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Spectroscopic investigation of water-soluble polyaniline copolymers
Synthetic Metals 130 (2002) 27–33
Spectroscopic investigation of water-soluble polyaniline copolymers Bidhan C. Roya,*, Maya Dutta Guptab, Leena Bhoumikb, Jayanta K. Rayb,1 a
Department of Chemistry, North Dakota State University, P.O. Box 5516, Fargo, ND 58105, USA b Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Received 3 July 2001; received in revised form 19 November 2001; accepted 6 May 2002
Abstract The chemical copolymers of m-aminobenzene sulfonic acid with O-anisidine (POABSA) and O-toluidine (POTBSA) have been synthesized. Polymers are water soluble and self-doped. The peak in IR spectra at 1100–1160 cm1 shows the presence of SO3 group in the polymer backbone. Self-doped copolymers exhibit a strong and wide range of polaron absorption at 772–943 nm in m-cresol, due to the conformational changes induced by polymer solvent interaction. Copolymers also display a strong polaron transition in m-cresol with LiCl. Salt acts as a pseudo-doping agent and extends ‘coil-like’ conformations. The highest thermal activation is monitored at 70 8C, above that temperature (<100 8C) overcompensation of thermal activation effect is observed. The electrical conductivity of copolymers is low (104 to 105 S cm1), caused by strong steric effect between SO3 and OMe/Me groups. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Water-soluble; Solvatochromism; Thermochromism; UV–VIS/NIR
1. Introduction Conjugated polymers [1–4] have drawn attention as nonconventional electrically conducting materials. Among these polymers, polyaniline (PANI) has attracted a special attention due to its high stability toward air and moisture, high electrical conductivity [5] and unique redox properties [6]. However, its intractability resulting from the stiffness of backbone and H-bonding interactions between adjacent chains restricts not only the industrial implications but also determination of molecular structure of this conducting polymer. There are several ways to improve the solubility. The most fruitful approach was to synthesize the sulfonated polyanilines [7–23]. Regardless of their low conductivity, these PANI derivatives are drawing significant attention as their processibility is greatly improved due to the presence of the SO3 groups. From a practical point of view, sulfonated polyanilines are interesting because of their unusual electroactive properties, self-doping affinity, characteristic optical properties and potential technological applications [24–26]. In this paper, we demonstrate the synthesis and spectroscopic properties of water soluble poly(O-anisidineco-m-aminobenzene sulfonic acid) (POABSA) and poly* Corresponding author. Tel.: þ1-701-231-8552; fax: þ1-701-231-8831. E-mail addresses: [email protected] (B.C. Roy), [email protected] (J.K. Ray). 1 Co-corresponding author.
(O-toluidine-co-m-aminobenzene sulfonic acid)(POTBSA). Precisely, we are focusing on solvatochromic, thermochromic and salt effects. 2. Experimental 2.1. Materials and instrumentation Aniline, m-aminobenzene sulfonic acid (ABSA), O-anisidine(OA) and O-toluidine (OT), m-cresol, N-methyl pyrrolidinone (NMP), were purchased from Aldrich. Aniline was doubly distilled under vacuo prior to polymerization. All monomers (ABSA, OA, OT) were crystallized in suitable solvents. Reagent grade dimethylsulfoxide (DMF), ammonium persulfate, ammonium hydroxide, sodium hydroxide, LiCl and protonic acid were used as received from SD Chemicals. All aqueous solutions were prepared in distilled water. All experiments were also carried out at room temperature (30 8C) unless specified otherwise. The IR spectra of polymer samples in KBr pellets were recorded on a Perkin-Elmer (Model 883) infrared spectrometer using 2–4 mg dry solid powder. The UV–VIS/NIR (near-IR) absorption spectra of dilute polymer solutions were recorded on a Shimadzu UV-3100 spectrometer. The thermal analyses (TG, DTA) of the polymer powders were achieved on a Shimadzu DT-40 unit with heating 10 8C min1. The electrical conductivity of polymer pellets was measured by the two-probe technique using a Philips PR 9500 bridge.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 0 8 - X
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B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
Scheme 1.
2.2. Synthesis Copolymers of m-aminobenzene sulfonic acid with Oanisidine and O-toluidine were prepared by chemical oxidation of respective comonomers in various mole ratios in the feed [1,13,14] (Scheme 1). Ammonium persulfate was used as an oxidant in an aqueous acid (1 M HCl) medium. The O-anisidine or Otoluidine/m-aminobenzene sulfonic acid (ABSA) comonomer solutions (4:4) were prepared by dissolving 20.3 mmol of anisidine or toluidine and 20.3 mmol of m-aminobenzene sulfonic acid in 300 ml of 1 M HCl with constant stirring. The reaction mixture was then cooled to 5 8C in an ice water bath. The precooled solution of ammonium persulfate 9.27 g (20.3 mmol) in 150 ml of 1 M HCl was added dropwise for 20 min. The resultant mixtures were further stirred at that temperature for another 4 h and filtered off. Black solid was washed with 1 M HCl (5 times) until the filtrate turned colorless, then washed with ethanol and dried under vacuo. Yield: 2.1 g (42%, POABSA-4) and 1.3 g (27%, POTBSA-4). Experimental details and yields are also provided in Table 1. The ratio in the Table 1 indicates the molar equivalent of O-anisidine/
O-toluidine and aminobenzene sulfonicacid in the feed composition. As a consequence, copolymer POABSA-4 or POTBSA-4 consists of higher mole ratio of ABSA (four times) in the feed composition than copolymer POABSA-1 or POTBSA-1. This fine polymer powder was again dissolved in 0.1 M of aqueous NH4OH or 0.1 M of aqueous NaOH solution in order to yield emeraldine base polymer. Polymer powder was almost (90%) soluble in a basic medium. The insoluble portion was filtered off and water was removed from the polymer solution by slowly evaporating at room temperature. Finally, it was stirred in ethanol for 6 h and dried under vacuo for 24 h. The copolymers (POABSA and POTBSA) have been synthesized by a chemical oxidative method using ammonium persulfate as an oxidant. Copolymers with 0.50 and 0.43 mol fraction of m-aminobenzene sulfonic acid content are almost soluble in aqueous solution of NaOH (0.1 M) and NH4OH (0.1 M) and moderately soluble in organic solvents such as DMF, DMSO and m-cresol. Water solubility gets reduced but solubility in organic solvents increases with decreasing concentration of m-aminobenzene sulfonic acid in the feed.
Table 1 Comonomer feed composition (mol ratio of OA/OT and ABSA), yield and conductivity of the copolymers Number
Copolymer (abbreviation)
Ratio
Yield (%)
Conductivity (S/cm)
1 2 3 4 5 6 7 8
Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-4)) Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-3)) Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-2)) Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-1)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-4)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-3)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-2)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-1))
4:4 4:3 4:2 4:1 4:4 4:3 4:2 4:1
42 48 55 61 27 41 50 65
1.8 4.5 1.3 8.5 2.7 8.5 2.5 8.6
10-5 10-5 10-4 10-4 10-5 10-5 10-4 10-4
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3. Results and discussion 3.1. Infrared spectra
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The peaks at 693 and 616 cm1 are assigned for the characteristic S–O and C–S stretching vibration, respectively. The C–H out-of-plane vibration (strong) exhibits at 795– 835 cm1 in both the acid and Na-salt for 1,4-disubstituted benzene rings.
The characteristic bands (Figs. 1 and 2) are observed at 1576–1586 and 1485–1490 cm1 due to benzenoid and quinoid ring stretching frequency [27,28], respectively in virgin POABSA and POTBSA. Again Na-salts of POABSA and POTBSA exhibit these characteristic stretching vibration at 1595–1605 and 1491–1505 cm1, respectively. The strong band for polymer POABSA at 1201–1222 cm1 can be ascribed to the characteristic aromatic C–O–C [–OMe, (asymmetric)] [29] stretching vibration while the symmetric band [29] (1120–1145 cm1) of the corresponding group may overlap with the S=O (symmetric) stretching band between 1100–1150 cm1 region. The symmetric S=O band of sulfonic acid for polymer POTBSA can assign at 1145– 1160 cm1 [29]. The band at 1300–1345 cm1 may be interpreted due to the SO2 or C–N asymmetric stretching vibration [16,17,29]. A strong absorption at 1123–1127 cm1 may be assigned to aromatic C–N stretching vibration [16].
The UV–VIS/NIR absorption spectra of polymers are shown in Figs. 3–10. In PANI–base, two characteristic transitions are monitored. The bands appear at 325 nm for p–p transition and at 624 nm for exciton transition [30–32]. The base form of copolymer (POABSA-4 Nasalts, Fig. 3a) exhibits similar absorption features at 287 and 540–552 nm [13–16]. The blue shift may be attributed to the lowering of the extent of conjugation caused by steric repulsion among –Me/–OMe and SO3H groups and adjacent phenyl ring hydrogens [13,28]. In water soluble portions, this hypsochromic shift is noticeable where bulky [33] electron-withdrawing SO3Na group’s population is high.
Fig. 1. IR spectra of (a) POABSA-4 Na-salt, (b) POABSA-4, (c) POABSA-2 and (d) POABSA-3.
Fig. 2. IR spectra of (a) POTBSA-4, (b) POTBSA-4 Na-salt, and (c) POTBSA-3.
3.2. UV–VIS/near-IR absorption spectra
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B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
Fig. 3. UV–VIS/NIR absorption spectra of (a) POABSA-4 Na-salt in H2O, (b) POABSA-3, (c) POABSA-2, and (d) POABSA-1 all in DMSO.
Fig. 4. UV–VIS/NIR absorption spectra of (a) POTBSA-2 and (b) POTBSA-1 both in DMSO.
Fig. 5. UV–VIS/NIR absorption spectra of (a) POABSA-3, (b) POABSA2, (c) POABSA-1, and (d) POABSA-2 and LiCl all in m-cresol.
Fig. 6. UV–VIS/NIR absorption spectra of (a) POTBSA-2, (b) POTBSA1, (c) POTBSA-3, and (d) POTBSA-2 and LiCl all in m-cresol.
Fig. 7. UV–VIS/NIR absorption spectra of (a) POABSA-2 in NMP, (b) POABSA-2 and LiCl in NMP, (c) POABSA-2 in m-cresol/NMP (2:1 (v/ v)), and (d) POABSA-2 in m-cresol/NMP (1:1 (v/v)).
Fig. 8. UV–VIS/NMR absorption spectra of (a) POTBSA in NMP, (b) POTBSA-2 and LiCl in NMP, (c) POTBSA-2 in m-cresol/NMP (2:1 (v/v)), and (d) POTBSA-2 in m-cresol/NMP (1:1 (v/v)).
B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
Fig. 9. UV–VIS/NIR absorption spectra of (a) POABSA-2 at 70 8C, (b) POABSA-2 at 100 8C, and (c) POABSA-2 at 150 8C all in m-cresol.
Fig. 10. UV–VIS/NIR absorption spectra of (a) POTBSA-2 at 70 8C, (b) POTBSA-2 at 100 8C, and (c) POTBSA at 150 8C all in m-cresol.
Upon protonation by self-doping process (as-synthesized polymer) the intensity of exciton transition drastically reduces. At higher protonation levels, the exciton band completely disappears. Concomitantly, two or more new absorption bands display at 404–446 and at 773–943 nm (Figs. 3–10). These bands may be assigned to the polaron transition in the doped form [7,13]. Partially soluble pristine copolymers in NMP exhibit only a strong p–p transition at 310 nm (Figs. 7a and 8a) without presence of exciton absorption. NMP is a poor solvent for this polymer. Therefore, it may serve as aggregated coil-like conformation by losing conjugation along the polymer backbone. The absorption in electronic spectra depends on the level of doping, extent of conjugation, nature of the polymer, polymersolvent interaction and nature of solvent. The strong second polaron (773–943 nm) transition is observed with variation of mole ratio in the feed (reduce the mole ratio of ABSA) and solvent (basic to acidic). As a matter of fact, it is observed that O-anisidine-based copolymers exhibit low energy transitions in both DMSO
31
(Fig. 3b–d) and m-cresol (Fig. 5a–c) at 852–943 nm compared to O-toluidine-based (Fig. 4a and b in DMSO and Fig. 6a–c in m-cresol) conducting copolymers (773– 850 nm). This indicates that O-anisidine-based copolymers are more delocalized than O-toluidine-based copolymers [20]. Due to the solvatochromic effect [34], the light blue solution of pristine copolymers in NMP converts into deep blue in DMSO, shifting again to deep green in m-cresol. Bathochromic shifts display at 773–865 nm while studying the interaction with relatively basic solvents (NMP and DMSO) and this red shift again enhances (796–943 nm) in presence of acidic solvent (m-cresol). It indicates that copolymer chains of POABSA and POTBSA may have different conformations and therefore different conjugation lengths in different solvents [35]. For instance, self-doped copolymer chains should have a more expanded-coil like conformation and longer conjugation length in a solvent such as m-cresol rather than in solvents like NMP and DMSO. It has been stated earlier that copolymers in NMP display only a strong p-p transition at 310 nm (Figs. 7a and 8a). There is no evidence for exciton transition. Here, NMP acts as a ‘poor’ solvent as far as solute–solvent interaction is concerned. But in the presence of LiCl (1–2% (v/w)), the light blue color of solution shifts to deep blue and in UV, a strong exciton band appears at 600–610 nm (Figs. 7b and 8b). It again suggests that the polymers interact with LiCl and become deaggregated. The disentangled chains can now be better solvated and thus adopt a more expanded-coil conformation as supported by the exciton transition at 600–610 nm when polymers were processed with LiCl [36]. A significant red shift is monitored in both p-p and polaron (more prominent) transitions by adding LiCl (Figs. 5d and 6d) to copolymer-m-cresol system. The LiCl acts as a pseudo-doping agent and promotes an ‘expanded coil’ conformation due to the electrostatic repulsion of similar charges. LiCl also hampers the intermolecular H-bonding, allowing the chain to expand [36] and induces the pseudo doped copolymers to exhibit a free-carrier tail indicative of increased delocalization of the electrons in the polaron conduction band [13]. Similar solvatochromism is observed while NMP is gradually added to a solution of copolymer, particularly POABSA in m-cresol, in which the color change is monitored from deep green to light blue. This phenomenon can be attributed to solvent polymer backbone interaction in different solvent environments [34]. UV-absorption pattern is again changed by adding NMP to the copolymer-m-cresol system (mixing exothermic) [Figs. 7c and d and 8c and d)]. By increasing the proportion of NMP in copolymer-m-cresol system, for POABSA-2 and POTBSA-2, one exhibits bathochromic shift only in lower proportion (ratio) of NMP content in the system (Fig. 5d). On the other hand, the polaron bands disappear gradually by adding higher proportion of NMP and a strong absorption due to exciton
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transition of the quinoid rings at 614–620 nm grows (1) at the same time. It is mentioned above that polymers are soluble in m-cresol and partially soluble in NMP; they interact readily in m-cresol rather than NMP to increase the degree of freedom and the entropy of solution [13]. NMP is a poor solvent for these polymers, where polymers more coil-like conformations by losing the planarity and the conjugation of the polymer system. While adding larger proportion of NMP in POTABS-m-cresol system, the polaron band exhibits a lower energy transition with increasing intensity of absorption than aforementioned transition [13]. It may be concluded that a less polar solvent like NMP promotes a thermodynamically more stable chain conformation and restricts the polymer to lower energy, high planarity states, which yields a red shift [13]. The copolymers (POABSA-2 and POTBSA-2) in m-cresol are green color and turned into deep green when it was warmed to 70 8C. It again changed to light blue at 200 8C. The temperature dependence of UV–VIS/NIR absorption spectra of copolymers is displayed in Figs. 9 and 10. The copolymers in m-cresol were heated at 70, 100, 150 and 200 8C for 10 min and then their UV–VIS/NIR spectra at room temperature were recorded. It is noticed that the polaron band transitions at 70 8C are more intense, and it shows bathochromic shift. Polaron transition drops gradually from 100 8C, mostly disappearing at 150 8C. Simultaneously a strong absorption due to exciton transition of quinoid ring grows at 614–640 nm. [20]. These UV-absorption patterns would indicate that the increase in polaron delocalization with temperature, from room temperature (30 8C) up to 70 8C (Figs. 9a and 10a), is due to the thermal activation, while the decrease of polaron delocalization with temperature from 100 8C (Figs. 9b and 10b) to 150 8C (Figs. 9c and 10c) is due to the overcompensation of the thermal activation effect by the loss of some polarons (partial permanent thermal dedoping effect) [20]. This thermochromism phenomenon is ascribed by Zheng et al. [34] to the rise in thermal expansion of average phenyl ring–torsion angles, the constant reduction in the band width and average energy of the valence band. 3.3. Thermal analysis (TG–DTA) Thermogravimetric analyses indicate (figures are not shown) that pristine copolymers are stable up to 420 8C (Table 2). Above that temperature, the polymers begin to decompose in an exothermic manner. It is previously stated that, first polymer powders begin to lose moisture and free dopants at 100 8C and in the second step weight loss between 150–200 8C is attributed to the expulsion of bonded water and dopants [27,28,37] (Table 2). Thermogram is exothermic over the entire range of analysis. It indicates the decomposition of some existing monomer and oligomer units, intra and inter chain H-bonding and decomposition or rearrangement of the side chains [21,37]. The strong exotherm at 450 8C is assigned to desulfonation [15] of SO3 groups.
Table 2 Thermal parameters of copolymers Polymer
PANI–HCl POABSA-4 POABSA-Na-4 POABSA-2 POTBSA-2
Weight loss(%) 25–150 8C
300–500 8C
11–12 9–10 7–8 9–10 9–10
48–50 58–60 36–38 45–47 46–48
Range of decomposition Temperature (8C) 100–556 170–605 130–585 120–616 170–620
The degradation of SO3 groups should be naturally faster than the corresponding SO3H group. It loses up to 70% weight when heated to 800 8C, with the residue of Na2SO4. The copolymers offer significant thermal stability as PANI. 3.4. Conductivity Generally, conducting polymers with substituents on their frameworks show lower conductivities compared to those of the original polyaniline (PANI). The room temperature conductivity of the aforementioned self-doped copolymer found by the two-probe technique is 10–4 S cm1, which is at least four-orders of magnitude lower than that of doped PANI. The conductivity of doped copolymer decreases as the mole-ratio of m-aminobenzene sulfonic acid increases. m-Aminobenzene sulfonic acid has dual effects on polymer backbone. The steric effect of –SO3H substituent (–SO3H > –MeO/–Me) might be increased due to the torsional angle between adjacent phenyl rings [34] thus disturbing the overlapping of orbitals between the phenyl pelectrons and the nitrogen lone pairs, hence results in lowering the degree of conjugation [13]. The red shift can be expected to decrease with increasing molar portion of ABSA in polymer backbone. Along with this, the electronic effect of the substituents would be taken into account. The electron donating character of the –OMe/–Me groups increases the electron density on the phenyl ring, enhancing the dipole moment. It will be a cause that decreases the energy level of LUMO [34] [specially, O-anisidine-based copolymers exhibit low energy transition at 852–943 nm (Fig. 3b–d) and Fig. 5a–c]. Though, the –SO3H group has strong doping induce features, but the deactivating effect due to the strong electron withdrawing character of the –SO3H group can be nullified the effect. Therefore, the steric and electronic effect of the –SO3H substituent essentially drops the conductivity.
4. Conclusion Poly(O-anisidine-co-m-aminobenzene sulfonic acid) (POAABS) and poly(O-toluidine-co-m-aminobenzene sulfonic acid) (POTABS) copolymers are obtained by chemical oxidative polymerization of the respective comonomers. Copolymers with 0.5 and 0.4 mol fraction of m-aminobenzene sulfonic acid are mostly soluble in NH4OH, NaOH
B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
(aqueous) and m-cresol solution and moderately soluble in organic solvents such as DMF, DMSO, NMP. IR data indicates the presence of SO3H group, and also head-to-tail coupling in the copolymer. Doped form of copolymers, especially those containing lower proportion (mole ratio) of acid (ABSA), exhibits a strong and wide range of polaron absorption at 772–943 nm with variation of solvents; that can be assigned to conformational change due to polymer–solvent interaction. The O-anisidine-based copolymers exhibit more delocalized electronic structure (bathochromic shift) than toluidine-based ones and may have different conformational and different conjugation length, in different solvents. Copolymer-NMP system only displays a strong p–p transition, but in presence of LiCl an exciton band also appears at 600–610 nm. LiCl induces a red shift in p–p and polaron transition in polymer-m-cresol system. An interesting solvatochromism is noted in polymer-m-cresol-NMP mixture, and polaron transition disappears when proportion of NMP, increases. The maximum thermochromism, is monitored in polymer-m-cresol system at 70 8C may be due to thermal activation, and overcompensation of thermal activation effect has started above 100 8C. Thermal analysis shows the polymers to be more resistant to thermal changes compared to polyaniline itself. The incorporation of OCH3/CH3 group definitely enhances conductivity, which is about 100 times higher than that of homopolymer poly(m-aminobenzene sulfonic acid)(PABS) [22]. Nevertheless, the conductivity does not exceed that of PANI due to the presence of bulky and insulating substituents impeding charge delocalization. Acknowledgements Financial support from DST and CSIR, New Delhi, are greatly acknowledged.
References [1] (a) A.G. MacDiarmid, L.S. Yang, W.S. Huang, B.P. Humphrey, Synth. Met. 18 (1987) 393; (b) W.S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc. Faraday Trans. 1 82 (1987) 2385; (c) Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91; (d) Y. Cao, P. Smith, A.J. Heeger, PCT Patent Application, WO 22/ 22911, 1992; (e) J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [2] (a) G.K. Noren, J.K. Stille, Macromol. Rev. 5 (1971) 385; (b) P. Kovacic, M.B. Jones, Chem. Rev. 87 (1987) 357; (c) P. Kovacic, J. Oziomek, J. Org. Chem. 29 (1994) 100; (d) D.M. McKean, J.K. Stille, Macromolecules 20 (1987) 1787. [3] (a) K. Samechika, T.Y. Yamamoto, A. Yamamoto, Polym. Prep. Jpn. 28 (1979) 966; (b) A.O. Oalit, A.J. Heeger, F. Wuld, Chem. Rev. 88 (1988) 183; (c) C.P. Andriux, P. Anderbert, P. Hapiot, J.M. Saveant, J. Am. Chem. Soc. 112 (1990) 2439. [4] (a) N. Toshima, S. Hara, Prog. Polym. Sci. 20 (1985) 155; (b) M.G. Kanatzidis, L.M. Tonge, T.K. Marks, J. Am. Chem. Soc. 109 (1987) 3797.
33
[5] A.G. MacDiarmid, S.L.Mu.M.L.D. Samasiri, W. Wu, Mol. Cryst. Liquid Cryst. 121 (1985) 187. [6] (a) D.-K. Moon, T. Maruyama, K. Osakada, T. Yamamoto, Chem. Lett. (1991) 1739; (b) D.K. Moon, M. Ezuka, T. Maruyama, K. Osakada, T. Yamamoto, Macromolecules 26 (1993) 364; (c) Y. Sun, A.G. MacDiarmid, A. J. Epstein, J. Chem. Soc. Chem. Commun. (1990) 529. [7] W. Yin, E. Ruckenstein, Macromolecules 33 (2000) 1129. [8] W. Liu, A. Anagnostopoulos, F.F. Bruno, K. Senecal, J. Kumar, S. Tripathy, L. Samuelson, L. Synth. Met. 101 (1999) 738. [9] S. Ito, K. Murata, S. Teshima, R. Aizawa, Y. Asako, K. Takahashi, B.M. Hoffman, Synth. Met. 96 (1998) 161. [10] L.A. Samuelson, A. Anagnostopoulos, K.S. Alva, J. Kumar, S.K. Tripathy, Macromolecules 31 (1998) 4376. [11] S. Balasubramanian, J. Kim, J. Kumar, D.Y. Kim, S.K. Tripathy, Macromol. Rapid Commun. 17 (1996) 853. [12] J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [13] J. Yue, Z.H. Wang, K.R. Cromack, A.J. Epstein, A.G. MacDiarmid, J. Am. Chem. Soc. 113 (1991) 2665. [14] P. Kathirgamanathan, P.N. Adams, K. Quill, A.E. Underhill, J. Mater. 1 (1991) 141. [15] M.T. Nguyen, P. Kasai, J.L. Miller, A.F. Diaz, Macromolecules 27 (1994) 3625. [16] M.T. Nguyen, A.F. Diaz, Macromolecules 27 (1994) 7003. [17] S.-A. Chen, G.-W. Hwang, J. Am. Chem. Soc. 116 (1994) 7939. [18] S.-A. Chen, L.C. Lin, Macromolecules 28 (1994) 1239. [19] X.-L. Wei, Y.Z. Wang, S.M. Long, C. Bobeczko, A.J. Epstein, J. Am. Chem. Soc. 118 (1996) 2545. [20] S.-A. Chen, C.W. Hwang, J. Am. Chem. Soc. 117 (1995) 10055. [21] S.C. Ng, H.S.O. Chan, H.H. Huang, P.K.H. Ho, J. Chem. Soc. Chem. Commun. (1995) 1327. [22] B.C. Roy, M. Dutta Gupta, L. Bhoumik, J.K. Ray, Synth. Met. 100 (1999) 245–248. [23] E.V. Strounina, L.A.P. Kane-Maguire, G.G. Wallace, Synth. Met. 106 (1999) 129. [24] J. Yue, A.J. Epstein, Z. Zhong, P.K. Gallagher, A.G. MacDiarmid, Synth. Met. 41 (1991) 765. [25] T.-C. Tsai, D.A. Tree, M.S. High, Ind. Eng. Chem. Res. 33 (1994) 2600. [26] Y. Yue, G. Gordon, A.J. Epstein, Polymer 33 (1992) 4409. [27] H.S.O. Chan, S.C. Ng, W.S. Sim, K.L. Tan, B.T.G. Tan, Macromolecules 25 (1992) 6029. [28] H.S.O. Chan, S.C. Ng, W.S. Sim, S.H. Seow, K.L. Tan, B.T.G. Tan, Macromolecules 26 (1993) 144. [29] (a) K. Nakanishi (Ed.), Infrared Absorption Spectroscopy, HoldenDay Inc., San Francisco, 1962, p. 36; (b) R.M. Silverstein, G.C. Bassler, T.C. Morrill (Eds.), Spectroscopic Identification of Organic Compounds, 5th Edition, Wiley, New York, NY, 1991, pp. 112–113, 129–130. [30] (a) H.S.O. Chan, S.C. Ng, P.K.H. Ho, Macromolecules 27 (1994) 2159; (b) S. Strafstrom, B. Sjogeren, Synth. Met. 29 (1989) E219; (c) C.B. Duke, A. Paton, E.M. Conwell, W.R. Salaneck, I. Lungstrom, J. Chem. Phys. 86 (1987) 3414. [31] A.J. Epstein, J.M. Ginder, F. Zuo, H.S. Woo, D.B. Tanner, A.F. Richter, M. Angelopoulos, W.S. Huang, A.G. MacDiamid, Synth. Met. 21 (1987) 63. [32] M. Wan, J. Polym. Sci., Polym. Chem. Ed. 30 (1992) 543. [33] H.S.O. Chan, P.K.H. Ho, S.C. Ng, B.T.G. Tan, K.L. Tan, J. Am. Chem. Soc. 117 (1995) 8517. [34] W.-Y. Zheng, K. Levon, J. Laakso, J.-E. Osterholm, Macromolecules 27 (1994) 7754. [35] Y. Xia, A.G. MacDiarmid, Macromolecules 27 (1994) 212. [36] (a) M. Angelopoutos, Y.-H. Liao, B. Furman, T. Graham, Macromolecules 29 (1996) 3046; (b) A.V. Saprigin, K.R. Brenneman, W.P. Lee, S.M. Long, R.S. Kohlman, A.J. Epstein, Synth. Met. 100 (1999) 55. [37] J.C. Chiang, A.G. MacDiarmid, Synth. Met. 13 (1986) 193.
Spectroscopic investigation of water-soluble polyaniline copolymers Bidhan C. Roya,*, Maya Dutta Guptab, Leena Bhoumikb, Jayanta K. Rayb,1 a
Department of Chemistry, North Dakota State University, P.O. Box 5516, Fargo, ND 58105, USA b Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Received 3 July 2001; received in revised form 19 November 2001; accepted 6 May 2002
Abstract The chemical copolymers of m-aminobenzene sulfonic acid with O-anisidine (POABSA) and O-toluidine (POTBSA) have been synthesized. Polymers are water soluble and self-doped. The peak in IR spectra at 1100–1160 cm1 shows the presence of SO3 group in the polymer backbone. Self-doped copolymers exhibit a strong and wide range of polaron absorption at 772–943 nm in m-cresol, due to the conformational changes induced by polymer solvent interaction. Copolymers also display a strong polaron transition in m-cresol with LiCl. Salt acts as a pseudo-doping agent and extends ‘coil-like’ conformations. The highest thermal activation is monitored at 70 8C, above that temperature (<100 8C) overcompensation of thermal activation effect is observed. The electrical conductivity of copolymers is low (104 to 105 S cm1), caused by strong steric effect between SO3 and OMe/Me groups. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Water-soluble; Solvatochromism; Thermochromism; UV–VIS/NIR
1. Introduction Conjugated polymers [1–4] have drawn attention as nonconventional electrically conducting materials. Among these polymers, polyaniline (PANI) has attracted a special attention due to its high stability toward air and moisture, high electrical conductivity [5] and unique redox properties [6]. However, its intractability resulting from the stiffness of backbone and H-bonding interactions between adjacent chains restricts not only the industrial implications but also determination of molecular structure of this conducting polymer. There are several ways to improve the solubility. The most fruitful approach was to synthesize the sulfonated polyanilines [7–23]. Regardless of their low conductivity, these PANI derivatives are drawing significant attention as their processibility is greatly improved due to the presence of the SO3 groups. From a practical point of view, sulfonated polyanilines are interesting because of their unusual electroactive properties, self-doping affinity, characteristic optical properties and potential technological applications [24–26]. In this paper, we demonstrate the synthesis and spectroscopic properties of water soluble poly(O-anisidineco-m-aminobenzene sulfonic acid) (POABSA) and poly* Corresponding author. Tel.: þ1-701-231-8552; fax: þ1-701-231-8831. E-mail addresses: [email protected] (B.C. Roy), [email protected] (J.K. Ray). 1 Co-corresponding author.
(O-toluidine-co-m-aminobenzene sulfonic acid)(POTBSA). Precisely, we are focusing on solvatochromic, thermochromic and salt effects. 2. Experimental 2.1. Materials and instrumentation Aniline, m-aminobenzene sulfonic acid (ABSA), O-anisidine(OA) and O-toluidine (OT), m-cresol, N-methyl pyrrolidinone (NMP), were purchased from Aldrich. Aniline was doubly distilled under vacuo prior to polymerization. All monomers (ABSA, OA, OT) were crystallized in suitable solvents. Reagent grade dimethylsulfoxide (DMF), ammonium persulfate, ammonium hydroxide, sodium hydroxide, LiCl and protonic acid were used as received from SD Chemicals. All aqueous solutions were prepared in distilled water. All experiments were also carried out at room temperature (30 8C) unless specified otherwise. The IR spectra of polymer samples in KBr pellets were recorded on a Perkin-Elmer (Model 883) infrared spectrometer using 2–4 mg dry solid powder. The UV–VIS/NIR (near-IR) absorption spectra of dilute polymer solutions were recorded on a Shimadzu UV-3100 spectrometer. The thermal analyses (TG, DTA) of the polymer powders were achieved on a Shimadzu DT-40 unit with heating 10 8C min1. The electrical conductivity of polymer pellets was measured by the two-probe technique using a Philips PR 9500 bridge.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 0 8 - X
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B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
Scheme 1.
2.2. Synthesis Copolymers of m-aminobenzene sulfonic acid with Oanisidine and O-toluidine were prepared by chemical oxidation of respective comonomers in various mole ratios in the feed [1,13,14] (Scheme 1). Ammonium persulfate was used as an oxidant in an aqueous acid (1 M HCl) medium. The O-anisidine or Otoluidine/m-aminobenzene sulfonic acid (ABSA) comonomer solutions (4:4) were prepared by dissolving 20.3 mmol of anisidine or toluidine and 20.3 mmol of m-aminobenzene sulfonic acid in 300 ml of 1 M HCl with constant stirring. The reaction mixture was then cooled to 5 8C in an ice water bath. The precooled solution of ammonium persulfate 9.27 g (20.3 mmol) in 150 ml of 1 M HCl was added dropwise for 20 min. The resultant mixtures were further stirred at that temperature for another 4 h and filtered off. Black solid was washed with 1 M HCl (5 times) until the filtrate turned colorless, then washed with ethanol and dried under vacuo. Yield: 2.1 g (42%, POABSA-4) and 1.3 g (27%, POTBSA-4). Experimental details and yields are also provided in Table 1. The ratio in the Table 1 indicates the molar equivalent of O-anisidine/
O-toluidine and aminobenzene sulfonicacid in the feed composition. As a consequence, copolymer POABSA-4 or POTBSA-4 consists of higher mole ratio of ABSA (four times) in the feed composition than copolymer POABSA-1 or POTBSA-1. This fine polymer powder was again dissolved in 0.1 M of aqueous NH4OH or 0.1 M of aqueous NaOH solution in order to yield emeraldine base polymer. Polymer powder was almost (90%) soluble in a basic medium. The insoluble portion was filtered off and water was removed from the polymer solution by slowly evaporating at room temperature. Finally, it was stirred in ethanol for 6 h and dried under vacuo for 24 h. The copolymers (POABSA and POTBSA) have been synthesized by a chemical oxidative method using ammonium persulfate as an oxidant. Copolymers with 0.50 and 0.43 mol fraction of m-aminobenzene sulfonic acid content are almost soluble in aqueous solution of NaOH (0.1 M) and NH4OH (0.1 M) and moderately soluble in organic solvents such as DMF, DMSO and m-cresol. Water solubility gets reduced but solubility in organic solvents increases with decreasing concentration of m-aminobenzene sulfonic acid in the feed.
Table 1 Comonomer feed composition (mol ratio of OA/OT and ABSA), yield and conductivity of the copolymers Number
Copolymer (abbreviation)
Ratio
Yield (%)
Conductivity (S/cm)
1 2 3 4 5 6 7 8
Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-4)) Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-3)) Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-2)) Poly(O-anisidine-co-m-aminobenzene sulfonic acid (POABSA-1)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-4)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-3)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-2)) Poly(O-toluidine-co-m-aminobenzene sulfonic acid (POTBSA-1))
4:4 4:3 4:2 4:1 4:4 4:3 4:2 4:1
42 48 55 61 27 41 50 65
1.8 4.5 1.3 8.5 2.7 8.5 2.5 8.6
10-5 10-5 10-4 10-4 10-5 10-5 10-4 10-4
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3. Results and discussion 3.1. Infrared spectra
29
The peaks at 693 and 616 cm1 are assigned for the characteristic S–O and C–S stretching vibration, respectively. The C–H out-of-plane vibration (strong) exhibits at 795– 835 cm1 in both the acid and Na-salt for 1,4-disubstituted benzene rings.
The characteristic bands (Figs. 1 and 2) are observed at 1576–1586 and 1485–1490 cm1 due to benzenoid and quinoid ring stretching frequency [27,28], respectively in virgin POABSA and POTBSA. Again Na-salts of POABSA and POTBSA exhibit these characteristic stretching vibration at 1595–1605 and 1491–1505 cm1, respectively. The strong band for polymer POABSA at 1201–1222 cm1 can be ascribed to the characteristic aromatic C–O–C [–OMe, (asymmetric)] [29] stretching vibration while the symmetric band [29] (1120–1145 cm1) of the corresponding group may overlap with the S=O (symmetric) stretching band between 1100–1150 cm1 region. The symmetric S=O band of sulfonic acid for polymer POTBSA can assign at 1145– 1160 cm1 [29]. The band at 1300–1345 cm1 may be interpreted due to the SO2 or C–N asymmetric stretching vibration [16,17,29]. A strong absorption at 1123–1127 cm1 may be assigned to aromatic C–N stretching vibration [16].
The UV–VIS/NIR absorption spectra of polymers are shown in Figs. 3–10. In PANI–base, two characteristic transitions are monitored. The bands appear at 325 nm for p–p transition and at 624 nm for exciton transition [30–32]. The base form of copolymer (POABSA-4 Nasalts, Fig. 3a) exhibits similar absorption features at 287 and 540–552 nm [13–16]. The blue shift may be attributed to the lowering of the extent of conjugation caused by steric repulsion among –Me/–OMe and SO3H groups and adjacent phenyl ring hydrogens [13,28]. In water soluble portions, this hypsochromic shift is noticeable where bulky [33] electron-withdrawing SO3Na group’s population is high.
Fig. 1. IR spectra of (a) POABSA-4 Na-salt, (b) POABSA-4, (c) POABSA-2 and (d) POABSA-3.
Fig. 2. IR spectra of (a) POTBSA-4, (b) POTBSA-4 Na-salt, and (c) POTBSA-3.
3.2. UV–VIS/near-IR absorption spectra
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Fig. 3. UV–VIS/NIR absorption spectra of (a) POABSA-4 Na-salt in H2O, (b) POABSA-3, (c) POABSA-2, and (d) POABSA-1 all in DMSO.
Fig. 4. UV–VIS/NIR absorption spectra of (a) POTBSA-2 and (b) POTBSA-1 both in DMSO.
Fig. 5. UV–VIS/NIR absorption spectra of (a) POABSA-3, (b) POABSA2, (c) POABSA-1, and (d) POABSA-2 and LiCl all in m-cresol.
Fig. 6. UV–VIS/NIR absorption spectra of (a) POTBSA-2, (b) POTBSA1, (c) POTBSA-3, and (d) POTBSA-2 and LiCl all in m-cresol.
Fig. 7. UV–VIS/NIR absorption spectra of (a) POABSA-2 in NMP, (b) POABSA-2 and LiCl in NMP, (c) POABSA-2 in m-cresol/NMP (2:1 (v/ v)), and (d) POABSA-2 in m-cresol/NMP (1:1 (v/v)).
Fig. 8. UV–VIS/NMR absorption spectra of (a) POTBSA in NMP, (b) POTBSA-2 and LiCl in NMP, (c) POTBSA-2 in m-cresol/NMP (2:1 (v/v)), and (d) POTBSA-2 in m-cresol/NMP (1:1 (v/v)).
B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
Fig. 9. UV–VIS/NIR absorption spectra of (a) POABSA-2 at 70 8C, (b) POABSA-2 at 100 8C, and (c) POABSA-2 at 150 8C all in m-cresol.
Fig. 10. UV–VIS/NIR absorption spectra of (a) POTBSA-2 at 70 8C, (b) POTBSA-2 at 100 8C, and (c) POTBSA at 150 8C all in m-cresol.
Upon protonation by self-doping process (as-synthesized polymer) the intensity of exciton transition drastically reduces. At higher protonation levels, the exciton band completely disappears. Concomitantly, two or more new absorption bands display at 404–446 and at 773–943 nm (Figs. 3–10). These bands may be assigned to the polaron transition in the doped form [7,13]. Partially soluble pristine copolymers in NMP exhibit only a strong p–p transition at 310 nm (Figs. 7a and 8a) without presence of exciton absorption. NMP is a poor solvent for this polymer. Therefore, it may serve as aggregated coil-like conformation by losing conjugation along the polymer backbone. The absorption in electronic spectra depends on the level of doping, extent of conjugation, nature of the polymer, polymersolvent interaction and nature of solvent. The strong second polaron (773–943 nm) transition is observed with variation of mole ratio in the feed (reduce the mole ratio of ABSA) and solvent (basic to acidic). As a matter of fact, it is observed that O-anisidine-based copolymers exhibit low energy transitions in both DMSO
31
(Fig. 3b–d) and m-cresol (Fig. 5a–c) at 852–943 nm compared to O-toluidine-based (Fig. 4a and b in DMSO and Fig. 6a–c in m-cresol) conducting copolymers (773– 850 nm). This indicates that O-anisidine-based copolymers are more delocalized than O-toluidine-based copolymers [20]. Due to the solvatochromic effect [34], the light blue solution of pristine copolymers in NMP converts into deep blue in DMSO, shifting again to deep green in m-cresol. Bathochromic shifts display at 773–865 nm while studying the interaction with relatively basic solvents (NMP and DMSO) and this red shift again enhances (796–943 nm) in presence of acidic solvent (m-cresol). It indicates that copolymer chains of POABSA and POTBSA may have different conformations and therefore different conjugation lengths in different solvents [35]. For instance, self-doped copolymer chains should have a more expanded-coil like conformation and longer conjugation length in a solvent such as m-cresol rather than in solvents like NMP and DMSO. It has been stated earlier that copolymers in NMP display only a strong p-p transition at 310 nm (Figs. 7a and 8a). There is no evidence for exciton transition. Here, NMP acts as a ‘poor’ solvent as far as solute–solvent interaction is concerned. But in the presence of LiCl (1–2% (v/w)), the light blue color of solution shifts to deep blue and in UV, a strong exciton band appears at 600–610 nm (Figs. 7b and 8b). It again suggests that the polymers interact with LiCl and become deaggregated. The disentangled chains can now be better solvated and thus adopt a more expanded-coil conformation as supported by the exciton transition at 600–610 nm when polymers were processed with LiCl [36]. A significant red shift is monitored in both p-p and polaron (more prominent) transitions by adding LiCl (Figs. 5d and 6d) to copolymer-m-cresol system. The LiCl acts as a pseudo-doping agent and promotes an ‘expanded coil’ conformation due to the electrostatic repulsion of similar charges. LiCl also hampers the intermolecular H-bonding, allowing the chain to expand [36] and induces the pseudo doped copolymers to exhibit a free-carrier tail indicative of increased delocalization of the electrons in the polaron conduction band [13]. Similar solvatochromism is observed while NMP is gradually added to a solution of copolymer, particularly POABSA in m-cresol, in which the color change is monitored from deep green to light blue. This phenomenon can be attributed to solvent polymer backbone interaction in different solvent environments [34]. UV-absorption pattern is again changed by adding NMP to the copolymer-m-cresol system (mixing exothermic) [Figs. 7c and d and 8c and d)]. By increasing the proportion of NMP in copolymer-m-cresol system, for POABSA-2 and POTBSA-2, one exhibits bathochromic shift only in lower proportion (ratio) of NMP content in the system (Fig. 5d). On the other hand, the polaron bands disappear gradually by adding higher proportion of NMP and a strong absorption due to exciton
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transition of the quinoid rings at 614–620 nm grows (1) at the same time. It is mentioned above that polymers are soluble in m-cresol and partially soluble in NMP; they interact readily in m-cresol rather than NMP to increase the degree of freedom and the entropy of solution [13]. NMP is a poor solvent for these polymers, where polymers more coil-like conformations by losing the planarity and the conjugation of the polymer system. While adding larger proportion of NMP in POTABS-m-cresol system, the polaron band exhibits a lower energy transition with increasing intensity of absorption than aforementioned transition [13]. It may be concluded that a less polar solvent like NMP promotes a thermodynamically more stable chain conformation and restricts the polymer to lower energy, high planarity states, which yields a red shift [13]. The copolymers (POABSA-2 and POTBSA-2) in m-cresol are green color and turned into deep green when it was warmed to 70 8C. It again changed to light blue at 200 8C. The temperature dependence of UV–VIS/NIR absorption spectra of copolymers is displayed in Figs. 9 and 10. The copolymers in m-cresol were heated at 70, 100, 150 and 200 8C for 10 min and then their UV–VIS/NIR spectra at room temperature were recorded. It is noticed that the polaron band transitions at 70 8C are more intense, and it shows bathochromic shift. Polaron transition drops gradually from 100 8C, mostly disappearing at 150 8C. Simultaneously a strong absorption due to exciton transition of quinoid ring grows at 614–640 nm. [20]. These UV-absorption patterns would indicate that the increase in polaron delocalization with temperature, from room temperature (30 8C) up to 70 8C (Figs. 9a and 10a), is due to the thermal activation, while the decrease of polaron delocalization with temperature from 100 8C (Figs. 9b and 10b) to 150 8C (Figs. 9c and 10c) is due to the overcompensation of the thermal activation effect by the loss of some polarons (partial permanent thermal dedoping effect) [20]. This thermochromism phenomenon is ascribed by Zheng et al. [34] to the rise in thermal expansion of average phenyl ring–torsion angles, the constant reduction in the band width and average energy of the valence band. 3.3. Thermal analysis (TG–DTA) Thermogravimetric analyses indicate (figures are not shown) that pristine copolymers are stable up to 420 8C (Table 2). Above that temperature, the polymers begin to decompose in an exothermic manner. It is previously stated that, first polymer powders begin to lose moisture and free dopants at 100 8C and in the second step weight loss between 150–200 8C is attributed to the expulsion of bonded water and dopants [27,28,37] (Table 2). Thermogram is exothermic over the entire range of analysis. It indicates the decomposition of some existing monomer and oligomer units, intra and inter chain H-bonding and decomposition or rearrangement of the side chains [21,37]. The strong exotherm at 450 8C is assigned to desulfonation [15] of SO3 groups.
Table 2 Thermal parameters of copolymers Polymer
PANI–HCl POABSA-4 POABSA-Na-4 POABSA-2 POTBSA-2
Weight loss(%) 25–150 8C
300–500 8C
11–12 9–10 7–8 9–10 9–10
48–50 58–60 36–38 45–47 46–48
Range of decomposition Temperature (8C) 100–556 170–605 130–585 120–616 170–620
The degradation of SO3 groups should be naturally faster than the corresponding SO3H group. It loses up to 70% weight when heated to 800 8C, with the residue of Na2SO4. The copolymers offer significant thermal stability as PANI. 3.4. Conductivity Generally, conducting polymers with substituents on their frameworks show lower conductivities compared to those of the original polyaniline (PANI). The room temperature conductivity of the aforementioned self-doped copolymer found by the two-probe technique is 10–4 S cm1, which is at least four-orders of magnitude lower than that of doped PANI. The conductivity of doped copolymer decreases as the mole-ratio of m-aminobenzene sulfonic acid increases. m-Aminobenzene sulfonic acid has dual effects on polymer backbone. The steric effect of –SO3H substituent (–SO3H > –MeO/–Me) might be increased due to the torsional angle between adjacent phenyl rings [34] thus disturbing the overlapping of orbitals between the phenyl pelectrons and the nitrogen lone pairs, hence results in lowering the degree of conjugation [13]. The red shift can be expected to decrease with increasing molar portion of ABSA in polymer backbone. Along with this, the electronic effect of the substituents would be taken into account. The electron donating character of the –OMe/–Me groups increases the electron density on the phenyl ring, enhancing the dipole moment. It will be a cause that decreases the energy level of LUMO [34] [specially, O-anisidine-based copolymers exhibit low energy transition at 852–943 nm (Fig. 3b–d) and Fig. 5a–c]. Though, the –SO3H group has strong doping induce features, but the deactivating effect due to the strong electron withdrawing character of the –SO3H group can be nullified the effect. Therefore, the steric and electronic effect of the –SO3H substituent essentially drops the conductivity.
4. Conclusion Poly(O-anisidine-co-m-aminobenzene sulfonic acid) (POAABS) and poly(O-toluidine-co-m-aminobenzene sulfonic acid) (POTABS) copolymers are obtained by chemical oxidative polymerization of the respective comonomers. Copolymers with 0.5 and 0.4 mol fraction of m-aminobenzene sulfonic acid are mostly soluble in NH4OH, NaOH
B.C. Roy et al. / Synthetic Metals 130 (2002) 27–33
(aqueous) and m-cresol solution and moderately soluble in organic solvents such as DMF, DMSO, NMP. IR data indicates the presence of SO3H group, and also head-to-tail coupling in the copolymer. Doped form of copolymers, especially those containing lower proportion (mole ratio) of acid (ABSA), exhibits a strong and wide range of polaron absorption at 772–943 nm with variation of solvents; that can be assigned to conformational change due to polymer–solvent interaction. The O-anisidine-based copolymers exhibit more delocalized electronic structure (bathochromic shift) than toluidine-based ones and may have different conformational and different conjugation length, in different solvents. Copolymer-NMP system only displays a strong p–p transition, but in presence of LiCl an exciton band also appears at 600–610 nm. LiCl induces a red shift in p–p and polaron transition in polymer-m-cresol system. An interesting solvatochromism is noted in polymer-m-cresol-NMP mixture, and polaron transition disappears when proportion of NMP, increases. The maximum thermochromism, is monitored in polymer-m-cresol system at 70 8C may be due to thermal activation, and overcompensation of thermal activation effect has started above 100 8C. Thermal analysis shows the polymers to be more resistant to thermal changes compared to polyaniline itself. The incorporation of OCH3/CH3 group definitely enhances conductivity, which is about 100 times higher than that of homopolymer poly(m-aminobenzene sulfonic acid)(PABS) [22]. Nevertheless, the conductivity does not exceed that of PANI due to the presence of bulky and insulating substituents impeding charge delocalization. Acknowledgements Financial support from DST and CSIR, New Delhi, are greatly acknowledged.
References [1] (a) A.G. MacDiarmid, L.S. Yang, W.S. Huang, B.P. Humphrey, Synth. Met. 18 (1987) 393; (b) W.S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc. Faraday Trans. 1 82 (1987) 2385; (c) Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91; (d) Y. Cao, P. Smith, A.J. Heeger, PCT Patent Application, WO 22/ 22911, 1992; (e) J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [2] (a) G.K. Noren, J.K. Stille, Macromol. Rev. 5 (1971) 385; (b) P. Kovacic, M.B. Jones, Chem. Rev. 87 (1987) 357; (c) P. Kovacic, J. Oziomek, J. Org. Chem. 29 (1994) 100; (d) D.M. McKean, J.K. Stille, Macromolecules 20 (1987) 1787. [3] (a) K. Samechika, T.Y. Yamamoto, A. Yamamoto, Polym. Prep. Jpn. 28 (1979) 966; (b) A.O. Oalit, A.J. Heeger, F. Wuld, Chem. Rev. 88 (1988) 183; (c) C.P. Andriux, P. Anderbert, P. Hapiot, J.M. Saveant, J. Am. Chem. Soc. 112 (1990) 2439. [4] (a) N. Toshima, S. Hara, Prog. Polym. Sci. 20 (1985) 155; (b) M.G. Kanatzidis, L.M. Tonge, T.K. Marks, J. Am. Chem. Soc. 109 (1987) 3797.
33
[5] A.G. MacDiarmid, S.L.Mu.M.L.D. Samasiri, W. Wu, Mol. Cryst. Liquid Cryst. 121 (1985) 187. [6] (a) D.-K. Moon, T. Maruyama, K. Osakada, T. Yamamoto, Chem. Lett. (1991) 1739; (b) D.K. Moon, M. Ezuka, T. Maruyama, K. Osakada, T. Yamamoto, Macromolecules 26 (1993) 364; (c) Y. Sun, A.G. MacDiarmid, A. J. Epstein, J. Chem. Soc. Chem. Commun. (1990) 529. [7] W. Yin, E. Ruckenstein, Macromolecules 33 (2000) 1129. [8] W. Liu, A. Anagnostopoulos, F.F. Bruno, K. Senecal, J. Kumar, S. Tripathy, L. Samuelson, L. Synth. Met. 101 (1999) 738. [9] S. Ito, K. Murata, S. Teshima, R. Aizawa, Y. Asako, K. Takahashi, B.M. Hoffman, Synth. Met. 96 (1998) 161. [10] L.A. Samuelson, A. Anagnostopoulos, K.S. Alva, J. Kumar, S.K. Tripathy, Macromolecules 31 (1998) 4376. [11] S. Balasubramanian, J. Kim, J. Kumar, D.Y. Kim, S.K. Tripathy, Macromol. Rapid Commun. 17 (1996) 853. [12] J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [13] J. Yue, Z.H. Wang, K.R. Cromack, A.J. Epstein, A.G. MacDiarmid, J. Am. Chem. Soc. 113 (1991) 2665. [14] P. Kathirgamanathan, P.N. Adams, K. Quill, A.E. Underhill, J. Mater. 1 (1991) 141. [15] M.T. Nguyen, P. Kasai, J.L. Miller, A.F. Diaz, Macromolecules 27 (1994) 3625. [16] M.T. Nguyen, A.F. Diaz, Macromolecules 27 (1994) 7003. [17] S.-A. Chen, G.-W. Hwang, J. Am. Chem. Soc. 116 (1994) 7939. [18] S.-A. Chen, L.C. Lin, Macromolecules 28 (1994) 1239. [19] X.-L. Wei, Y.Z. Wang, S.M. Long, C. Bobeczko, A.J. Epstein, J. Am. Chem. Soc. 118 (1996) 2545. [20] S.-A. Chen, C.W. Hwang, J. Am. Chem. Soc. 117 (1995) 10055. [21] S.C. Ng, H.S.O. Chan, H.H. Huang, P.K.H. Ho, J. Chem. Soc. Chem. Commun. (1995) 1327. [22] B.C. Roy, M. Dutta Gupta, L. Bhoumik, J.K. Ray, Synth. Met. 100 (1999) 245–248. [23] E.V. Strounina, L.A.P. Kane-Maguire, G.G. Wallace, Synth. Met. 106 (1999) 129. [24] J. Yue, A.J. Epstein, Z. Zhong, P.K. Gallagher, A.G. MacDiarmid, Synth. Met. 41 (1991) 765. [25] T.-C. Tsai, D.A. Tree, M.S. High, Ind. Eng. Chem. Res. 33 (1994) 2600. [26] Y. Yue, G. Gordon, A.J. Epstein, Polymer 33 (1992) 4409. [27] H.S.O. Chan, S.C. Ng, W.S. Sim, K.L. Tan, B.T.G. Tan, Macromolecules 25 (1992) 6029. [28] H.S.O. Chan, S.C. Ng, W.S. Sim, S.H. Seow, K.L. Tan, B.T.G. Tan, Macromolecules 26 (1993) 144. [29] (a) K. Nakanishi (Ed.), Infrared Absorption Spectroscopy, HoldenDay Inc., San Francisco, 1962, p. 36; (b) R.M. Silverstein, G.C. Bassler, T.C. Morrill (Eds.), Spectroscopic Identification of Organic Compounds, 5th Edition, Wiley, New York, NY, 1991, pp. 112–113, 129–130. [30] (a) H.S.O. Chan, S.C. Ng, P.K.H. Ho, Macromolecules 27 (1994) 2159; (b) S. Strafstrom, B. Sjogeren, Synth. Met. 29 (1989) E219; (c) C.B. Duke, A. Paton, E.M. Conwell, W.R. Salaneck, I. Lungstrom, J. Chem. Phys. 86 (1987) 3414. [31] A.J. Epstein, J.M. Ginder, F. Zuo, H.S. Woo, D.B. Tanner, A.F. Richter, M. Angelopoulos, W.S. Huang, A.G. MacDiamid, Synth. Met. 21 (1987) 63. [32] M. Wan, J. Polym. Sci., Polym. Chem. Ed. 30 (1992) 543. [33] H.S.O. Chan, P.K.H. Ho, S.C. Ng, B.T.G. Tan, K.L. Tan, J. Am. Chem. Soc. 117 (1995) 8517. [34] W.-Y. Zheng, K. Levon, J. Laakso, J.-E. Osterholm, Macromolecules 27 (1994) 7754. [35] Y. Xia, A.G. MacDiarmid, Macromolecules 27 (1994) 212. [36] (a) M. Angelopoutos, Y.-H. Liao, B. Furman, T. Graham, Macromolecules 29 (1996) 3046; (b) A.V. Saprigin, K.R. Brenneman, W.P. Lee, S.M. Long, R.S. Kohlman, A.J. Epstein, Synth. Met. 100 (1999) 55. [37] J.C. Chiang, A.G. MacDiarmid, Synth. Met. 13 (1986) 193.