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Secondary doping phenomena of two conductive polyaniline composites
Synthetic Metals 123 (2001) 47±52
Secondary doping phenomena of two conductive polyaniline composites Hong-Quan Xie*, Yong-Mei Ma, Jun-Shi Guo Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 16 July 1999; received in revised form 14 February 2000; accepted 15 September 2000
Abstract Effects of secondary doping of polyaniline (PAn) composites, prepared by in situ emulsion polymerization of aniline in the presence of chlorosulfonated polyethylene (CSPE) or styrene±butadiene±styrene triblock copolymer (SBS) and dodecylbenzenesulfonic acid (DBSA), on conductivity were studied. In situ observations of the secondary doping process using electronic spin resonance (ESR) and ultraviolet± visible±near infrared (UV±VIS±NIR) spectrophotometry were carried out. In situ observation of dedoping process of the two composites with or without previous secondary doping using UV±VIS spectrophotometry was also performed. The in situ observation of dedoping process with ammonia solution for both composites with or without previous secondary doping via UV±VIS spectrophotometry indicated that the secondary doping caused the absorption peak at 560 nm for benzoid±quinoid structure to shift to red and broaden to form a shoulder at 710 nm. This fact demonstrates that the p-conjugation of the benzoid±quinoid structure of PAn chains increases due to the expanding of the coiled PAn chains induced by the secondary doping. It was also found that the increase of conductivity of the PAn composites during removal of secondary dopant is accompanied by an obvious decrease of ESR signal. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Secondary doping; Dedoping; Composite; Conductivity
1. Introduction Cao et al. [1,2] found that polyaniline (PAn) doped by camphorsulfonic acid (CSA) and cast from m-cresol solution has a conductivity 103 times higher than that cast from chloroform solution and the conductivity of PAn-CSA ®lm cast from chloroform solution can be improved by treatment of the ®lm with m-cresol. The increase in conductivity is not necessarily contributed by the presence of m-cresol and keeps almost constant even after removal of m-cresol. MacDiamid and Epstein [3] de®ned this doping as a secondary doping, as it can affect the arrangement and conformation of the macromolecular chains of PAn. Xia et al. [4] demonstrated that secondary doping can expand the coiled PAn-CSA chains and causes an increase of p-conjugation and a change of polarons from localized state to delocalized state, via in situ observation of electron and conformational change of PAn-CSA ®lm induced by mcresol vapors, using an ultraviolet±visible±near infrared (UV±VIS±NIR) spectrophotometric method. Cao et al. [5], using UV±VIS spectrophotometry, showed that the solid-state properties of PAn-CSA processed from solution
are determined by the conformation of PAn chains in solution which, in turn, is determined by the interaction between the PAn chains, the counterion, solvent and the co-solvent. Cao and Heeger [6] indicated that the magnetic susceptibility of PAn-CSA is independent on temperature, which implies a metallic state of PAn-CSA. More recently, Menon et al. [7] and Kohlman and Epstein [8] reviewed the metal± insulator transition in doped conducting polymers. This paper deals with the effect of secondary doping on conductivity for PAn/(styrene±butadiene±styrene) triblock copolymer (SBS) [9] or PAn/chlorosulfonated polyethylene (CSPE) [10] composites prepared by in situ emulsion polymerization of aniline in the presence of DBSA and SBS or CSPE as well as the in situ observation of the secondary doping process and dedoping process using UV±VIS±NIR, UV±VIS spectrophotometry and electronic spin resonance (ESR), in order to know further the effect of secondary doping and dedoping on the conformation of PAn chains in the composites. 2. Experimental 2.1. Materials
*
Corresponding author. Tel.: 86-27-87548486; fax: 86-27-87545438. E-mail address: [email protected] (H.-Q. Xie).
All reagents and solvents used were chemically pure. Aniline was distilled under reduced pressure and stored in
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 7 7 - 4
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a refrigerator before use. Dodecylbenzenesulfonic acid (DBSA) of industrial grade with sulfonic acid content of 0.31 mol/100 g was manufactured by Nanjing Alkylbenzene Factory and was puri®ed by extraction with toluene. CSPE of 40 type containing 33±37% Cl and 0.8±1.2% S was supplied by Jilin Chemical Industry Co. SBS was made by Yueyang Petroleum Chemical Co. with styrene content of about 30% and Mw of 1:2 105 . 2.2. In situ emulsion polymerization [9,10] An amount of 93 g CSPE (or SBS), 18.1 ml aniline (An) and 97.6 g DBSA were dissolved in 800 ml xylene with stirring. After mixing with 100 ml distilled water during vigorous stirring to form an emulsion, 100 ml aqueous solution of ammonium persulfate (Ox) was added dropwise into the emulsion during stirring at room temperature. Concentration of Ox depends on the molar ratio of Ox=An 0:5. The in situ emulsion polymerization was carried out at 258C for 12 h. After deemulsifying and washing the emulsion with acetone/water (1/1, v/v) to remove excess DBSA and Ox, xylene solution of the composite containing DBSA-doped PAn and CSPE (or SBS) was obtained. The xylene solution was cast on glass and dried under infrared lamp and under vacuum at 408C to form a composite ®lm. The secondary doping was carried out by dipping the composite ®lm fully in m-cresol, so as to make it semi-swollen, followed by drying under infrared light and then vacuum drying at 408C to constant weight. The secondary doping for in situ observation was performed by moistening the composite ®lm with enough m-cresol, immediately before in situ observation. 2.3. In situ observation during secondary doping and dedoping of the composites
Fig. 1. Conductivity of PAn/CSPE composites before (b) and after secondary doping (a).
emulsion polymerization in the presence of DBSA, due to secondary doping with m-cresol, respectively. It is evident that secondary doping can enhance the conductivity of PAn/ CSPE and reduce the percolation threshold more effectively than that of PAn/SBS. This fact may be contributed to the interaction between ±SO2Cl groups of CSPE chains and NH± groups of PAn chains in the PAn/CSPE composite, which causes more uniform distribution of PAn chains than PAn/SBS and more dif®culty to form conductive route via self-assembly [11]. However, during secondary doping the m-cresol molecules weaken the interaction between CSPE and PAn and expand the coil-like form of PAn chains, resulting in self-assembly of conductive routes. On the
UV±VIS±NIR spectra were taken by a Shimadzu UV3100 spectrophotometer. ESR spectra were recorded by a JES-FEIXG ESR spectroscope in the atmosphere of puri®ed nitrogen, using intensity of central magnet at 0.336 T and Mn2/MgO as reference standard. UV±VIS spectra were taken by a UV-240 spectrophotometer. 2.4. Measurement PAn content of the composites was determined by elemental analysis, which was performed using a Carlo Erba MOD 1106 apparatus. Conductivity was measured with four-probe method or high impedance method. 3. Results and discussion 3.1. Effect of secondary doping on conductivity of PAn/ CSPE and PAn/SBS composites Figs. 1 and 2 illustrate the increase of conductivity of PAn/CSPE and PAn/SBS composites, obtained by in situ
Fig. 2. Conductivity of PAn/SBS composites before (b) and after secondary doping (a).
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Thus, in the case of secondary doping of PAn/CSPE composite the expanded coil-like conformation of PAn chains and the p-conjugation length of PAn chains increases with increasing time of secondary doping. As the m-cresol molecules diffuse into the polymer ®lm, solvate and plasticize the polymer chains, then a conformational change of PAn chains from coil-like form to expanded coil-like form occurs. Since this conformational change reduces the defects between the repeating units of the PAn chains, the conjugation length is increased. Change in the UV±VIS±NIR spectrum of PAn/ SBS during secondary doping (not indicated here) was similar to that of PAn/CSPE. Fig. 3. Change in UV±VIS±NIR spectra during treatment of PAn/CSPE composites with m-cresol: (a) 0 min; (b) 5 min; (c) 30 min.
contrary, no interaction occurs between SBS and PAn chains before secondary doping. 3.2. In situ observation of secondary doping for the two kinds of composites by UV±VIS±NIR spectrophotometry Fig. 3 shows that after moistening the PAn/CSPE composite ®lm with enough m-cresol, the absorption peak at about 780 nm decreases gradually, whereas the absorption at 1100±2000 nm increases sharply. Xia et al. [4] using UV±VIS±NIR spectrophotometric method in their in situ observation of electron and conformational changes of CSA-doped PAn induced by m-cresol vapor indicated that absorption peak at 780 nm is consistent with a coil-like conformation and a localized polaron structure (shorter conjugation length) and the absorption curve at 1100±2600 nm is consistent with an expanded coil-like conformation and a delocalized polaron structure (larger conjugation length).
3.3. In situ observation of dedoping process and redoping process of the composites via UV±VIS spectrophotometry In order to know what is the change of conformation during dedoping of PAn-DBSA chains in the two composites, in situ observation of the change of UV±VIS spectra during dedoping was carried out. Fig. 4(A) and (B) represent the changes of UV±VIS spectra during dedoping of PAn/ SBS composite with concentrated ammonia solution and 10% NaOH solution, respectively. At the beginning of dedoping process, the characteristic absorption peak at 780 nm disappears quickly, whereas the peak at 560 nm appears. The former represents the localized polaron structure and the latter denotes the benzoid±quinoid structure. However, the change of absorption peak at 560 nm with time differs with different dedoping agents. Using concentrated ammonia as the dedoping agent the peak at 560 nm increases, but the position of peak does not change. It implies that no change of conformation of benzoid±quinoid structure occurs during dedoping process. However, using 10% NaOH solution as the dedoping agent, the absorption
Fig. 4. Change in UV±VIS spectra during dedoping of PAn/SBS composite films with ammonia (A) and NaOH (B) solutions: (1) 0 min; (2) 5 min; (3) 20 min; (4) 60 min; (5) 7 h; (6) 24 h; (7) 48 h.
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peak at 560 nm shifts to blue with broadening of the peak. This change is probably due to the twist of PAn chains during dedoping, thus weakening the p-conjugation between benzoid and quinoid structures, or due to the chemical degradation of PAn chains. Dedoping of PAn/CSPE composite with concentrated ammonia solution showed the same change in UV±VIS spectra as that of PAn/SBS, whereas in the case of dedoping of PAn/CSPE composite with 10% NaOH solution, the disappearance of the peak at 780 nm and the appearance of the peak at 560 nm seems to be slower than dedoping with concentrated ammonia, probably due to slower diffusion of NaOH molecules than that of ammonia. If the PAn/SBS composite with previous secondary doping is dedoped with concentrated ammonia, the in situ observation of the dedoping process by UV±VIS spectrophotometry is shown in Fig. 5. At the end of dedoping there appears a shoulder at about 700 nm. This implies that the benzoid and quinoid structure and the p-conjugation of the PAn chains increases, reducing the difference of pq pb orbit energy, resulting in red-shift of the absorption peak. This fact also demonstrates that secondary doping enhances the conjugation length of PAn chains. But if the dedoping agent is 10% NaOH solution, the absorption peak at 560 nm shifts to blue. The latter fact demonstrates that the p-conjugated plane conformation twists. Dedoping of the PAn/CSPE composite with previous secondary doping showed a similar change in UV±VIS spectra, but the absorption at 560±710 nm is stronger than that of PAn/SBS, which indicates that secondary doping expands the coil-like chains more thoroughly in CSPE than in SBS. When the dedoped PAn/CSPE composites with or without previous secondary doping are redoped with 1.4 M HCl
solution, the in situ observation of UV±VIS spectra during redoping is shown in Fig. 6. It can be seen that after redoping with HCl solution for 7 h (curve 5), the dedoped PAn/CSPE composite with previous secondary doping shows an absorption change from a ¯attened peak at 580±710 nm to a peak at 740 nm, whereas the PAn/CSPE composite without previous secondary doping shows an absorption change from a peak at 580 nm to a peak at 670 nm. At the end of redoping for 48 h, the absorption peaks in the spectra of the two composite ®lms are close to each other at about 760 nm. It seems that the composite ®lm with previous secondary doping is easier to be doped, in other words, the expanded coil-like conformation of PAn chains are more easily to be redoped than the coil-like conformation of PAn chains. 3.4. In situ observation of secondary doping for the two kinds of composites via ESR Pure CSPE, SBS and m-cresol display no ESR signals. Fig. 7 shows that during secondary doping of the composite the central position of the ESR peak does not change and g value is 2.0032. It seems that secondary doping does not affect the type of charge carriers. But the intensity change of ESR peak differs for different composites. In the case of secondary doping for PAn/CSPE composite the intensity of ESR peak (a) ®rst decreases to a minimum value (b), then gradually increases to a constant value (c) at 30 min. The constant intensity is higher than that of the composite ®lm before treatment with m-cresol (a). After the composite ®lm was dried, the intensity of the ESR peak decreases markedly. The peak-to-peak linewidth DHpp is 2.8 G and does not change. ESR signals of the composite re¯ect directly the charge carriers in PAn chains of the composite, since SBR and CSPE display no ESR signals. The above phenomena
Fig. 5. Change in UV±VIS spectra during dedoping of PAn/SBS composite films, which were treated with m-cresol before dedoping, with ammonia (A) and NaOH (B) solutions (the footnotes are the same as in Fig. 4).
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Fig. 6. Change in UV±VIS spectra during redoping with 1.4 M HCl solution for PAn/CSPE composite films, which were treated with m-cresol (A) or without m-cresol (B) followed by dedoping with ammonia solution (the footnotes are the same as in Fig. 4).
Fig. 7. Change in ESR spectra during treatment of PAn/CSPE (A) and PAn/SBS (B) composite films with m-cresol: (a) 0 min; (b) 5 min; (c) 30 min; (d) after removal of m-cresol.
may probably be explained as follows: at the beginning of secondary doping, the composite ®lm solvates slightly, which favors the interaction between PAn chains and formation of bipolarons, resulting in reduction of ESR signals. With the increase of solvation, the interaction between PAn chains decreases, meanwhile the p-conjugation of the PAn chain increases, which may favor the formation of polarons and increases the ESR signal. The conductivity of this semi-swollen composite ®lm was measured to be less than 10 5 S/cm, which rose to 3 S/m after removal of m-cresol. This work indicates that the increase of conductivity of PAn is accompanied by an evident decrease of ESR signals. It may be considered that during evaporation of m-cresol, the interaction between PAn chains was enhanced, resulting in self-assembly of conductive routes between the PAn chains.
In the case of secondary doping for PAn/SBS composites, the ESR peak becomes widened gradually, while the intensity of ESR signal changes moderately. After removal of mcresol the width of ESR peak increases further to 5.6 G. This fact probably implies that secondary doping enhances the spin±spin correlation of the charge carriers of the PAn chains, i.e. solvation of PAn chains with m-cresol induces the expanding of coiled PAn chains and self-assembly of PAn chains into conductive routes simultaneously. 4. Conclusion The conductivity of the two kinds of composites increases after secondary doping and the increase is more obvious for PAn/CSPE than for PAn/SBS. The in situ observation of
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dedoping process with ammonia solution for both composites without previous secondary doping via UV±VIS spectrophotometry indicated that the absorption at 780 nm for localized polarons disappears rapidly, whereas the absorption peak at 560 nm for benzoid±quinoid structure increases gradually. For both composites with previous secondary doping during dedoping with concentrated ammonia the absorption peak at 560 nm for benzoid±quinoid structure shifts to red and broadens to form a shoulder at 710 nm. This fact demonstrates that the p-conjugation of the benzoid± quinoid structure of PAn chains increases due to the expanding of the coiled PAn chains induced by the secondary doping. It was also found that the increase of conductivity of the PAn composites during removal of secondary dopant is accompanied by an obvious decrease of ESR signal.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91. Y. Cao, A.J. Heeger, Synth. Met. 52 (1992) 193. A.G. MacDiamid, A.J. Epstein, Synth. Met. 69 (1995) 85. Y. Xia, A.G. MacDiamid, A.J. Epstein, Macromolecules 27 (1994) 7212. Y. Cao, P. Smith, C. Yang, Synth. Met. 69 (1995) 191. Y. Cao, A.J. Heeger, Synth. Met. 52 (1992) 193. R. Menon, C.D. Yoon, D. Moses, A.J. Heeger, in: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, 2nd Edition, 1997, Chapter 2, p. 27. R.S. Kohlman, A.J. Epstein, in: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, 2nd Edition, Marcel Dekker, New York, 1997, Chapter 3, p. 85. H.Q. Xie, Y.M. Ma, J.S. Guo, Polymer 40 (1998) 261. H.Q. Xie, Y.M. Ma, J. Appl. Polym. Sci. 76 (2000) 845. W.Y. Zheng, R.H. Wang, K. Levon, Macromol. Chem. Phys. 196 (1995) 2443.
Secondary doping phenomena of two conductive polyaniline composites Hong-Quan Xie*, Yong-Mei Ma, Jun-Shi Guo Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 16 July 1999; received in revised form 14 February 2000; accepted 15 September 2000
Abstract Effects of secondary doping of polyaniline (PAn) composites, prepared by in situ emulsion polymerization of aniline in the presence of chlorosulfonated polyethylene (CSPE) or styrene±butadiene±styrene triblock copolymer (SBS) and dodecylbenzenesulfonic acid (DBSA), on conductivity were studied. In situ observations of the secondary doping process using electronic spin resonance (ESR) and ultraviolet± visible±near infrared (UV±VIS±NIR) spectrophotometry were carried out. In situ observation of dedoping process of the two composites with or without previous secondary doping using UV±VIS spectrophotometry was also performed. The in situ observation of dedoping process with ammonia solution for both composites with or without previous secondary doping via UV±VIS spectrophotometry indicated that the secondary doping caused the absorption peak at 560 nm for benzoid±quinoid structure to shift to red and broaden to form a shoulder at 710 nm. This fact demonstrates that the p-conjugation of the benzoid±quinoid structure of PAn chains increases due to the expanding of the coiled PAn chains induced by the secondary doping. It was also found that the increase of conductivity of the PAn composites during removal of secondary dopant is accompanied by an obvious decrease of ESR signal. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Secondary doping; Dedoping; Composite; Conductivity
1. Introduction Cao et al. [1,2] found that polyaniline (PAn) doped by camphorsulfonic acid (CSA) and cast from m-cresol solution has a conductivity 103 times higher than that cast from chloroform solution and the conductivity of PAn-CSA ®lm cast from chloroform solution can be improved by treatment of the ®lm with m-cresol. The increase in conductivity is not necessarily contributed by the presence of m-cresol and keeps almost constant even after removal of m-cresol. MacDiamid and Epstein [3] de®ned this doping as a secondary doping, as it can affect the arrangement and conformation of the macromolecular chains of PAn. Xia et al. [4] demonstrated that secondary doping can expand the coiled PAn-CSA chains and causes an increase of p-conjugation and a change of polarons from localized state to delocalized state, via in situ observation of electron and conformational change of PAn-CSA ®lm induced by mcresol vapors, using an ultraviolet±visible±near infrared (UV±VIS±NIR) spectrophotometric method. Cao et al. [5], using UV±VIS spectrophotometry, showed that the solid-state properties of PAn-CSA processed from solution
are determined by the conformation of PAn chains in solution which, in turn, is determined by the interaction between the PAn chains, the counterion, solvent and the co-solvent. Cao and Heeger [6] indicated that the magnetic susceptibility of PAn-CSA is independent on temperature, which implies a metallic state of PAn-CSA. More recently, Menon et al. [7] and Kohlman and Epstein [8] reviewed the metal± insulator transition in doped conducting polymers. This paper deals with the effect of secondary doping on conductivity for PAn/(styrene±butadiene±styrene) triblock copolymer (SBS) [9] or PAn/chlorosulfonated polyethylene (CSPE) [10] composites prepared by in situ emulsion polymerization of aniline in the presence of DBSA and SBS or CSPE as well as the in situ observation of the secondary doping process and dedoping process using UV±VIS±NIR, UV±VIS spectrophotometry and electronic spin resonance (ESR), in order to know further the effect of secondary doping and dedoping on the conformation of PAn chains in the composites. 2. Experimental 2.1. Materials
*
Corresponding author. Tel.: 86-27-87548486; fax: 86-27-87545438. E-mail address: [email protected] (H.-Q. Xie).
All reagents and solvents used were chemically pure. Aniline was distilled under reduced pressure and stored in
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 7 7 - 4
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a refrigerator before use. Dodecylbenzenesulfonic acid (DBSA) of industrial grade with sulfonic acid content of 0.31 mol/100 g was manufactured by Nanjing Alkylbenzene Factory and was puri®ed by extraction with toluene. CSPE of 40 type containing 33±37% Cl and 0.8±1.2% S was supplied by Jilin Chemical Industry Co. SBS was made by Yueyang Petroleum Chemical Co. with styrene content of about 30% and Mw of 1:2 105 . 2.2. In situ emulsion polymerization [9,10] An amount of 93 g CSPE (or SBS), 18.1 ml aniline (An) and 97.6 g DBSA were dissolved in 800 ml xylene with stirring. After mixing with 100 ml distilled water during vigorous stirring to form an emulsion, 100 ml aqueous solution of ammonium persulfate (Ox) was added dropwise into the emulsion during stirring at room temperature. Concentration of Ox depends on the molar ratio of Ox=An 0:5. The in situ emulsion polymerization was carried out at 258C for 12 h. After deemulsifying and washing the emulsion with acetone/water (1/1, v/v) to remove excess DBSA and Ox, xylene solution of the composite containing DBSA-doped PAn and CSPE (or SBS) was obtained. The xylene solution was cast on glass and dried under infrared lamp and under vacuum at 408C to form a composite ®lm. The secondary doping was carried out by dipping the composite ®lm fully in m-cresol, so as to make it semi-swollen, followed by drying under infrared light and then vacuum drying at 408C to constant weight. The secondary doping for in situ observation was performed by moistening the composite ®lm with enough m-cresol, immediately before in situ observation. 2.3. In situ observation during secondary doping and dedoping of the composites
Fig. 1. Conductivity of PAn/CSPE composites before (b) and after secondary doping (a).
emulsion polymerization in the presence of DBSA, due to secondary doping with m-cresol, respectively. It is evident that secondary doping can enhance the conductivity of PAn/ CSPE and reduce the percolation threshold more effectively than that of PAn/SBS. This fact may be contributed to the interaction between ±SO2Cl groups of CSPE chains and NH± groups of PAn chains in the PAn/CSPE composite, which causes more uniform distribution of PAn chains than PAn/SBS and more dif®culty to form conductive route via self-assembly [11]. However, during secondary doping the m-cresol molecules weaken the interaction between CSPE and PAn and expand the coil-like form of PAn chains, resulting in self-assembly of conductive routes. On the
UV±VIS±NIR spectra were taken by a Shimadzu UV3100 spectrophotometer. ESR spectra were recorded by a JES-FEIXG ESR spectroscope in the atmosphere of puri®ed nitrogen, using intensity of central magnet at 0.336 T and Mn2/MgO as reference standard. UV±VIS spectra were taken by a UV-240 spectrophotometer. 2.4. Measurement PAn content of the composites was determined by elemental analysis, which was performed using a Carlo Erba MOD 1106 apparatus. Conductivity was measured with four-probe method or high impedance method. 3. Results and discussion 3.1. Effect of secondary doping on conductivity of PAn/ CSPE and PAn/SBS composites Figs. 1 and 2 illustrate the increase of conductivity of PAn/CSPE and PAn/SBS composites, obtained by in situ
Fig. 2. Conductivity of PAn/SBS composites before (b) and after secondary doping (a).
H.-Q. Xie et al. / Synthetic Metals 123 (2001) 47±52
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Thus, in the case of secondary doping of PAn/CSPE composite the expanded coil-like conformation of PAn chains and the p-conjugation length of PAn chains increases with increasing time of secondary doping. As the m-cresol molecules diffuse into the polymer ®lm, solvate and plasticize the polymer chains, then a conformational change of PAn chains from coil-like form to expanded coil-like form occurs. Since this conformational change reduces the defects between the repeating units of the PAn chains, the conjugation length is increased. Change in the UV±VIS±NIR spectrum of PAn/ SBS during secondary doping (not indicated here) was similar to that of PAn/CSPE. Fig. 3. Change in UV±VIS±NIR spectra during treatment of PAn/CSPE composites with m-cresol: (a) 0 min; (b) 5 min; (c) 30 min.
contrary, no interaction occurs between SBS and PAn chains before secondary doping. 3.2. In situ observation of secondary doping for the two kinds of composites by UV±VIS±NIR spectrophotometry Fig. 3 shows that after moistening the PAn/CSPE composite ®lm with enough m-cresol, the absorption peak at about 780 nm decreases gradually, whereas the absorption at 1100±2000 nm increases sharply. Xia et al. [4] using UV±VIS±NIR spectrophotometric method in their in situ observation of electron and conformational changes of CSA-doped PAn induced by m-cresol vapor indicated that absorption peak at 780 nm is consistent with a coil-like conformation and a localized polaron structure (shorter conjugation length) and the absorption curve at 1100±2600 nm is consistent with an expanded coil-like conformation and a delocalized polaron structure (larger conjugation length).
3.3. In situ observation of dedoping process and redoping process of the composites via UV±VIS spectrophotometry In order to know what is the change of conformation during dedoping of PAn-DBSA chains in the two composites, in situ observation of the change of UV±VIS spectra during dedoping was carried out. Fig. 4(A) and (B) represent the changes of UV±VIS spectra during dedoping of PAn/ SBS composite with concentrated ammonia solution and 10% NaOH solution, respectively. At the beginning of dedoping process, the characteristic absorption peak at 780 nm disappears quickly, whereas the peak at 560 nm appears. The former represents the localized polaron structure and the latter denotes the benzoid±quinoid structure. However, the change of absorption peak at 560 nm with time differs with different dedoping agents. Using concentrated ammonia as the dedoping agent the peak at 560 nm increases, but the position of peak does not change. It implies that no change of conformation of benzoid±quinoid structure occurs during dedoping process. However, using 10% NaOH solution as the dedoping agent, the absorption
Fig. 4. Change in UV±VIS spectra during dedoping of PAn/SBS composite films with ammonia (A) and NaOH (B) solutions: (1) 0 min; (2) 5 min; (3) 20 min; (4) 60 min; (5) 7 h; (6) 24 h; (7) 48 h.
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peak at 560 nm shifts to blue with broadening of the peak. This change is probably due to the twist of PAn chains during dedoping, thus weakening the p-conjugation between benzoid and quinoid structures, or due to the chemical degradation of PAn chains. Dedoping of PAn/CSPE composite with concentrated ammonia solution showed the same change in UV±VIS spectra as that of PAn/SBS, whereas in the case of dedoping of PAn/CSPE composite with 10% NaOH solution, the disappearance of the peak at 780 nm and the appearance of the peak at 560 nm seems to be slower than dedoping with concentrated ammonia, probably due to slower diffusion of NaOH molecules than that of ammonia. If the PAn/SBS composite with previous secondary doping is dedoped with concentrated ammonia, the in situ observation of the dedoping process by UV±VIS spectrophotometry is shown in Fig. 5. At the end of dedoping there appears a shoulder at about 700 nm. This implies that the benzoid and quinoid structure and the p-conjugation of the PAn chains increases, reducing the difference of pq pb orbit energy, resulting in red-shift of the absorption peak. This fact also demonstrates that secondary doping enhances the conjugation length of PAn chains. But if the dedoping agent is 10% NaOH solution, the absorption peak at 560 nm shifts to blue. The latter fact demonstrates that the p-conjugated plane conformation twists. Dedoping of the PAn/CSPE composite with previous secondary doping showed a similar change in UV±VIS spectra, but the absorption at 560±710 nm is stronger than that of PAn/SBS, which indicates that secondary doping expands the coil-like chains more thoroughly in CSPE than in SBS. When the dedoped PAn/CSPE composites with or without previous secondary doping are redoped with 1.4 M HCl
solution, the in situ observation of UV±VIS spectra during redoping is shown in Fig. 6. It can be seen that after redoping with HCl solution for 7 h (curve 5), the dedoped PAn/CSPE composite with previous secondary doping shows an absorption change from a ¯attened peak at 580±710 nm to a peak at 740 nm, whereas the PAn/CSPE composite without previous secondary doping shows an absorption change from a peak at 580 nm to a peak at 670 nm. At the end of redoping for 48 h, the absorption peaks in the spectra of the two composite ®lms are close to each other at about 760 nm. It seems that the composite ®lm with previous secondary doping is easier to be doped, in other words, the expanded coil-like conformation of PAn chains are more easily to be redoped than the coil-like conformation of PAn chains. 3.4. In situ observation of secondary doping for the two kinds of composites via ESR Pure CSPE, SBS and m-cresol display no ESR signals. Fig. 7 shows that during secondary doping of the composite the central position of the ESR peak does not change and g value is 2.0032. It seems that secondary doping does not affect the type of charge carriers. But the intensity change of ESR peak differs for different composites. In the case of secondary doping for PAn/CSPE composite the intensity of ESR peak (a) ®rst decreases to a minimum value (b), then gradually increases to a constant value (c) at 30 min. The constant intensity is higher than that of the composite ®lm before treatment with m-cresol (a). After the composite ®lm was dried, the intensity of the ESR peak decreases markedly. The peak-to-peak linewidth DHpp is 2.8 G and does not change. ESR signals of the composite re¯ect directly the charge carriers in PAn chains of the composite, since SBR and CSPE display no ESR signals. The above phenomena
Fig. 5. Change in UV±VIS spectra during dedoping of PAn/SBS composite films, which were treated with m-cresol before dedoping, with ammonia (A) and NaOH (B) solutions (the footnotes are the same as in Fig. 4).
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Fig. 6. Change in UV±VIS spectra during redoping with 1.4 M HCl solution for PAn/CSPE composite films, which were treated with m-cresol (A) or without m-cresol (B) followed by dedoping with ammonia solution (the footnotes are the same as in Fig. 4).
Fig. 7. Change in ESR spectra during treatment of PAn/CSPE (A) and PAn/SBS (B) composite films with m-cresol: (a) 0 min; (b) 5 min; (c) 30 min; (d) after removal of m-cresol.
may probably be explained as follows: at the beginning of secondary doping, the composite ®lm solvates slightly, which favors the interaction between PAn chains and formation of bipolarons, resulting in reduction of ESR signals. With the increase of solvation, the interaction between PAn chains decreases, meanwhile the p-conjugation of the PAn chain increases, which may favor the formation of polarons and increases the ESR signal. The conductivity of this semi-swollen composite ®lm was measured to be less than 10 5 S/cm, which rose to 3 S/m after removal of m-cresol. This work indicates that the increase of conductivity of PAn is accompanied by an evident decrease of ESR signals. It may be considered that during evaporation of m-cresol, the interaction between PAn chains was enhanced, resulting in self-assembly of conductive routes between the PAn chains.
In the case of secondary doping for PAn/SBS composites, the ESR peak becomes widened gradually, while the intensity of ESR signal changes moderately. After removal of mcresol the width of ESR peak increases further to 5.6 G. This fact probably implies that secondary doping enhances the spin±spin correlation of the charge carriers of the PAn chains, i.e. solvation of PAn chains with m-cresol induces the expanding of coiled PAn chains and self-assembly of PAn chains into conductive routes simultaneously. 4. Conclusion The conductivity of the two kinds of composites increases after secondary doping and the increase is more obvious for PAn/CSPE than for PAn/SBS. The in situ observation of
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dedoping process with ammonia solution for both composites without previous secondary doping via UV±VIS spectrophotometry indicated that the absorption at 780 nm for localized polarons disappears rapidly, whereas the absorption peak at 560 nm for benzoid±quinoid structure increases gradually. For both composites with previous secondary doping during dedoping with concentrated ammonia the absorption peak at 560 nm for benzoid±quinoid structure shifts to red and broadens to form a shoulder at 710 nm. This fact demonstrates that the p-conjugation of the benzoid± quinoid structure of PAn chains increases due to the expanding of the coiled PAn chains induced by the secondary doping. It was also found that the increase of conductivity of the PAn composites during removal of secondary dopant is accompanied by an obvious decrease of ESR signal.
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