- Email: [email protected]
Polyaniline in the conducting state in neutral medium
Synthetic Metals, 45 (1992) 349-352
349
Short Communication
P o l y a n i l i n e in t h e c o n d u c t i n g s t a t e in n e u t r a l m e d i u m Soumyadeb Ghosh, B. Vishalakshi and V. Kalpagam* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 550012 (India) (Received May 8, 1991; in revised form October 4, 1991; accepted October 16, 1991)
Abstract A composite of polyaniline-carboxymethylcellulose has been prepared and it has been found by electronic spectroscopy that polyaniline in the composite exists in the conducting state even when the material is equilibrated with neutral aqueous medium. The phenomenon is attributed to the Donnan effect.
Introduction Polyaniline (PANi)-polyelectrolyte composites show interesting and useful properties [1-3]. Anionic polyelectrolytes act as the dopant for the 'aciddoped' [4] polyaniline as well as the matrix for the conducting polymer, resulting in a crosslinked polymeric material with good mechanical properties. These materials also allow high ionic mobility when equilibrated with aqueous medium, making them highly suitable as electrode materials. From their cyclic voltammetric studies, Orata and Buttry [3] have further shown that PANi-Nafion composite has a higher electrical conductivity than pure PANi a t p H 2. We have prepared polyaniline-carboxymethylcellulose (PANi-CMC) composite films which swell in aqueous medium. The electronic spectra of such films, equilibrated in water at different pH, show that PANi exists in the conducting form even in almost neutral medium. This has been attributed to the Donnan effect [5].
E x p e r i m e n t a l and results PANi, in the emeraldine form, was prepared by the standard chemical method [4] of oxidative polymerization of aniline using (~1-I4)2S208 at a *Author to whom correspondence should be addressed.
0379-6779/92/$5.00
© 1 9 9 2 - Elsevier Sequoia. All rights reserved
350
t e m p e r a t u r e between - 5 and - 8 °C. The green precipitate was treated with NH4OH and washed with water to obtain the PANi in polyemeraldine base (PEB) form. The deep blue pow de r of PEB was washed thoroughly with CHC18 to drain out the low molecular weight oligomers. IR spectra of the product, r eco r d ed using a Perkin-Elmer Model 597 spectrometer, were c o m p a r e d with the spectra of the PEB already report ed in the literature [6]. Sodium carboxymethylcellulose (NaCMC), obtained from BDH Chemicals Ltd., was found to have a viscosity average molecular weight (/~0 of 4.3 × 105 and a degree of substitution (by carboxyl group) of 0.74. Films of NaCMC were cast from its aqueous solution on glass slides and were then swollen in formic acid. A PANi solution ( ~ 1%0) in formic acid was spread evenly over the swollen film such that the PANi content in the material was below 10%. Excess formic acid was evaporated at 50 °C under low pressure. The films were then kept in water for at least 10 h and the water was frequently changed to wash off all the acid. A pure PANi film was also cast from formic acid on a glass slide for comparison and was treated in the same manner. The composite films were equilibrated with aqueous medium of different pH for at least 24 h. UV-Vis spectra of the films were taken using an Hitachi Model U-3400 s p e c t r o p h o t o m e t e r and are shown in Fig. 1. The lower energy p e a k for the film equilibrated at pH 6.5 has a Am~ value of 860 nm com pared to a value of 589 nm for the pure PANi film equilibrated with the same medium. This shows that even in neutral medium the PANi in the composite exists in the conducting state. It is well known that as the polyaniline changes fr o m the nonconducting to the conducting form with protonation, the Am~ value of the lower energy electronic transition peak red shifts from around 600 n m t o > 7 5 0 nm [7].
~
Q
\\
jJ
.: \ , 1
"
//" /,/~ "..•-
....%...
\ \ ~ c l
[
//
\\
J 400
~ ....... I 600 Wovelength
I
I\~ e 800
(nm)
Fig. 1. UV-Vis spectra of PANi-CMC composite films equilibrated with aqueous medium of (a) pH 6.5, Co) pH 6.7, (c) 2% NaCI solution at pH 6.6, (d) buffer solution of pH 7.0 and (e) pure PANi film equilibrated with the same medium as (a).
351
It may be most appropriate to compare these results with the results obtained from a similar study done on a PANi-cellulose composite by Weiss et al. [8]. For the composite film equilibrated at pH 7, they obtained a ~m~x at around 550 nm, and a Am~> 800 nm was obtained only at pH < 3. The dramatic change in property, brought about by the presence of carboxyl groups in the former material, can be attributed to the Donnan effect [5]. In the prepared PANi-CMC composite, the amount of CMC is much greater than the amount of PANi, giving rise to a net negative charge on the film due to the ionization of the carboxyl groups. To balance this excess charge, the actual proton concentration inside the film attains a higher value than that in the external solution. Hence, the PANi present inside the film becomes protonated even when the film is equilibrated with a neutral medium. A detailed study correlating the difference between the internal and external pH with the Donnan potential, for a system of 'acidic oxycellulose' equilibrated with aqueous medium of different ionic strength, is reported by Neale and Standring [9]. To test our explanation further, the films were kept in aqueous solutions of high ionic strength for the same duration as above. The spectrum of the film equilibrated with a 2% NaC1 solution of pH 6.6 has Am~ at 689 nm, and for the film equilibrated with the buffer solution of pH 7 the value is 603 nm. At higher ionic strength of the medium, the protons are replaced from the film by other exchangeable cations, increasing the pH inside it. This deprotonates the PANi to its nonconducting state. When observed under an optical microscope, the prepared films showed inhomogeneity. Three different phases could be identified: a colourless region of the polyelectrolyte matrix, a green continuous and homogeneous phase of the PANi-CMC complex and a phase of dark green particles of PANi-CMC. The presence of PANi in different environments makes its degree of doping also inhomogeneous. This is reflected in the broadness of the peaks in the electronic spectra of the composite material compared to that of pure PANi (compare Fig. l(e) with the others). It should be noted that the advantage of the Donnan effect can be obtained only in the absence of any cations in the solution that may exchange with the protons. However, if the cations present in the solution have a low diffusion coefficient in the composite compared to protons then the Donnan effect can be tapped and the PANi can be used even at higher pH. Due to this kinetic factor, Orata and Buttry [3] could observe the Doiman effect even at quite high ionic strength in their cyclic voltammetric studies on PANi-Nafion composite. A semipermeable coating on the composite may further help to decrease or stop the replacement of protons by other cations from the material. Apart from PANi-polyelectrolyte composites, the Donnan effect can also be observed in 'self-doped' polyanilines [10, 11] and 'self doping' should be applicable only under the conditions mentioned above. The present paper not only shows a way to extend the applicability of PANi at higher pH, but also emphasizes the need for application of general principles of polymer science to the field of conducting polymers. More
352
detailed studies on the PANi-polyelectrolyte complexes are being done and the results will be reported later.
Acknowledgement Financial support from the Centre for Scientific and Industrial Research (CSIR), India, is gratefully acknowledged.
References G. Bidan and B. Ehui, J. Chem. Soc., Chem. Commun., (1989) 1568. B. D. Malhotra, S. Ghosh and R. Chandra, J. Appl. Polym. Sci., 40 (1990) 1049. D. Orata and D. A. Buttry, J. Electroanal. Chem., 257 (1988) 71. J.-C. Chiang and A. G. MacDiarmid, Synth. Met., 13 (1986) 193. C. Tan_ford, Physical Chemistry of Macromolecules, Wiley, New York, 1961. J. Tang, X. Jing, B. Wang and F. Wang, Synth. Met., 24 (1988) 231. Y. Cao, P. Smith and A. J. Heeger, Synth. Met., 32 (1989) 263. H. Weiss, O. Pfefferkron, G. Kotora and B. D. Humphrey, J. Electrochem. Soc., 136 (1989) 3711. 9 S. M. Neale and P. T. Standring, Proc. Roy. Soc. London, Set. A, 213 (1952) 530. 10 J. Yue and A. J. Epstein, J. Am. Chem. Soc., 112 (1990) 2800. 11 P. Hany, E. M. Geni~s and C. Santier, Synth. Met., 31 (1989) 369. 1 2 3 4 5 6 7 8
349
Short Communication
P o l y a n i l i n e in t h e c o n d u c t i n g s t a t e in n e u t r a l m e d i u m Soumyadeb Ghosh, B. Vishalakshi and V. Kalpagam* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 550012 (India) (Received May 8, 1991; in revised form October 4, 1991; accepted October 16, 1991)
Abstract A composite of polyaniline-carboxymethylcellulose has been prepared and it has been found by electronic spectroscopy that polyaniline in the composite exists in the conducting state even when the material is equilibrated with neutral aqueous medium. The phenomenon is attributed to the Donnan effect.
Introduction Polyaniline (PANi)-polyelectrolyte composites show interesting and useful properties [1-3]. Anionic polyelectrolytes act as the dopant for the 'aciddoped' [4] polyaniline as well as the matrix for the conducting polymer, resulting in a crosslinked polymeric material with good mechanical properties. These materials also allow high ionic mobility when equilibrated with aqueous medium, making them highly suitable as electrode materials. From their cyclic voltammetric studies, Orata and Buttry [3] have further shown that PANi-Nafion composite has a higher electrical conductivity than pure PANi a t p H 2. We have prepared polyaniline-carboxymethylcellulose (PANi-CMC) composite films which swell in aqueous medium. The electronic spectra of such films, equilibrated in water at different pH, show that PANi exists in the conducting form even in almost neutral medium. This has been attributed to the Donnan effect [5].
E x p e r i m e n t a l and results PANi, in the emeraldine form, was prepared by the standard chemical method [4] of oxidative polymerization of aniline using (~1-I4)2S208 at a *Author to whom correspondence should be addressed.
0379-6779/92/$5.00
© 1 9 9 2 - Elsevier Sequoia. All rights reserved
350
t e m p e r a t u r e between - 5 and - 8 °C. The green precipitate was treated with NH4OH and washed with water to obtain the PANi in polyemeraldine base (PEB) form. The deep blue pow de r of PEB was washed thoroughly with CHC18 to drain out the low molecular weight oligomers. IR spectra of the product, r eco r d ed using a Perkin-Elmer Model 597 spectrometer, were c o m p a r e d with the spectra of the PEB already report ed in the literature [6]. Sodium carboxymethylcellulose (NaCMC), obtained from BDH Chemicals Ltd., was found to have a viscosity average molecular weight (/~0 of 4.3 × 105 and a degree of substitution (by carboxyl group) of 0.74. Films of NaCMC were cast from its aqueous solution on glass slides and were then swollen in formic acid. A PANi solution ( ~ 1%0) in formic acid was spread evenly over the swollen film such that the PANi content in the material was below 10%. Excess formic acid was evaporated at 50 °C under low pressure. The films were then kept in water for at least 10 h and the water was frequently changed to wash off all the acid. A pure PANi film was also cast from formic acid on a glass slide for comparison and was treated in the same manner. The composite films were equilibrated with aqueous medium of different pH for at least 24 h. UV-Vis spectra of the films were taken using an Hitachi Model U-3400 s p e c t r o p h o t o m e t e r and are shown in Fig. 1. The lower energy p e a k for the film equilibrated at pH 6.5 has a Am~ value of 860 nm com pared to a value of 589 nm for the pure PANi film equilibrated with the same medium. This shows that even in neutral medium the PANi in the composite exists in the conducting state. It is well known that as the polyaniline changes fr o m the nonconducting to the conducting form with protonation, the Am~ value of the lower energy electronic transition peak red shifts from around 600 n m t o > 7 5 0 nm [7].
~
Q
\\
jJ
.: \ , 1
"
//" /,/~ "..•-
....%...
\ \ ~ c l
[
//
\\
J 400
~ ....... I 600 Wovelength
I
I\~ e 800
(nm)
Fig. 1. UV-Vis spectra of PANi-CMC composite films equilibrated with aqueous medium of (a) pH 6.5, Co) pH 6.7, (c) 2% NaCI solution at pH 6.6, (d) buffer solution of pH 7.0 and (e) pure PANi film equilibrated with the same medium as (a).
351
It may be most appropriate to compare these results with the results obtained from a similar study done on a PANi-cellulose composite by Weiss et al. [8]. For the composite film equilibrated at pH 7, they obtained a ~m~x at around 550 nm, and a Am~> 800 nm was obtained only at pH < 3. The dramatic change in property, brought about by the presence of carboxyl groups in the former material, can be attributed to the Donnan effect [5]. In the prepared PANi-CMC composite, the amount of CMC is much greater than the amount of PANi, giving rise to a net negative charge on the film due to the ionization of the carboxyl groups. To balance this excess charge, the actual proton concentration inside the film attains a higher value than that in the external solution. Hence, the PANi present inside the film becomes protonated even when the film is equilibrated with a neutral medium. A detailed study correlating the difference between the internal and external pH with the Donnan potential, for a system of 'acidic oxycellulose' equilibrated with aqueous medium of different ionic strength, is reported by Neale and Standring [9]. To test our explanation further, the films were kept in aqueous solutions of high ionic strength for the same duration as above. The spectrum of the film equilibrated with a 2% NaC1 solution of pH 6.6 has Am~ at 689 nm, and for the film equilibrated with the buffer solution of pH 7 the value is 603 nm. At higher ionic strength of the medium, the protons are replaced from the film by other exchangeable cations, increasing the pH inside it. This deprotonates the PANi to its nonconducting state. When observed under an optical microscope, the prepared films showed inhomogeneity. Three different phases could be identified: a colourless region of the polyelectrolyte matrix, a green continuous and homogeneous phase of the PANi-CMC complex and a phase of dark green particles of PANi-CMC. The presence of PANi in different environments makes its degree of doping also inhomogeneous. This is reflected in the broadness of the peaks in the electronic spectra of the composite material compared to that of pure PANi (compare Fig. l(e) with the others). It should be noted that the advantage of the Donnan effect can be obtained only in the absence of any cations in the solution that may exchange with the protons. However, if the cations present in the solution have a low diffusion coefficient in the composite compared to protons then the Donnan effect can be tapped and the PANi can be used even at higher pH. Due to this kinetic factor, Orata and Buttry [3] could observe the Doiman effect even at quite high ionic strength in their cyclic voltammetric studies on PANi-Nafion composite. A semipermeable coating on the composite may further help to decrease or stop the replacement of protons by other cations from the material. Apart from PANi-polyelectrolyte composites, the Donnan effect can also be observed in 'self-doped' polyanilines [10, 11] and 'self doping' should be applicable only under the conditions mentioned above. The present paper not only shows a way to extend the applicability of PANi at higher pH, but also emphasizes the need for application of general principles of polymer science to the field of conducting polymers. More
352
detailed studies on the PANi-polyelectrolyte complexes are being done and the results will be reported later.
Acknowledgement Financial support from the Centre for Scientific and Industrial Research (CSIR), India, is gratefully acknowledged.
References G. Bidan and B. Ehui, J. Chem. Soc., Chem. Commun., (1989) 1568. B. D. Malhotra, S. Ghosh and R. Chandra, J. Appl. Polym. Sci., 40 (1990) 1049. D. Orata and D. A. Buttry, J. Electroanal. Chem., 257 (1988) 71. J.-C. Chiang and A. G. MacDiarmid, Synth. Met., 13 (1986) 193. C. Tan_ford, Physical Chemistry of Macromolecules, Wiley, New York, 1961. J. Tang, X. Jing, B. Wang and F. Wang, Synth. Met., 24 (1988) 231. Y. Cao, P. Smith and A. J. Heeger, Synth. Met., 32 (1989) 263. H. Weiss, O. Pfefferkron, G. Kotora and B. D. Humphrey, J. Electrochem. Soc., 136 (1989) 3711. 9 S. M. Neale and P. T. Standring, Proc. Roy. Soc. London, Set. A, 213 (1952) 530. 10 J. Yue and A. J. Epstein, J. Am. Chem. Soc., 112 (1990) 2800. 11 P. Hany, E. M. Geni~s and C. Santier, Synth. Met., 31 (1989) 369. 1 2 3 4 5 6 7 8