Chem. Mater. 1992,4, 583-588
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Electrosynthesis of Polypyrrole in a Nematic Liquid Crystal Walter Torres and Marye Anne Fox* Department of Chemistry and Biochemistry University of Texas, Austin, Texas 78712 Received September 13, 1991. Revised Manuscript Received December 9, 1991 Pyrrole was anodically polymerized on platinum and indium-tin oxide electrodes in the nematic liquid crystal 4cyano-4'-pentylbiphenyl containing an electrolyte. The nematic mixture was oriented with respect to the opposite working and counterelectrodes, which were separated by a 13-pm spacer, by previous silanization of the electrodes or by applying a sufficiently high electric field. Orientation of the nematic polymerization mixture induces shifta in the voltammetric peaks and the absorption spectral bands of the resulting films to more negative potentials and higher wavelengths, respectively. Some anisotropic conductivity was attained in films prepared in an oriented nematic environment. However, measurementa along and across the f h surface showed only small conductivitydifferences, and no evidence for crystallinity was obtained.
Introduction Toward the Synthesis of Anisotropic Conducting Polymers. The essential feature required for an organic conducting polymer is an electronic band structure which permits electronic delocalization.' In neutral conjugated organic polymers, charge carriers generated when these materials undergo reversible chemical or electrochemical charge-transfer reactions induce changes in conductivity of several orders of magnitude. Qpical conductivity values obtained in common conjugated polymers (in the oxidized state) such as polyacetylene, polypyrrole, and polythiophene are of the order of 101-103 mho cm-lS2 In these polymeric materials, the finite size of the chains (which determines the extent of the conjugation) and the level of structural disorder (such as cross-links and sp3 defects) limit the attainable conductivity. The magnitude of conductivity in such materials depends on the charge carrier density and the mobility of these carriers. In conducting polymers, the carrier densities are relatively high (in the range 1018-1022~arriers/cm-~, compared to loz4and 1Ol8 carrier~/cm-~ in typical metals and semiconductors, respectively),3but the mobilities are lo& because of resistive backscattering and localization of charge carriers produced by the high doping level5and by structural disorder. Thus, to attain higher conductivities in conjugated polymers, higher charge carrier mobilities (delocalization of carriers produced by extension of the conjugation and suppression of scattering6) are needed. Higher carrier mobilities are attainable by molecular design,' longer polymer chains? and/or by polymer chain alignment in the solid ~ t a t e . ~ Much work has been done on the design and synthesis of new monomeric units bearing appropriate side groups to improve specific electronic or mechanical properties of (1) For a comprehensive review, see: Skotheim, T. J., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986.
(2) These values are higher than the conductivities of inorganic semiconductors (10-5-10-3 mho cm-') but lower than thcae of common metals (104-106mho cm-'). (3) Typical carrier densities in inorganic semiconductors are in the range 1d~a-10'8 (4) Typical carrier mobilities in conducting polymers are 10*lO-z cmz V" 5-I. Mobilities in inoreanic semiconductors are about 10L105 cmz s-1. (5) A typical conducting polymer requires ca. 25% 'doping" (1 counterion/5 monomer units) to attain quasi-metallic conductivities. In a typical doped inorganic semiconductor, the amount of doping is of the order of a few ppm. (6) Winokur, M.; Moon, Y. B.; Heeger, A. J. Phys. Reu. Lett. 1987,58, 2329. (7) (a) Jenekhe, S. A. Nature (London) 1986,322,345. (b) Jenekhe, S. A. Macromolecules 1986,19,2663. (c) Jenekhe, S. A. Polym. P r e p . 1986, 17, 74. (8) (a) Heeger, A. J. Faraday Discuss. Chem. SOC. 1989,88, 203. (b) Heeger, A. J.; Wudl, F. Chem. Reu. 1988, 88, 183. Patil, A. 0.; (9) Kivelson, S.; Heeger, A. J. Synth. Met. 1988,22, 371. I
the resulting polymers.'O However, only recently have efforts been made to obtain a specific polymer orientation at the microscopic level." To reach this goal, two synthetic approaches have been employed (a) polymerization within a spatially restricted host lattice such as a clay12 or a zeolite13and (b) polymerization in oriented liquidcrystal s01vents.l~ Chemical syntheses of p o l y p y r r ~ l e , ~PqlY~~J~~ thiophene,'*13a polyaniline,'" and p ~ l y f u r a n composites '~ in clays'" and zeolites'%J%have been described. Isolated single polymer chains (or "molecular wires") can be prepared in small zeolite channels by this scheme, but conductivity measurements indicate that the resulting polymer/zeolite composites are in~u1ators.l~~ The chemical synthesis of polyacetylene in a 4-(trans4-n-propylcyclohexy1)alkoxybenzenenematic liquid-crystal medium containing a soluble Ti(OC,H9)4-A1(C2HS)3 Ziegler-Natta catalyst has also been acc~mplished.'~~ High orientation of the liquid crystal (LC) matrix was induced by either gravity flow of the solution or by applying magnetic fields higher than 5 kG. The resulting polyacetylene films have conductivities of the order of 104mho cm-'. Fibrillar orientation was determined by electronic micro~copy.'~~ Recently, a procedure by which pyrrole (or thiophene) is polymerized in a nematic liquid crystal by electrochemical means was patented.15 A 2 mA cm-2constant current was applied between two Pt foil electrodes (separated by 0.1 cm) immersed in 0.1 M pyrrole in 0.3 M tetrabutylperchlorate/4-cyano-4'-(trans-4-pentylcyclo~ammonium ~~ ~
(10) For a comprehensive review, see: Garnier, F.; Horowitz, G.; Roncali, J.; Lemaire, M. Ber. Bunsenges. Phys. Chem. 1988,92,1261 and references therein. (11) Macroscopic fibrillar morphologies can also be induced by conventional techniques such as mechanical stretching. (12) (a) Soma, Y.; Soma, M.; Harada, I. Chem. Phys. Lett. 1983,99, 153. (b) Soma, Y.; Soma, M.; Harada, I. J. Phys. Chem. 1985,89, 738. (c) Soma, Y.; Soma, M.; Fumkawa, Y.; Harada, I. Clays Clay Miner. 1987, 35, 53. (d) Kanatzidis, K. G.; Tonge, L. M.; Marks, T. J.; Marcy, H. 0.; Kannewurf, C. R. J. Am. Chem. SOC.1987, 109, 3797. (e) Brandt, P.; Fischer, R. D.; Marchez, E. S.; Calleja, R. D. Angew. Chem., Znt. Ed. Engl. 1989,28, 1265. (13) (a) Chao, T. H.; Erf, H. A. J. Catal. 1986,100,492. (b) Enzel, P.; Bein, T. Chem. Commun. 1989, 1326. (c) Bein, T.; Enzel, P. Angew. Chem.,Znt. Ed. Engl. 1989,28,1692. (d) Bein, T.; Enzel, P. Mol. Cryst. Liq. Cryst. 1990,181,315. (e) Hwang, B.; Chon, H. Zeolites 1990,10,101. (14) (a) Araya, K.; Mukoh, A.; Narahara, T.; Shirakawa, H. Synth. Met. 1986,14,199. (b) Akagi, K.; Shirakawa, H.; Araya, K.; Mukoh, A.; Narahara, T. Polym. J. 1987, 19, 185. (c) Akagi, K.; Katayama, S.; Shirakawa, H.; Araya, K.; Mukoh, A.; Narahara, T. Synth. Met. 1987,17, 241. (d) Shirakawa, H.; Akagi, K.; Katayama, S.; Araya, K.; Mukoh, A.; Narahara, T. J.Macromol. Sci.: Chem. 1988, A25, 643. (e) Akagi, K.; Ito, M.; Katayama, S.; Shirakawa, H. Mol. Cryst. Liq. Cryst. 1989, 172, 115. (15) Naarmann, H.; Portugall, M.; Hisgen, B. (BASF A.-G., Fed. Rep. Ger.) Ger. Offen. DE 3,533,252, 1987; Chem. Abstr. 106(26): 225801~.
08974756f 92f 2804-0583$03.00f 0 0 1992 American Chemical Society
584 Chem. Mater., Vol. 4, No. 3, 1992
Torres and Fox
hexyl)biphenyl. Polypyrrole films with conductivities of 370 mho cm-' (in one direction) and 70 mho cm-l (measured at 90° with respect to the first direction) were claimed, although no details of the measurements were discussed. No procedure to align the liquid-crystalline phase with respect to the electrode plane was described nor was any independent evidence of alignment of the polypyrrole chains given. Here we report the anodic synthesis of polypyrrole films on modified Pt and indium-tin oxide electrodes in 4cyano-4'-pentylbiphenyl (K-15, a nematic liquid crystal) containing tetrabutylammonium tetrafluoroborate and pyrrole. The opposing working and counterelectrodes, which were separated by ca. 13 pm, were pretreated with a silane layer that orients the liquid-crystal mixture perpendicular to these electrodes (as observed by optical experiments) to test whether this anisotropic environment could influence orientation and crystallinityin the resulting polymer. The voltammetric and conductivity properties of films produced in K-15depended on the pretreatment of the electrodes and the magnitude of the applied electric field in the nematic medium during polymerization, although no evidence of crystallinity in the resulting polymers was found.
Experimental Section Chemicals. The liquid crystal4-cyanc4-pentylbiphenyl (K-15, BDH) was used as received. Tetrabutylammonium tetrafluoroborate (TBATFB, Southwestern Chemicals, 99%) was recrystallized from ethyl acetate/ether, vacuum dried at 70 "C for 72 h, and stored under vacuum at room temperature. Pyrrole (Aldrich, 99%) was first distilled and then passed through a neutral alumina column before use. Dichlorodimethylsilane (Fluka)was distilled before use. ACM-72 (a surfactant solution commercially used to induce parallel alignment, Atomergic) was used as received. Alumina was stored at 140 "C. Toluene was dried over MgSO,.
Surface Pretreatment for Alignment of the Nematic Liquid Crystal. Glass Slides. Glass slides were cleaned in hot HN03 for 15 min, rinsed with deionized water, and dried at 140 "C for 4 h. Surfaces that induce perpendicular orientation were obtained by dip-coating the clean glass slides with 5% (v/v) dichlorodimethylsilane/toluene under nitrogen at room temperature for 10 min, rinsing with toluene, and drying at 140 "C for 2 h.% To induce parallel orientation, clean glass slides were dip-coated with 0.10% (v/v) ACM-72ltolueneat room temperature for 3 min, rinsed with water, rubbed on a 0.05pm diamond paste slurry, rinsed with deionized water, and dried at 140 "C for 2 h. Indium-"in Oxide Electrodes. hdium-tin oxide coated glass slides (ITO, Delta Technologies) were cleaned in ethanol at 60 "C for ca. 30 min, rinsed with ethanol and deionized water, dried at 140 "C for 2 h, and immersed in hot 5% (v/v) silane/toluene under N2 for 2 h, following a procedure for silanization of Sn02 electrodes.16 Platinum Electrodes. Pt disk electrodes (area = 0.10 mm2) were polished with 1-,0.3-, and 0.05-pm alumina. Platinum oxide electrodes (PtO)17were prepared by holding polished Pt electrodes at +1.9 V v9 SCE in 0.5 M H8O4 for 5 min, cycling between +1.4 and 0.0 V for ca. 1h, and holding at +1.4 V for ca. 5 min. PtO electrodes were washed with deionized water, vacuum-oven dried at 50 "C for 30 min, and dipped in 5% dichlorodimethylsilane/toluene under N2 for 5 min. The silanized PtO electrodes were washed with toluene and acetonitrile and dried under a stream of N2 before use. (16) Cieslinaki, R C.; Armstrong,N. R. J. Electrochem. Soc. 1980,127, 2605. (17) (a) Lenhard, J. R; Murray, R. W. J. Electrocmal.Chem. 1977,78, 195. (b)Wrighton, M. S.;Palazzotto, M. C.; Bocarsly, A. B.; Bolts, J. M.; Fischer, A. B.; Nadjo, L. J . Am. Chem. SOC.1978, 100, 72. (c) Angerstan-Kozlowska, H.; Conway, B. E.; Sharp,B. A. J . Electroanal. Chem. 1973, 43, 9.
Figure 1. Textures of nematic liquid crystal mixtures containing 0.10 M pyrrole in 40 mM TBATFB/K-15 observed with a cross-polarized microscope at 24 f 1 "C. Samples were placed between (a) untreated glass slides and (b) glass slides treated with ACM-72. Gold Electrodes. Gold electrodes were prepared by vapor deposition of ca. 1000-hhick gold films onto ca. 100-&thick chromium layers previously deposited onto clean glass slides (to improve adhesion to the gold). To prepare gold oxide electrodes (AuO), gold electrodes were immersed in hot chromic acid18 for ca. 5 min, rinsed with deionized water and methanol, and dried with a heat gun.lg The dried oxide-coated electrodes were then dipped in 5 % dichlorodimethylsilanein toluene for ca. 20 min, rinsed with toluene, and dried under a stream of NPm Determining the Nematic-Isotropic Transition of LC Phases. To determine the characteristic temperature at which a nematic-isotropic phase transition occurs, samples of K-15 containing 0.040 M TBATFB and a variable concentration of pyrrole (in the range 0 . 2 0 M) were sandwiched between clean glass slides, separated by a 13-pm polyethylene spacer (First Brands),and heated (and m l e d ) in the temperature range 22-38 "C at 2 "C/min, while being observed with a Leitz Laborlux D cross-polarized optical microscope (in the transmission mode) equipped with a hot stage and a Cannon A-1 SLR camera. Below a critical temperature T:, characteristic of each sample composition, irregular multicolor textures in the sample indicated that the sample was in the nematic phase and that no bulk orientation of the sample with respect to the glass surfaces had occurred. Above Tl,the disappearance of birefringence (the sample would (18) Chromic acid was freshly prepared as described in: The Chemist's Companion; Wiley and Sons: New York, 1974.
(19) For an alternative procedure to prepare gold oxide surfaces, see: Ttimkvist, C.; Liedberg, B.; Lundstriim, I. Langmuir 1991, 7, 479. (20) Widrig, C. A.; Majda, M. Anal. Chem. 1987,59, 754.
Electrosynthesis of Polypyrrole
a
-
Chem. Mater., Vol. 4, No. 3, 1992 585
LC =Liquid crystal phase oxidelayer
silane layer
onecompartment threeelectrode cell was used,with a Ag/AgNOS electrodeB and a Pt wire used as reference and counter electrode, respectively). The two-electrode configuration cell (Figure 2b) used to prepare samples in K-15 for UV-vis, X-ray diffraction, and conductivity measurements was made with two clean I T 0 (or two silanized ITO, exposed area = 1cm2)electrodes separated by a 13-pm polyethylene spacer. Synthesis of Polypyrrole in K-15.The anodic syntheses of polypyrrole were done in K-15 containing 0.10 M pyrrole and 40 mM TBATFB under potentiostatic conditions at 24 1OC. For syntheses on Pt disk (or silanized PtO) electrodes, the total deposited anodic charge in every case was 12 2 pC, which produces a ca. 50-nm-thick film as estimated from profilometry measurements on thicker polypyrrole f h s , assuming a linear relationship between the anodic charge passed for polymerization and the thickness of the resulting film. The polymer-coated electrodes were then dipped in acetonitrile for ca. 1ha and transferred to a monomer-free 0.40 M TBATFB/acetonitrile solution for cyclic voltammetric inspection. Prior to the experiments, the cell was bubbled with nitrogen for 15 min. Experiments were run under nitrogen. For polymerizations on IT0 (or silanized ITO), Figure 2b, so as to permit in situ observation by a cross-polarized microscope, the total anodic charge passed was 12 0.1 mC. UV-Vis Absorbance and X-ray Diffraction Experiments. Absorbance spectra in the UV-vis region were measured with a Hewlett-Packard 8451-A diode array single-beam spectrophotometer. X-ray diffraction experiments were carried out with a Philips VS1.3 instrument. The scattered intensity data were collected using Cu Ka radiation (X-ray generator operated at 40 kV and 40 mA) over the angular range 2-60" 219in steps of 0.2". Conductivity Measurements. For conductivity measurements, polymer samples (area = 1 cm2,thickness = 2 pm) were peeled from the I T 0 electrode. In linear four-point probe conductivity measurements, with an Alessi 614-5 connected to a Hewlett-Packard 6186-C dc current source, the freestanding films were mounted with epoxy to a glass slide. In two-point probe measurements, the films were secured between Pt foils with silver paint. Values of conductivity were calculated from current-voltage curves.26
*
*
r
Polyethylene
1-
1111
*
Au
b
I
eilanelayer
IT0
1
I
Figure 2. Electrochemical cells used for the synthesis of polypyrrole in K-15 (a) three-electrode configuration; (b) two-electrode configuration. turn black) indicated the transition of the sample to the isotropic phase. Orientation. To determine the alignment of nematic phases, samples of either K-15 alone or K-15 containing 0.040 M TBATFB 0.10 M pyrrole were sandwiched between treated glass plates (or between silanized I T 0 electrodes) separated by a 13-pm polyethylene spacer at 24 1 "C) and observed with a crosspolarized optical microscope to check for the presence or absence of birefringence. Samples of either K-15 alone or samples of K-15 containing 0.040 M electrolyte 0.10 M pyrrole between clean untreated glass slides (or between clean I T 0 electrodes) exhibited irregular birefringenttextures in the nematic when observed with the cross-polarized microscope, Figure la, indicating no bulk orientation of the sample with respect to the surfaces. Samples between glass surfaces treated with ACM-72 (Figure lb) showed birefringent areas, as threadlike double lines (that correspond to inversion walls), indicating parallel orientation of the sample.21 Samples between silanized glass slides (or between silanized I T 0 electrodes) looked totally black when observed with the crosspolarized microscope. Perpendicular orientation of the samples was confirmed when, by slightly touching the cover glass of the sample, a characteristic flashlike brightness was observed, indicating disturbance of the perpendicular orientation of the sample as a response to the locally applied pressure.22 Cells. Either a three-electrode or a two-electrode configuration cell was used for the electrosynthesis of polypyrrole. For electrosynthesis in K-15 in the three-electrode configuration cell (Figure 2a), the working electrode was a Pt (or a silanized PtO) electrode, the quasi-reference electrode was a silver wire, and the counter electrode was an Au (or a silanized AuO) electrode. (For cyclic voltammetric experiments in acetonitrile, a conventional
+
+
(21) Nehring, J.; Saupe, A. J.Chem. Soc., Faraday Trans. 2 1972,68, 1.
(22) Saupe, A. In Liquid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A,, Eds.; John Wiley and Sons: New York, 1974; Chapter 2, Vol 1.
Results and Discussion Nematic Liquid Crystals. The utility of nematic liquid crystals as reaction media stems from the anisotropic properties of these materials.26 The nematic liquid crystalline (NLC) phase is usually formed by molecules with elongated shapes. In a nematic phase, the orientation of the long molecular axis fluctuates around a preferred direction called the director L so that the phase has uniaxial symmetry with respect to all physical proper tie^.^' Although the direction of L changes from place to place over macroscopic volumes, LCs can be oriented over thicknesses as large as 100 pm by interactions of the LC phase with a solid surface (wall effects) or by interaction with an electric or magnetic field.28 The mean orientation of the LC molecular axis relative to a solid surface depends on the ratio of the surface tension of the LC, yl,and that of the solid, y8,29and/or (23) Because of the size restrictions of the cells, a silver wire wa8 used for polymerizations in K-15. Cyclic voltammetry experiments in acetonitrile, however, are reported vs Ag/AgNO,. (24) This procedure was sufficient to completely remove the liquid crystal as indicated by cyclic voltammetry of the film. (25) (a) ValdBs, L. B. Proc. Inst. Radio Eng. 1964,42,420. (b) Uhlir, A. Bell Syst. Tech. J. 1966, 105. (26) (a) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Germany, 1988, p 263 and references therein. (b) Scheffer, J., Tetrahedron 1987, 43, 1197 and references therein. (27) Meier, G.; Sackmann, E.; Grabmaier, J. G. Applications of Liquid Crystals; Springer-Verlag: New York, 1975. (28) (a) Cognar, J. J. Mol. Cryst. Liq. Cryst., Suppl 1 1982, 1. (b) Herino, R. J. Chem. Phys. 1981, 74, 3016. (c) Kuhn, W.L.; Finlayson, B. A. Mol. Cryst. Liq. Cryst. 1976, 36, 307. (29) According to the empirical Friedel-Creagh-Kmetz rule, when yl > yI,the LC molecules will orient parallel to the solid surface, but when yo < yI,a perpendicular orientation of the LC molecules will be preferred.
586 Chem. Mater., Vol. 4, No.3, 1992
Torres and Fox
t
n
50 nA
I/
J,
/d 2.5
1.5
0.5
0.5
1.5
1.5
1.0
0.5
0.0
Volts vs AgiAgNU3
Volts vs Ag
Figure 3. Cyclic voltammogram of 1.0 mM K-15 on a Pt electrode (area = 0.10 mm2) in 0.40 M TBATFB/acetonitrile at 50 mV/s at room temperature under nitrogen.
Figure 4. Linear potential sweep curves (sweep rate = 20 mV s at 24 1 "C,under nitrogen) for the anodic oxidation of 0.10 pyrrole: (a) on silanized PtO with a silanized AuO counter electrode in 40 mM TBATFB/K-15; (b) on Pt with a silanized AuO counter electrode in 40 mM TBATFB/K-15; (c) on Pt with an Au counter electrode in 40 mM TBATFB/K-15; (d) on Pt in 40 mM TBATFB/acetonitrile.
specific bonding interactions at the interface.30 The relative orientation of a NLC with respect to an electric field depends on the sign of the LC dielectric anisotropy Ae = ell - eL, where ell and el refer to the components of the dielectric constant of the LC along and perpendicular to the molecular axis. The sign of A€ depends on the molecular polarizabilities and the value and angular position of the permanent electric dipole moment. When in the presence of an electric field, the LC molecules tend to orient parallel to the field (after some threshold field value) if A€ > 0 and perpendicular to the field if At < 0.31 A LC solvent can induce orientation of molecular solutes that do not form a LC phase by t h e m s e l v e ~ . ~For ~ instance, pyrrole as well as thiophene and furan dissolved in some nematics orient with their molecular planes parallel to the long axis of the nematic long axis.33334 The parameters that determine the orientation of solutes in liquid crystals are not well understood. Several theoretical models to explain solute-liquid crystal interactions are based on dispersion forces,35polarization anisotropy,36size and moments of inertia of the and interactions of the molecular quadrupole moment of the solute with the average electric field gradient in the liquid crystalline environment .3'3b934,4'3 Studies involving the use of LCs as solvents for electrochemistry are relatively new.4l Although LCs have low
*
XI
1978, 54, 667. (32) (a) Meibom, S.;Snyder, L. C. Science 1968,162,1337. (b) Snyder, L. C.; Meibom, S.Mol. Cryst. Liq. Cryst. 1969, 7,181. ( c ) Balon, W. E.; Brown, G. H. Mol. Cryst. Liq. Cryst. 1969, 6, 155. (d) Saeva, F. D.; Sharpe, P. E.; O h , G. E. J. Am. Chem. SOC.1975, 97, 204. (e) Verbit, L.; Halbert, T. R.; Patterson, R. B. J. Org. Chem. 1975, 40, 1649. (33) (a) Diehl, P.; Khetrapal, C. L. NMR-Basic Principles and Progress; Diehl, P.; Fluck, E., Eds.; Springer-Verlag: Berlin, 1969; Vol 1. (34) (a) Bags, J. M.; Fhhka",E. J.; Randall, E. W. J.Magn. Reson. 1975,17,55. (b) Viiniinen, T.; Jokisaari, J.; Kiiiiriiiinen, A.; Lounila, J. J. Mol. Struct. 1983, 102, 175. (35) (a) Saupe, A. Mol. Cryst. 1966,1, 527. (b) Weaver, A.; Van der
dielectric constants and high viscosities, which make electrochemical experiments difficult to perform, these materials may provide an interesting medium for investigating differences in electrochemical reactivity, provided the liquid-crystal phase is oriented with respect to the electrode surface.31* Hence, the rate of mass transport of a redox species, which affects the faradaic current response at an electrode, should depend on the relative orientation of the LC phase a t the electr~de.'~ Suitability of K-15as a Polymerization Solvent. A cyclic voltammogram of K-15 in acetonitrile, Figure 3, shows one poorly resolved irreversible oxidation wave at ca. +1.6 V vs Ag/AgN03 and a quasi-reversible reduction peak at about -1.9 V. Other investigators have used K-15 as a solvent for cyclic voltammetry experiments on some common redox couples, such as ferrocene and TCNQe31 For the purpose of electrochemical polymerization of pyrrole, K-15 offers a suitable potential range since pyrrole can be polymerized at potentials lower than +1 V vs Ag/AgNO3. Nematic Mixtures of (Pyrrole + Electrolyte) in K-15.Neat K-15 is a nematic liquid crystal in the temperature range 22-35 "C. The ability of K-15 to dissolve an electrolyte (TBATFB) and pyrrole and simultaneously remain in the nematic phase was confirmed by observing these mixtures between clean glass slides with the crosspolarized microscope at 24 f 1 "C. Thus, mixtures of TBATFB (in the concentration range 0-50 mM) in K-15 and mixtures containing pyrrole (in the concentration range 04.20 M) in 0.040 M TBATFB/K-15 exhibited textures that are characteristic of unaligned nematic liquid crystals,43Figure la. Bulk orientation of nematic samples containing 0.10 M pyrrole + 40 mM TBATFB in K-15 was obtained when this mixture was placed between treated glass slides or treated IT0 electrodes. Textures characteristic of a parallel alignment4 were observed for samples between either glass surfaces or IT0 electrodes treated with ACM-72, Figure lb. Silanized surfaces (glass or ITO) induced the black texture characteristic of a perpendicular orientat i ~ n . Presumably, ~~ in these nematic mixtures pyrrole
1982, 77, 5386. (37) Robertson, J. C.; Yim, C. T.; Gilson, D. F. R. Can. J. Chem. 1971, 49, 235. (38) (a) Anderson, J. M. J. Magn. Reson. 1971,4, 231. (b) Samulksi, E. T. Ferroelectrics 1980, 30, 83. (39) Weaver, A.; Van der Est, A. J.; Rendell, J. C. T.; Hoataon, G. L.; Bates, G. S.; Burnell, E. E. Liq. Cryst. 1987, 2, 633. (40)Patey, G. N.; Burnell, E. E.; Snijders, J. G.; De Lange, C. A. Chem. Phys. Lett. 1983, 99, 271.
(41) (a) Serra, A. M.; Mariani, R. D.; Abrulia, H. D. J. Electrochem. SOC. 1986,133,2226. (b) Mariani, R. D.; AbruAa, H. D. Electrochim. Acta 1987, 32, 319. (c) Oyama, N.; Osaka, T.; Okajima, T.; Maruyama, T.; Ohnuki, Y. J. Electroanal. Chem. 1985, 187, 79. (d) Mariani, R. D.; Abruh, H. D. J. Electrochem. SOC.1989,136, 113. (42) S w m a n , A. RCA Reu. 1974,35,600. (43) Priestley, E. B.; Wojtowicz, P. J.; Sheng, P. Ed. Introduction to Liquid Crystals; Plenum Press: New York, 1974. (44) Collings, P. J. Liquid Crystals: Nature's Delicate Phase of Matter; Princeton University: Princeton, NJ, 1990.
(30) (a) Sanda, P. N.; Dove, D. B.; Ong, H. L.; Jansen, S. A.; Hoffman, R. Phys. Reu. A 1989,39,2653. (b) Hauck, G.; Kwwig, H. D. Mol. Cryst. Liq. Cryst. 1990, 179, 435. (31) (a) Kaito, A.; Wang, Y. K.; Hsu, S. L. Anal. Chim.Acta 1986,189, 27. (b) Hakemi, H.; Jagodzinski, E. F.; Du PrC, D. B. J. Chem. Phys. 1983,78,1513. (c) Cummins, P. G.; Dunmur, D. A,; Laidler, D. A. Mol. Cryst. Liq. Cryst. 1975, 30, 109. (d) Coles, H. J.; Jennings, B. R. Mol. Phys. 1978,36,1661. (e) Parneix, J. P.; Chapoton, A. Acta Phys. Pol. A
Est, A. J.; Rendell, J. C. T.; Hoataon, G. L.; Bates, G. S.; Burnell, E. E. Liq. Cryst. 1987,2, 633. (36) Snijders, J. G.; De Lange, C. A,; Burnell, C. A. J. Chem. Phys.
Chem. Mater., Vol. 4, No. 3, 1992 587
Electrosynthesis of Polypyrrole
A
orients with its molecular plane parallel to the long axis of K-15.33 However, no assumption can be made about whether there is a preferred orientation for the C2axis of pyrrole. Thus, even in this oriented medium, pyrrole molecules may freely rotate and translate in any plane perpendicular to the electrodes. Synthesis of Polypyrrole in K-15. Figure 4 shows linear sweep voltammetric curves between 0.0 and +1.4 V VE Ag for the oxidation of pyrrole in K-15 (curves a-c) and in acetonitrile (curve d) at 20 mV/s. For curve a, both the working and counter electrodes were silanized.23For curve b, the working electrode was an untreated Pt electrode, I . . . , . , I and for curve c, the working and counter electrodes were 0.5 0.0 - 0.5 - 1.0 - 1 . 5 untreated Pt and Au surfaces, respectively. The onset for Volts vs Ag/AgNO 3 the oxidation of pyrrole in K-15 is at ca.+0.9 V vs Ag. In Figure 5. Cyclic voltammograms of polypyrrole in 0.40 M acetonitrile, the onset is at ca. +0.8 V. TBABTFB/acetonitrile at 20 mV s at room temperature under High overpotentials for the polymerization onset and low nitrogen. Films were synthesized (a) on Pt in 0.10 M pyrrole polymerization rates in K-15 are expected because of the + 40 mM TBATFB/acetonitrile at +1.0 V w Ag; (b)on Pt in 0.10 high resistivity and viscosity of the medium and the low M pyrrole + 40 mM TBATFB/K-15 at +1.0 V va Ag; (c) on Pt mobility of the monomer. At 24 "C, the two components in 0.10 M pyrrole + 40 mM TBATFB/K-15 at +1.3 V vs Ag; (d) on silanized PtO in 0.10 M pyrrole + 40 mM TBATFB/K-15 at of the dielectric constant for K-15, e,, and el, are 16 and +1.0 V vs Ag. 8.2, and the viscosity components,.VlIand vl, are 28 and 39 cP, respectively. For acetonitrile at 20 "C, 71 = 0.376 to +1.4 V vs Ag are represented by curve d. For curves CPand c = 37.5. b and c, no perturbation of the cyclic voltammogram is For curve a, we assume that silanization of both the Pt observed when the potential is scanned from -1.0 to -2.3 and Au surfaces induces a perpendicular orientation of the V, indicating that no K-15 is trapped in any of these films. nematic phase as was observed by polarized microscopy The possibility that the negative potential shift of the for silanized IT0 and glass surfaces. For the film produced oxidation/reduction peaks observed on going from curve in curve b, since the untreated Pt surface was not hydroa to d is a consequence of the local dielectric properties phobic the nematic phase was not expected to be perof the resulting films was tested by polymerizing pyrrole pendicularly oriented at the interface. For curve c, the in a solvent of low dielectric constant, dichloromethane, nematic phase is presumably not oriented, as neither the and in a solvent of high viscosity, cyclohexanol. When Pt disk nor the Au surfaces were pretreated. The different formed by electropolymerizationon Pt in dichloromethane current responses in curve a and b indicate that, when the (e = 9.08 at 20 "C, v = 0.449 CPat 20 "C) in the potential silanized PtO electrode is used, the mass transport and/or +0.9 to +1.2 VI polypyrrole films show voltammetric range interfacial electron-transfer rates decrease, presumably as waves at -0.30 V/-0.40 V w Ag/AgNO,. Polypyrrole films a consequence of the flow restrictions imposed by the generated in the more viscous cyclohexanol (e = 15.0 at 20 anisotropic environment. The small difference in current "C, 7 = 45.0 CPat 20 "C) showed cyclic voltammetric peaks response between b and c indicates that the orientation at -0.25/-0.35 V. Thus, the peak shift observed in Figure at the hydrophobic silanized AuO counter electrode pro5 (ca. 100 mV from curve a to curve b) may relate to the duces little effect on the rate of mass-transport to the low dielectric constant of K-15.49 For films polymerized working electrode. on Pt in K-15 (curves b and c), the positions of the anCyclic Voltammetry of Polypyrrole in Acetonitrile. odic/cathodic waves are a function of the polymerization Figure 5 shows cyclic voltammograms of polypyrrole films potentials. in 0.20 M TBATFB/acetonitrile at 20 mV/s. These films Published spectroscopic studies indicate that, at a were synthesized in 0.10 M pyrrole + 40 mM TBATFB/ threshold field of ca. 1000 V/cm, the long axis of K-15 acetonitrile (curve a) and in 0.10 M pyrrole + 40 mM orients parallel to the field.50 Indeed, we observe, by TBATFB/K-15 (curves b-d) at 24 f 1 "C. Curve a corpolarized microscopy, orientation of a reacting mixture of responds to a film prepared on a Pt disk at +1.0 V w AgSB pyrrole + electrolyte in K-15 between two IT0 electrodes Oxidation and reduction waves for this film are centered a t ca. -0.15 and -0.20 V vs Ag/AgN03, respe~tively.~~,~'induced by the applied field. Since the changes in peak position from curve b to c cannot be attributed to differThe films in curves b and c were prepared on Pt at +1.0 ences in solvent dielectric, they are likely to reflect oriand +1.3 V vs Ag, respectively. The oxidation/reduction entation of the polymerization medium induced by the waves are at -0.25 V/-0.35 V and -0.48 V/-0.53 V vs applied electric field. In accord with this conclusion, the Ag/AgN03.4S The film in curve d was synthesized on a film in curve d was synthesized in an oriented medium silanized PtO electrode at +1.0 V vs Ag. Oxidation/reinduced by the silane layers on both the working and duction waves are at -0.50/-0.55 V. Films synthesized on counter electrodes. The peaks of the film in curve d are silanized PtO at any fixed potential in the interval +0.9 within A20 mV of those of the film in curve c, as would be expected if the peak shifta from c w e b to c were caused (45) I T 0 electrodes were passivated by treatment with ACM-72. Siby differences in the orientation of the polymerization lanized IT0 electrodes, however, remained conducting and could be used medium. The peaks of the curves c and d are narrower for further electrochemical experiments. than those of curves a and b, suggesting that the molecular (46) Films polymerized potentiostatically at any fixed value in the ,
interval +0.8 to +1.3 V vs Ag exhibited cyclic voltammetric oxidation/ reduction waves at -0.14 0.06 V/-0.20 0.04 V vs Ag/AgNO,. (47) Voltammograms of film prepared on a silanized PtO electrode at the same polymerization potential are superimposable on curve a. (48) For film synthesized in the interval +0.9 to +1.1 V vs Ag, the corresponding cyclic voltammetric oxidation/reduction waves were at -0.28 0.10/-0.35 0.06 V. Film prepared in the interval +1.2 to +1.4 V showed cyclic voltammetric wave8 at -0.48 h 0.02/-0.54 0.01 V.
*
*
*
I
.
.
,
,
I
.
.
.
.
,
.
(49) In all experiments, the same electrolyte was used as it is known that the electrochemistry of polypyrrole can be quite sensitive to the mobilities of counterions in and out of the film. (See, for instance, ref 1, P 96.) (50) Kaito, A.; Wang, Y. K.; Hsu, S.L. Anal. Chim. Acta 1986, 189, 27.
Torres and Fox
588 Chem. Mater., Vol. 4, No. 3, 1992
T a b l e I. Conductivity of Polypyrrole F i l m s P r e p a r e d in Acetonitrile and in K-15n polymerization conditions applied electrode solvent potential IT0 acetonitrile + 2.0
IT0 IT0
silanized IT0
K-15 K-15 K-15
+ 2.0 + 4.0 + 2.0
conductivity, mho/cm (n = 3) perpendiparallelb culd 323 22 380 39 410 f 22 479 f 35
370 f 34 364 47
*
579 f 28 572 f 52
Prepared on a two-electrode cell. Measured by the four-probe method. Measured by the two-probe method.
Wavelength (nm)
Figure 6. W-vis absorption spectra of reduced polypyrrole films prepared at +2.0 V: (a) o n IT0 in 40 m M TBATFB/K-15; (b) o n silanized IT0 in 40 m M TBATFB/K-15.
weight distribution of the films may be narrower in c and d than in a and b. Effect of the Applied Electric Field during Polymerization on the Properties of the Films. When the polymerization of pyrrole was conducted on optically transparent silanized IT0 in a two-electrode cell (Figure 2b) observed with the cross-polarized microscope at potentials higher than +1.5 V, the characteristic black texture indicated perpendicular orientation of the sample. No texture changes were observed in the system for polymerizations at any fixed potential in the range +1.5 to 4.0 V, indicating that the bulk of the sample remained oriented during polymerization. These films showed cyclic voltammetric waves in acetonitrile at -0.48 V f 0.02/-0.55 f 0.01 V vs Ag/AgNO,. When two untreated I T 0 electrodes were used, a threshold voltage region (3.0 to 3.2 V), at which the sample turns black, was observed. A t potentials lower than +3 V, the observed textures of the polymerization mixtures were like that in Figure la, giving polymeric films which showed cyclic voltammetric peaks at -0.23 f 0.03/-0.34 f 0.02 V vs Ag/AgNO,. Films polymerized at potentials in the range 3.2 to 4.2 showed voltammetric peaks in acetonitrile at -0.47 f 0.03/-0.54 f 0.01 V vs Ag/AgNO,, in good agreement with the changes observed in Figure 5, curves b and c. Figure 6 shows absorption spectra of reduced polypyrrole fiims prepared on a clean IT0 (curve a) and on silanized I T 0 electrode (curve b) at 2.0 V, followed by a discharge in 0.40 M TBATFB/acetonitrile at -0.5 V vs Ag/AgNO, for ca. 10 min. The ,A, at 380 nm observed in curve a corresponds to the interband transition of p ~ l y p y r r o l e . ~ ~ Films synthesized on either silanized IT0 or untreated IT0 at +3.8 V show instead a A- at ca. 460 nm. This red shift, curve b, indicates an extension of the c ~ n j u g a t i o n ~ ~ (51)Yakushi, K.; Lauchlan, L. J. Clarke, T. C.; Street, G. B. J. Chem. Phys. 1983, 79,4774. (52) The shift of the absorption maximum to longer wavelengths is in agreement with the 'reciprocal rule" which states that many properties of polymers tend to vary linearly with the reciprocal of chain length. For further discussion, see: (a) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J. Am. Chem. SOC.1991,113,600. (b) Lahti, P. M.; Obrzut, J.; Karaaz, F. E. Macromolecules 1987,20,2023. (c) Duke, C. B.; Paton, A.; Salaneck, W. R. Mol. Cryst. Liq. Cryst. 1982,83, 177. (d) Diaz, A. F.; Crowley, J.; Bargon, J.; Gardini, G. P.; Torrance, J. B. J. Electroanal. Chem. 1981, 121, 355.
as might be reasonably expected from directional orientation of the polymerization mixture. Searching for Anisotropy: Conductivity and X-ray Diffraction Experiments. Conductivities measured along (parallel) and across (perpendicular) the surface of films prepared on IT0 are shown in Table I. The observed conductivity values, representing the average of three samples, are consistently lower for the films along the surface than for across the surface, although the magnitude of the difference is modest. The perpendicular-to-parallel conductivity ratio for the samples prepared on IT0 in acetonitrile and on IT0 in nonoriented K-15 at +2.0 V is ca. 1.2. The conductivity ratio for the samples prepared in oriented K-15 (on I T 0 at +4.0 V and on silanized I T 0 at 2.0 V) is ca. 1.6. Thus, the degree of anisotropy induced by medium order, while real, is probably small. Accordingly, X-ray diffraction patterns obtained for films prepared in oriented K-15 on IT0 at +4.0 V and on silanized IT0 at +2.0 V are totally superimposable on that of a film produced in nonoriented K-15 on a clean IT0 slide and are totally featureless. Both the films prepared in acetonitrile and in K-15 show very compact nodular morphologies as determined by scanning electron microscopy. Although the nodule size depends on the polymerization solvent, these micrographs do not indicate a higher degree of order in the films prepared in K-15. Thus, there is no evidence for either short- or long-range alignment of the polymer chains in either film.
Conclusions Pyrrole can be electrochemically polymerized in a nematic liquid-crystal matrix. Orientation of the nematic polymerization mixture induces changes in the voltammetric response, the absorption spectra, and the conductivity of the resulting films. However, measurements along and across the film surface showed only small conductivity differences. Since the polypyrrole films obtained in the oriented nematic showed no evidence of crystallinity, this matrix acts, at best, as only an inefficient template for forming anisotropic conducting polymeric films. Acknowledgment. This work was supported by the
U.S.Department of Energy, Office of Basic Energy Sciences, and to the Robert A. Welch Foundation. We are grateful to Mr. Mark Arendt for preparing the gold electrodes and to Drs.Chang-jin Lee, Jon Merkert, and David Collard for helpful discussions. Registry No. TBATFB, 429-42-5; polypyrrole (homopolymer), 30604-81-0; 4-cyano-4'-pentylbiphenyl,40817-08-1.
