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Comparative studies of chemically synthesized polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid
European Polymer Journal 40 (2004) 379–384 www.elsevier.com/locate/europolj
Comparative studies of chemically synthesized polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid Milind V. Kulkarni, Annamraju Kasi Viswanath
*
Photonics and Advanced Materials Laboratory, Centre for Materials for Electronics Technology, (C-MET), Panchawati, Off Pashan Road, Pune 411 008, India Received 6 June 2003; received in revised form 18 September 2003; accepted 3 October 2003
Abstract Polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid (p-TSA) were synthesized by in situ chemical polymerization method using ammonium per sulphate as an oxidizing agent. This is a novel polymerization process for the direct synthesis of emeraldine salt phase of the polymer. The polymers were characterized by using UV–Vis and FT-IR spectroscopy, SEM, elemental analyzer, TGA/DSC and conductivity measurements. Thermal analysis shows that poly(o-toluidine) is less thermally stable compared to polyaniline. The less conductivity in poly(o-toluidine) is due to the cumulative steric as well as electronic effect of the bulky methyl substituent present on the benzene ring. High temperature conductivity measurements show Ôthermal activated behavior’. 2003 Published by Elsevier Ltd. Keywords: Polyaniline; Poly(o-toluidine); Chemical polymerization; Characterization
1. Introduction The field of conducting polymers has been flourishing rapidly day by day and these materials are becoming indispensable for this century. Polyaniline, one of the most promising conducting polymers, is inherently brittle and poor in processibility due to its insolubility in common organic solvents [1,2]. This problem has been overcome to some extent by using substituted derivatives of anilines such as toluidines, anisidines, N-methyl or N-ethyl anilines etc. [3–5]. The polymers of the substituted aniline exhibit greater solubility but the conductivity is found to be slightly lower. However, efforts have been made to improve the processibility of these polymers, in recent years, by using a functionalized protonic acid, which makes polyaniline conducting as well as renders the resulting polyaniline complex soluble in organic solvents like toluene, chloroform, xylene, m-cresol etc. [6,7]. *
Corresponding author. E-mail address: [email protected] (A.K. Viswanath).
0014-3057/$ - see front matter 2003 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2003.10.007
A large number of papers are available reporting the studies on substituted derivatives of polyaniline [3–5, 8–10]. This paper describes a novel polymerization process for the direct synthesis of the conducting emeraldine salt phase of the polyaniline and poly(o-toluidine) without the need of post doping treatment. We have taken up a systematic investigation of conducting polymers and polymers doped with suitable molecules for the development of humidity sensors [11–13]. In the present work, we report the synthesis and characterization of p-toluene sulphonic acid doped polyaniline and poly (o-toluidine). The results are explained on comparative basis. Effect of methyl substituent (present at ortho position in poly(o-toluidine) on the polymer properties has been discussed in detail.
2. Experimental All the chemicals and monomers (aniline and o-toluidine) used were of AR grade and used as received. All the solutions were prepared using doubly distilled
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water. The polymerization was initiated by the dropwise addition of the oxidizing agent in an acidified solution (containing 1 M p-TSA) of monomer (aniline or otoluidine) under constant stirring at 0–5 C. The monomer to oxidizing agent ratio was kept as 1:1. After complete addition of the oxidizing agent, the reaction mixture was kept under stirring for 24 h. The greenishblack precipitate of the polymer was isolated by filtration and conditioned by washing and drying in an oven. UV–Visible spectra of the polymer solution in m-cresol were recorded by using Hitachi-U3210 spectrophotometer in the range of 300–900 nm. FT-IR spectra of the polymer were taken on a Perkin–Elmer-Spectrum 2000 spectrophotometer between 400 and 4000 cm 1 . The samples were prepared in the pellet form using spectroscopic grade KBr powder. The elemental composition of the material was estimated by using CE Instruments EA-1110 CHNSO analyzer. Morphological studies were performed with the help of Philips XL-30 scanning electron microscope. Thermogram of the polymer sample was recorded using Mettler–Toledo 851 thermogravimetric analyzer in presence of N2 atmosphere from RT to 900 C with a heating rate of 10 C/min. DSC studies were performed using Mettler-Toledo 821 system in presence of N2 atmosphere up to 500 C at a heating rate of 10 C/min. The room temperature conductivity was measured by two probe technique. Dry powdered samples were made into pellets using a steel die having 1.5 cm diameter in an hydraulic press under a pressure of seven tons. Temperature dependent electrical conductivity of the polymer samples was measured using a laboratory made set up. The electrical contacts were made by using platinum foils. The controlled heating of the sample was carried out by the heater placed near the sample. The change in resistance was recorded with increase in the temperature. The conductivity values were calculated directly from the measured resistance and sample dimensions.
extended tail at 820 nm representing the conducting emeraldine salt phase of the polymer [14]. On comparison of the two spectra, it is observed that in poly(o-toluidine) the peak at 320 nm is found to be very sharp with suppression of the peak at 420 nm as compared to polyaniline. Also the spectra shows a blue shift compared to polyaniline which can be attributed to the presence of bulky methyl substituent on the benzene ring which induces the additional deformation along the polymer backbone, owing to an increase in the steric hindrance. This results in a decrease of the degree of the conjugation [15]. Thus, the absorption maxima in poly(o-toluidine) located at relatively shorter wavelength is due to the nonplaner conformation of the polymer backbone. The FT-IR spectra of polyaniline and poly(o-toluidine) doped with PTSA are presented in Fig. 2(a) and (b) and the peak positions related to the corresponding chemical bonds are listed in the Table 1. The presence of the two bands in the vicinity of 1500 and 1600 cm 1 are assigned to the nonsymmetric C6 ring stretching modes. The higher frequency vibration at 1600 cm 1 has a major contribution from the quinoid
3. Results and discussion Fig. 1 shows the optical absorption spectra of the polyaniline and poly(o-toluidine) doped with PTSA. The spectra were recorded by using m-cresol as a solvent. In case of polyaniline (Fig. 1(a)) the spectrum shows two peaks at 320 nm at 420 nm together with an extended tail with increasing absorption at 850 nm, characteristic of an extended coil conformation. whereas, in poly(otoluidine) (Fig. 1(b)) the polymer exhibits a sharp peak at 320 nm and small peaks at 420 and 820 nm. The peak at 320 nm corresponds to the p–p transition of the benzenoid rings while, the peak at 420 nm can be assigned to the localized polaron bands which are the characteristics of protonated polyaniline together with
Fig. 1. UV–Visible spectra of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA acid recorded using m-cresol as a solvent.
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381
Table 1 Characteristic frequencies of chemically synthesized p-toluene sulphonic acid doped polyaniline and poly(o-toluidine) Wavenumber (cm 1 ) Polyaniline
Fig. 2. FT-IR spectra of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
rings while, the lower frequency mode at 1500 cm 1 depicts the presence of benzenoid ring units. The presence of these two bands clearly shows that the polymer is composed of the amine and imine units. Further this also supports our UV–Visible characterization where the different phases are observed in the spectrum. The presence of vibration band of the dopant ion and other characteristic bands confirm that the polymer is in the conducting emeraldine salt phase. The elemental analysis was carried out in order to know about the composition of the polymer with various elements such as C, H, N and S present in the polymer matrix. Table 2 shows the values for the emeraldine salt phase of the polymer. The higher values of %C and %H in poly(o-toluidine) are obviously due to the presence of the methyl substituent on the benzene ring. However, the higher value of %S in polyaniline confirms the efficient doping of the polymer chains especially iminic nitrogen atom which is further supported by conductivity measurements discussed in later section. The conductivity of polyaniline doped with PTSA was found to be quite higher than that of poly(o-toluidine) doped with PTSA.
Band characteristics
Poly(o-toluidine)
805.91
808.16
–
879.26
558.52
563.65
1031.75
1005.06
1123.64
1111.55
1299.10
1314.28
1485.23
1492.70
1572.11
1587.62
–
2923.50
3232.47
3217.71
3440.83
3440.60
Paradisubstituted aromatic rings indicating polymer formation Due to the methyl group attached to the phenyl ring C–H out of plane bending vibration Due to SO3 group of the p-TSA C–H in plane bending vibration Aromatic C–N stretching indicating secondary aromatic amine group C–N stretching of benzenoid rings C–N stretching of quinoid rings C–H stretching due to substituent methyl group The aromatic C–H stretching >N–H stretching vibration
Table 2 Elemental composition of polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid Polymer
%C
%H
%N
%S
Polyaniline Poly(o-toluidine)
59.84 64.56
5.19 5.89
9.31 8.62
5.6 4.54
SEM studies also reveal a variation in the morphological structure of the two polymers. Fig. 3(a) and (b) shows the SEM photographs of polyaniline-PTSA and poly(o-toluidine)-PTSA respectively. Polyaniline doped with PTSA exhibits a sponge like structure whereas poly(o-toluidine) shows the presence of needle shaped small chopped fibers comprising of a whole polymer matrix. Presence of small chopped fibers in poly(otoluidine) can be explained by considering the steric contribution of the methyl group. The methyl group present at the ortho position of the benzene ring leads to a distortion in the polymer chains and restrict the polymer growth in a linear fashion, which in turn results
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Fig. 4. Thermograms of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
Fig. 3. Scanning electron micrographs of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
in a breakdown of the polymer chain into small fragments and this appears as small chopped or needle shaped fibers in the micrograph. The thermal stability of the polymers was evaluated with the help of thermogravimetric analysis. Fig. 4(a) and (b) represents the thermograms of polyaniline and poly(o-toluidine) doped with PTSA respectively. The thermal behavior of poly(o-toluidine) is similar to that of polyaniline and exhibits a three stage decomposition pattern. In the first stage, the weight loss observed upto 110 C is due to the loss of water molecules present in the polymer matrix. The second stage loss observed from 100 to 320 C is attributed to the loss of the dopant (dedoping) from the polymer chains. While, the third stage loss from 320 C onwards is responsible for the complete degradation and decomposition of the polymer backbone [16]. From the comparison of the thermograms of polyaniline and poly(o-toluidine) it is observed that the first stage of decomposition is found to be identical whereas, variation is observed in the second and third step of decomposition. The second stage loss, due to thermal dedoping is very sharp in poly(o-toluidine) and shows a real step at about 250 C with sudden weight loss upto about 400 C. Whereas, a very small weight loss is ob-
served in polyaniline over the same temperature range. The 70% of the original weight is observed upto 520 C in the thermogram of polyaniline while, only 40% of the original weight is stable upto 520 C in case of poly (o-toluidine) as observed from the third step in the thermogram. Also it is observed that the 50% of the original weight is stable upto 800 C in the polyaniline. However only 30% of the original weight is observed upto 800 C in poly(o-toluidine). In poly(o-toluidine) major weight loss is observed in a very small temperature range from 320 to 520 C on the other hand it is found to be extended over a broad temperature range from 320 to 800 C in polyaniline. This clearly indicates that poly (otoluidine) is thermally less stable as compared to unsubstituted polyanilines. This may be due to the presence of an alkyl substituent in the backbone which gives the polymer with lots of defect sites, which in turn facilitates the loosening of the polymer chains and hence the degradation and decomposition of the polymer backbone at lower temperature. Thus poly(o-toluidine) is less themally stable compared to polyaniline. Fig. 5 shows the DSC thermogram of polyaniline and poly(o-toluidine) doped with PTSA. The thermograms of both the polymers exhibit an endotherm which completes at 120 C. This is attributed to the expulsion of the loosely bound water molecules present in the polymer matrix [17]. The peak corresponding to this weight loss is found to be very sharp in polyaniline as compared with poly(o-toluidine). The second endothermic peak, at 210 C is due to thermal dedoping of the p-toluene sulphonic acid from the polymer chains. This
M.V. Kulkarni, A.K. Viswanath / European Polymer Journal 40 (2004) 379–384
Fig. 5. Differential scanning calorimeter (DSC) thermograms of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
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the benzene ring induces additional deformation along the polymer backbone. This in turn results in a decrease of the degree of conjugation and hence a decrease in conductivity. Consequently, the torsion angle between the repeat units is greater in alkyl substituted polyaniline. This explains the noticed decrease in the conductivity [22]. Fig. 6 shows the temperature dependent conductivity of polyaniline and poly(o-toluidine) doped with p-TSA. From the figure it is observed that the conductivity is found to increased with increase in the temperature from RT to 120 C. This increase in conductivity with increasing temperature is the characteristic of Ôthermal activated behavior’ and is observed in protonated polyaniline prepared from general polymerization method and is in good agreement with the literature. The possible explanation for increase in conductivity is the increase of efficiency of charge transfer between the polymer chain and the dopant with increase in the temperature [23,24]. It can also been suggested that, the thermal curing affects the chain alignment of the polymer, which leads to the increase of conjugation length and that brings about the increase of conductivity. Also there had to be molecular rearrangement on heating, which made the molecular
peak also found to be sharp in case of polyaniline. While, it is broad and appears like a hump in case of poly(o-toluidine). These observations support our thermogravimetric analysis results discussed in earlier section. The exothermic peak at about 370 C suggests the interchain crosslinking and thermally effected morphological changes [18–21]. The thermogram also confirms the absence of any glass transition (Tg ) or melting (Tm ) for both the systems and is in good agreement with the literature [17]. The room temperature solid state conductivities were measured on pressed pellets having a diameter of 1.5 cm using two probe technique. Table 3 gives the values for the same. The conductivity of poly(o-toluidine) is about three orders of magnitude less as compared to polyaniline. The lower conductivity relative to polyaniline may be explained by an increase of the interchain distance and diluting effect of the charge carriers caused by the presence of bulky methyl group in the polymer. The methyl substituent present at the ortho position of
Table 3 Room temperature conductivity values of polyaniline and poly(o-toluidine) doped with p-TSA Polymer
Conductivity S/cm
Polyaniline-p-TSA Poly(o-toluidine)-p-TSA
1.46 1.93 · 10
3
Fig. 6. Temperature dependent conductivity plots of polyaniline (a) and poly(o-toluidine) doped with p-TSA.
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conformation favourable for electron delocalization [25]. 4. Conclusions Polyaniline and poly(o-toluidine) having conducting emeraldine salt phase were synthesized by in situ chemical oxidative polymerization method using p-TSA as a dopant. The similarities between the polyaniline and poly(o-toluidine) doped with p-TSA are also evident in the UV–Visible and FT-IR spectroscopic measurements. The steric effect of the methyl substituent is mainly responsible for the decrease in the extent of p electron conjugation which is reflected as a blue shift in peak position in the UV–Visible spectrum. Thermal studies revealed that poly(o-toluidine) is thermally less stable and undergoes fast thermal decomposition over a short thermal range compared to polyaniline. The less conductivity in poly(o-toluidine) is due to the cumulative electronic as well steric effect of the methyl group. Thus the improvement in the conductivity of poly(o-toluidine) to make them suitable for various technological applications should be the goal of the future.
Acknowledgements Authors are thankful to Dr. B.K. Das, Executive Director, C-MET, Pune for his constant encouragement. The authors would like to thank Dr. Tanay Seth, Dr. Sunit Rane, Mrs. Rina Gorte, and Mr. R. Marimuthu for their help in characterization. References [1] Angelopoulos M, Ray A, MacDiarmid AG, Epstein AJ. Synth Met 1987;21:12. [2] Andreatta A, Cao Y, Chiang JC, Heeger AJ, Smith P. Synth Met 1988;26:383.
[3] Dao LH, Leclerc M, Guay J, Chevalier JW. Synth Met 1989;E29:377. [4] D’Aparno G, Leclerc M, Zotti G. J Electroanal Chem 1993;35:145. [5] Mortimer RJ. Mater Chem 1995;5:969. [6] Cao Y, Smith P, Heeger AJ. Synth Met 1992;48:91. [7] Cao Y, Smith P, Heeger AJ. Synth Met 1993;55–57:3514. [8] Mattoso LHC, Bulhoes LOS. Synth Met 1992;52:171. [9] Chapman SE, Bilingham NC, Armes SP. Synth Met 1993;55–57:995. [10] Anand J, Palaniappan S, Sathyanarayana DN. Synth Met 1994;66:129. [11] Somani PR, Viswanath AK, Aiyer RC, Radhakrishnan S. Sens Actuators B 2001;80:141. [12] Somani PR, Viswanath AK, Aiyer RC, Radhakrishnan S. Org Electron 2001;2:83. [13] Somani PR, Marimuthu R, Viswanath AK, Radhakrishnan S. Polym Degrad Stab 2003;79:77. [14] Athawale AA, Kulkarni MV, Chabukswar VV. Mater Chem Phys 2002;73:106. [15] Leclerc M, D’Aparno G, Zotti G. Synth Met 1993;55– 57:1527. [16] Patil SF, Bedekar AG, Patil RC, Agashe CH. Ind J Chem 1994;33(A):580. [17] Palaniappan S, Narayana BH. J Polym Sci A Polym Chem 1994;32:2431. [18] Shenglong W, Fosong W, Xiaohui G. Synth Met 1986;79:11. [19] Sharma AL, Saxena V, Annapoorni S, Malhotra BD. J Appl Polym Sci 2001;81:1460. [20] Wei Y, Tang GW, Husch KF, Sherr EM, MacDiarmid AG, Epstein AJ. Polymer 1992;33:314. [21] Chan HSO, Ng SC, Sim WS, Seo WSH, Tan BTG. Macromolecules 1993;26:144. [22] D’Aparno G, Leclerc M. J Electroanal Chem 1993;351:145. [23] Zuo F, Angelopoulos M, MacDiarmid AG, Epstein AJ. Phys Rev B 1987;36:3475. [24] Kobayashi A, Ishikawa H, Amano K, Satoh M, Hasegawa E. J Appl Phys 1993;74(1):296. [25] Han MG, Im SS. Polymer 2000;41:3253.
Comparative studies of chemically synthesized polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid Milind V. Kulkarni, Annamraju Kasi Viswanath
*
Photonics and Advanced Materials Laboratory, Centre for Materials for Electronics Technology, (C-MET), Panchawati, Off Pashan Road, Pune 411 008, India Received 6 June 2003; received in revised form 18 September 2003; accepted 3 October 2003
Abstract Polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid (p-TSA) were synthesized by in situ chemical polymerization method using ammonium per sulphate as an oxidizing agent. This is a novel polymerization process for the direct synthesis of emeraldine salt phase of the polymer. The polymers were characterized by using UV–Vis and FT-IR spectroscopy, SEM, elemental analyzer, TGA/DSC and conductivity measurements. Thermal analysis shows that poly(o-toluidine) is less thermally stable compared to polyaniline. The less conductivity in poly(o-toluidine) is due to the cumulative steric as well as electronic effect of the bulky methyl substituent present on the benzene ring. High temperature conductivity measurements show Ôthermal activated behavior’. 2003 Published by Elsevier Ltd. Keywords: Polyaniline; Poly(o-toluidine); Chemical polymerization; Characterization
1. Introduction The field of conducting polymers has been flourishing rapidly day by day and these materials are becoming indispensable for this century. Polyaniline, one of the most promising conducting polymers, is inherently brittle and poor in processibility due to its insolubility in common organic solvents [1,2]. This problem has been overcome to some extent by using substituted derivatives of anilines such as toluidines, anisidines, N-methyl or N-ethyl anilines etc. [3–5]. The polymers of the substituted aniline exhibit greater solubility but the conductivity is found to be slightly lower. However, efforts have been made to improve the processibility of these polymers, in recent years, by using a functionalized protonic acid, which makes polyaniline conducting as well as renders the resulting polyaniline complex soluble in organic solvents like toluene, chloroform, xylene, m-cresol etc. [6,7]. *
Corresponding author. E-mail address: [email protected] (A.K. Viswanath).
0014-3057/$ - see front matter 2003 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2003.10.007
A large number of papers are available reporting the studies on substituted derivatives of polyaniline [3–5, 8–10]. This paper describes a novel polymerization process for the direct synthesis of the conducting emeraldine salt phase of the polyaniline and poly(o-toluidine) without the need of post doping treatment. We have taken up a systematic investigation of conducting polymers and polymers doped with suitable molecules for the development of humidity sensors [11–13]. In the present work, we report the synthesis and characterization of p-toluene sulphonic acid doped polyaniline and poly (o-toluidine). The results are explained on comparative basis. Effect of methyl substituent (present at ortho position in poly(o-toluidine) on the polymer properties has been discussed in detail.
2. Experimental All the chemicals and monomers (aniline and o-toluidine) used were of AR grade and used as received. All the solutions were prepared using doubly distilled
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water. The polymerization was initiated by the dropwise addition of the oxidizing agent in an acidified solution (containing 1 M p-TSA) of monomer (aniline or otoluidine) under constant stirring at 0–5 C. The monomer to oxidizing agent ratio was kept as 1:1. After complete addition of the oxidizing agent, the reaction mixture was kept under stirring for 24 h. The greenishblack precipitate of the polymer was isolated by filtration and conditioned by washing and drying in an oven. UV–Visible spectra of the polymer solution in m-cresol were recorded by using Hitachi-U3210 spectrophotometer in the range of 300–900 nm. FT-IR spectra of the polymer were taken on a Perkin–Elmer-Spectrum 2000 spectrophotometer between 400 and 4000 cm 1 . The samples were prepared in the pellet form using spectroscopic grade KBr powder. The elemental composition of the material was estimated by using CE Instruments EA-1110 CHNSO analyzer. Morphological studies were performed with the help of Philips XL-30 scanning electron microscope. Thermogram of the polymer sample was recorded using Mettler–Toledo 851 thermogravimetric analyzer in presence of N2 atmosphere from RT to 900 C with a heating rate of 10 C/min. DSC studies were performed using Mettler-Toledo 821 system in presence of N2 atmosphere up to 500 C at a heating rate of 10 C/min. The room temperature conductivity was measured by two probe technique. Dry powdered samples were made into pellets using a steel die having 1.5 cm diameter in an hydraulic press under a pressure of seven tons. Temperature dependent electrical conductivity of the polymer samples was measured using a laboratory made set up. The electrical contacts were made by using platinum foils. The controlled heating of the sample was carried out by the heater placed near the sample. The change in resistance was recorded with increase in the temperature. The conductivity values were calculated directly from the measured resistance and sample dimensions.
extended tail at 820 nm representing the conducting emeraldine salt phase of the polymer [14]. On comparison of the two spectra, it is observed that in poly(o-toluidine) the peak at 320 nm is found to be very sharp with suppression of the peak at 420 nm as compared to polyaniline. Also the spectra shows a blue shift compared to polyaniline which can be attributed to the presence of bulky methyl substituent on the benzene ring which induces the additional deformation along the polymer backbone, owing to an increase in the steric hindrance. This results in a decrease of the degree of the conjugation [15]. Thus, the absorption maxima in poly(o-toluidine) located at relatively shorter wavelength is due to the nonplaner conformation of the polymer backbone. The FT-IR spectra of polyaniline and poly(o-toluidine) doped with PTSA are presented in Fig. 2(a) and (b) and the peak positions related to the corresponding chemical bonds are listed in the Table 1. The presence of the two bands in the vicinity of 1500 and 1600 cm 1 are assigned to the nonsymmetric C6 ring stretching modes. The higher frequency vibration at 1600 cm 1 has a major contribution from the quinoid
3. Results and discussion Fig. 1 shows the optical absorption spectra of the polyaniline and poly(o-toluidine) doped with PTSA. The spectra were recorded by using m-cresol as a solvent. In case of polyaniline (Fig. 1(a)) the spectrum shows two peaks at 320 nm at 420 nm together with an extended tail with increasing absorption at 850 nm, characteristic of an extended coil conformation. whereas, in poly(otoluidine) (Fig. 1(b)) the polymer exhibits a sharp peak at 320 nm and small peaks at 420 and 820 nm. The peak at 320 nm corresponds to the p–p transition of the benzenoid rings while, the peak at 420 nm can be assigned to the localized polaron bands which are the characteristics of protonated polyaniline together with
Fig. 1. UV–Visible spectra of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA acid recorded using m-cresol as a solvent.
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381
Table 1 Characteristic frequencies of chemically synthesized p-toluene sulphonic acid doped polyaniline and poly(o-toluidine) Wavenumber (cm 1 ) Polyaniline
Fig. 2. FT-IR spectra of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
rings while, the lower frequency mode at 1500 cm 1 depicts the presence of benzenoid ring units. The presence of these two bands clearly shows that the polymer is composed of the amine and imine units. Further this also supports our UV–Visible characterization where the different phases are observed in the spectrum. The presence of vibration band of the dopant ion and other characteristic bands confirm that the polymer is in the conducting emeraldine salt phase. The elemental analysis was carried out in order to know about the composition of the polymer with various elements such as C, H, N and S present in the polymer matrix. Table 2 shows the values for the emeraldine salt phase of the polymer. The higher values of %C and %H in poly(o-toluidine) are obviously due to the presence of the methyl substituent on the benzene ring. However, the higher value of %S in polyaniline confirms the efficient doping of the polymer chains especially iminic nitrogen atom which is further supported by conductivity measurements discussed in later section. The conductivity of polyaniline doped with PTSA was found to be quite higher than that of poly(o-toluidine) doped with PTSA.
Band characteristics
Poly(o-toluidine)
805.91
808.16
–
879.26
558.52
563.65
1031.75
1005.06
1123.64
1111.55
1299.10
1314.28
1485.23
1492.70
1572.11
1587.62
–
2923.50
3232.47
3217.71
3440.83
3440.60
Paradisubstituted aromatic rings indicating polymer formation Due to the methyl group attached to the phenyl ring C–H out of plane bending vibration Due to SO3 group of the p-TSA C–H in plane bending vibration Aromatic C–N stretching indicating secondary aromatic amine group C–N stretching of benzenoid rings C–N stretching of quinoid rings C–H stretching due to substituent methyl group The aromatic C–H stretching >N–H stretching vibration
Table 2 Elemental composition of polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid Polymer
%C
%H
%N
%S
Polyaniline Poly(o-toluidine)
59.84 64.56
5.19 5.89
9.31 8.62
5.6 4.54
SEM studies also reveal a variation in the morphological structure of the two polymers. Fig. 3(a) and (b) shows the SEM photographs of polyaniline-PTSA and poly(o-toluidine)-PTSA respectively. Polyaniline doped with PTSA exhibits a sponge like structure whereas poly(o-toluidine) shows the presence of needle shaped small chopped fibers comprising of a whole polymer matrix. Presence of small chopped fibers in poly(otoluidine) can be explained by considering the steric contribution of the methyl group. The methyl group present at the ortho position of the benzene ring leads to a distortion in the polymer chains and restrict the polymer growth in a linear fashion, which in turn results
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Fig. 4. Thermograms of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
Fig. 3. Scanning electron micrographs of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
in a breakdown of the polymer chain into small fragments and this appears as small chopped or needle shaped fibers in the micrograph. The thermal stability of the polymers was evaluated with the help of thermogravimetric analysis. Fig. 4(a) and (b) represents the thermograms of polyaniline and poly(o-toluidine) doped with PTSA respectively. The thermal behavior of poly(o-toluidine) is similar to that of polyaniline and exhibits a three stage decomposition pattern. In the first stage, the weight loss observed upto 110 C is due to the loss of water molecules present in the polymer matrix. The second stage loss observed from 100 to 320 C is attributed to the loss of the dopant (dedoping) from the polymer chains. While, the third stage loss from 320 C onwards is responsible for the complete degradation and decomposition of the polymer backbone [16]. From the comparison of the thermograms of polyaniline and poly(o-toluidine) it is observed that the first stage of decomposition is found to be identical whereas, variation is observed in the second and third step of decomposition. The second stage loss, due to thermal dedoping is very sharp in poly(o-toluidine) and shows a real step at about 250 C with sudden weight loss upto about 400 C. Whereas, a very small weight loss is ob-
served in polyaniline over the same temperature range. The 70% of the original weight is observed upto 520 C in the thermogram of polyaniline while, only 40% of the original weight is stable upto 520 C in case of poly (o-toluidine) as observed from the third step in the thermogram. Also it is observed that the 50% of the original weight is stable upto 800 C in the polyaniline. However only 30% of the original weight is observed upto 800 C in poly(o-toluidine). In poly(o-toluidine) major weight loss is observed in a very small temperature range from 320 to 520 C on the other hand it is found to be extended over a broad temperature range from 320 to 800 C in polyaniline. This clearly indicates that poly (otoluidine) is thermally less stable as compared to unsubstituted polyanilines. This may be due to the presence of an alkyl substituent in the backbone which gives the polymer with lots of defect sites, which in turn facilitates the loosening of the polymer chains and hence the degradation and decomposition of the polymer backbone at lower temperature. Thus poly(o-toluidine) is less themally stable compared to polyaniline. Fig. 5 shows the DSC thermogram of polyaniline and poly(o-toluidine) doped with PTSA. The thermograms of both the polymers exhibit an endotherm which completes at 120 C. This is attributed to the expulsion of the loosely bound water molecules present in the polymer matrix [17]. The peak corresponding to this weight loss is found to be very sharp in polyaniline as compared with poly(o-toluidine). The second endothermic peak, at 210 C is due to thermal dedoping of the p-toluene sulphonic acid from the polymer chains. This
M.V. Kulkarni, A.K. Viswanath / European Polymer Journal 40 (2004) 379–384
Fig. 5. Differential scanning calorimeter (DSC) thermograms of polyaniline (a) and poly(o-toluidine) (b) doped with p-TSA.
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the benzene ring induces additional deformation along the polymer backbone. This in turn results in a decrease of the degree of conjugation and hence a decrease in conductivity. Consequently, the torsion angle between the repeat units is greater in alkyl substituted polyaniline. This explains the noticed decrease in the conductivity [22]. Fig. 6 shows the temperature dependent conductivity of polyaniline and poly(o-toluidine) doped with p-TSA. From the figure it is observed that the conductivity is found to increased with increase in the temperature from RT to 120 C. This increase in conductivity with increasing temperature is the characteristic of Ôthermal activated behavior’ and is observed in protonated polyaniline prepared from general polymerization method and is in good agreement with the literature. The possible explanation for increase in conductivity is the increase of efficiency of charge transfer between the polymer chain and the dopant with increase in the temperature [23,24]. It can also been suggested that, the thermal curing affects the chain alignment of the polymer, which leads to the increase of conjugation length and that brings about the increase of conductivity. Also there had to be molecular rearrangement on heating, which made the molecular
peak also found to be sharp in case of polyaniline. While, it is broad and appears like a hump in case of poly(o-toluidine). These observations support our thermogravimetric analysis results discussed in earlier section. The exothermic peak at about 370 C suggests the interchain crosslinking and thermally effected morphological changes [18–21]. The thermogram also confirms the absence of any glass transition (Tg ) or melting (Tm ) for both the systems and is in good agreement with the literature [17]. The room temperature solid state conductivities were measured on pressed pellets having a diameter of 1.5 cm using two probe technique. Table 3 gives the values for the same. The conductivity of poly(o-toluidine) is about three orders of magnitude less as compared to polyaniline. The lower conductivity relative to polyaniline may be explained by an increase of the interchain distance and diluting effect of the charge carriers caused by the presence of bulky methyl group in the polymer. The methyl substituent present at the ortho position of
Table 3 Room temperature conductivity values of polyaniline and poly(o-toluidine) doped with p-TSA Polymer
Conductivity S/cm
Polyaniline-p-TSA Poly(o-toluidine)-p-TSA
1.46 1.93 · 10
3
Fig. 6. Temperature dependent conductivity plots of polyaniline (a) and poly(o-toluidine) doped with p-TSA.
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conformation favourable for electron delocalization [25]. 4. Conclusions Polyaniline and poly(o-toluidine) having conducting emeraldine salt phase were synthesized by in situ chemical oxidative polymerization method using p-TSA as a dopant. The similarities between the polyaniline and poly(o-toluidine) doped with p-TSA are also evident in the UV–Visible and FT-IR spectroscopic measurements. The steric effect of the methyl substituent is mainly responsible for the decrease in the extent of p electron conjugation which is reflected as a blue shift in peak position in the UV–Visible spectrum. Thermal studies revealed that poly(o-toluidine) is thermally less stable and undergoes fast thermal decomposition over a short thermal range compared to polyaniline. The less conductivity in poly(o-toluidine) is due to the cumulative electronic as well steric effect of the methyl group. Thus the improvement in the conductivity of poly(o-toluidine) to make them suitable for various technological applications should be the goal of the future.
Acknowledgements Authors are thankful to Dr. B.K. Das, Executive Director, C-MET, Pune for his constant encouragement. The authors would like to thank Dr. Tanay Seth, Dr. Sunit Rane, Mrs. Rina Gorte, and Mr. R. Marimuthu for their help in characterization. References [1] Angelopoulos M, Ray A, MacDiarmid AG, Epstein AJ. Synth Met 1987;21:12. [2] Andreatta A, Cao Y, Chiang JC, Heeger AJ, Smith P. Synth Met 1988;26:383.
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