-
J . Phys. Chem. 1991, 95, 10151-10156
mutant. It may well be that the Phe(L)181 Tyr mutation in Rb. capsulatus primarily affects IP, but this does not appear to be the case for Tyr(M)210 Phe in Rb. sphaeroides.
-
Acknowledgment. We thank Drs. Larry Takiff, Kathy Meehan, Aland Chin, and Tom Haw of Polaroid Corp. for their generous
10151
gift of the laser diodes used in these experiments. This work was supported by grants from the NIH FIRST and a Research Career Development Award (C.C.S.), the donors of the Petroleum Research Fund (C.C.S.), administered by the American Chemical Society, and the National Science Foundation Biophysics Program (S.G.B.).
Structural Study of Polyaniiine Films in Reprotonatlon/Deprotonation Cycles K. G. Neoh,* E. T. Kang, Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 051 1
and K. L. Tan Department of Physics, National University of Singapore, Kent Ridge, Singapore 051 I (Received: May 10, 1991)
The structures of emeraldine base films, cast from N-methylpyrrolidinone solution, after being subjected to varying levels of reprotonation and repeated reprotonation/deprotonation are studied using X-ray photoelectron spectroscopy and infrared absorption spectroscopy. Two acids, HCl and HCIO,, have been tested, and the results indicate that with the latter the amine units of the film can be protonated much more readily than those of the powder sample. For highly protonated films, structural modifications resulting in a higher intrinsic oxidation state are apparent after one reprotonation/deprotonationcycle. The structural modifications have also resulted in the solubility of the films being drastically reduced. By the third reprotonation cycle, the interactions of the anions with the then structurally modified polymer films have resulted in chlorine species of distinctly different nature from those of the first two cycles. In contrast, the powder samples after three reprotonation/ deprotonation cycles show no significant deviation from the pristine base.
Introduction The structure of the polyaniline family can be denoted as [(-BNHBNH-),_A-BN=N-),l,, where B represents a benzenoid ring while Q represents a quinoid ring. The oxidation state of the polymer can be varied from the fully reduced state ( Y = 0) or leucoemeraldine (LM) to the fully oxidized state ( Y = 1) or pernigraniline. The 50% oxidized polymer has been termed emeraldine (EM) base.' This family of polymers has attracted considerable interest because of its unique redox properties. The electrical conductivity of the polymer can be increased from about IO-'' S/cm to over 1 S/cm either by varying the number of electrons per repeat unit through oxidative doping or by the addition of protons associated with the nitrogen sites.'" Another factor which may further enhance the use of polyaniline in potential electronic applications is its solution processibility. It has recently been reported that both EM base and its protonated form can dissolve completely in concentrated HzS04.' The use of protonic acid dopants of large molecular size such as toluene-psulfonic acid in the chemical synthesis of polyaniline has also ( I ) Chiang. J . C.; MacDiarmid, A. G.Synth. Met. 1986, 13, 193. MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synfh.Met. 1987, 18. 285. (2) MacDiarmid, A. G.; Chiang, J. C.; Halpern, M.; Huang, W. S.; Mu, S. L.; Somasiri, N. L. D.; Wu, W.; Yaniger, S. 1. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (3) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. Interfacial Elecfrochem. 1980, 1 1 1 , 1 1 1 . (4) Leclerc, M.; Guay, J.; Dao, L. H. Macromolecules 1989, 22, 649. ( 5 ) Ray, A.; Asturias, G.E.; Kerschner, D. L.; Richter, A. F.; MacDiarmid, A. G.;Epstein, A. J. Synfh. Met. 1989, 29, E141. (6) Nakajima, T.; Harada, M.; Osawa, R.; Kawagoe, T.; Furukawa, Y.; Harada, 1. Macromolecules 1989, 22, 2644. (7) Andretta, A.; Cao, Y.; Chiang, J. C.; Heeger, A. J.; Smith, P. Synth. Mer. 1988, 26. 383.
0022-3654/91/2095-10151$02.50/0
resulted in a salt soluble in common organic solvents.8 The simultaneous polymerization and oxidation of aniline by copper( 11) perchlorate also resulted in a conductive polymer which is soluble in Furthermore, EM base is partially soluble in THF, DMF, and DMSO and completely soluble in N-methylpyrrolidinone (NMP).'O The polyanilines and their complexes have been extensively studied by spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS)"-'s and infrared (IR) absorption spectros~ o p y . l ~ -Recent '~ XPS studies*&*) have demonstrated that the (8) Li, S.; Cao, Y.; Yue, Z . Synth. Mef 1987, 20, 141. (9) Inoue, M.; Navarro, E. R.; Inoue, M. B. Synfh. Met. 1989, 30, 199. (10) Angelopoulous, M.; Asturias, G.E.; Ermer, S. P.; Ray, A,; Scherr, E. M.; MacDiarmid, A. G.;Aktar, M.; Kiss, 2.;Epstein, A. J. Mol. Crysf. Liq. Crysf. 1988, 60, 151. (1 1) Salaneck, W. R.; Lundstrom, I.; Hjertberg, T.; Duke, C. B.; Conwall, E.; Paton, A.; MacDiarmid, A. G.;Somasiri, N. L. D.; Huang, W. S.; Richter, A. F. Synfh. Met. 1987, 18, 291. (12) Snauwaert, R.; Lazzaroni, R.; Riga, J.; Verbist, J. J. Synth. Mct. 1987, 18, 335. (13) Munro, S. H.; Parker, D.; Eaves, J. G.Springer Ser. Solid-State Sci. 1987, 76, 251. (14) Mirrezaei, S. R.; Munro, H. S.; Parker, D. Synth. Met. 1987,26, 169. ( 1 5 ) Hagiwara, T.; Demura, T.; Iwata, K. Synfh. Mer. 1987, 18, 317. (16) Ohsada, T.; Ohnuki, Y.; Oyama, N.; Katagiri, G.;Kamisako, K J . Electroanal. Chem. Interfacial Electrochem. 1985, 161, 399. (17) Kim, Y. H.; Foster, C.; Chiang, J.; Heeger, A. J. Synfh. Met. 1988, 25, 49. (18) Cao, Y.; Li, S.; Xue, Z . ; Guo, D. Synth. Met. 1986, 16, 305. (19) Tang, J.; Jing, X.; Wang, B.; Wang, F. Synth. Met. 1988, 24, 231. (20) Tan, K. L.; Tan, 8 . T. G.;Kang, E. T.; Neoh, K. G.Phys. Reo. B 1989, 39, 8070. (21) Kang, E. T.; Neoh, K. G.;Khor, S. H.; Tan, K. L.; Tan, B. T. G. J. Chem. Soc., Chem. Commun. 1989, 695. (22) Kumar, S. M.; Gaillard, F.; Bouyssoux, G.:Sartre, A. Synth. Mer. 1990.36, 111.
0 1991 American Chemical Society
10152 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
TABLE 11: XPS Results of EM-HCI Powders and Films for (a) First and (b) Second Reprotonation Cycles (1 M HCI Used)
TABLE I: Protonation Levels of Polyaoiline Salt Films and Distribution of Nitrogen Species upon Deprotonation
sample
Nt/N
salt CIOi/N'
IC
-
-
2 3 4
0.23 0.50 0.85
0.18 0.50 0.88
Neoh et al.
N=b/N
base NH/N
Nt/N
0.37 0.36 0.48 0.49
0.50 0.50 0.41 0.37
0.13 0.14 0.1 I 0.14
"Based on the corrected chlorine to nitrogen core level spectra area ratios and includes the Clod* component. *Neutral imine structure. EM base film treated with deionized water. proportions of the quinoid imine, benzenoid amine, and positively charged nitrogen can be determined quantitatively and unambiguously from the properly deconvoluted N Is XPS core level spectrum. In view of the potential importance of polyaniline films, this paper reports on a detailed study of the structures of polyaniline films after being subjected to progressive and repeated reprotonation/deprotonation. The differences between films and powders are examined. XPS is used as the primary analytical tool in this study. Experimental Section
Emeraldine salt was prepared by the oxidative polymerization of aniline by ammonium persulfate in HCI according to the method reported in the literature.' It was converted to EM base by treatment with excess 0.5 M NaOH, then washed with deionized water until neutral, and dried by pumping under reduced pressure. EM base was dissolved in NMP, and free-standing films were cast from this solution. The thickness of the films was of the order of IO pm. Two different sets of experiments were carried out: (i) reprotonation to varying degrees and (ii) cyclic reprotonation/deprotonation. The reprotonation of the EM base samples (powder and film) to varying degrees was carried out by equilibrating them in either HCI or HCIO4 of various concentrations for 1 h. The films were wiped dry and then pressed between two pieces of filter paper before being subjected to further drying under reduced pressure. Excess acid solution was also removed from the powder samples by lightly pressing them between filter papers before pumping to dryness. These two different acids were chosen so that the effects of the different anions can be explored. Subsequent deprotonation and reprotonation cycles were carried out as described above. The XPS measurements were made on a VG ESCALAB MkII spectrometer with a Mg Ka X-ray source (1253.6-eV photons). The polymer samples in both powder or film forms were mounted on the standard sample studs by means of double-sided adhesive tape. A take-off angle of 7 5 O was used in all XPS runs. The X-ray source was run at 12 kV and 10 mA. The pressure in the analysis chamber was maintained at IO-* mbar or lower during measurements. To compensate for surface charging effects, all binding energies were referenced to the C Is neutral carbon peak at 284.6 eV. In spectral deconvolution, the full width at half-maximum (fwhm) of the Gaussian peak components was kept constant in a particular spectrum. The peak area ratios for various elements were corrected by experimentally determined instrumental sensitivity factors and may be subjected to a maximum of *lo% error. The IR absorption spectra were measured on a Perkin-Elmer Model 682 spectrophotometer. The measurements were carried out directly on films while the powder samples were dispersed in KBr. The electrical conductivities ( 0 ) of the samples in the form of films or pellets formed by the compression of powders were measured using both the standard collinear four-probe and twoprobe techniques. Results and Discussion Effects of Protonation Levels. The protonation levels of the
EM films can be readily determined from the XPS N 1s and CI (23) Snauwaert, P.;Lazzaroni, R.;Riga, J.; Verbist, J. J.; Gonbeau, D. J . Chem. Phys. 1990, 92, 2187.
surface proportion of stoichiometryb N=O NH Nt CI-/Nc -CI/N (a) First Reprotonation Cycle powder film
0.10 0.06
0.31 0.33
0.04 0.05
powder
(b) Second Reprotonation Cycle 0.08 0.64 0.28 0.29 0.05 0.64 0.31 0.29
0.04 0.06
film
0.60 0.62
0.30 0.32
Neutral imine structure. Based on the corrected chlorine to nitrogen core level spectra area ratios. eIncludes the CI* species. TABLE 111: XPS Results of EM-HCIO, Powders and Films for (a) First and (b) Second Reprotonation Cycles (1 M HCIO, Used)
proportion of surface stoichiometry* N=" NH N+ CIO;/NC (a) First Reprotonation Cycle powder film
powder film
-
0.53 0.50
0.47
0.50
0.57 0.50
(b) Second Reprotonation Cycle 0.52 0.48 0.59 0.52 0.48 0.51
Refer to Table 11. Includes C104*species. 2p core level spectra. The results are summarized in Table I. Figure la, c, and e show the N 1s XPS core level spectra of the EM-HCIO., salt films obtained from reprotonation with 0.02, 1.O, and 3.0 M HC104 and correspond to samples 2-4 of Table I, respectively. The spectrum in Figure l a can be resolved into four component peaks. The two lowest energy components with binding energies (BE) centered at 398.2 f 0.1 and 399.4 f 0.1 eV are attributed to the neutral imine (N=) and amine (NH) nitrogens, respectively, in agreement with previously reported results.2G23 Based on the fixed fwhm approach in peak synthesis used in the present work, the high-BE tail has been resolved as the two highest energy peaks, separated by 1.5 and 3 eV from the amine peak, respectively. This high-BE tail is best assigned to positively charged nitrogen species (N+) with a continuous BE distribution, as a result of charge nonuniformity. As the molarity of the acid is increased, the high-BE tail becomes more prominent and the imine component disappears. This is consistent with the postulate that protonation occurs preferentially at the imine units.' The N+/N ratios of the three salt films are given in Table I. When all the imine units are protonated, an N+/N ratio of 0.5 is expected. This case is exemplified by sample 3 (Figure IC; Table I). When the concentration of HC104 is increased to 3 M, the N+/N ratio approaches 0.9 (sample 4,Figure le). This implies the protonation of almost all of the amine units as well. From the N Is core level spectrum of this sample (Figure le), it is interesting to note that the N + component now exists predominantly as a single species. For N + / N of 0.5 or less, charge nonuniformity results in a continuous distribution of energy, as suggested by the continuous high-BE tail. In view of the high proportion of positively charged nitrogen in sample 4, all the N+ atoms may be more uniformly charged and in a more similar environment than those of the salts having protonation level less than 50%. This would account for the predominance of a single species of positively charged nitrogen with a more distinct positive BE shift from the amine nitrogens in the former. Furthermore, as a result of the reduced conjugation and the disruption of the polaron lattice in sample 4, its u is not increased significantly over that of sample 3 (-4 S/cm) even though the N + / N ratio is substantially higher in the former. The use of HCI as the reprotonating acid results in a substantially lower proportion of positively charged nitrogen. The N 1s core level spectra of the EM base powders after reprotonation by 1 M HCI and 1 M HC10, are shown in Figure 2, parts a and b, respectively, while those of the corresponding films are given
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10153
Polyaniline Films in Reprotonation/Deprotonation
201
m
210
n3
CI 2p
201
396
393
LO2
UK
396
W
399
Lo5
BINDING
ENERGY (eV1 Figure 1. N Is XPS core level spectra of EM films: (a) reprotonated with 0.02 M HC10, (sample 2), (c) reprotonated with 1.0 M HCIO, and rinsed with 0.1 M HClO (sample 3), (e) reprotonated with 3.0 M HClO (sample 4). and (b. d, and 9 samples 2-4 after treatment with 0.5 M NaOH. BINDING
I
I
I
396
399
102
105
196
399
102
207
10s
BINDING ENERGY lev1 Figure 2. N Is XPS core level spectra: (a and b) EM base powder reprotonated by 1.O M HCI and 1 .O M HC10, and rinsed with 0.1 M of the respective acid. (c and d) EM base film reprotonated in the same manner.
in Figure 2c,d. (For ease of comparison, Figure I C is reproduced as Figure 2d.) It can be seen that the spectra and composition of the powder and film reprotonated by a particular 1 M acid are similar (Tables 11 and 111). Since HCI is a volatile acid, it is possible that loss of dopant occurs under the ultrahigh-vacuum environment during XPS measurements. Elemental analyses of the EM-HCI and EM-HC10, salts (powders) indicate CI/N ratios of 0.46 and 0.53, respectively. The corresponding values obtained from XPS (see Tables I1 and 111 and discussion below) are 0.35 and 0.57, respectively. The significant deviation in the CI/N ratios of the EM-HCI salt obtained by the two different methods confirms a loss of the “dopant” during the XPS measurements. It should be pointed out that the volatile species removed from the HC1-protonated sample may not be just HCI but may also be chlorine since the loss of chloride species in each case is also accompanied by an increased in neutral amine nitrogens (Table 11). Thus, the protonation level of the EM-HCI salt as determined by XPS analysis is lower than the actual value, which is probably quite similar to that of the EM-HCI04 salt.
ENERGY(eV)
Figure 3. CI 2p XPS core level spectra of EM base films reprotonated by (a) HCI and (b) HC10,.
This is supported by the similarity of the u of the two salts in powder form, which is 1-2 S/cm when 1 M acid is used. When the strength of HCI is increased, the proportion of chlorine covalently bonded to the polymer (as determined from the CI 2p core level spectrum) increases. On the other hand, the use of excess 3 M HClO, in reprotonating the base powder results in a substantial amount of HC104 absorbed on the polymer surface upon drying of the sample, but no significant protonation of the amine units has occurred. The CIOL/N and N+/N ratios of this sample are 1.0 and 0.53, respectively, and may be compared with the corresponding values for the film (sample 4) in Table I. This indicates that the amine units in the film are more susceptible to protonation than those of the powder. However, the possibility that the 3 M HC104is acting as an oxidizing agent and changing the oxidation state of the initial EM base films should also be considered. Any changes in the oxidation state of the salt films would be reflected in the imine/amine ratio of the films after deprotonation. The XPS results obtained from the deprotonated films (discussed in detail below) indicate that the use of 3 M HC104 increases the oxidation state by only a small extent over that when 1 M HC104 is used. The C12p core level spectra of the EM-HCI samples are best resolved into three spin-orbit-split doublets (CI 2pIl2and CI 2p3 J, with the BE for the CI 2p3/2peaks lying at about 197.1, 19d.6, and 200.1 eV. This is illustrated by the CI 2p spectrum of the EM-HCI film in Figure 3a. The BE of the CI 2p3/2components at about 197.1 and 200.1 eV are attributable to the ionic (C1-) and covalent (-C1) species, respectively.” The chlorine species with the intermediate BE of 198.6 eV (Cl*) probably arises from the charge-transfer interaction between the chlorine dopant and the highly conductive polymer chain. The reduced negative BE shift of the CI* species compared to the CI- species suggests the presence of the chloride anion in a more positive environment. A similar intermediate species has been reported earlier for HCI-protonated p ~ l y a n i l i n eand ~ ~ also for polypyrrole chloride complexes.25 The C1 2p core level spectra of the HC104protonated samples indicate two envelopes at 199 and 207 eV. The higher BE envelope is attributed to the perchlorate species and can be resolved into two doublets, with the C1 2p3l2peaks lying at 207.4 and 208.9 eV (Figure 3b). These peaks are at(24) Urdal, K.; Hasan, M. A.; Nilsson, J. 0.; Salaneck; W. R.; Lundstrom, 1.; MacDiarmid, A. G.; Ray, A.; Angelopoulous, A. Electronic Properties of Conjugated Polymers; Springer-Verlag: Berlin, 1989; p 262. ( 2 5 ) Tan,K. L.; Tan, B. T. G.; Kang, E. T.;Neoh, K. G.; Ong,Y. K. Phys. Rev. B 1990, 42, 1563.
10154 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
I
I
,
3%
399
101
LO5
BINDING ENERGY (cV)
Figure 4. N Is XPS core level spectra: (a) EM-HCI04 powder (N+/N = 0.50) after treatment with 0.5 M NaOH, (b) EM-HCI film (N+/N = 0.32) after treatment with 0.5 M NaOH.
tributed to the C104- anion and the presence of the C104-anion in a more positive environment (CIO,*), respectively, similar to that described earlier for the chloride anion. The satellite lines from the peak in the 207-eV region as well as the presence of any CI-, CI*, or -CI species will contribute to the envelope in the 199-eV region. Since the intensity of the lower BE envelope is very much less than that of the higher BE envelope, the proportions of CI-, Cl*, and 4 1 are not calculated for the EM-HC10, salts. It can be seen from Tables 1-IIT that the CI-/N or C104-/N ratios generally agree closely with the N + / N ratio, which is to be expected for maintaining charge neutrality. The only exception is the EM-HCIO, salt powder obtained with 1 M HC10, (Table 111) where the exem Cloy anions are due to the retention of some HCIO, on the surfaces of the particles. As mentioned earlier, the use of HClO, of strength higher than 1 M will result in the CIO,-/N ratio being much higher than the N + / N ratio. In the instances where the reprotonation of EM base powder and film samples results in salts with similar structure and degree of protonation, the deprotonation of these salts can give rise to bases with different structures. For example, both the EM-HCIO, salt film and powder obtained from reprotonation with 1 M HCIO, have an N+/N of about 0.5. Upon deprotonation, the former (sample 3 base film, Figure Id) has distinctly higher imine/amine ratio than the corresponding base powder (Figure 4a) whose imine/amine ratio is identical to that of the pristine base. In the case of films, the electronic structures of the resulting bases are dependent on the degree of protonation in the salts. This is illustrated by the N Is core level spectra of the base films in Figure lb,d,f. From these figures, it is obvious that the proportion of the imine units in the base increases with the level of protonation of the salt prior to deprotonation. The proportions of the various nitrogen species of these base films are compared with those of the EM base film treated with deionized water (sample 1) in Table I. The composition of sample 1 is similar to that of the pristine EM base powder. When the base film is protonated to less than 20% and then subsequently deprotonated, the composition of the resulting base remains similar to that of the pristine base. The oxidation state, as given by the imine/amine ratio, of the samples 3 and 4 bases is significantly higher than that of the pristine base. Although the protonation level of sample 4 is very much higher than that of sample 3 (85% versus 50%),the intrinsic structures of these two bases are quite similar. This supports the postulate that amine units are protonated in sample 4 and compensation with NaOH results in the recovery of a large fraction of the neutral amine units. The progressive increase in oxidation state from sample 1 base to sample 4 base is accompanied by an increase in the BE separation between the imine and amine component peaks in the N Is core level spectrum. These separations are 1.20,
Neoh et al. 1.30, 1.45, and 1.50 eV for sample 1 and the bases of samples 2-4, respectively. The BE separation is also 1.20 eV for the pristine base and the base from the deprotonation of the salt powder with 50% protonation (Figure 4a). It appears that the BE separation increase is due to the shifting of the amine peak to a higher BE while the imine component remains at 398.2 eV. When HCI is used as the protonating acid for the films, the bases after deprotonation also show differences with the pristine base, albeit not to the same extent as when HClO, is used. Figure 4b shows the N 1s core level spectrum of the base of the EM-HCI salt film (which was obtained with 1 M HCI), and this figure may be compared with Figure Id. The imine/amine ratio of the former is almost unity, and the BE separation between these two components is 1.25 eV. Finally, in the N 1s core level spectra of the bases, a residual high-BE tail attributable to surface oxidation remains. Alternatively, the component at -402 eV has been suggested to be a shake-up satellite associated with the imine units.23 Although the differences between the pristine base and the films after the reprotonation/deprotonation cycle are most obvious from the N 1s core level spectra, careful examination of the C 1s core level spectra reveals that the fwhm line width of the C 1s component peaks of the latter has decreased. In the pristine base and the powder sample after the reprotonation/deprotonation cycle the fwhm line width of the main C 1s component peak is 1.70 eV while the corresponding value for the sample 4 base film is 1.55 eV. The decrease in the line width of the C 1s signal of poly(P-dimethylpyrrole) perchlorate relative to that of unsubstituted polypyrrole has been attributed to the crystallographically more aligned chains in the former, which is supported by electron diffraction studies.26 X-ray diffraction studies have also established the presence of a higher degree of crystallinity in EM base film cast from N M P solutions as compared to the amorphous powder and the change in crystalline structure upon protonation and d e p r ~ t o n a t i o n . ~It~is possible that the differences in crystalline structure result in the charged state, Le., the polaron and bipolaron structures, of the films and powders being different. These dissimilarities become more prominent in the more highly protonated samples such that compensation of such films with NaOH results in a restructuring of the quinoid imine and benzenoid amine units. The structural differences between the deprotonated salt films and the pristine base are also manifested in the IR absorption spectra. Figure 5a shows the spectrum of sample 1 (base film treated with deionized water), which is essentially identical to that of the pristine base powder with the exception of the small absorption band at 1680 cm-] attributable to the C=O structure of the residual N M P in the film. This small amount of N M P is not readily removable even after a few hours soaking in water or alcohol and is probably retained in the matrix of the film. Surface analysis of the base film by XPS does not indicate any significant differences in the C 1s or N 1s spectra of this film and the base powder. Parts b-d of Figure 5 are the IR absorption spectra of the base of samples 2-4 (in Table I), respectively. In Figure 5a,b, the aromatic ring stretching at 1600 and 1500 cm-' have a relative (1600/ 1500 cm-I) intensity of slightly less than unity. After deprotonation of the heavily protonated EM salt films, the 1 6 0 0 - ~ m -band ~ associated with the quinoid ( Q ) ring19 has broadened and increased in intensity relative to the 1500-cm-' band attributed to the benzenoid (B) ring (Figure 5c,d). This difference, ascribed to the oxidation of some of the B units to Q units, is entirely consistent with the higher imine/ amine ratio in the N 1s XPS core level spectra of these films. The absorption band in the 1700-cm-I region is no longer discernible in the spectra of these films, indicating the displacement of the N M P molecules by the protonating acid. The IR absorption spectra of these samples also show an increase in intensity and broadening of the bands in the C-N stretching region (1400-1240 ( 2 6 ) Pfluger, P.; Street, G . B. J . Chem. Phys. 1984,80, 544. (27) Josefowicz, M. E.; Laversanne, R.; Javadi, H.H. S.;Epstein, A. J.; Pouget, J. P.;Tang, X.;MacDiarmid, A. G . Phys. Reu. B 1989, 39, 12958.
Polyaniline Films in Reprotonation/Deprotonation
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10155
MICRONS 3
6
I
,
8
I
I2 1
fbl
I
I 196
399
U2
kM
3%
199
w)2
LO5
BllOlNG ENERGY lev)
Figure 6. N 1s XPS core level spectra of films in second reprotonation (with 1 M acid)/deprotonation cycle: (a) EM-HCI, (b) EM-HCIO,, (c) base of (a), and (d) base of (b).
COO0
3m
Moo
IW
lm
m
WAVENUMBER (un-lt
Figure 5. IR absorption spectra of (a) sample 1 and (W)samples 2-4 after treatment with 0.5 M NaOH.
cm-l) for aromatic amines. The three bands in this region at 1380, 1315, and 1240 cm-' are assigned to the different arrangements of the Q and B structures in the polyaniline chains, and as a result of the orientation of the hybrid orbits on the N atom, the C-N band has different chemical en~ir0nments.l~ The broad nature of these bands implies that they contain more than one vibrational mode. Thus, the 1R analysis also indicates that the quinoid imine and benzenoid amine units have undergone structural changes after the reprotonation/deprotonation cycle. Furthermore, the aromatic ring deformation bandi9 in the 500-cm-I region is much enhanced for these originally heavily protonated samples. The 1160-cm-I band, attributed to another characteristic mode of the quinoid imine structure, shows a slight broadening in Figure 5b and has become very broad in Figure Sc,d. In EM salt samples (films and powders), this band is very intense and broad and is considered as a measure of the degree of delocalization of electrons on the polyaniIine.l9 In the present base films, the cause of the broadening of this band is not known. There are also two small new bands at 745 and 695 cm-I in Figure 5c,d, which coincide with the absorption bands for 1,2-substitution on the benzene ring.19 The 830-cm-I band, characteristic of 1,4-substitution, is dominant in all the polyaniline salt and base samples. The reprotonation and deprotonation of the polyaniline films may have induced deviations from the predominant head to tail coupling of the aniline rings to coupling in the ortho positions of some rings which probably have been from different chains. It is observed that after EM base is cast as film from N M P solution, its solubility is reduced such that it cannot be totally redissolved in NMP. Furthermore, when the highly protonated films are deprotonated, the solubility further decreases and they become almost completely insoluble in N M P and other common organic solvents. This would be consistent with cross-linking and/or an increase in chain length. The former phenomenon would also be consistent with the appearance of 1,2-substitution bands in the IR spectrum. There is an extra band at 620 cm-l in Figure 5c,d and is attributable to residual C104- anions in samples 3 and 4 base films, consistent with a residual CI04-/N ratio of -0.03 obtained from the XPS analysis of these samples. It should be emphasized that, in contrast to the base films, the base powders after the reprotonation/de-
io6
MI
I 210
n3
ElNOlNG ENERGY 1 H ) Figure 7. C1 2p XPS core level spectra of EM films after third-time reprotonation by (a) HCI and (b) HCI04.
protonation cycle possess IR absorption spectra that are identical to that of the pristine base. Reprotonation/Deprottion Cycles. in view of the differences between the films and powders after the first reprotonation/deprotonation cycle, it would be interesting to investigate their behavior in subsequent reprotonation/deprotonation cycles. These tests were conducted with the films and powders which were treated with 1.0 M acid in the first reprotonation. The XPS N 1s core level spectra of the second-time HCI-and HCIO,-reprotonated films are given in Figure 6a,b. A comparison of these spectra with those of the first-time reprotonated films (Figure 2c,d) indicates that there are no apparent differences. The C12p core level spectra of the first-time and second-time reprotonated films are also similar. No significant differences are observed in the XPS core level spectra of the powder samples also. The compositions of the salt films and powders after the second reprotonation cycles are summarized in Tables I1 and 111. It can be concluded that the structural changes in the films after the first reprotonation/deprotonationcycle do not appear to influence the second reprotonation to any significant extent, and although the imine/amine ratios of these films have increased after the first cycle, the degree of protonation does not increase. There is also no substantial deviation in u of the films and powders after the different cycles.
J. Phys. Chem. 1991, 95, 10156-10157
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The N 1s core level spectra of the films after the second reprotonation cycle are shown in Figure 6c,d. The imine/amine ratios of the base films remain substantially higher than that of the pristine base, and the BE separation between these two components further increases slightly, being 1.30 and 1.50 eV, respectively, with HCI and HC104 as the protonating acid. When the base samples are reprotonated for the third time, the C1 2p spectra of the films (Figure 7) show distinct differences from those after the first reprotonation (Figure 3) regardless of the type of acid used. The CI 2p spectra in Figure 7 cannot be properly curve fitted, assuming only the species C1-, Cl*, and -Cl or C104- and C104* at the BE’S discussed earlier. The number of chemical states of chlorine appears to have increased, indicating some changes in the intrinsic structure of the polyaniline chains. This phenomenon is not observed with the powder samples, and the CI 2p core level spectra of these samples after the third reprotonation are not substantially different from those after the first reprotonation. The chemical states of the chlorine in the bulk environment are not determined and may show variations from those on the surface. Such variations may also be manifested differently in powders and films due to the difference in specific surface areas. In spite of the changes observed in the CI 2p core
level spectra of the films, the N+/N ratios are reduced by about only 10%from the values obtained in the first reprotonation cycle and the u’s of these films are also not significantly lower than the values obtained after the first cycle. Conclusion
X-ray photoelectron spectroscopy analysis of emeraldine base films cast from NMP solutions and reprotonated by HClO, has established that, in addition to the imine units, a substantial proportion of the amine units can be protonated. Deprotonation of the salt films reveals structural changes which are dependent on the extent of reprotonation and the acid used in the process. These modifications drastically reduce the solubility of the base films but do not significantly affect the subsequent reprotonation process and the conductivity of the resulting salts. In the third reprotonation cycle of these films, changes in the chemical state of the anions become apparent. In the case of powder samples, the reprotonation/deprotonation process appears to be reversible over the three cycles tested with no apparent change in the structure of the polyaniline. Registry No. HCI, 7647-01-0; HC104, 7601-90-3; aniline (homopolymer), 25233-30-1.
Measurements of ‘H Spin Diffusion and Cross Polarization Rates in Annealed (Monoclinic) and Quenched (Smectic) Isotactic Polypropylene Hiroshi Tanaka* Macromolecular Research Laboratory, Faculty of Engineering, Yamagata University, Yonezawa, Yamagata, 992 Japan
and Shiro Maeda Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Fukui University, Fukui, 910 Japan (Received: June 12, 1991)
Measurements of the relaxation time constants characterizing proton spin diffusion, Td, and cross polarization, TCH,have been made on annealed (monoclinic) and quenched (smectic) isotactic polypropylene. Td for methyl protons, Td(CH3), is longer than Tadfor methylene and methine protons, Td(CH,) and Td(CH), respectively, and Td(CH2) is longer than Td(CH). Td for the quenched sample is slightly longer than that for the annealed sample. The difference, however, is relatively small. There is a definite difference in TCHof methylene, TcH(CH,), methine, TcH(CH), and methyl carbons, Tc,(CH,). The order of TCH for the three carbons is TcH(CH3)> TcH(CH) > TcH(CH2). There is no substantial difference i n TCH for both quenched and annealed samples.
Introduction It is well-known that the proton spin diffusion occurs in entire
portions of the sample, but with a limit of diffusion length.l.2 In a plot of intensity vs time for the measurement of proton spinlattice relaxation time, T I ,of semicrystalline polymers consisting of crystalline and amorphous regions, the signal intensity often decays exponentially, indicating that the proton spin diffusion is effectively operating. Recently, Wu et aL3 have reported the two-stage feature of cross relaxation in the depolarization experiment for glycine and terephthalic acid and derived the following equation for the depolarized 13Cmagnetization of the I3CH group during high-speed spinning at the magic angle:
( 1 ) McCall,
D.W . Acc. Chem. Res. 1971, 4 , 223.
(2) McBrierty, V . J. Faraday Discuss. Chem. Soc. 1979, 68, 78 (3) Wu,X.;Zhang, S.; Wu,X . Phys. Rev. E 1988, 37,9827.
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where M,,is the depolarized 13Cmagnetization for ”CH group, R is the spin diffusion rate among protons, and t is the time of depolarization. If we assume that the above equation is applicable to semicrystalline polymers, we can obtain information on spin diffusion from the first term of the equation and on cross depolarization from the second term. In this paper we describe some preliminary results of a relaxation time constant characterizing proton spin diffusion, Td, defined as Td = l / R , for methylene, methine, and methyl protons, where R is the spin diffusion rate among protons in the above equation. In addition, an estimation of a cross depolarization time constant, TcH, for methylene, methine, and methyl carbons was also carried out using above equation. The results obtained are informative and will serve as a basis for more detailed work. Exoerimental Section
The polymer used in this study was isotactic polypropylene extracted with boiling n-heptane. The molecular weight char@ 1991 American Chemical Society
