Structural, Electronic, and Morphological Changes in Poly

Structural and electronic transitions in poly(phenylenesulfide phenyleneamine) (PPSA) upon electrochemical doping have been investigated. The results ...
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J. Phys. Chem. B 2001, 105, 2191-2196

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Structural, Electronic, and Morphological Changes in Poly(phenylenesulfide phenyleneamine) upon Electrochemical Doping Guofeng Li, Mira Josowicz, and Jirˇ´ı Janata* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400

Klaus Mu1 llen Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ReceiVed: October 31, 2000; In Final Form: December 5, 2000

Structural and electronic transitions in poly(phenylenesulfide phenyleneamine) (PPSA) upon electrochemical doping have been investigated. The results indicate that polarons are the predominant charge defects at low doping levels, yielding an electronically and mechanically stable material. The electrochemical doping at high potentials induces a transition of the polarons to a bipolaron state. Due to the heterogeneous nature of the polymer chain and different oxidation potentials associated with aniline and phenylene sulfide units, the formation of polaron and bipolaron is distinguished as two separated steps. The dopant perchlorate ion exists in the polymer matrix, not only as charge neutralizer but also as a ligand that is simultaneously Coulombically bound to the positively charged S or N sites on one polymer chain and hydrogen bonded to the N-H group on the neighboring chain. The formation of such perchlorate anion centered Coulombic/hydrogen-bonded complexes has a major impact on the electrochemical activity and the morphology of the doped polymer. Also due to stabilization of the dopant ions, the doped polymer can be retained in a stable and desirable oxidation state.

I. Introduction Chemical, thermal, and electronic stability as well as processibility are highly desirable properties of conducting polymers that are intended for dry-state applications. Such applications include non-silicon-based electronics, optoelectronics, protective coatings, and solid-state chemical sensors. Recently synthesized1-3 poly(phenylenesulfide phenyleneamine) (PPSA) is an alternating copolymer (Figure 1) that promises to combine the best electronic and structural properties of polyaniline (PANI)4-6 as well as the thermal and mechanical stability of poly(phenylene sulfide) (PPS).7,8 In contrast to these two intensively studied polymers, PPSA exhibits excellent chemical stability and high solubility in cyclohexanone, THF, DMF, DMSO, and NMP.9 In its undoped state, the polymer is completely amorphous and electrically nonconducting. Up to now, the doping of PPSA has been carried out chemically, using conventional oxidants such as bromine, iodine, SbCl5, and FeCl3. Preliminary studies have shown that best results were obtained with SbCl5 in chloroform and FeCl3 in acetone/chloroform, yielding stable conductivities of 0.2 and 1.4 S/cm, respectively.9 However, the structural and electronic changes in the PPSA upon doping remain relatively unexplored. Although recent efforts have been made to use PPSA as a hole injection promotor in multilayer LED devices,10 this new addition to the family of conducting polymers has yet to demonstrate more technological advantages over its predecessors to gain more attention. The objective of this study was to investigate the possibility of controlling the level of doping and thus the value of work function of PPSA by electrochemical means. For the above * Corresponding author. Tel: (404) 894-4828. Fax: (404) 894-8146. E-mail: [email protected].

Figure 1. Chemical structure of poly(phenylenesulfide phenyleneamine) (PPSA).

prospect, PPSA is an excellent candidate material as a matrix for selective layers of solid-state chemical sensors. However, in this paper, we focus our investigation on the nature of the structural and electronic transitions that evolve upon the electrochemical doping of PPSA with lithium perchlorate. The morphological changes in the doped polymer, as well as their implications for the electrochemical behavior and other materials properties, will also be elucidated. II. Experimental Section Sample Preparation. PPSA polymer sample was dissolved in cyclohexanone (typical concentration ∼ 5 mg/mL) and then cast onto various substrates for different applications. The film thickness was measured by a Dektak II profilometer using a diamond stylus under a 10 mg load. Electrochemical Measurements. The voltammetric apparatus was a CH Instruments model 660 electrochemical workstation. The cyclic voltammetry (CV) and square-wave voltammetry (SWV) were done at room temperature in a threeelectrode cell using a platinum foil as counter electrode and a silver wire as reference electrode. A platinum disk electrode (φ ) 1.5 mm, Bioanalytical Systems) or platinum-coated silicon substrate (active area 1.0 × 0.15 cm) with spin-cast PPSA film

10.1021/jp004005u CCC: $20.00 © 2001 American Chemical Society Published on Web 02/21/2001

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Figure 3. Typical cyclic voltammogram of PPSA in 50 mM LiClO4/ AcCN. Inset: Square wave voltammogram of PPSA thin film (800 Å) in 0.2 M LiClO4/AcCN. Figure 2. IR spectra of (a) dry PPSA film and (b) PPSA film freshly cast from cyclohexanone solution. These spectra have been offset for clarity.

was used as the working electrode. The silver wire reference electrode was calibrated with the ferrocene/ferrocenium (Fec/ Fec+) reference redox system following the IUPAC recommendation.11 The E1/2 of 5 mM Fec/Fec+ in 0.25 M LiClO4/ acetonitrile was 0.54 V. The standard redox potential of the Fec/Fec+ system in acetonitrile vs the SCE electrode is 0.307 V.12 The CV measurements were done in a 0.2 M LiClO4/ acetonitrile solution in the potential range of -0.2 to 1.8 V with a scan rate of 10 mV/S. The SWV studies were performed with 4 mV increments and 25 mV of amplitude at the frequency of 15 Hz. The same setup was used for the electrochemical doping of PPSA. However, PPSA dry films deposited on the platinum meshes were used so that UV-vis-NIR and IR transmission spectra could be taken after the doping. Spectroscopic Measurements. Infrared measurements were performed in the frequency range of 400-4000 cm-1 using a Biorad FTS-6000 Fourier transform infrared (FTIR) spectrometer attached with a UMA-500 infrared microscope and a slideon Ge crystal ATR accessory. The optical absorption spectra were measured with a Shimadzu UV-3101PC UV-vis-NIR scanning spectrophotometer in the range of 190-3200 nm (∼50 000-3125 cm-1). Materials Characterization. Modulated differential scanning calorimetry (DSC) was carried out under nitrogen on a TA Instruments model 2920 DSC with a heating rate of 5 °C/min. The transmission electron microscopy (TEM) was performed on a JEOL JEM-100CX II electron microscope. Resistance Measurements. The doped PPSA film was sandwiched between the platinum substrate13 and a polished silver electrode with constant pressure applied to ensure good electrical contact. The dc resistance was recorded with a Hewlett-Packard (model no. 34401A) multimeter. III. Results A. Properties of the Undoped PPSA. Due to the weak absorption of the C-S stretching vibration and the variability in its position (∼700-600 cm-1),14 the infrared spectra of the undoped PPSA only manifest the characteristics of the aniline building block of the polymer, a para-substituted aromatic secondary amine, as suggested by the peak assignments indicated in Figure 2. A comparison of the infrared spectra of the PPSA film freshly prepared from cyclohexanone with the vacuum oven dried PPSA film reveals

the ability of the N-H functional groups to form strong hydrogen bonds. In the dry PPSA film, the polymer chains are randomly linked to each other by interchain hydrogen bonding between N-H groups. The band assigned to the N-H stretch vibration is a single sharp peak at 3393 cm-1. As for the freshly prepared film, the presence of the residual solvent is indicated by the additional C-H stretching vibrational bands at ∼ 2900 cm-1 and the absorption band at 1700 cm-1 (νCdO). Due to the presence of the residual solvent, part of the N-H absorption band shifts and forms a slightly broadened shoulder at 3350 cm-1. The red shifts observed in the frequencies of both vibrations (νN-H of PPSA and νCdO of cyclohexanone) in comparison to their “free” counterparts suggest that the residual solvent molecules are hydrogen-bonded to the polymer chain at the N-H sites. The tendency of the N-H group to form strong hydrogen bonds in fact plays an important role in the behavior of PPSA, which will be discussed later in more detail. PPSA in its undoped state is electrically insulating. The UVvis-NIR absorption spectrum is relatively simple, displaying only a π f π* absorption band centered at 331 nm, which originates from the conjugated phenyl ring. The DSC measurement shows that PPSA is fully amorphous with a glass transition temperature (Tg) of 145 °C, which is higher than that of PPS (89 °C)15 but lower than that of PANI in its emeraldine form (220 °C).16 The variances in Tg are due to the difference in the degree of the interchain hydrogen bonding as well as backbone chain stiffness of the polymer.9 B. Electrochemical Doping of PPSA. Figure 3 is a typical cyclic voltammogram of the PPSA film cast on a platinum disk electrode. In general, the polymer is oxidized during the positive potential scan. The oxidation of the PPSA is accompanied by the influx of the ClO4- ions into the film to maintain the overall charge neutrality. When the potential is swept in the negative direction, the oxidized form of the polymer is reduced and the ClO4- ions should be expelled back into the solution. The as-cast undoped PPSA film shows an unusual electrochemical behavior. More specifically, at the first cycle, the oxidation occurs as a relatively sharp, prominent peak centered at considerably higher potential E ) 1.48 V, while the reduction peak is centered at 0.75 V. Notice that, on the same first cycle, the charge under the reduction peak is much smaller than the preceding oxidation peak. The disparity between the anodic and the cathodic charge suggests that a large amount of ClO4- ions remains in the polymer after the completion of the first cycle. Subsequently in the second and following cycles, the oxidation

Electrochemical Doping of PPSA begins at more negative potential, while the position of the reduction peak remains unchanged. Contrary to what was observed in the first cycle, the oxidation and reduction peaks are now more symmetrical with respect to the amount of the exchanged charge and the position of the peaks, indicating more reversible ion exchange. However, for the subsequent cycles, a gradual decrease in the magnitude of both the oxidation and the reduction peak is seen in CV. After about a dozen cycles, both the oxidation and reduction peaks disappear. It is also observed that the shape and position of the first oxidation peak varies from 1.4 to 1.8 V with the scan rate and, more importantly, with the morphology of the film. With slower scan rates, the first oxidation peak appears at less positive potentials. For thick films and/or dry film containing less residual solvent, it shifts toward the positive end of the potential range. This behavior indicates that the influx of the ClO4- ion into the polymer matrix is hindered at the first cycle due to the tight packing of the polymer chains caused by the interchain N-H hydrogen bonding. As for the cycles beyond the first scan, with remaining ClO4- ions and solvent molecules wedged between the polymer chains, the polymer matrix is relatively “open”, and therefore, the influx of the ClO4- ion appears at less positive potential. Due to the low conductivity of the thick undoped film initially cast on the Pt electrode, as well as the compact nature of the polymer matrix caused by the interchain hydrogen bonding, it is rather difficult to distinguish these microscopic electrochemical processes in the CV. As a result, the CV in Figure 3 exhibits poor peak separation, asymmetric shape, and large peak width. Also during CV measurements, very slow scan rate has to be used to avoid capacitive charging. Consequently, the constant loading of perchlorate anion into the polymer matrix, especially at high potentials, gives rise to poor mechanical properties of the doped film. One solution to this problem is to cast thin films and to use pulse voltammetric techniques such as square wave voltammetry (SWV) instead of CV. Figure 3 inset shows the SWV of a PPSA film. The overall pattern observed in the SWV is similar to that of the thick film studied by CV. However, the shape of the redox peaks is quite symmetric, and the peaks are better resolved especially at the first oxidation scan. As shown in the inset of Figure 3, two oxidation waves, one centered at 1.03 V (peak b) and the other one centered 1.28 V (peak c), clearly suggest two distinctive oxidation steps in PPSA. The full detail of the electrochemical behavior of PPSA and the merit of applying SWV to the study of conducting polymer are discussed elsewhere.17

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Figure 4. (A) FTIR ATR spectra of PPSA in three redox states as indicated in Figure 3 inset: (a) undoped; (b) polaron; (c) bipolaron state. The spectra have been offset for clarity. (B) Comparison of the chemical structure of PPSA at undoped, polaron, and bipolaron states.

Figure 5. UV-vis-NIR spectra of a PPSA film at low levels of doping.

vibrations suggest further configurational changes in the aromatic ring. These IR results confirm that two structural transitions take place in PPSA upon electrochemical dopings the formation of polaron at a low level of doping, followed by the transition from polaron to bipolaron as illustrated in Figure 4B. The repeating unit of PPSA in its polaron state is singly positively charged at the sulfur with one ClO4--ion Coulombically attached. In its bipolaron state, the repeating unit is positively charged on both the sulfur and nitrogen, with one ClO4- ion attached on each site.

C. Structural Transitions in PPSA.

D. Electronic Transitions in PPSA.

To understand the peculiar electrochemical behavior of PPSA, the electrochemical studies were complemented with optical and spectroscopic measurements. Figure 4A shows the infrared spectra of the PPSA film at different stages of the first oxidation cycle as portrayed in Figure 3 inset. In comparison to the IR spectrum of the undoped PPSA (spectrum a in Figure 4A), the spectrum taken at the peak of first oxidation wave (spectrum b) shows several new features. The most noticeable ones are the following: CdS stretch vibration at 1074 cm-1; sharp absorption band at 1550 cm-1 due to the formation of phenyl radical; decrease in intensity of the skeletal vibrations (1500 and 1590 cm-1) of the phenyl ring, indicating a configurational change within the phenyl ring. Further, a new absorption peak at 1650 cm-1 (νCdN) starts to emerge in the spectrum taken at the peak of the second oxidation wave (spectrum c). In addition, the decrease in the peak intensity and the splitting of skeleton

The optical changes in PPSA were also investigated using UV-vis-NIR spectroscopy. At the low level of doping, as shown in Figure 5, there is a decrease in the intensity of the π f π* absorption, coinciding with the loss of the aromatic identity as previously observed in the IR data. No shift in the peak position is observed for this particular transition, suggesting that, in the undoped state, the phenyl rings are isolated from each other with no electronic conjugation along the polymer chain. Meanwhile, a new peak that absorbs at 409 nm begins to appear as the level of doping increases. It is proposed that this optical absorption is due to the n f π* transition from the nonbonding sulfur lone electron pair to the conduction band as the conjugation along the chain increases. In addition, there is a very broad band in the NIR region. It can be dissected into two absorption bands resulted from the introduction of two localized interband energy levels within the

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Figure 6. Band diagram of PPSA at undoped, polaron, and bipolaron states.

Figure 8. (A) Changes in the N-H stretch vibration upon doping. (B) Schematic representation of the two types of interchain ClO4hydrogen/Coulombic bonded “bridges” in doped PPSA.

Figure 7. Film resistance of PPSA during the first oxidation cycle as shown in the inset of Figure 3. Inset: Film resistance of PPSA as a function of the charge injected during the first oxidation cycle.

band gap. The absorption band at 995 nm is the subgap transition between two localized energy levels (excitation 4 in Figure 6). The second absorption band at 1370 nm is the electronic transition from the valence band to the lower subgap energy level (excitation 5). The weaker absorption at 575 nm results from the electronic transition from the valence band to the higher subgap energy level (excitation 3). With the formation of bipolaron at higher doping levels, the two subgap states do not contain electrons and therefore there is no observable subgap transition. However, two broad electronic transitions from the valence band to the gap states are expected along with the interband transition. The energy level of the lower subgap state is slightly higher in the bipolaron state; hence, the excitation, ′5, occurs at higher energy and the broad band located in the NIR region now shifts to lower wavelength. On the basis of the above discussion, the energy band diagrams of the polaron and bipolaron states are schematically illustrated in Figure 6. The electronic structure in the polaron state can be well resolved for each excitation. However, due to interference from the polaron excitations and peak broadening at higher doping levels,18-20 it becomes increasingly difficult to accurately resolve the electronic structure of doped PPSA in its bipolaron state. Our measurements on the electronic properties of the doped PPSA film further validate the polaron-bipolaron model. Figure 7 shows the resistance of a PPSA film at different stages of the first oxidation cycle as portrayed in the Figure 3 inset. A decrease in the film resistance was first observed due to the accumulation of polaron concentration in the polymer, with the optimal conductivity recorded in the order of magnitude of 10-5 S/cm. The trend was then reversed at around 1.25 V where the polaron to bipolaron transition starts to occur. A similar pattern in the changes of the film resistance of PANI upon electrochemical cycling has been previously reported.21 Further, the amount of electrons extracted from the polymer during the oxidation can be integrated from the voltammogram. A replot of the film resistance against the amount of charge injected during the oxidation is presented in the Figure 7 inset, in which

nearly identical pattern was observed. The observation implies that the amount of charge injected into the polymer during the oxidation is in fact a direct indication of the charge carrier concentration in the doped polymer, and the doping level in PPSA can be quantitatively adjusted electrochemically through controlling the doping potential. This provides an effective method for fine-tuning the electronic properties such as conductivity and work function in the doped PPSA. The details of these findings are reported elsewhere.22 E. Morphological Changes in PPSA. Along with the changes seen in the aromatic ring configuration and chain conjugation, a progressing red shift in the frequency of the νN-H absorption band is also evidenced in the IR spectra. The subtle changes in N-H stretch vibration upon doping contain rich information. Three types of hydrogen bondings are present, as indicated in Figure 8A. As the doping progresses, the interchain N-H hydrogen bonds (3393 cm-1, peak a) gradually break up. The entrainment of the solvent (acetonitrile) molecules and ClO4- ions manifests itself by the appearance of new hydrogen bondings between the N-H and acetonitrile (3345 cm-1, peak b) or between the N-H and the ClO4- ion (3258 cm-1, peak c). Further increase in the doping level eventually shifts both the N-H stretching absorptions at 3393 and 3345 cm-1 to 3258 cm-1, as the ClO4- ion accumulates in the polymer. Since the ClO4- ions are Coulombically bound to the positively charged S or N sites, the hydrogen bonding between the ClO4- ion and the N-H group is most likely to be formed in an interchain fashion. Therefore, the ClO4- ion in the doped polymer acts as a ligand, forming an interchain “bridge” that binds the neighboring chains together as schematically illustrated in Figure 8B. The formation of the above proposed ClO4--centered hydrogen/ Coulombic bonding “bridges” has a major impact on the electrochemical behavior and morphological changes in PPSA. With the presence of those additional ClO4- ions in the doped polymer, the chains become much more mobile due to considerably smaller degree of interchain hydrogen bonding. In other words, oxidation of PPSA to the polaron stage or beyond favors the formation of a networked structure linked together by the ClO4--centered hydrogen/Coulombic bonding “bridges”. As a result, domains of networked complexes appear, in which the ClO4- ions are trapped and hence the polymer can be no longer fully reduced to its initial undoped state. The markedly lower mobility of ClO4- ions in this tightly networked complex

Electrochemical Doping of PPSA

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Figure 9. Comparison between the differential scanning calorimetry thermograms of undoped and doped PPSA.

Figure 11. Resistance of a doped PPSA film (10 µm thick) measured at the end of each oxidation (upper panel) and reduction (lower panel) scans after different numbers of cycles within the potential range from -0.2 to 1.7 V.

Figure 10. (A) TEM image of the doped PPSA in its polaron state and (B) selected area diffraction (SAD) pattern of the crystallite at the center of the TEM image.

contributes to the large separation of the anodic and cathodic peaks in the first cycle of the CV and to the abnormally large widths of the subsequent oxidation/reduction peaks. In the following cycling the continuous and alternating influx and expulsion of ClO4- ions serve as a self-organizing process for the networked complex, resulting in a gradual decrease in the magnitude of both the oxidation and reduction peaks after the first cycle in CV. The effect of changing morphology on the electrochemical behavior of PPSA is also evident from the CV. With increasing number of cycles the peak separation increases while the peak current decreases for both the anion insertion (oxidation) and anion expulsion (reduction). The DSC measurements performed on the doped PPSA in its polaron state confirm the improved chain mobility. As shown in Figure 9, a broad crystallization exotherm at Tc ) 200 °C replaces the glass transition previously seen in the DSC thermogram of the undoped PPSA. One way to prove the existence of this networked hydrogen/Coulombic bonded complex is to use high-resolution transmission electron microscope (HRTEM) to look for crystalline domains in the polymer after thermal treatment at Tc. The TEM image of the doped PPSA in its polaron state after heat treatment reveals dispersed dark spots (domains that are doped with ClO4- ions) in the film with the size of around 1µm (Figure 10A). The electron diffraction pattern (Figure 10B) of a large domain located in the center of the TEM image verifies the formation of the polycrystallites in

the doped PPSA. Similar observation of ordered structure in the emeraldine form of polyaniline has been reported.23-25 But the nature of the ordering is believed to be quite different due to the uniquely formed Coulombic/hydrogen-bonded networked complex in the doped PPSA. Previous study on PPSA in its undoped state has shown that it is completely amorphous and crystallization cannot be achieved even by annealing the polymer at temperatures above the glass transition.15 Clearly the influx of the perchlorate ions and the occurrence of the Coulombic/ hydrogen-bonded networked complex have great impact on the morphological changes observed in the doped polymer. These morphological changes are also reflected in the increase of resistance upon continuing cycling as shown in Figure 11. IV. Discussion PPSA was shown to be an electron-rich material. Electrochemical doping (p-type doping) of PPSA removes electrons from its extended π-bonded system, which entails the incorporation of perchlorate ions as dopant into the polymer matrix to maintain the overall charge neutrality. The structural and electronic transitions in the PPSA upon electrochemical doping can be characterized by the polaron-bipolaron model, which is commonly observed in other electronically conductive organic polymers.8,26,27 However, the doping in the PPSA polymer system also showed some distinctions. Due to the heterogeneous nature of the polymer chain and the different oxidation potentials associated with aniline and phenylenesulfide units, the formation of the polaron and bipolaron are distinguished as two separated steps as demonstrated in the CV and the IR results. Also, the introduction of the heteroatoms into the polymer chain causes significant changes in the behavior of these two individual building blocks. For example, the reduced form of PANI is difficult to obtain because the phenyleneamine units can be easily oxidized into a quinone structure. However, the aniline units of PPSA in the reduced form are protected due to the presence of the phenylene sulfide units with lower oxidation potential. The behavior of the phenylene sulfide units in PPSA is also different from what is normally seen in PPS. The

2196 J. Phys. Chem. B, Vol. 105, No. 11, 2001 formation of carbon-carbon bonds bridging the sulfur linkages to form thiophene rings, which leads to a polybenzothiophene structure,8,28 can no longer be seen in the doping process of PPSA due to the presence of those N heteroatoms. In the doped state of PPSA, the perchlorate anions are electrostatically bonded to positively charged S or N sites on the polymer chain. Meanwhile, as indicated by the IR results, those perchlorate anions are hydrogen bonded to the N-H groups on the neighboring chain. The electrochemically induced formation and reorganization of these ClO4- Coulombic/ hydrogen-bonding “bridges” lead to a networked structure in the polymer matrix, in which the dopant anion cannot be electrochemically exchanged. This, in turn, means that the ClO4ions bound in these sites represent a permanent level of doping. Nonetheless, the prerequisite for this multiple binding of the perchlorate ion is the favorable spatial configuration of the two polymer chains. Obviously, there are many chains where such bridging is not possible. Yet, the ClO4- can and must interact Coulombically with the positively charged S and N sites on the doped polymer. In such case, when the ClO4- ions are not hydrogen bonded, they can be readily exchanged when the doped polymer is reduced (peak d in the Figure 3 inset). The number of these sites decreases as the polymer undergoes dynamic self-organization as discussed above. V. Conclusion The structural and electronic transitions taking place in PPSA upon doping can be described by the common polaronbipolaron model. The somewhat unusual electrochemical behavior of PPSA during doping was rationalized on the basis of the conformational changes induced by the dopant anion. The formation a ClO4--centered hydrogen/Coulombic bonded complex leads to irreversible incorporation of the ClO4- ion, which cannot be electrochemically exchanged. The free energy of formation of the “binding site” is so large that repetitive potential cycling induces conformational changes favoring this type of interchain bond. This effect is the basis for electrochemical tuning of conductivity and work function of these materials. Furthermore, the electrochemical doping can also be carried out with other anions, such as CCl3COO-, which yields similar electrochemical behavior and spectroscopic results. The impact of the anion type in the doped PPSA on the resulting materials properties is yet to be investigated. Changing the anion type in the doped PPSA can be a potentially useful method to modulate its materials properties in another dimension. The doping mechanism found in PPSA can exist also in polyaniline and other conducting polymers, under carefully selected conditions that allow this type of interchain bonding. Indeed, our preliminary experiments have shown that selforganized doping and conformational stabilization of polyaniline

Li et al. emeraldine base takes place and leads to an electronically important form of this ubiquitous material. Results of those findings will be reported in the upcoming publication. Acknowledgment. The funding for this work was provided by the NSF (Grant CHE-9816017). G.L. thanks Yolande Berta for her assistance with TEM measurements and J. L. Musfeldt for her helpful suggestions. References and Notes (1) Wang, L.; Soczka-Guth, T.; Havinga, E.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1495-1497. (2) Klarner, G.; Leuninger, J.; Former, C.; Soczka-Guth, T.; Mu¨llen, K. Macromol. Symp. 1997, 118, 103-109. (3) Wang, L.; Jing, X.; Wang, F.; Zhang, J.; Wang, R.; Soczka-Guth, T.; Mu¨llen, K. Synth. Met. 1999, 101, 320-320. (4) Ginder, J. M.; Epstein, A. J.; MacDiarmid, A. G. Synth. Met. 1989, 29, E395-400. (5) Genies, E. M.; Boyle, A.; Lapkowski, M.; Tsintavis, C. Synth. Met. 1990, 36, 139-82. (6) Gospodinova, N.; Terlemezyan, L. Prog. Polym. Sci. 1998, 23, 1443-1484. (7) Edmonds, J. T., Jr.; Hill, H. W., Jr. U.S. Patent 3 354 129, 1967. (8) Schacklette, L. W.; Elsenbaumer, R. L.; Chance, R. R.; Eckhardt, H.; Frommer, J. E.; Baughman, R. H. J. Chem. Phys. 1981, 75, 19191927. (9) Leuningher, J.; Wang, C.; Soczka-Guth, T.; Enkelmann, V.; Pakula, T.; Mu¨llen, K. Macromolecules 1998, 31, 1720-1727. (10) Tak, Y. H.; Ba¨ssler, H.; Leuninger, J.; Mu¨llen, K. J. Phys. Chem. B 1998, 102, 4887-4891. (11) Kuta, G. J. Electrochim. Acta 1984, 29, 869-873. (12) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; p 701. (13) Chinn, D.; Dubow, J.; Liess, M.; Josowicz, M.; Janata, J. Chem. Mater. 1995, 7, 1504-1509. (14) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; John Wiley & Sons: New York, 1998; p 106. (15) Leuningher, J.; Wang, C.; Soczka-Guth, T.; Enkelmann, V.; Pakula, T.; Mu¨llen, K. Macromolecules 1998, 31, 1720-1727. (16) Wie, Y.; Jang, G. W.; Hsueh, K. F.; Scherr, E. M.; MacDiarmid, A. G.; Epstein, A. J. Polymer 1992, 33, 314-332. (17) Li, G.; Josowicz, M.; Janata, J. J. Electrochem. Soc., accepted. (18) Bre´das, J. L.; The´mans, B.; Fripiat, J. G.; Andre´, J. M. Phys. ReV. B 1984, 29, 6761-6773. (19) Sun, Z. W.; Frank, A. J. J. Chem. Phys. 1991, 94, 4600-4608. (20) Bre´das, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309-315. (21) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441-1447. (22) Li, G.; Josowicz, M.; Janata, J. Synth. Met., submitted. (23) Mazerolles, L.; Rolch, S.; Colomban, P. Macromolecules 1999, 32, 8504-8508. (24) Wan, M.; Zhu, C.; Yang, J.; Bai, C. Synth. Met. 1995, 69, 157158. (25) Nicolau, Y. F.; Djurado, D. Synth. Met. 1993, 55, 394-401. (26) Bre´das, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309-315. (27) Epstein, A. J.; MacDiarmid, A. G. J. Mol. Electron. 1988, 4, 161165. (28) Bre´das, J. L.; Elsenbaumer, R. L.; Chance, R. R.; Silbey, R. J. Chem. Phys. 1983, 78, 5656-5662.