulphonic acid,26 respectively, to yield semicrystal- line PEEK of high molecular weight. This strategy worked remarkably well in this case also. Polymerization of tsrt-butylhydroquinone and the bishalide 12 was conducted in DMAc-toluene (2: 1) in the presence of excess anhydrous potassium carbonate. Very-high-molecular-weight polymer 14c could be synthesized by carrying out the polymerization reaction at 160 "C for a period of 8 h (Scheme 3). The polymer exhibited unlimited solubility in dichloromethane, tetrahydro- furan, trichloromethane and DMAc at room temperature (Table 2).The high-molecular-weight nature of the polymer was assessed by intrinsic viscosity measurements in trichloro- methane at 25 "C (q= 154 cm3 g- I) (Table 3). Fingernail creas- able films of 14c could be obtained upon solvent casting or compression moulding. The poly(ary1 ether-bisketone) 14c exhibited a well defined glass-transition temperature of 206 "C. This value is ca. 30°C higher than that observed for corre-sponding amorphous poly(ary1 ether-ketone), prepared from tert-butylhydroquinone and 4,4'-difl~orobenzophenone.~~~~~ Once again, the higher value can be attributed to the presence of a rigid biphenyl linkage and two keto groups in the repeat unit of poly(ary1 ether-bisketone) 14c.The linear nature and the structure of the repeat unit of 14c was verified by solution (CDC1,) 13C NMR analysis. As in the case of 14a, the calculated and the observed positions of the absorbances were in close agreement.22 The quarternary and the primary carbon atoms of the tert-butyl group exhibited two absorbances at 35.03 and at 30.05 ppm, respectively. Finally, a singlet at 1.38 ppm due to the tert-butyl group in the 'H NMR (CDC1,) spectrum and a peak at 1651 cm-' due to the keto group and at 1233 cm-I due to the ether linkage in the IR spectrum (film), confirmed the structure of 14c. trans De-tert-butylation 14c was carriea out in trifluorome- thanesulphonic acid in the presence of toluene according to a previously reported procedure.26 Trifluoromethanesul- phonic acid acts as the Friedel-Crafts catalyst for the removal of the tert-butyl group as a carbocation, which is subsequently trapped by the aromatic coreagent, toluene.In addition, trifluoromethanesulphonic acid also acts as the solvent for the resulting semicrystalline polymer. The reaction was remarkably effective in completely removing the tert-butyl substituent from polymer 14c (Scheme 4). The toluene layer was analysed by 'H NMR spectroscopy to detect and measure the isomeric ratio of resulting m-and p-tert-butyltoluene. This was done by measuring the intensities of the singlets at 1.25 and at 1.26 ppm due to the tert-butyl groups of meta and para isomers, respectively.The ratio of the isomeric byproduct was determined to be 80:20. Evidence for the complete removal of the tert-butyl group from 14c, came from the 'H NMR of the resulting semicrystalline polymer, 14e, in trifluo- romethanesulphonic acid. Deuterium oxide was used as the internal lock. No residual alkyl groups could be detected in the spectrum. As expected, the resulting semicrystalline polymer was insoluble in all common solvents. It was only soluble in strong protic solvents. The high molecular weight of 14e was deter- mined by intrinsic viscosity measurements in concentrated sulphuric acid at 25 "C (q = 190 cm3 g- '). This is significantly higher than that (q=28 cm3 g-') obtained for the oligomer prepared from the attempted one-step synthesis of 14e.The DSC thermogram of 14e contains a q at 184 "C, a crystallization exotherm (-6.4 J g-') at 206 "C (T,) and a melting point endotherm (27.2 J g-') at 390.5 "C (T,).The q and T, values are ca. 40 and 60 "C, respectively, higher than those reported for PEEK., Upon quench cooling and reheating, polymer 14e exhibits a well defined q at 192 "C, a T, at 285 "C (-8.8 J g-') and a T, at 372 "C (5.2 J g-'). J. MATER. CHEM., 1991, VOL. 1 14c trifluromethanesulphonicacid toluene, 25 "C,18 hI 14e + 0.H. meta :para = 20:80t Scheme 4 From our observations related to hydroquinone functional poly(ary1 ether-bisketone)s 14e, and from the solubility behav- iour of model compound 13b, it was expected that the polymer 14f derived from biphenol and the bisketo functional bishal- ides 11 or 12 would be semicrystalline in nature and would exhibit a higher melting point than 14e.This expectation was consistent with the fact that the melting point of conventional biphenol functional poly(ary1 ether-ketone) derived from biphenol and 4,4'-difluorobenzophenone is ca. 80 "C higher than that of the corresponding hydroquinone functional poly- mer.3,27 Therefore, the direct one-step synthesis of 14f essen-tially would be an impossible task. We have recently reported both homogeneous and controlled heterogeneous hydrolysis of poly(ary1 ether-ketimine)s as alternative routes for the synthesis of high-molecular-weight, semicrystalline poly(ary1 ether-ketone)~.~',~~The latter procedure, which involves the synthesis of amorphous poly(ary1 ether-ketimine) prepolymer followed by hydrolysis in water with a stoichiometric amount of hydrochloric acid to generate poly(ary1 ether-ketone) of semicrystalline morphology, was utilized in the present instance. The required bisketimine functional difluoride mono- mer 15 was synthesized by the reaction of 12 with aniline in the presence of molecular sieves in satisfactory yield (Scheme 5).29 Data from elemental analysis was consistent with the desired structure of the monomer.In addition, the 13C NMR (CDC13) of the monomer revealed two peaks at 166.52 and 166.67 ppm due to the imine groups. This is due to the fact that the two N-substituted rings (ring B) can remain in cis and trans configuration with respect to the fluorine substituted rings (ring A).This can give rise to three possible combinations, namely, cis-cis, trans-trans and cis-trans isomers, resulting in two absorbances due to the imine carbon atom in the 13C NMR of 15. The presence of two absorbances due to the fluorine atoms in the I9F NMR of 15 corroborated the findings from the 13C NMR analysis. Poly- merization of biphenol with 15 was carried out in the presence of potassium carbonate (excess) in DMAc-toluene (2 : 1) using a standard procedure (Scheme 3).30 Owing to the low solu- bility of the bisphenoxide derived from biphenol, an increase in solution viscosity was not observed even after prolonged heating at 165 "C.High-molecular-weight polymer 14d could be synthesized, however, by adding diphenyl sulphone to the reaction mixture and conducting the reaction at 230 "C for a period of 3 h (Scheme 3). The polymer was precipitated from the resulting viscous solution by pouring the mixture into acetone while hot. The polymer was collected by filtration, and extracted with acetone, water and acetone, in that order. It was dried at a reduced pressure at 50 "C for 8 h. The coagulated polymer was fibrous and light-yellow. Intrinsic viscosity measurements (99 cm3g-' in CHC13 at 25 "C) indi- cated the high molecular weight of the polymer. Polymer 14d exhibited solubility behaviour typical of amorphous poly(ary1 2.2 eq 12 chlorobenzene 3A sieves A, 10 hI N0N-15 Scheme 5 ether)s.It was soluble in dipolar aprotic solvents such as dimethyl sulphoxide (DMSO) and DMAc, and in chlorinated hydrocarbons at room temperature (Table 2). Upon solution casting or compression moulding, fingernail creasable films could be obtained. The fact that the imine linkages were not hydrolysed to keto groups under basic reaction conditions2' during the polymer synthesis was verified by solution 13C NMR (CDCl,) of the polymer. Absence of peaks due to aromatic keto carbonyl around 190 ppm and the presence of two absorbances at 167.42 and 167.55 ppm due to the imine linkages confirmed this. The presence of the imine group in the polymer backbone was further confirmed by the IR analysis (film) of the polymer.The IR spectrum of 14d (Fig. 3) exhibits a peak as a weak shoulder at 1620 cm- due to the imine linkages. DSC analysis of the polymer revealed a well defined T, at 211 "C. A melting-point endotherm was not 1620 -1592-u c=c L 1 I I 1784 1675 1565 1455 wavenumber/cm-' Fig. 3 IR spectrum (film) of poly(ary1 ether-bisketimine) 14d J. MATER. CHEM., 1991, VOL. 1 observed. These observations further confirmed the amorph- ous nature of 14d. Hydrolysis of the ketimine group was accomplished by heating a slurry of the polymer in water in a Parr reactor containing a stoichiometric amount of hydro- chloric acid (Scheme 6). This procedure was remarkably effec- tive despite its heterogeneous nature. Aniline hydrochloride, the water-soluble byproduct of the reaction, could be removed easily by simple filtration.Any last traces of the residual salt could be removed by extracting the hydrolysed polymer with water in a Soxhlet apparatus for 24 h. Cleavage of the ketimine group was conveniently estimated by elemental analysis for nitrogen. From earlier findings temperature has been identified as the most critical factor for efficient hydrolysis of the ketimine functional polymers.27 Hydrolysis was very slow at or below the Tp of the polymer. For complete hydrolysis in a reasonable amount of time, it was necessary to raise the hydrolysis temperature at least 70 "C above the Tp of the polymer. Accordingly, polymer 14d was subjected to hetero- geneous hydrolysis at 300 "C [90 "C above the Tp (211 "C) of 14dl for 1.5 and 24 h.The resulting polymers were insoluble in all common solvents at room temperature and upon warming (Table 2). The thermal behaviours of the polymers were identical also. In addition, elemental analysis revealed the absence of nitrogen in both instances. This established the fact that the quantitative hydrolysis could be achieved in 1.5 h. The DSC thermogram of the polymer derived from 1.5 h hydrolysis (14f)reveals a Tgat 219 "C in the first heating. This is essentially identical to the value for the precursor ketimine functional polymer. The sample also exhibited a small melting endotherm at 353 "C (1.9 J g-') in addition to a larger endotherm at 469 "C (40.7 J g-') in the first heat.Similar multiple melting transitions have also been reported for PEEK.31 After it was quench cooled and reheated, the same sample exhibited a well defined T at 219 "C, a crystalliz- ation exotherm at 375 "C (-12.9 J g-P) and a melting endo- therm at 459 "C (14.8 J g-I). The thermal behaviour of 14f demonstrates that it is possible to synthesize high-melting- point semicrystalline poly(ary1 ether-bisketone). On the other hand, the observed melting point is very close to the tempera- ture required for the onset of decomposition (Table 4). This may limit the practical utility of 14f. Nevertheless, it will be possible to introduce a suitable comonomer, such as 4,4- dichlorodiphenyl sulphone so as to lower the melting point 14d 300 "C, HCI.H20. 1.5 h 14f Scheme 6 J. MATER. CHEM., 1991, VOL. 1 while retaining a reasonable degree of crystallinity in the 0.08 r polymer. DMTA and Thermogravimetric Analysis of Poly(ary1 ether-bisketone)s and Poly(ary1 ether-bisketimine) 14d The dynamic mechanical thermal analysis of these polymers (14a-d) corroborated the data obtained from DSC measure-ments. The dynamic mechanical behaviour for the polymers is shown in Fig.4. These data clearly show the high Tg and good dimensional stability exhibited by these materials and also indicate a glassy morphology for this polymeric system (14a-d). In addition, a major low-temperature secondary relaxation (p transition) in the vicinity of -100 "C can also be seen in all cases.Fig. 5 illustrates one such typical transition for 14a. Such transitions have been investigated by dynamic me~hanical~~-~~ The observation ofand NMR techniq~es.~' h ____22_2_ a- I1 > !.u- 8.2- I\ I1 rn --00 II11 -120 -70 -20 30 80 130 180 230 TI"C t I n 341 0.121 i I/ -120 -70 -20 30 80 130 180 230 T/"C Fig. 4 (a)Storage modulus (bending) versus temperature. (6) tan 6 versus temperature for various amorphous poly(ary1ether-bisketone)s and poly(ary1 ether-bisketimine) 14d 0.05 DQ -5 0.04 c 0.03 --0.02 -0.01 -1 50 -1 00 -50 T/"C Fig. 5 tan 6 versus temperature for 14a a p relaxation is believed to be associated with polymers that ran pvhihit diirtile defnrmatinn 32 In spite of the high Tg of these materials, the amorphous poly(ary1 ether-bisketone)s and the poly(ary1ether-bisketim-inpi\ *A1 r* r >I 1_1* * iun rgn 1np TnPrmnTnrmPn nv rnmnrpccinn mniiininu in.LAW,, "Y) "Ic~1--II-vIv'-II"u -J """jf'-"""" lllVllllllb ---contrast to other conventional high-performance rigid-rod polymers.Samples were moulded at ca. 70 "C above the glass-transition temperature. That this can be done is reflective of the excellent thermal stability of these materials. The semicrys-talline poly(ary1 ether-bisketone)s, 14e and 14f, were not compression moulded because of the practical high-tempera-ture limitation of the press. The thermal stability of the polymers 14a-f was further affirmed by thermogravimetric analysis in both the isothermal and the variable-temperature modes.The variable-temperature thermograms of the poly-mers are shown in Fig. 6. From examination of the figure it is apparent that the poly(ary1 ether-bisketone)s demonstrate 110 80 60 50 50 250 450 650 E T/"C Fig. 6 TG thermograms (weight loss uersus temperature) for various poly(ary1 ether-bisketone)s and poly(ary1 ether-bisketimine) 14d Table 4 Thermogravimetric analysis (nitrogen) of poly(ary1 ether-bisketone) and poly(ary1 ether-bisketimine) 14 polymer wt. loss on isothermal ageing at 400 "C (wt.% h-') polymer onset of decomposition temperature/ "C %residue at 750 "C 14a 0.86 512 64.63 14b 1.80 510 56.60 14C 0.96 506 53.21 14d 0.52 49 1 68.74 14e 0.76 516 51.68 14f 0.26 514 64.09 J.MATER. CHEM., 1991, VOL. 1 very good thermal stability with polymer decomposition temperature in excess of 500 "C, except for the poly(ary1 ether- bisketimine) 14d, which exhibits an onset of decomposition temperature at 491 "C (Table4). This is presumably because of the presence of the imine (-C=N-) groups. Among the poly(ary1 ether-bisketone)s, polymer 14c with the tert-butyl substituent is of slightly lower stability than the other poly- mers. From an examination of the data in Table 4, it is clear that the polymers 14a-f decompose with a significant amount (> 50%) of char yield at 750 "C. This would suggest a high level of flame-retardant characteristic^.^^ Isothermal TG (400 "C, 1 h, under a nitrogen atmosphere) was also used to assess the thermal stability of the polymers (Table4).The data consistent with the variable TG scans, demonstrate that the poly(ary1 ether-bisketone)s and the poly(ary1 ether- bisketimine), 14d, are materials of exceptional thermal stab- ility. In all cases, >I% weight loss was observed, except for the bisphenol-S functional poly(ary1 ether-bisketone) 14b. The polymer exhibits a 1.8% weight loss, possibly due to its highly polar nature, resulting in higher moisture uptake under ambi- ent conditions. Conclusions A series of high-molecular-weight poly(ary1 ether-bisketone)s have been synthesized by the reaction of 4,4'-bis(p-fluoroben- zoy1)biphenyl with suitable bisphenols.Model compound studies indicated that both 4,4'-bis( p-fluorobenzoy1)biphenyl and 4,4'-bis(p-chlorobenzoyl)biphenyl were suitable mono-mers for polymer synthesis with bisphenol-A and 4,4'-dihydroxybiphenyl. For more acidic phenols such as hydro- quinone and 4,4'-dihydroxybenzophenone, 4,4'-bis(p-fluoro- benzoy1)biphenyl as the monomer would be required for the synthesis of high-molecular-weight polymers. The amorphous poly(ary1 ether-bisketone)s exhibited high glass-transition temperatures; some of the highest Ts known for the poly(ary1 ether-ketone) family of macromolecules. In addition, these materials showed improved solvent resistance over that observed for the monoketone analogues. The semicrystalline poly(ary1 ether-bisketone)s were synthesized from amorphous high-molecular-weight polymeric precursors by post-removal of bulky substituents from the polymer backbone.Semicrystal- line hydroquinone functional polymer was synthesized by the removal of the bulky tert-butyl substituent, whereas the biphenol functional polymer was synthesized by the hetero- geneous hydrolysis of amorphous poly(ary1 ether-bisketimine) precursor. Both polymers showed very high melting points; significantly higher than the commercially important PEEK. In addition, all synthesized poly(ary1 ether-bisketone)s possess excellent thermal stability, as evidenced from both variable- temperature and isothermal thermogravimetric analysis. Fur- thermore, the polymers afford tough films upon compression moulding.The polymers display show a major secondary relaxation similar to that observed for other ductile engineer- ing thermoplastics. The authors wish to acknowledge partial support under PRF Grant #2 I 166-AC7, administered by the American Chemical Society, Grant #628 1 I made available to DKM by the Depart- ment of Chemistry and the Michigan Polymer Consortium, Grant #42368, from the FRCE committee, Central Michigan University, and the Research Excellence Fund from the State of Michigan. Partial support for the purchase of the GE QE- 300 NMR Spectrometer used in this work was provided by NSF/I LI grant #USE-8852049. References 1 C. P. Smith, Chemtech, 1988, 290. 2 R. N. Johnson, A. G. Farnham, R. A. Callendinning, W. F.Hale and C. N. J. Merian, J. Polym. Sci., Polym. Chem. Ed., 1967, 5, 2375. 3 T. E. Atwood, D. A. Barr, T. King, A. B. Newton and J. B. Rose, Polymer, 1977, 18, 359. 4 T. E. Atwood, P. C. Dawson, J. L. Freeman, L. R. J. Hoy, J. B. Rose and P. A. Stanliland, Polymer, 1981, 22, 1096. 5 P. C. Dawson and D. J. Bundell, Polymer, 1980, 21, 577. 6 R. Rigby, Adv. Polym. Technol., 1982, 2, 163. 7 C. M. Chan and S. Venkatraman, Polym. Mater. Sci. Eng., 1986, 54, 37. 8 D. K. Mohanty, S. D. Wu and J. E. McGrath, Polym. Prep. Am Chem. SOC., 1988, 29, 1, 352. 9 S. D. Wu, J. L. Hedrick, D. K. Mohanty, B. Carter, G. L. Wilkes and J. E. McGrath, Znt. SAMPE Symp., 1986, 31, 933. 10 D. K. Mohanty and J. E. McGrath, in Adv. Polym. Synth., ed. B.M. Cullbertson and J. E. McGrath, Plenum Press, New York- London, 1985, 31, p. 113. 11 J. L. Hedrick, D. K. Mohanty, B. C. Johnson, R. Viswanathan, J. A. Hinkley and J. E. McGrath, J. Polym. Sci., Polym. Chem. Ed., 1986, 24, 287. 12 P. M. Hergenrother and B. J. Jensen, Polym. Prep., 1985, 21, 2283. 13 P. M. Hergenrother, N. T. Wakelyn and S. J. Havens, J. Polym. Sci., Polym. Chem. Ed., 1987, 25, 1093. Hans R. Kricheldorf, Ulrich Delius and Kai Uwe Tonnes, New Polym. Mater., 1988, 1, 127. M. Winkler, P. Itteman and G. Heinz, Ger. Offen., DE 3738749, 1989, BASF-AF. V. Janson, S. Moore and J. B. Mazzanti, Eur. Pat. Appl., Ep 298771, 1989, Raychem Corp. 1. Colon and G. T. Kwiatkowski, J. Polym. Sci., Polym. Chem. Ed., 1990, 28, 367.D. C. Cummings, R. S. Mani, P. B. Balanda, B. A. Howell and D. K. Mohanty, J. Macromol. Sci., Polym. Chem. Ed., 1991, 28, 793. 19 J. F. Bunnet, Annu. Rev. Phys. Chem., 1963, 14, 271. 20 R. G. Pearson and J. Songstad, J. Am. Chem. SOC., 1967, 89, 1827; R. G. Pearson, H. Sobel and J. Songstad, J. Am. Chem. Soc., 1968, 90, 319; P. L. Block and G. M. Whiteside, J. Am. Chem. Soc., 1974, 96, 2826. 21 M. Fujio, R. T. McIver Jr. and R. W. Taft, J. Am. Chem. SOC., 1981, 103, 4017. 22 G. C. Levy, R. L. Lichter and G. L. Nelson, in Carbon-13 Nuclear Magnetic Spectroscopy, John Wiley, New York, 2nd edn., 1980, p. 111. 23 T. C. Stening, C. P. Smith and P. Kimber, J. Mod. Plast., 1981, 11, 86. 24 J. B. Rose, US Pat., 4, 419, 486, 1983. 25 D. K. Mohanty, T. S. Lin, T. C. Ward and J. E. McGrath, Znt. SAMPE Symp. Exp., 1986, 31, 945. 26 W. Risse and D. Y. Sogah, Macromolecules, 1990, 23, 4029. 27 B. E. Lindfors, R. S. Mani, J. E. McGrath and D. K. Mohanty, Macromol. Chem., Rapid Commun., 1991, 12, 337. 28 D. K. Mohanty, R. C. Lowery, G. D. Lyle and J. E. McGrath, Int. SAMPE Symp. Exp, 1987, 32, 408. 29 S. Patai, in The Chemistry of Carbon-Nitrogen Double Bond, Interscience, New York, 1970, p. 64. 30 D. K. Mohanty, J. S. Senger, C. D. Smith and J. E. McGrath, SAMPE Symp. Exp., 1988,33,970. 31 D. J. Bundell and B. N. Osborn, Polymer, 1983, 24, 953. 32 L. M. Robeson, A. G. Farnham and J. E. McGrath, in Molecular Basis of Transitions and Relaxation, ed. D. J. Meier, Gordon and Breach, New York, 1978, p. 405. 33 G. Allen, J. McAinsh and G. M. Jetts, Polymer, 1971, 18, 85. 34 J. Heigboer, Br. Polym. J., 1969, 1, 3. 35 J. J. Dumais, A. L. Cholli, L. W. Jelenski, J. L. Hedrick and J. E. McGrath, Macromolecules, 1986, 19, 1884. 36 D. W. vanKrevelen, Polymer, 1975, 16, 516. Paper 1102307A; Received 16th May, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100977

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, l(6) 989-996 Correlation of Surface Acidity with Electrical Conductivity of Silica-supported Heteropoly Compounds studied by the Complex- impedance Method Naoto Azuma,* Reiji Ohtsuka, Yoshio Morioka, Hiroko Kosugi and Jun-ichi Kobayashi Department of Applied Chemistry and Materials Technology, Faculty of Engineering, Skizuoka University, Hamamatsu 432, Japan A.c. electrical measurements and impedance analysis have been carried out to characterize silica-supported heteropoly compounds (HPC), viz. 12-molybdophosphoric acid (H,PMo,,O,), 12-tungstophosphoric acid (H,PW,,O,,) and their sodium, potassium, and caesium salts under various conditions of humidity. The absorption of water was found to enhance the electrical conductivity of silica-supported HPC.In H,PMo,,O,, and its salts supported on silica, complex-impedance plots showed one semicircular arc due to adsorbed water. For H,PW,,O, and its salts supported on silica, the plots showed two arcs; these are ascribed to fast and slow relaxation processes of orientation polarization for conductive species, supposedly protons in occluded and adsorbed water, respectively. As the relative humidity was increased, the conductivity increased logarithmically, showing humidity-sensing characteristics. The conduction behaviour was affected by the loaded amount of HPC and the substitution of cations in HPC. The activation energy for electrical conduction of low-loaded heteropoly acid (HPA) on silica was lower than that for bulk HPA. It changed both with the amount of loaded HPA and with the substitution for hydrogen ions in the loaded HPA.The acidity function, H,, of the silica-supported HPC has been examined also in connection with their electrical behaviour. The electrical conductivity increased with increasing acidity. A linear relationship was found between the electrical conductivity and the acidity function. Keywords: Supported heteropoly compound; Acidity; Ionic conductivity; Complex impedance spectroscopy Inorganic ion-exchangers are used as solid electrolytes in many electrical devices, such as chemical sensors and fuel cells. The electrical conductivity of the ion-exchanger is depen- dent both on the concentrations of ionic and electronic conductors involved and on their mobilities. Dissociative ions, such as protons, play an important role in the electrical behaviour, and these are affected by the surface properties and structure. In order to clarify the electricai properties and develop applications to electrical devices, it is very important to elucidate the interactions between the conductive species and the material.Previous studies of chemical sensors based on ion-exchangers, however, have been concerned with the charac- terization of the conduction mechanism, the sensitivity, and the level of electrical resistance. They have been performed mainly with unmodified bulk materials. However, insufficient information is available on surface properties and structures to understand the electrical behaviour.The relationship between electrical conduction and surface states has been studied in only a few cases, and so the dependence of electrical conduction on surface acidity or structure in silica-supported heteropoly acids (HPAs), e.g. 12-molybdophosphoric acid (H3PMo12040)and 12-tungstophosphoric acid (H3PW12040), and their salts (heteropoly compounds; HPC) will be reported here. In recent years, HPCs, have received considerable attention in the hydration, dehydration, and dehydrogenation of organic compounds' -6 since SOH10 catalysts, which were developed for the synthesis of acrylonitrile by Standard Oil of Ohio, had been applied to the oxidation of a$-unsaturated aldehydes.' Some HPA catalysts not only behave as strong acids, but also play roles as oxidizing agents.* Recently, it has been reported that H3PMoI2O4,, and H3PW12040 show high protonic conducti~ities.~ HPCs may be used as chemical sensors in many electrical devices.We have recently studied HPAs, such as H3PW12040, H3PM012040 and their salts, supported on silica in an effort to gain a better understanding of their reactivities and structures."-'* If the HPAs are loaded onto inorganic supports such as silica or alumina, their mechanical and thermal toughness do not increase, but the solubility of supported HPAs in water decreases. Furthermore, it is easy to modify the surface characteristics of supported HPAs by changing the supported amount and the extent of cation exchange for hydrogen ions in the HPA.The purpose of this work is to characterize the electrical properties of silica-supported H3PMo12040, H3PW 12040 and their sodium, potassium, and caesium salts. The efiect of surface modification on the electrical behaviour is also investigated. A.c. electrical measurements and complex impedance analyses are used to construct an equivalent-circuit representation for the silica-supported heteropoly compounds. The relation between electrical behaviour and surface acidity is also investi- gated. This paper also describes several experiments designed to help determine the conduction mechanism in HPCs and to assess their usefulness as humidity sensors. Experimental Preparation of Samples The raw materials used in preparing the specimens were the HPAs 12-molybdophosphoric acid (H3PMo12040) and 12- tungstophosphoric acid (H3PW 12040), obtained from Japan New Metals and the cation-exchanged HPAs.The silica used was AEROSIL-200 (Aerosil Nippon; denoted throughout as SiO2-20O) which had a surface area of ca. 200m2 g-I. Solutions or emulsions of the HPCs were prepared as follows. Potassium salt emulsions of the HPAs were prepared by slow addition of an aqueous potassium carbonate solution to an HPA solution. Sodium salt solutions and caesium salt emul- sions were obtained by a similar procedure. The samples supported on SiO2-2O0 were obtained as described previously. lo For completeness, it is described briefly here. The support was calcined at 773 K for 3 h and then impregnated with the required amount of HPC solution.The resulting solution was dried at 383 K for 24 h. The solid thus obtained was ground and then calcined at 573 K for 3 h. The amount of supported HPC, rn [in mmol HPC g- '(support)], ranged from 0.05 to 0.45. The HPC samples supported on Si0,-200 will be denoted hereafter as H3PMo12040/Si02-200, M,P3 ~,Mo120,0/Si02-200, H3PW 12040/Si02-200 or M,P3 -,,W 12040/Si02-200, respectively, where M stands for sodium, potassium or caesium, and n denotes the number of substituted hydrogen ions. Fig. 1 shows schematically the surface structure of the HPC/Si02-200 sample. The SiO2-2O0 particle is considered to be a spherical body with no micro- pores. The particle size is ca. 14 nm, calculated from the surface area (200m2 g-') and the density (2.2g~m-~) of SiO2-2O0.For the formation of the first monolayer of HPC on the surface of the silica support, a critical value of m= 0.30 can be deduced, using the effective cross-section per HPC molecule." The structures of loaded HPC on SiO2-2O0 support are also shown in Fig. 1. The surface acidity of the sample is affected by the amount and species of the HPC supports. The acidity function of the sample was measured by means of a Hitachi model 340 spectrophotometer equipped with a head-on detector at room temperature. The quartz cell had a light pathlength of 0.5 mm. The sample was dispersed in dry decalin, and dicinnamylidene- acetone (DCA) was used as an acid-base indicator.DCA adsorbed on H2S04/Si02 and Si0,-200 was adopted as the standard for the acid and base colour, respectively. The acidity function was estimated from the changes in intensities of absorption bands at 400 and 560nm. Details have been described previously." Form of Element and Electrode The samples were pressed into cylindrical pellets (1 3 mm in diameter, 2 mm in thickness) at 1000 kgcm-2, and then calcined at 573 K for 1 h. For electrical measurements, vapour- deposited gold electrodes were applied on both surfaces of the samples. The shapes and positions of the evaporated electrodes were controlled by a thin metal mask. The forms of electrodes and elements are shown in Fig. 2. Two electrodes (inner and outer) were made up on a surface of the element, and one electrode on another surface.The outer electrode served as an earthed guard ring in electrical measurements. This form of electrode can eliminate edge and fringe effects caused by leakage electric fields in electrical measurements. Conductive paint (DOTITE D-500, Fujikura Kasei) was used to make the electrical connection between the evaporated Au electrodes and the Cu lead wires, which were connected to a measuring device. The electrical conduction was measured loaded HPC low-loaded HPC high-loaded HPC 0ca. 14 nm Si02Si02 support 0 : HPC molecule J. MATER. CHEM., 1991, VOL. 1 G H / samp e-I I I+ electrical lead L Fig. 2 Construction of electrode and sample between electodes H and L (Fig. 2).The outer electrode G was joined to the ground terminal of an electrical device. Electrical Measurements It is well known that for poorly conducting materials, perma- nent changes in physical and chemical properties, caused by e.g. dielectric breakdown and electrolysis, can be induced by applying a d.c. voltage. Silica-supported HPC samples, used in this study, showed high resistivity in dry atmospheres. Therefore, we used a complex-impedance method' in order to estimate the electrical conductivity of the silica-supported HPC at various humidities. The frequency dependencies of the impedance, 2, and loss tangent, tan 6, were measured at at least nine frequencies between 100 Hz and 100 kHz with a commercial LCR meter (AG-43 1 1, Ando Electric) under an applied a.c.voltage of 1 V. The variation of impedance with relative humidity (r.h.) were measured in the humidity range 30-90% r.h. at 323 K. The change of electrical conductivity with temperature was deter- mined at 60% r.h. and five temperatures between 303 and 343 K. In order to obtain the equivalent values in each measurement, we waited for at least 6 h after having set a sample in a test chamber. The results were obtained automati- cally with a personal computer system (PC 9801 VX2, Nippon Electric), which was connected to the LCR meter by an IEEE- 488 standard digital interface. l4 In the above measurements, we used a commercial humidity generator (ETAC JLH-400- 20, Kusumoto Kasei). Results and Discussion Complex Impedance Analysis of Silica-Supported HPC in the Presence of Adsorbed Water Complex Impedance Diagram of Supported HPC The resistance of silica-supported H3PMo12040 and H3PW12040(rn =0.05) was beyond the limit of our measure- ments (>lo0 MQ).The measurements were made at room temperature after the samples had been dried at 383 K. The bulk conductivity of supported HPA/Si02-200 is inherently low, and the remaining intrinsic surface protons do not Fig. 1 Schematic illustration for supported HPC on silica-200 and contribute to any detectable surface conductivity under the stacking structure of supported HPC dry atmosphere. When the samples had been left to stand J. MATER. CHEM., 1991, VOL. I (ca. 80% r.h.) for some time before measurements, the resist- ance of the samples became low enough to make our measure- ments possible.The electrical conductivity of supported HPA/Si02-200 is enhanced by the water molecules in the ambient atmosphere. Complex-impedance analysis was applied to elucidate the electrical behaviour of several of the samples. The complex impedance, Z(o),of a material at an applied angular frequency can be represented as follows: Z(o)=Z’(o)+jZ”(w) (1) where Z’(w)and Z”(o)stand for the real part (resistance) and the imaginary part (reactance) of the impedance, respectively. Plots of Z’(o)us. Z(o)suggest one or more possible equival- ent circuits leading to the circuit parameters, i.e. resistance Iand cond~ctance,’~ for a system. Fig.3 shows the typical complex-impedance plots for the 0.05 mmol g- ‘(support) H3PMo ,,0,,/Si0,-200 element as functions of the angular frequencyf=o/2n in 30, 40, 50, and 60% r.h. at 323 K. A computer program was used to fit the best arc to the data, which is shown by the solid lines in Fig. 3. These profiles are semicircular arcs. The impedance of H3PMo12040/Si02-200 decreased with increasing relative humidity. The electrical behavour of H3PMo12040/Si02-200 was found to be closely related to the humidity. Fig. 4 shows the complex-impedance plots for 0.05 mmol g -‘(support) H3PW120,0/Si02-200 in 50% r.h. at 323 K. The diagram comprises two neighbouring arcs, which are attributed to fast and slow relaxation processes of orientation polarization for conductive species, respectively.In the low-humidity region, the high-frequency spur in Fig. 4 diminished or disappeared 1.5 c:2 1.0> ru 0.5 0 0.5 1.0 1.5 2.0 2.5 Z ’/MQ Fig. 3 Complex-impedance plots for supported H3PMo,,0, on silica-200 of 0.05 mmol g -‘(support) measured at various humidities at 323 K. 0,30; 0,40; A,50; 0,60% r.h. 10 1 5 10 15 20 25 Z’jhnQ Fig. 4 Complex-impedance plot for supported H3PW,,0, on silica- 200 of 0.05 mmol g-’(support); relative humidity was 50% at 323 K 99 1 from the complex-impedance diagram. The low-frequency spur, however, prevailed at all humidities. The loci of the complex-impedance plots exhibited significant humidity dependence. For the higher-loaded HPA on silica, there appeared to be another low-frequency tilted spike at the right- hand sides of the arcs in Fig.3 and 4. The spike may be interpreted as representing the specimenlelectrode interface process. As shown in Fig. 3 and 4, the loci of complex-impedance plots are not complete semicircles. The loci in Fig. 4 cross each other. If the electrical behaviour of specimen HPA/Si02- 200 involves a single relaxation process of orientation polariz- ation of the conductive species and the electrodes are complete blocking electrodes, the complex-impedance diagram should be a complete ~emicircle’~ and the spike should be a perpen- dicular line to the Z’(o) axis. Therefore, in this case, there may exist either another relaxaiton process of orientation polarization or some other conductive species responsible for the electrical conduction in HPA/Si02-200.The causes of the distributed relaxation processes and the appearance of the tilted spike in the complex-impedance plots have not yet been identified. One plausible cause is that the supported HPA crystalline sizes are distributed around the most probable size, since HPC molecules tend to aggregate in an earlier stage of loading, even under low The coarse surface of the sample and the specimen/electrode interface cause the current inhomogeneity in the a.c. electrical measurement. l6 Such a possibility is supported by the following experimental result. When measurements were made on uniform films of highly dispersed HPA particles prepared by the sol-gel method, the loci of the complex-impedance plots were complete semicircles.These results will be reported in detail elsewhere. Similar results were also obtained for the complex-impedance plots of other samples of H3PMo12040/Si02-200 and H3PW12040/ SiO2-2O0 with different degrees of loading and for cation- exchange ones (M,H3 -,PMo12040/Si02-200 and M,H3 -,PW 12040/Si02-200). The nature of absorbed water in HPC/Si02-200 is classified into four categories: occlusion as pseudo-liquid into the HPC m~lecules;’~adsorption as a multilayer on the surface of HPC; adsorption as a multilayer on the SiO2-2O0 support; and condensation as a liquid at the contact zone of SiO2-2O0 particles (cf. Fig. 1). Under these experimental conditions, the last form of water is negligible, since condensation as a liquid at the contact zone of SiO2-2O0 particles does not occur below 90% r.h., which can be calculated by the Kelvin equa- tion and the meniscus radius at the contact zone.Furthermore, it was found that the resistance of silica support (SiO2-2O0) without HPC, in which the absorbed water exists only in the form of adsorption as a multilayer on the SiO2-2O0 support, was much higher than that of silica-supported HPC by ca. 3 orders of magnitude over these experimental humidity ranges. For electrical conduction, water adsorption on the SiO2-2O0 support seems to be less pronounced in HPC/Si02-200 samples. Thus, it is concluded that both the water occlusion as pseudo-liquid into HPC molecules and the water adsorp- tion as a multilayer on the surface of HPC are pronounced for ionic conduction of HPC-supported SiO2-2O0 sample under these experimental conditions.The arcs in Fig. 3 and 4 are correlated with the electrical behaviour of the above two absorption methods of water. The change of water content in variously loaded HPA/ SiO2-2O0 samples was investigated vs. humidity at 323 K. It has been reported for HPA that a maximum of 30 water molecules can be contained per Keggin unit,’* made up of [MO,,PO~~]~-or [W12P040]3- anions.lg At low humidity (30% r.h.), the water content was ca. 1 water molecule per Keggin unit for any loaded HPA/Si0,-200 specimen. With an increase in relative humidity, the water content was found to increase gradually.It reached a maximum of ca. 18-30 water molecules per Keggin unit at 90% r.h. For the same amount of loaded HPA on SiO2-2O0 support, the effect of occluded water in H3PW 12040/Si02-200 is more significant than that in H3PMo12040/Si02-200 since the growth of HPA crystal in H3PW12040/Si02-200 exceeds that in H3PMo12040/Si02-200,as shown by the X-ray diffraction patterns of these samples. Furthermore, in a highly loaded H3PMo,2040/Si02-200 sample, in which the growth of the supported H3PMo12040 crystal and the amount of occluded water is high compared to the sample in Fig. 3, there appeared another high-frequency spur at the left-hand sides of the arcs in Fig. 3. Thus, the high-frequency spur in Fig.4 may be linked to the behaviour of the occluded water in the H3PW12040 crystal in H3PW12040/Si02-200. The low-frequency spur in Fig.4 and the arcs in Fig. 3 indicate the behaviour of the adsorbed water on the surface of HPA. Equivalent Circuit for Supported HPC We suggest a total equivalent circuit representation of H3PMo12040 and H3PW12040/Si02-200 as depicted in Fig. 5 and 6, respectively. The complex-impedance diagrams are also shown. The equivalent circuit associated with loop A in these figures consists of resistance R,, and capacitance Cwl, rep-resenting the supported HPC which contains some adsorbed water in HPC/Si02-200. Loop A in Fig. 6 consists of resist- ance Rw2 and capacitance Cw2,representing the HPC which contains some occluded water in M,H3 -,PW 12040/Si02-200.Spike B is associated with the electrical process of the elec- trode/specimen interface and is represented by constant phase- angle impedance 2, and capacitance for electrode polarization Ci.?The electrode/specimen interface process in the a.c. electri- cal measurement seems to be less pronounced in this frequency range and for this applied voltage since the impedance of the samples did not change with time and spike B only appeared for the element of higher-loaded HPA on silica. Thus, we will not consider the specimen/electrode interface process in the following paragraphs. The impedance 2 of the equivalent circuit for M,H3 ~,PMo12040/Si02-200 (Fig. 3) is given by where o is the angular frequency, R,, and CW1or cb are the resistance and capacitance of the adsorbed water on the surface of supported M,H3 -,PMo120q0 crystal or silica sup- port, respectively.In this equation, we do not consider the bulk resistance of the silica support. The contribution of resistance Rb, the bulk resistance, to the a.c. electrical process can be ignored in these systems because the silica support is an insulator. Rewriting eqn. (2) leads to eqn. (3) =(Rw1/2)2 (3)(2'-17,,/2)2 +zr2 This equation shows that the impedance diagram of H3PMo12040/Si02-200becomes a semicircle of radius RW1/2. The loci of Fig. 3 are approximately described by the above equation. From the above relationships, we obtained t Impedance Z, is expressed by: 2, =K,W-~[cos(pz/2)-j sin(pn/2)] where w is the angular frequency, K, and p are independent of o and lpl<1.18 J.MATER. CHEM., 1991, VOL. 1 t-IoopA -+ spikeB -Fig. 5 Equivalent-circuit representation and impedance plot of M,H, -,PM0,,0,~/Si0~-200. R, ,=resistance for adsorbed water; CW1=capacity for adsorbed water; Rh =bulk resistance; Cb =capaci- tance of silica support; Z, =constant-phase angle impedance; Ci = specimen/electrode interfacial capacity Rw2 Rw1 cb I I Fig. 6 Equivalent-circuit representation and impedance plot of M"H3 -,P~Wl,0,,/Si0,-200. -R,, =resistance for -adsorbed- water; CW1=capacity for adsorbed water; R,, =resistance for condensed water; C,, =capacity for condensed water; Rb=bulk resistance; Cb= the capacitance af silica support; Z, =constant phase-angle impedance; Ci specimen/electrode interfacial capacity the resistance, capacitance values and conductivity of the system.$ The impedance 2 of the equivalent circuit for M,H3 ~,PW,2040/Si02-200 (Fig. 6) can also be obtained.It $ Eqn. (2) indicates that the resistance values of the system are derivable from the circular-arc intercepts on the Z axis; the capaci- tance values (the sum of CW1 and C,) can be derived from the frequency (f) at the peaks of the circular arcs (w,) and the resistance (R)by using the following relation, which can be derived also from eqn. (1): = /c(cwl +ch) Rwll The d.c. conductivity of the sample, 0, can be obtained from the resistance by correcting for specimen geometry.J. MATER. CHEM., 1991, VOL. 1 is expressed as follows: z=CRwl+Rw2+j~RW1Rw2Kwl+ Cw2)l/ c1 -W2RwlRw2(CwlCw2 + CbCwl’ cbcw2) +jo(CwlRwl +Cw2Rw2 +CbRwl + CbRw2)l (4) where o is the angular frequency, Rwl or Rw2 and Cwl or Cw2 stand for the resistance and capacitance of surface- adsorbed water or occluded water in crystalline M,H3 -,PW12O40 in the M,H, -nPW12040/Si02-200 speci- men. Cb is the capacitance of the silica support. In this case, although the impedance shows a more complex form com- pared with eqn. (2), the complex-impedance diagram for M,H3 -,PW 12040/Si02-200 can be explained. The resistance and capacitance for H3PW12040/Si02-200 can also be obtained in the same way as for the H3PMo12040/Si02-200 system above.Typical parameters were Rwl= 17.2 MR, Cwl=23.1 pF, Rw2=5.4 MQ Cw2=4.9 pF for H3PW12040/Si02-200 (rn= 0.05, 323 K, 80% r.h.). In the humidity range 30-90% r.h., the conductance of HPC/Si02-200 samples was almost constant. The resistances, however, changed with relative humidity. In order to compare the M,H3 -,PW 12040/Si02-200 specimen with the M,H3 -,,PM0~~0~~/Si0~-200 one, we will consider only the low-frequency spur A in Fig. 5 and 6. Variation of Electrical Conductivity of Supported HPC with Humidity Fig. 7 shows plots of resistivity us. relative humidity for the H,PM0~,0~~/Si0~-200samples as a function of amount supported. As the relative humidity increases from 30 to 90% r.h., the magnitude of the resistivity decreases logarithmi- O\ -lo6 lo5 -0 E q4 -.$I0.-CI v).-v)L lo3 --\lo2 \ 0 10’ -30 40 50 60 70 80 93 relative humidity (YO) Fig.7 Plots of resistivity against relative humidity for the as-prepared H,PM0,,0~~/Si0~-200with various loadings of H,PMol,04, meas-ured at 323 K. Values of mlmmol g-’(support): 0,0.05; A, 0.1; 0, 0.25; V, 0.45 993 cally. The behaviour of the electrical conduction depends upon the amount of H3PMo12040 supported. The electrical conduction increases with an increase in the amount of loaded HPA. The electrical behaviour of highly loaded (rn=0.45) H3PMo12040/Si02-300 is different from that of other low- loaded H,PMo12040/Si02-200 samples. In a previous study,12 we clarified by EPR the difference in the structures of loaded H,PMO,~O~~ silica supports between high- on loaded and low-loaded ones (cJ: Fig. 1).The H~PMo~~O~~ molecules tend to aggregate in an early stage of loading, owing to the network formation of H3PMo12040 and/or the stacking of on themselves. The difference in the electrical behaviour of highly loaded H,PMo 12040/Si02-200, compared with other loaded samples, is due to the structural difference in HPC/Si02-200. Similar results were also obtained for the other HPC/Si02-200 samples. Observation of conductivity changes with temperature is essential for an understanding of conductivity phenomena. When electrical conductivity, 6, is supported by ionic charge carriers, it varies approximately exponentially with the absol- ute temperature, T,” i.e.CT =go exp(-E/RT) (5) where R is the gas constant, E stands for the activation energy, and go is a constant. Fig. 8 shows the Arrhenius plots for electrical conduction in H,PMo12040/Si02-200 at various loading levels. Other silica-supported HPC plots gave negative temperature coefficients similar to those in this figure. From these results, it was concluded that ionic conduction occurs in the HPC/Si0,-200 system. Dissociative protons produced in adsorbed water may be taken as charge carriers in the adsorbed water-HPC/Si02-200 system, since bulk HPA is a good protonic condu~tor.~ Table 1 shows the activation energies for the electrical 0-0-0-0-0 I I I 1 1 2.9 3.0 3.1 3.2 3.3 103 K/T Fig.3 Arrhenius plots for the electrical conductivities of H3PMo,,040/Si0,-200. Values of m/mmol g-’(support): 0,0.05; A, 0.10; 0,0.25; V,0.45 Table 1 Activation energies for the electrical conduction of supported H3PMo12040 on silica-200 sample m"/mmol g- '(support) Eb/kJ mol-' 3PMo 1Z040 0.05 7.1 0.10 9.2 0.25 16.3 0.45 16.5 H3PM012040 bulk 15.5' J. MATER. CHEM., 1991, VOL. 1 1o8 1 lo7 -5 56210 -.-c. v).-v)E -lo5 0 lo4 t \ 30 40 50 60 70 relative humidity (%) The measuring temperature range is 303-343 K in 60% r.h. " m= amount of HPC supported. E =activation energy; error =ca. +1 kJ mol-'. 'From ref. 9. conduction obtained from Fig. 8. The activation energy for bulk H3PMo12040 is also shown.The activation energy for rn=0.05 and 0.10 is lower than that for bulk H3PMo12040. The activation energy for the electrical conduction, however, increases with the amount of loaded H3PMo12040, becoming closer to the activation energy for bulk H3PMo12040. The reason for the lower activation energy for electrical conduction in lower-supported HPA compared to that in bulk HPA is not yet clear. Since the humidity sensing properties of various loaded H3PMo12040/Si02-200 specimens show the same SiO2-2O0 (cf: Fig. 7), a charge in the conductive species responsible for the electrical conduction process is unlikely in this system. Thus, the difference of the activation energy is ascribed to the difference of the surface-stacking structure of supported H3PMo12040 on silica.There may arise a potential barrier for the electrical conduction with increased loadings of HPA on Si02-200. Table 2 lists activation energies for the electrical conduction of supported H3PMo12040, Na3PMo12040. K3PMo12040, and on SiO2-2O0. Since the activation energies for the H3PMo12040/Si02-200 series vary with the loaded amount, we include the data for rn=0.05 in this table. The activation energy is changed when cations are substituted for hydrogen ions in HPA. Replacement of the cation in HPC by H, K, and Cs ions increases the activation energy for electrical conduction in HPC/Si02-200 in this order. The lowest activation energy for Na3PMo 12040/Si02-200 com- pared to other cation-exchanged HPC/Si02-200, may be due to the participation of some other conductive species, such as Na' ions, in the electrical conduction, since Na3PMo12040 is a hydrated, water-soluble ~alt.~.~~ Correlation between the Electrical Conductivity and the Acidity Function H,, of supported HPC Fig.9 shows plots of resistivity us. relative humidity for silica- supported K,H3 -,PM012040 (n= 1-3) samples. The electrical conductivity decreases with an increase of the number, n, of cations substituted for hydrogen ions in H,PMO,~O~~. A similar trend was observed for the Cs,H3 -nPMo12040/Si02- tendency except for high-loaded (rn=0.45) H3PMo12040/Fig. 9 Plots of resistivity us. relative humidity for m =0.05 mmol g-'(support) HPC supported on SiO2-2O0 measured at 323 K.0, KH,PMol,0,0/Si02-200; 0, K,HPMol,040/Si0,-200; A, K3PMo,z0,,/Si02-200 sample, however, there was not a clear correlation between the conductivity and the number of cations substituted for hydrogen ions. Fig. 10 shows plots of resistivity us. relative humidity for silica-supported Na3PMo12040, K3PMo12040 and samples. The behaviour for the H3PMo12040/Si02-200 sample is also shown in this figure. When the hydrogen ions in HPA were completely substituted with sodium, potassium, or caesium cations, the electrical conductivity of the specimens decreased in this order. Further- I o9 I O8 5 lo7 5> c.-.-> -6.5 10 200 sample examined. For the Na,H3 -,PM0,~0~~/Si0~-200 Table 2 Activation energies for the electrical conduction of supported HPC on silica-200 ~~~ sample E"/kJ mol-' H3PMo,2040/Si02-200 7.1 Na3PMo,,0,,/Si0,-200 3.6 K3PMo 12040/Si02-200 9.2 Cs3PMo 12040/Si0,-200 10.4 The amount of HPC supported is 0.05 mmol g- '(support).The measuring temperature range is 303-343 K in 60% r.h. " E =acti-vation energy; error =ca. f1 kJ mol- '. I o5 \\ 6 I o4 30 40 50 60 70 relative humidity (%) Fig. 10 Plots of resistivity vs. relative humidity for m=0.05mmol g-'(support) HPC supported on Si0,-200 measured at 323 K. 0, H3PMo12040/Si0,-200; A, Na3PMo120,0/Si0,-200;0,K,PMo1,0,,/Si0,-200; V, Cs3PMo1,0,,/Si0,-200 J. MATER. CHEM., 1991, VOL. 1 more, the electrical behaviour of Na3PMo12040/Si02-200 differs from that of H3PMo12040/Si02-200, K3PMoI2O40/ SiO2-2O0 and Cs3PMo12040/Si02-200.The surface acidity, which is a measure of the tendency to donate the proton, of HPC/Si0,-200 depends both on the number of cations substituted for hydrogen ions and the species of substituted cations. Previously, we have presented a quantitative evaluation of the acidity of HPC supported on silica (Si0,-200) using the acidity function, Ho.ll The value of Ho for the H3PMo12040/Si02-200 samples was found to increase with the amount of support. It was also affected by the substitution of hdyrogen ions in HPA. Thus, it seems that the change in electrical conduction in HPC/Si02-200 with the number and species of cations substituted for hydrogen ions may correspond to the change in the surface acidity (Br~nsted acidity) of HPC/Si02-200.We have assumed that Ho is closely related to the electrical conduction of ion- exchangers such as HPC. Hereafter, we discuss the surface electrical conductivity for HPA/Si02-200 samples with various loadings in relation to the Ho value. Fig. I1 shows the electrical conductivity of supported H3PMo12040/Si02-200 against Ho. The solid line was drawn to fit the plots. The figure shows a good linear relationship between the electrical conductivity and Ho. The electrical conductivity increases with increasing Ho. Thus, it is con- cluded that the surface acidity affects the electrical conduction of supported HPA. It is seen from Fig. 11 that the relation between the surface acidity and the electrical conductivity can be empirically formulated as follows: logo=-A&+B (6) where A and B are constants.The slope A of the solid line in Fig. 11 may reflect the mobility of electrical conductors (protons). The constant B may reflect the concentration of electrical conductors. Fig. 12 shows plots of electrical conductivity against the acidity function Ho for various cation-exchanged MnH3-nPMo12040/Si02series. In this case, it is also clear that the surface acidity of HPC/Si02-200 has an influence on the electrical conductivity; plots for H3PMo12040/Si02-200, K,H3 -,PM0~~0~~/Si0~-200, and Cs,H3 -nPMo12040/Si02- 200 are superimposed on each other with a slope of -1.9, / P0 A/ 0/ I 6" 1ii4 r 5 r I G .-:lo 5 .-c0 7J 0 1Ci6 I ci7 -2 -3 -4 acidity function H, Fig.12 Relation between the electrical conductivity for HPC/Si02- 200 in 60% r.h. at 323 K and the acidity function H,. 0, H,PMo O4,/Si0,-2O0; A, Na,H, -,PMo ,O4,/SiO,-20O; 0, K,H, ~,PMo,,04,/Si0,-20~ V,Cs,H, -,PMo,,04,/Si0,-200 whereas a plot for Na,H3 ~,PMo12040/Si02-200 gives a sep- arate line with a slope of -0.64. Fig. 13 shows a plot of the electrical conductivity us. Ho for the M,H3 -,PW 12040/Si02- 200 series. The behaviour shows the same tendency as that for the M,H3 -,PM0~~0~~/Si0~-200 series. Plots for K,H -,PW 040 and Cs,H -,P W 2040/Si02- H PW 040, 200 fall approximately on a straight line with a slope of -2.0. The behaviour of the Na,H3 -,PW 12040/Si02-200 sample, however, differs from other samples with a slope of -0.76./ / / 10" -Ln -3n -4-2 -3 -4 acidity function H, acidity function H, Fig. 13 Relation between electrical conductivity for HPC/Si0,-200 Fig. 11 Relation between electrical conductivity for H,PMO,,O~~/ in 60% r.h. at 323 K and the acidity function Ho. 0, H3PW12040/Si0,-200 in 60% r.h. at 323 K and the acidity function H,. Values SiO2-2O0; A, Na,H3~,PW,,04,/Si0,-200;0,K,H, -nPW12040/ of m/mmol g-'(support): 0,0.05; A, 0.1; 0,0.25; V, 0.45 SiO2-2O0; V, Cs,H, -,PW,2040/Si02-200 The electrical conductivity of sodium cation-substituted HPC/Si0,-200 is larger than that of other cation-substituted HPC/SiO,-200, when compared at the same H, value.The slopes of plots for sodium cation-substituted HPC/Si02-200 in Fig. 12 and 13 are small, compared with the slopes for other HPC/Si0,-200. In sodium cation-substituted HPC/ Si0,-200, it is assumed that the substituted cations can dissolve into the adsorbed water and participate in the electri- cal conduction. Such a possibility is supported by the fact J. MATER. CHEM., 1991, VOL. 1 7 Standard Oil (Sohio), Jpn. Pat., 10 308, 1960. 8 I. V. Kozhevnikov and K. I. Matveev, Appl. Catal., 1983,5, 135. 9 0.Nakamura, T. Kodama, 1. Ogino and Y. Miyake, Chem. Lett., 1979, 1, 17. 10 R. Ohtsuka, Y. Morioka and J. Kobayashi, Bull. Chem. SOC. Jpn., 1989, 62, 3195. 11 R. Ohtsuka, Y. Morioka and J. Kobayashi, Bull. Chem. SOC. Jpn., 1990, 63, 2071.12 R. Ohtsuka and J. Kobayashi, Bull. Chem. SOC.Jpn., 1990, 63, 2076. 13that the electrical conductivity of Na,H, -,,PMo~~O~,/S~O~- 200 is affected by the number of hydrogen ions which have been replaced by sodium. It decreased with decreasing number of n (cJ Fig. 12 and 13). Furthermore, the electrical conduc- tivity for HPC/Si02-200 substituted by low concentrations of sodium cations approached that for HPC/Si02-200 exchanged by other cations. References K. Urabe, K. Fujita and Y. Izumi, Shokubai (Catalyst), 1980, 22, 223; Y. Onoue, Y. Mizutani, S. Akiyama, Y. Izumi and H. Ihara, Shokubai (Catalyst), 1976, 18, 180. T. Okuhara, A. Kasai, N. Hayakawa, Y. Yoneda and M. Misono, J. Catal., 1983, 83, 121. M. Ai, J. Catal., 1981, 71, 88.M. Akimoto, Y. Tsuchida, K. Sat0 and E. Echigoya, J. Catal., 1981, 72, 83. H. Hayashi and J. B. Moffat, J. Catal., 1982, 77, 473. Y. Izumi, R. Hasebe and K. Urabe, J. Catal., 1983, 84, 40%. S. H. Chu and M. A. Seitz, J. Solid State Chem., 1978, 23, 297; J. G. Thevenin and R. H. Muller, J. Electrochem. SOC.,1987, 1-34, 273. 14 ANSI/IEEE Std, 488 Standard Digital Interface for Programm- able Instrumentation, 1975, revised 1978, 1980. 15 M. Watanabe. S. Oohashi, K. Sanui, N. Ogata, T. Kobayashi and Z. Ohtaki, Macromolecules, 1984. 17, 2908. 16 P. K. Botteleberghs, Solid Electrolytes, General Principles, Characterisation, Materials, Applications, ed. P. Hagenmuller and W. Van Gool, Academic Press, New York, 1978, p. 145; M. Watanabe, Dodensei Kobunshi, ed. N. Ogata, Kodansya, Tokyo, 1990, p. 40. 17 G. M. Brown, M. R. Noe-Spirlet, W. R. Busing and H. A. Levy, Acta Crystallogr., Sect. B, 1977, 33, 1038; J. B. Moffat, Poly-hedron, 1986, 5, 261. 18 G. A. Tsigdinos, Ind. Eng. Chem. Prod. Res. Dev., 1974, 13, 267. 19 J. F. Keggin, Nature (London), 1983, 131,908; H. A. Lavy, F. A. Agron and M. D. Danford, J. Chem. Phys., 1959, 30, 1486. 20 L. Glasser, Chem. Reu., 1975, 75(1), 21. 21 M. Misono, N. Mizuno, K. Katamura, A. Kasai, Y. Konishi, K. Sakata, T. Okuhara and Y. Yoneda, Bull. Chem. SOC. Jpn., 1982, 55, 400. Paper 1/02400K; Received 22nd May, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100989

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 997-999 Trivalent Boron as Acceptor Chromophore in Asymmetrically Substituted 4,4'-Biphenyl and Azobenzene for Non-linear Optics Minh Lequan,* Rose Marie Lequan and Kathleen Chane Ching Laboratoire de Chimie et d'Electrochimie des Materiaux Moleculaires, Ecole Superieure de Physique et de Chimie lndustrielle de Paris, CNRS UA 429, 10 Rue Vauquelin, 75231 Paris, Cedex 05, France [4'-( Di methyl am ino) bi phenyl-4-ylldi mesi tyl borane (BN B) and [4-(d imethyl am ino) phenylazophenyl-4-yl]d imesityl-borane (BNA) have been prepared and studied by a solvatochromic technique. The high second-order hyperpolarisability coefficients PCTdetermined by this technique, 37 and 240 x esu,t respectively, show that this new family of materials is of interest for non-linear optics.Keywords: Boron; Solvatochromic technique; Non-linear optical material The search for materials possessing a large hyperpolarisability coefficient, /j, for potential applications in non-linear optics has been developed extensively in recent years. So far, exper- imental efforts have been focused on benzene, stilbene or azo dyes substituted by organic donor and acceptor moieties, the latter being generally limited to nitro, nitrile, pyridinium or sulphone groups. The use of Lewis acids as electron-acceptor chromophores has been suggested recently by Kanis et al.' These novel groups should be interesting because, according to the theor- etical calculations of the authors, they could enhance the hyperpolarisability coefficient.However, Zheng Yuan et al. * have published the results of their investigation on a series of boron compounds prepared principally by addition of borane to acetylenic derivatives. In a previous paper3, the use of the phosphonium cation Ph2MeP+ or phosphine oxide Ph,P-+O as electron-acceptor groups combined with amine or borate anions as electron donors, was mentioned. We now propose a trivalent boron species as the acceptor chromophore and a dimethylamino group as the donor, substituted in positions 4 and 4 of a biphenyl (BNB) or azobenzene (BNA) backbone. The results will be compared with those obtained for phosphorous derivatives. BNB BNA The present paper reports the synthesis of these compounds, the results of solvatochromic studies in both absorption and emission spectroscopy and the determination of hyperpolaris- ability properties in solution.Experimental Synthesis Fluorodimesitylborane was synthesised by Brown's meth~d.~ The 4'-bromodiphenyl-4-ylamine was obtained by reduction of the corresponding nitro derivative5 by NaBH, in ethanol according to the procedure described in the literature.6 The amino group was methylated into a dimethylamino group by trimethyl pho~phate.~ The [4'-(dimethylamino)biphenyl-4-yl]dimesitylborane (BNB) was prepared according to the method used for aryl- dimesitylboranes.8 6.5 cm3 of BuLi (1.5 mol dm-3 in hexane) was added to a solution of 2.76 g mol) of 4'-bromobiphe- nyl-4-yldimethylamine in 50 cm3 of distilled and deaerated tetrahydrofuran (THF) at -60 "C under argon.The tempera- ture was allowed to reach -40 "C then 2.6 g of fluorodimes- itylborane in 10cm3 of THF were introduced. The mixture was heated under agitation at 50 "C for 3 h. The reaction was hydrolysed and the product extracted with ether. The organic solution was dried over magnesium sulphate and the solvent evaporated to yield the crude product which was recrystallised in acetonitrile to give 2.9 g, m.p. 156 "C. Elemental analysis confirmed the expected formula. dH in CD,COCD,: 2.9 (NCH,); 2.3 (p-CH3); 2.03 (0-CH3); 6.81 HA; 7.63 HB; JHAHB =9 Hz; 7.66 HA,; 7.5 HB'; JHA,HB, =9 Hz. The (4'-bromophenylazophenyl-4-yl)dimethylaminewas prepared by electrophilic substitution of N-dimethylaniline by the diazonium salt derived from p-bromoaniline.The [4'- (dimethylamino)phenylazophenyl-4-yl]dimesitylborane(BNA) was synthesised by a similar procedure to that used for BNB. Recrystallisation in acetone gave red crystals, m.p. 180 "C. Satisfactory elemental analysis was obtained. 6, in CD3COCD3: 3.1 (NCH3); 2.26 (p-CH,); 2.00 (0-CH,); 6.83 HA; 7.86 HB; JHAHa 9.3 Hz; 7.8 1 HA-; 7.58 HB,; JHA,Hs, =8.4 Hz. Dipole Moment Determination The permittivities E for different concentrations of solutes in toluene (0.1-0.02 mol dmP3) were derived from the measured capacitance C, of solutions by the use of a homemade dipole meter equipped with a cylindrical nickel-plated condenser cell. The cell's constants were determined for four weakly polar reference solvents, these being carbon tetrachloride, cyclohex- ane, dioxane and toluene (HPLC grade).Measurement of the capacitance of the cell filled with reference solvents and knowledge of the corresponding relative permittivity of sol- vents, gives the cell's constants via linear regression C, =AE+ B which allows calculation of the permittivity E of each solution. The refractive indices were measured with an Abbe refrac- tometer. The temperature control for both dipole meter and refractometer was achieved by circulating water from a ther- moregulator adjusted to 20 +0.5 "C. The dipole moments pg were calculated using the Guggen- heim-Debye eq~ation.~ UV-VIS Absorption and Fluorescence Absorption and fluorescence spectra were recorded with an UVIKON 860 spectrometer and a FICA 55 fluorimeter.All solvents used were HPLC grade purchased from Aldrich and kept dried over molecular sieves. Results and Discussion The contribution of a trivalent boron chromophore as an acceptor group in the delocalisation of the lone pair from the amino group, in the ground state, can be estimated from the corresponding lLma,measured in UV- VIS absorption and compared with the withdrawing power of phosphine oxide P+O, the phosphonium cation P+ and the nitro group. In this case, the electron donor is the N,N-dimethylamino group for all the compounds. The dimesityl boron chromophore is seen to be as good an electron attractor as a nitro group.When the aromatic rings of biphenyl are separated by a diazo N=N bridge the delocalisation is greater and the absorption reaches 440 nm. In contrast, the phosphine -P< is a very poor donor group (Table 1). Moreover, a red shift appeared clearly for each compound when the polarity of solvents was increased. In fluorescence spectra, the red shift was also observed for BNB (Table2; Fig. 1). By contrast, no fluorescence was detected for BNA, the result of either a non-radiative deactivation or an emission in the IR region. This sensitivity to the polarity of the solvent allowed investigation of these new materials by a solvato-chromic technique. The variation of their dipole moment between the excited and the ground states was determined according to Varma and Groenen's method." Two parameters Xij and Yij were defined for various couples of solvents i and j, Xij depending on the relative permittivities and refractive indices and Yij the wavenumber corresponding to the maxima of the absorption or fluorescence bands of the solute and the same refractive indices (Fig.2). Xij= Si- Sj/Qi-Qj with Si=(E~-1)/(2~,+1) Qi= (n? -1)/(2n?+1) y..IJ =v.J -fi/Q.1 J-Q . vi = 107/~,,,~, Table 1 UV-VIS absorption of 4X (C,H,),N(Me), in ethanol Br 308 Ph,P+O 340 Ph, MeP 360+ Mes,B (BNB) 373 NO2 373.911 Mes, BC,H,N= NC,H,N( Me), 440 Ph,MeP'(C,H,),PPh, 2753 J. MATER. CHEM., 1991, VOL. I Table 2 Absorption (abs.) and emission (emiss.) of BNB and BNA in solvents with increasing polarity 'TI,, solvent abs.(BNB) emiss. abs. (BNA) cyclohexane 367.5 -426.5 dioxane 375 -438.7 carbon tetrachloride 369.5 -432.2 toluene 378 -440.5 ethyl acetate 373.7 489 440 dichloromethane 376 48 8 446.2 acetone -505 448 ethanol 373.2 495 440 acetonit rile 373.7 51 1 448 dimethylformamide 383 513 457.5 dimethyl sulphoxide 386.2 515 468 Plotting Yij us. Xij gave a straight he, the slope, b, of which is from absorption data and be, =2peAp/hca; (2) from emission data, where pg is the ground-state dipole moment, pe the excited-state dipole moment, Ap=pe -pg, h= Planck's constant, c =the velocity of light, a. =the radius of the spherical cavity occupied by the solute in the solvent (Onsager's cavity) which remains unchanged for the ground and excited states.pg was determined from dielectric measurements by the use of the Guggenheim-Debye eq~ation:~ pi =(9k,T/4dV*)[3/(~0 +2)(ni +2)]A where E =relative permittivity of solutions determined from 1 350 400 500 600 A/nm Fig. 1 (a) UV absorption spectrum for BNB in acetonitrile, (b) UV absorption spectrum for BNA in acetonitrile. Molar absorption coefficients: (a) 28 000dm3 cm-' mol-'; (b)37 500 dm3 cm-' mol-'. (c) fluorescence spectrum of BNB, in (------) ethyl acetate, (----) ethanol, (-) acetonitrile, (----) dimethyl sulphoxide J. MATER. CHEM., 1991, VOL. 1 the cell's constant equation, E, =relative permittivity of pure nsolvent (t~luene),'~ =refractive index of solutions, no= refractive index of pure solvent (t~luene),'~ NA=Avogadro's number, kB=Boltzmann's constant, A =slope of the curve (E -n')=/lC) with C in mol cmP3 and pg in D.? For BNB, pe can be estimated from the experimental value of pg (2.0 D) and eqn.(3) deduced from eqn. (1) and (2) pe =(bern/babs)Pg (3) Hence pe= 11 D and Ap=9 D. This calculation does not take account of Onsager's cavity for BNB; nevertheless, it can be calculated from eqn. (1) (a, = 4.0A) and compared to the value determined by immersion of a molecular model in water (a, =4.1 A).'. This result shows that the immersion technique may be a valuable method for estimation of the solvent cavity. In the case of BNA for which fluorescence data cannot be obtained, the parameter a, was determined by this method (ao=4.2 A).Using eqn. (1) we obtained Ap= 15 D for an experimentally measured value of pg=3.2 D. The hyperpolarisability coefficients can be now calculated from the dipole-moment variation of BNB and BNA in making use of Blombergen's equation, PCT =(3fi2e2/2m)F(@/fAlL (12) where e=the charge of an electron, m=the mass of an 70 000' 30 000 -1 0000 's-50000 . . * . . '. . . . . . ' ' * . .' -20 -15 -10 -5 0 5 10 15 20 150 50 -50 -1 50 -1 0 -5 0 5 10 ,-= -20 -1 0 0 10 20 Xij Fig. 2 Yij us. Xijobtained for BNB (a)and (b),and BNA (c).Linear-regression curves: (a)Yij= 17810+2818Xij,R=0.91 (absorption data); (b)Yij=5844 +15570Xij, R =0.99 (fluorescence data) (c)Yij= 20770 +5531Xi,, K =0.96 (absorption data) t 1 Dz3.335 64 x lop3' C In.electron,f= the oscillator strength derived from the area under the absorption band and F(o)=the frequency factor depen- dent on the transition energy of the solute and the laser energy, in the present case 1.17 eV. For BNB, f=0.63 (CH,Cl,), PCT=37 x lop3' esu CGS and Po= 16.5 x esu (at zero frequency). For BNA, f=0.90 (CH,CI,) PCT= 2lO~lO-~~esu CGS and ~,=50x10-30esu. These results show clearly that the trivalent boron chromo- phore behaves as a very good electron acceptor, as good as the nitro group. It is of interest to compare the experimental hyperpolarisability coefficient of BNB (37 x lop3' esu) to the theoretical value of the corresponding nitro compound calcu- lated by Morley et al." (31 x lop3' esu), recalling that PCT obtained by the solvatochromic method represents ca.75% of the overall effect. Moreover, a considerable enhancement of the molecular effect occurs when the two aromatic rings of the biphenyl are separated by an unsaturated bond, here, a diazo chain. The mesityl boron derivatives are stable to air, insensitive to light and can be recrystallised easily in organic solvents. X-Ray structure is under investigation, nevertheless one might expect, as is frequently the case for organic crystals, a centro- symmetric space group. However, the utility of such molecules in non-linear optics results in the possibility of incorporating organic functional groups, e.g. hydroxy or halogen groups, either in N-substituted alkyl chains or in aromatic mesityl groups.These functional groups allow attachment of the new chromophores to a polymeric matrix and polarisation under electric field to obtain a non-centrosymmetric material. Conclusions We have shown the preparation of a new family of organic materials possessing an important dipole-moment variation when excited by electromagnetic radiation. The use of a trivalent boron chromophore as an electron acceptor gives very promising results for obtaining high hyperpolarisability coefficients. The biphenyl derivatives and their homologues may be a good choice for potential applications in the field of non-linear optics. References 1 D. R. Kanis, M. A. Ratner and T. J. Marks, Chem. Muter., 1991, 3, 19.2 Zheng Yuan, N. J. Taylor, T. B. Marder, I. D. Williams, S. K. Kurtz and Lap-tak Cheng, J. Chem. SOC.,Chem. Commun., 1990, 1489. 3 K. Chane Ching, M. Lequan and R. M. Lequan, J. Chem. SOC., Faraday Trans., 1991, 14, 2225. 4 H. C. Brown and V. H. Dodson, J. Am. Chem. SOC.,1957, 79, 2302. 5 R. J. W. Le Fevre and E. E. Turner, J. Chem. SOC., 1926, 2041. 6 K. Hanaya, T. Muramatsu, H. Kudo and Y. L. Chow, J. Chem. SOC., Perkin Trans. I, 1979, 2409. 7 C. C. Barker and A. Stamp, J. Chern. SOC.,1961, 3445. 8 J. C. Doty, B. Babb, P. J. Grisdale, M. Glogowski and J. L. R. Williams, J. Organometal. Chem., 1972, 38, 229. 9 E. A. Guggenheim, Trans. Faraday SOC., 1949, 45, 714. 10 C. A. G. 0.Varma and E. J. J. Groenen, Recl. Trav. Chim. Pays- Bas, 1972, 91, 296. 11 J. 0.Morley, V. C. Docherty and D. Pugh, J. Chem. SOC.,Perkin Trans. 2, 1987, 1351. 12 N. Blombergen and Y. R. Shen, Phys. Rev., 1964, 37, 133; J. L. Oudar and D. S. Chemla, J. Chem. Phys., 1977, 66, 2664. 13 C. Reichardt, Solvents and Solvent EfSects in Organic Chemistry, VCH, Weinheim, 1988. 14 J. Simon and D. Nakache, personal communication, 1990. Paper 1/02431K; Received 23rd May, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100997

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 1001-1005 1001 Luminescence Spectra of Pure and Doped GaBO, and LiGaO, Gradus J. Dirksen, Arthur N. J. M. Hoffman, Teus P. van de Bout, Maurice P. G. Laudy and George Blasse* Debye Research Institute, Utrecht University, Solid State Chemistry Department, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands The luminescences of GaBO, and LiGaO, are reported. The former has Gal" in octahedral sites (calcite structure), the latter in tetrahedral sites (ordered wurtzite structure). The Gall' co-ordination number has the same influence on the luminescence as in the case of 0x0-do complexes: the lower the co-ordination number, the higher the energy of the first absorption band, the larger the Stokes shift, and the less mobile the excited state.The broad- band spectra with large Stokes shift indicate that the nature of the optical transition involved is complicated. Dopants in GaBO, are Eu"'and Cr"'. They are characterized by magnetic-dipole emission due to the inversion symmetry site in calcite. The Cr"' ion yields ,E emission at 4.2 K, in contrast to Cr"' in isomorphous ScBO, where it yields 4T, emission. The dopant in LiGaO, is Fell' which yields an intense deep-red emission with pronounced vibrational structure. Keywords: Luminescence; Gallium(Ir1) ; Gallium borate ; Lithium gallium dioxide 1. Introduction LiGaO, was prepared by dispersing P-Ga203 in a solution of LiOH with or without 0.5 atom.% iron. The dispersion In a recent paper, one of us has summarized evidence for the was dried and the residue firedin air twice after milling.The occurrence of luminescence from complexes consisting of a final firing temperature was 800 "C. central d" metal ion surrounded by oxygen ions in solids Samples were checked by X-ray powder diffraction using and molecules. Examples are Zn"-O" in Zn,O(a~etate),~ Cu-Kcr radiation. and ZII~O(BO~),,~ Cd"-O" in CaO,, and In"'-O" in Y203.5 Luminescence measurements were performed using aLater we extended this series with the luminescence of InB0,.6 Perkin-Elmer MPF-44B spectrofluorometer equipped with The relevant luminescence consists of a strongly Stokes- a liquid-helium cryostat. Excitation measurements in theshifted broad emission band, the nature of which is unknown, 190-250 nm region were performed with a Perkin-Elmerbut consists undoubtedly of a considerable amount of charge- MPF-3 spectrofluorometer equipped with a deuterium lamp.transfer character.' There is a similarity with the well known Diffuse reflection spectra were measured on a Perkin-Elmer luminescence of 0x0-complexes of the do metal ions (such as Lambda-7 spectrometer. vanadate and tungstate). In order to characterize this luminescence further, we investigated two simple gallium oxides, viz. GaB0, and LiGaO,. The luminescence of GaB0, was mentioned super- 3. Results ficially in the literat~re,~ but not much is known about this 3.1 GaB03 compound at all. It turns out to have the calcite crystal structure, i.e. the Gat" ions are six-co-ordinated by oxygen According to its X-ray diffractogram GaBO, has the calcite structure.The lattice parameters are a=4.57 A and c= 14.22ions which belong to triangular borate groups. It is isomorph- A. These values are comparable to those of CrBO, (4.57 and ous with the previously investigated JnBO,.The luminescence 14.23 A) and smaller than those of InB03 (4.82 and 15.45 of two dopant ions was also investigated in GaB03, viz. that A).'' All samples contain two second phases in small amounts, of Cr"' and Eu"'. The crystal structure of LiGa02 is an ordered variant of viz. P-Ga,O, and a borate glass phase (see below). In According to the diffuse reflection spectrum the optical wurtzite with Ga"' ions in tetrahedral co-~rdination.~,~ absorption edge of GaB0, is at 250 nm at 300 K.This implies this way the influence of the co-ordination number of Ga"' on the luminescence can be investigated. Since the Fe"' ion that upon host lattice excitation the small amount of P-Ga2O3 is known to be a very efficient activator in LiGa02,10 it was does not interfere, since its absorption edge is at the same Here already we note that the coincidence of also investigated in more detail. p~sition.'~*'~ It turns out that GaB0, as well as LiGaO, show efficient the optical absorption edge of GaB0, and P-Ga203 is rather luminescence at room temperature. It is surprising that the peculiar, since, generally, that of the borate is at higher energy efficient luminescence of such simple compounds has been than that of the corresponding oxide in view of the eiectro- overlooked for such a long time.negative character of the borate group. Upon excitation with 250nm radiation GaBO, shows an intense luminescence with high quantum efficiency (270%). 2. Experimental The emission consists of a broad band. At 300 K the emission maximum is at 460 nm. At 4.2 K, however, there is a different High-purity starting materials were used, among others emission band with a maximum at 375 nm (see Fig. 1). Both P-Ga203 (99.999%). Since this is not very reactive, it was emissions have an excitation spectrum which consists of a dissolved in concentrated KOH solution and precipitated as band coinciding with the absorption in the diffuse reflection the oxalate. This was fired in air with boric acid in order to spectrum.The excitation band maximum is at 250 nm. Upon obtain GaBO,. The final firing temperature was 825 "C.The increasing the temperature from 4.2 to 300 K, the ultraviolet dopants Cr,03 and Eu,03 were added in a concentration of emission quenches at ca. 150 K in favour of the blue emission. 0.5 atom.%. At 4.2 K the blue emission can be excited selectively by \ \ \ 300 500 wavelength/nm Fig. 1 Luminescence spectra of GaBO,. EX =Excitation spectrum of the emission of GaBO, at 4.2 K; qr gives the relative quantum output in arbitrary units. EM =Emission spectra of GaBO, at 4.2 K (LHT) and 290 K (RT) under 250 nm excitations; CD gives the relative spectral radiance in arbitrary units radiation with A I260 nm. The similarity between the lumi- nescence properties of GaB0, and P-Ga203 is ~triking.'~.' Therefore we follow the interpretation given for P-Ga203, viz.the ultraviolet emission is intrinsic and the blue emission is extrinsic. The Stokes shift of the intrinsic emission is ca. 13 500 cm-'. 3.2 GaB0, :Ed1' The luminescence properties of GaB0, :Eu"' are very similar to those of ScBO, :Eu"' which has the calcite structure also.I4 The Eu"' ion occupies a site with inversion symmetry so that 5Do-7F1 emission dominates. The other emission transitions occur as broad and weak features, since they can only take place as vibronic transitions. In GaBO, :Eu"' the europium emission can hardly be excited in the host lattice. Excitation with 250 nm yields mainly the blue/UV broad emission band on which the Eu"' lines are just observable.The excitation spectrum of the Eu"' emission does not show the host lattice excitation band. The study of GaB0,:Eu"' showed, however, that our sample contains a glassy second phase. This is most clear upon excitation into the 7Fo-5D2 transition of Eu"' which is forbidden in GaBO,. This excitation yields an Eu"' emission which is very similar to that in borate gla~ses:'~~'~ it consists of very broad lines and the 5D,-7F2 emission dominates. The broadness of the lines is due to inhomogeneous broadening in the glass phase. Whereas X-ray diffraction reveals one second phase, viz. P-Ga203, optical spectroscopy reveals another, viz. a borate glass. Fortunately, it is possible to separate the Eu"' emissions in the glass phase and the calcite phase, since the electric-dipole transitions dominate in the former and the magnetic-dipole transitions in the latter.Spectroscopically we did not observe any evidence for Eu"' in the B-Ga203 second phase. 3.3 GaBO, :Cr"' The luminescence properties of Cr"' in the calcite structure have been reported for ScB0, by several author^'^^'^ and the material has been proposed as a laser host. In ScB03 :Cr"' the chromium emission consists of the broad-band 4T2+4A2 emission, down to 4.2 K. 'This is due to a weak crystal field as is evident from the maximum of the 4A2-4T2 absorption band at 15 750 ~m-'.'~ J. MATER. CHEM., 1991, VOL. 1 In GaBO, :Cr"' the crystal field is stronger; the absorption maximum is now at 17 100 cm-' and at 4.2 K the emission consists of the 2E-4A2 transition (see Fig.2). The stronger crystal field is, of course, due to the smaller cation site in GaBO, (the ionic radii for six co-ordination are 0.615, 0.620 and 0.745 8, for Cr"', Ga"' and Sc"', respectively). At room temperature the chromium emission contains also the 4T2- 4A2 band emission, with a maximum at ca. 710 nm. Table 1 shows the spectral features observed for the emission of GaB0,:Cr"' at 4.2 K together with their assignment. The vibronic lines are relatively strong (see Fig. 2) which is due to the fact that the pure electronic line is electric-dipole forbidden 760 700 waveIe ng thin rn Fig. 2 Emission spectrum of GaBO, :Cr at 4.2 K Table 1 The vibronic side lines in the 2E-+4A2 emission of Cr"' in GaBO, at 4.2 K, and an assignment.Frequencies for ScB0,:Cr"' are given for c~mparison'~ GaBO, : Cr"' ScBO, :Cr"' position relative to vibronics in zero-phonon lineicm -assignment" 4~2-4~, emission 14 435 0-0 -65 V1 40 -180 '6 I70 -292 v4 320 -360 -430 v3 405 -662 V1 610 -710 v1 +Vl -850 pair line -960 pair line -1250 v3(B0,3 -1 1200 vl, lattice mode; v1 -6, vibrational modes of Cr06 octahedron; v,(BO,,-), asymmetrical borate stretching vibration J. MATER. CHEM., 1991, VOL. 1 due to the inversion centre at the cation site of the calcite structure. It can only occur as a magnetic-dipole transition. In Table 1 the vibronic lines are tabulated and tentatively assigned.The dominating vibronics are ascribed to coupling with the CrO, vibrational modes. It is interesting to note that in ScBO, :Cr these were also observed, although the emission transition is a different one, viz. 4T2-+4A2.However, in both cases the ungerade v3, v4 and v6 are necessary to break through the parity selection rule. Table 1 shows that in ScBO, these Cr06 vibrations are at lower frequency, which is to be expected in view of the larger Cr-0 distance. In addition we observed other side lines, uiz. one due to coupling with a lattice mode, vl, and one due to coupling with the asymmetric borate stretching vibration v,. The latter is very weak. Two lines are ascribed to pair lines since they are relatively sharp and their intensity varies upon changing the excitation wavelength.Actually, 1 -0.99512=6% of the Cr"' ions are expected to have a nearest Cr"' neighbour if we assume a statistical distribution of Cr"' on the gallium sites and 12 nearest cation neighbours. So the presence of pair lines is not unexpected. Fig. 3 shows the excitation spectrum of the Cr"' emission of GaBO, :Cr"' at room temperature. It shows the well known transitions 4A2-'4T2, 4T1 (F) and 4T1 (P) in sequence of increasing energy. The excitation band at the highest energy is the same as observed for the host-lattice emission and is ascribed to host-lattice excitation. At 4.2 K this host-lattice excitation band disappears. For comparison we investigated also P-Ga203 :Cr"'.Its luminescence characteristics are very similar to those of GaB0,:Cr"' with two exceptions: (i) the 4.2 K emission spectrum shows a much stronger zero-phonon line in the 2E-4Az emission for /I-Ga,O3:Cr1'' owing to the absence of inversion symmetry at the metal-ion sites of P-Ga203; (ii) host-lattice excitation is more effective in P-Ga203 :Cr"' than in GaB0, :Cr"'. At 4.2 K the excitation spectrum of the Cr"' emission of B-Ga2O3 :Cr"' contains the host-lattice excitation band, whereas that of GaBO, :Cr"' does not. Our samples of GaB0,:Cr"' contain, according to X-ray diffraction, a small amount of P-Ga203. In view of the results for GaBO, :Eu"' they may also contain a certain amount of the glass phase.The emission spectra at 4.2 K do not give any evidence for Cr"' in a glass phase, which is expected to give a broad-band 4T2-4A2 emission.16-18 This holds for every reasonable excitation wavelength. Therefore the Cr"' ion prefers the crystalline phases above the glass phase, as is well known (see e.g. ref. 18). Under a suitable excitation 1 300 600 wave lengthin m Fig. 3 Excitation spectrum of the Cr"' emission of GaBO,: Cr at 4.2 K. The excited levels of Cr"' have been indicated. H means host- lattice excitation wavelength it is possible to observe the Cr"' emission of 8-Ga203:Cr"'. It is easy, however, to account for this impurity. 3.4 LiGaO, The compound LiGaO, has an ordered wurtzite structure. Its diffuse reflection spectrum shows that the optical absorp- tion edge is at ca.215 nm at 300 K. The compound shows efficient luminescence at room temperature if excited with sufficiently short wavelengths. Fig. 4 shows the emission and excitation spectrum of LiGaO, at 290 K. The emission consists of a broad band with a maximum at 360 nm. The corresponding excitation band has a maximum at 220 nm which corresponds nicely with the diffuse reflection spectrum. From these values a Stokes shift of ca. 18 000 cm-' is derived. Note the higher energy positions of the spectral bands and the larger Stokes shift in comparison with the luminescence of GaBO, (section 3.1). 3.5 LiGaO, :Fe"' Rabatin" has claimed efficient and deep-red Fe"' emission from LiGa02:Fe"' and this was confirmed in the present study.Here we present only data which were not given by Rabatin, viz. the 4.2 K emission spectrum with a rich vibrational structure (see Table 2 and Fig. 5) and the Fe"' excitation spectrum. The latter spectrum consists of the well known crystal field transitions within the 3d5 configuration, the Fe"'-O" charge-transfer band at 260 nm, and the LiGaO, excitation band. The latter two have about the same intensity. 250 400 wave1 en gth in m Fig. 4 Emission and excitation spectra of the luminescence of LiGaO, at 290 K Table2 The vibronic side lines in the 4T1-6A1emission of Fe"' in LiGaO, at 4.2 K, and an assignment position relative to zero-phonon line/cm- assignment" 14 085 0-0 -1 10 Vl -225 v4 -325 -430 v3 -515 -635 -725 V1 1'1 +v, -900 '1 +'4 -1005 -1065 v1 +v3 -1120 a vl,lattice mode; v1 ,3 ,,, vibrational modes of FeO, tetrahedron I I I 4 760 700 wavelength/nm Fig.5 Emission spectrum of LiGaO, :Fe at 4.2 K In view of the Fe" concentration in LiGaO, (0.5 atom.%) this points to a restricted amount of energy transfer from host to activator. The 4.2 K emission spectrum consists of an intense zero- phonon line followed by a large number of side bands. These are tabulated in Table2 which contains also a possible assignment. The latter needs a more extended study for it to be confirmed. 4. Discussion Here we wish to concentrate on the gallate luminescence. The dopant emissions are essentially known and understood, and were treated above.It has been shown before that there exists an analogy between the luminescences of oxo-d10 and oxo- do complexes.' The present results underline this statement as will be shown now without entering into the problem of the nature of the optical transitions involved. We use the short-hand notation introduced above, uiz. oxo-d" and oxo- do for complexes such as Ga"' (d'") [O"In and Wv' (do) [O" I,, respectively. The luminescence properties of oxo-do complexes have been reviewed in ref, 20. Their emission and excitation spectra are characterized by very broad bands with Stokes shifts of 1-2 eV. Generally speaking the octahedral oxo-d" complexes have a smaller Stokes shift than the tetrahedral ones: for example, the WO: -octahedron in ordered perovskites shows a Stokes shift of 12 000 ern-', and the WOZ-tetrahedron in scheelites has 16 000 cm-'.' Finally, the optical absorption J. MATER.CHEM., 1991, VOL. 1 edge shifts to higher energies if the co-ordination number decreases. These properties are clearly evident in the present investi- gation. All gallate spectra observed are of the broad-band type (Fig. 1 and 4) and the Stokes shift of the emission is large (see Table 3). Since we now have available data for a compound with six-co-ordinated Ga"', uiz. GaBO,, and another with four-coordinated Ga"', uiz. LiGaO,, it is possible to evaluate the influence of the co-ordination number on the luminescence properties.Clearly the Stokes shift is larger for tetrahedral than for octahedral co-ordination (see Table 3). From a comparison of the excitation maxima for GaBO, and LiGa02 it is clear that the optical absorption edge is at lower energy for the case of six co-ordination (see Table 3). However, it should be realized that interaction between the optical centres involved may, in the case of a solid, lead to energy-band formation, so that the absorption edge shifts to lower energies. In case of the oxo-do complexes this is nicely illustrated by examples like TiO, and WO, where the edge shifts into the visible region. This effect plays most probably an important role in the case of In203 also (see Table 3). The comparison of InBO, and In203 leads immediately to the question why the optical absorption edges of GaBO, and /?-Ga203 are about the same.Above it was already noted that this coincidence is unexpected in view of the different nature of the borate and the oxygen ligands. However, in /?-Ga203 only 50% of the Ga'" ions are in six co-ordination, the others being in four co-ordination. In addition, both type of ions form layers. A nice presentation of the crystal structure has been given by Clark.21 In the octahedral layers each gallate octahedron has oxygen ions in common with four other octahedra. In GaB03 each Ga"' ion has 12 Ga"' nearest neighbours. This might bring the optical absorption edge to a lower value than expected at first sight. Unfortunately, we could not find reliable absorption data for a-Ga203 with all Ga"' ions in octahedral co-ordination. However, for SrGa12019 this edge is lower than for /?-Ga203.In SrGal2OI9 with magnetoplumbite structure there are spinel layers in which every octahedrally co-ordinated Ga"' ion has six Ga"' neighbours, a higher number than in P-Ga203. In view of our data we assume that the gallate tetrahedra in p-Ga203 show optical absorption at energies above the absorp- tion edge, so that it cannot be observed. In spite of the qualitative nature of the discussion it is clear that the analogy between the luminescence properties of oxo- d" and oxo-do complexes is striking. It is interesting to note that recently Nikol and Vogler2, observed for Sb"' and Bi"' chloro complexes the same spectral dependence on co- ordination number.In order to discuss the mobility of the excited state it is useful to consider how far the intrinsic excitation energy can be transferred to the impurity centres. Let us again first summarize the situation for the oxo-do complexes which has Table 3 Some data on the luminescence of compounds containing d" metal ions compounda excitation maximum/1O3 cm-' excitation maximum/103 cm- ' Stokes shift/103 cm-' ref. 40.0 26.5 13.5 this work 40.0 26.5 13.5 13 37.0 22.2 14.8 19 InBO, (6) ca. 45 33 ca. 12 6 InZ03 (6) 25' ca. 15 ca. 10 5 LiGaO, (4) Zn,O(acetate),c (4)Zn,O(B02)6 (4) 45.0 40.0 46.3 27.0 22.5 26.9 18.0 17.5 19.4 this work 3 2 a The co-ordination number of the d'O ion is given in parentheses; indirect band gap'; molecular complex in solution.J. MATER. CHEM., 1991, VOL. 1 been well studied.,, If the Stokes shift is large, the excitation is completely localized and the total amount of energy transfer restricted. However, if the Stokes shift is not too large, the excitation energy is mobile at room temperature and reaches the impurities from where efficient emission occurs. An example of the former is YNb04: Eu (niobate Stokes shift 16 000 cm-'), and of the latter YVO, :Eu (vanadate Stokes shift 10 000 cm-')',23 The same situation prevails for the 0x0-d" complexes. The compounds GaB03 and P-Ga203 are nice examples. At 4.2 K the gallate group shows intrinsic ultraviolet emission. At higher temperatures this emission disappears in favour of a blue emission.As has been shown for P-Ga203, this is due to energy migration to the blue-emitting centre.13 Unfortu- nately the nature of the centre is unknown. Harwig and Kellendonk have proposed that we are dealing with a gallate group with an oxygen vacancy.24 This is reminiscent of similar defect centres in 0x0-do complexes.' The presence of Eu"' or Cr"' in GaB0, can hardly compete with the blue centres for the excitation energy, the presence of Cr"' in P-Ga203 can at least trap part of the excitation energy. In contrast to these observations In,O,:Eu"' is able to show efficient luminescence upon host-lattice e~citation.~ This shows that the mobility of the excited state in In,O, is much higher, in agreement with the broader energy bands, which in turn are responsible for an absorption edge at low energy.In In203, with C-type lanthanide oxide structure, every In"' has 12 In"' neighbours which seem to be the basis for our observations. It is interesting that the case of Mn" in SrGa1201, shows an amount of energy transfer from host to activator" which is in between that in GaB0,:Eu"' and that in In,O,:Eu"'. This is to be expected from the position of the absorption edge. In ZII,O(BO~)~ with a larger Stokes shift, energy transfer to Mn" does not seem to take place., The same is expected to be the case for Fe"' in LiGaO,. Actually some transfer was observed in that system. However, it should be realized that each Ga"' ion in LiGaO, has six Ga"' neighbours and that the Fe"' concentration is 0.5 atom.%.Consequently 1 -0.9956=3% of the Ga"' ions has an Fe"' neighbour to which one-step energy transfer is possible. This accounts for the greater part of the transfer observed. All these results are additional evidence for the earlier statement' that 0x0-d" complexes have a localized excited state which is able to give efficient broad-band emission. At higher temperatures this excited state may become mobile depending on the amount of relaxation as measured by the Stokes shift. Therefore, this emission has also been indicated as self-trapped exciton emission (see e.g. ref. 13). These con- siderations, however, do not solve the problem of the nature of the optical transition.For that purpose, at least, a detailed energy-level calculation of an 0x0-d" complex has to be performed. References 1 G. Blasse, Chem. Phys. Lett., 1990, 175, 237. 2 H. Kunkely and A. Vogler, J. Chem. SOC., Chem. Commun., 1990, 1204. 3 A. Meijerink, G. Blasse and M. Glasbeek, J. Phys. Condensed Matter, 1990, 2, 6303. 4 H. Lange, Techn. Wiss. Abh. Osram Ges. (Munchen), 1969, 10, 87. 5 H. Yamamoto and K. Urabe, J. Electrochem. Soc., 1982, 129, 2069. 6 G. Blasse and L. H. Brixner, Muter. Chem. Phys., 1991, 28, 275. 7 G. Blasse, J. Inorg. Nucl. Chem., 1967, 29, 266. 8 R. Hoppe, Bull. SOC. Chim. Fr., 1965, 1115. 9 M. O'Keeffe and B. G. Hyde, Acta Crystallogr, Sect. B, 1978, 34, 3519. I0 J. G. Rabatin, J. Electrochem. SOC., 1978, 125, 920. 11 0. Muller and R. Roy, The Major Ternary Structural Families, Springer-Verlag, Berlin, 1974. 12 G. Blasse and A. Bril, J. Phys. Chem. Solids, 1970, 31, 707. 13 T. Harwig, F. Kellendonk and S. Slappendel, J. Phys. Chem. Solids, 1978, 39, 675. 14 G. Blasse and G. J. Dirksen, Inorg. Chim. Acta, 1988, 145, 303. 15 J. W. M. Verwey, G. J. Dirksen and G. Blasse, J. Non-cryst. Solids, 1988, 107, 49. 16 J. W. M. Verwey and G. Blasse, Muter. Chem. Phys., 1990, 25, 91. 17 S. T. Lai, B. H. T. Chai, M. Long and R. C. Morris, IEEE J. Quantum Electron., 1986, 22, 1931. 18 G. Boulon, Muter. Chem. Phys., 1987, 16, 301. 19 J. M. P. J. Verstegen, J. Solid State Chem., 1973, 7, 468. 20 G. Blasse, Structure Bonding, 1980, 42, 1. 21 G. M. Clark, The Structures of Non-molecular Solids, Applied Science, London, 1972. 22 H. Nikol and A. Vogler, J. Am. Chem. SOC., to be published. 23 G. Blasse, in Handbook on the Physics and Chemistry of the Rare Earths, ed. K. A. Gschneidner Jr. and L. Eyring, North Holland, 1979, ch. 34. 24 T. Harwig and F. Kellendonk, J. Solid State Chem., 1978, 24, 255. Paper 1/024816; Received 28th May, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101001

出版商:RSC    

年代:1991

数据来源: RSC