Polymer International 40 (1996) 201-205

Com parat ive Studies of Electrochemical ly Deposited Poly(o-toluidine) and

Poly(m-toluidine) Films

A. G. Bedekar*, S. F. Patil & R. C. Patil

Department of Chemistry, University of Pune, Pune 41 1 007, India

(Received 11 January 1996; accepted 1 March 1996)

Abstract: Under galvanostatic deposition conditions poly(o-toluidine) exhibits a higher rate of polymerization than poly(rn-toluidine). This observation is sup- ported by results obtained by different characterization techniques such as spec- trophotometry, scanning electron microscopy and thermogravimetric analysis. The monomer concentration was found to be the predominant parameter in obtaining selectively a conducting salt phase in both cases. However, the mor- phology of these polymeric films does not reveal any particular relationship with monomer concentration; instead a mixed morphology, i.e. a combination of granules and fibres, is observed. Finally, the thermal stability of poly(rn-toluidine) is lower than that of poly(o-toluidine) with a shift of 190°C in the final decompo- sition temperature.

Key words: poly(o-toluidine), poly(m-toluidine), monomer concentration, mor- phology, polymerization.

I NTRO D UCTlON

The properties of polyaniline and its derivatives have been studied extensively. The materials are used in dif- ferent applications to meet the current needs of industry.' Derivatives of polyaniline may be used as an alternative to polyaniline in certain specific applications. Among these, poly(o-toluidine) and poly(m-toluidine) are two materials whose properties can often be tailor- made to meet certain requirements. Very few workers have, however, evaluated the properties of these

In the present work, a comparative study has been undertaken to investigate the influence of the position of the methyl substituent on the rate of poly- merization. One of the deposition parameters, i.e. the monomer concentration, has been optimized to obtain a greater fraction of the conducting emeraldine salt.

* To whom correspondence should be addressed.

EXPERIMENTAL

All solutions, using AR grade chemicals, were prepared in doubly distilled water. Appropriate amounts of monomer were added to IM HCl to prepare electrolyte solutions of different monomer concentrations. The films were deposited on SnO, : F coated glass elec- trodes (sheet resistance = 10 !2/cm2) under galvanostatic conditions using a three-electrode system, with Pt and SCE as the other two electrodes. The deposited films were washed and dried before subjecting them to differ- ent characterization techniques. The optical absorption spectra were scanned on a UV-visible double beam spectrophotometer (Hitachi, model 330) over the range 300-900 nm. The morphological features were examined using a scanning electron microscope (Stereoscan 120, Cambridge Instruments Ltd, UK). Before performing the SEM study, the films were coated with a thin layer of gold (thickness - 20 nm) using a sputter-coater unit

20 1 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

202 A. G. Bedekar, S . F . Patil, R . C. Patil

(E-5O00, BioRad, UK). Weight losses of the polymers were recorded on a thermogravimetric analyser (Perkin-Elmer, model TGA-7) in an 0' atmosphere at a scanning rate of 10"C/min.

RESULTS AND DISCUSSION

The optical absorption spectra of pdy(o-toluidine) and poly(m-toluidine) films as a function of monomer con- centration during deposition are presented in Fig. 1. Notable differences in the spectra are observed with respect to the enhancement in the total absorbance of poly(o-toluidine) film compared with that of poly(m- toluidine) at a fixed monomer concentration. Further, poly(o-toluidine) was found to yield films at low monomer concentrations (0.2 M), while in the case of poly(m-toluidine) film was obtained only at a minimum concentration of 0.4 M. Apart from this, selectivity in the formation of a conducting phase in poly(o-toluidine) films decreased with increasing monomer concentration, while the reverse was observed in the case of poly(m- toluidine). Comparing the number of phases formed in each case, it is noted that two phases are observed in

Monomer 2.5-1 Concentration

-. - 0.4 M

- Wavelength,h (nm) ---)

Fig. 1. Optical absorption spectra of (a) poly(o-toluidine) and (b) poly(m-toluidine) films as a function of monomer concen- tration during deposition. Current strength applied = 1 mA; sheet resistance of electrode = 10C2/cmz; time of deposition-

(a) poly(o-toluidine) = 5 min, (b) poly(m-toluidine) = 20min.

the case of poly(o-toluidine), with peaks appearing at -450 and - 850 nm, whereas for poly(m-toluidine) an additional phase is seen by a peak at -560nm. The peak at -450nm corresponds to the generation of radical cations, while the peaks at - 850 and - 560 nm represent the formation of emeraldine salt and per- nigraniline base, re~pectively.~ The above results can be explained by considering the position of the methyl sub- stituent, as well as the difference in the rate of the poly- merization process. From the results obtained, it appears that the methyl group, when present at the meta position, sterically hinders the polymerization process leading to a slower rate of polymer deposition on the electrode surface.' Because of this, good quality film cannot be obtained at the concentration of 0 . 2 ~ . On the other hand, good quality adherent poly(m-tolu- idine) films are obtained especially at higher concentra- tions of monomer (0.8 and 1.0~), which is seen by the sharpening of the peak at - 850 nm with monomer con- centration (Fig. 1 b). Contrary to this, poly(o-toluidine) yields a sharp peak at - 850 nm for 0.2 M monomer con- centration. Further increase in concentration leads to a flattening of this peak in the optical absorption spectra (Fig. la).

It has been established previously that the initially formed pernigraniline base undergoes self-reduction, as well as oxidizing the incoming monomer molecules to form a conducting phase, together with some other lower oxidation state Owing to the slower rate of deposition in the case of poly(m-toluidine), the initially formed fully oxidized pernigraniline has SUE- cient time to undergo conversion to a conducting emer- aldine salt phase in greater amounts thus exhibiting a prominent peak at - 850 nm. For poly(o-toluidine), since the rate of deposition is high, there is insufficient time for conversion of pernigraniline base to emeraldine salt. As a result, accumulated pernigraniline on the elec- trode surface along with intermediate oxidation state species, show their presence by flattening the spectra, especially at higher monomer concentrations. On the other hand at the lower concentration of O - ~ M , this process of conversion of pernigraniline base to emer- aldine salt is seen to be in equilibrium, which is demon- strated by the presence of the peak at - 850 nm.

To understand the differences in the magnitude of the rate of deposition, as well as the effect of the position of the methyl substituent on morphology, the morphologi- cal features of the two polymers deposited at two differ- ent monomer concentrations were compared. Poly(o- toluidine) exhibits a fibrillar type of patterning, while in poly(m-toluidine) the morphology is granular, although a small fraction of fibres is also noticed in a few cases. Fibre formation is predominantly seen in the case of poly(o-toluidine), especially at higher concentrations of monomer (0.8 M, Fig. 2a). The fibres of poly(o-toluidine) are seen to extend up to a length of 2000pm, and are clearly observed under a low magnification of 50 x (Fig.

POLYMER INTERNATIONAL VOL. 40. NO. 3, 1996

Electrochemically deposited poly(o-toluidine) and poly(m-toluidine) films 203

Fig. 2. Scanning electron micrographs of poly(o-toluidine) films taken at a magnification of (a) 50 x and (b) 1-07K x . Monomer concentration = 0-8 M; current strength applied = 1 mA; sheet resistance of electrode = 100/cm2;

time of deposition = 20 min.

2a). The process of fibre formation progresses via linking-up of closely spaced small net-like fibres inter- connected with each other.This is mainly observed at a higher magnification of 1.07K x (Fig. 2b). Contrary to this, poly(m-toluidine) films under identical conditions exhibit granular morphology (Fig. 3). The morphology is less clear at lower magnification (Fig. 3a) than at higher magnification where larger granules can be observed embedded in interwoven stellate-shaped chan- nels (Fig. 3b). Further growth is found to proceed through the previously grown channel giving a smooth appearance to the film.

From the above results, it appears that the formation of seeds of the polymer on the surface by adsorption of monomer, or short oligomers, acts as a precursor step in the polymerization process. Subsequently, poly- merization continues through the increasing chain length of adsorbed o1igomer.l' A changeover from

Fig. 3. Scanning electron micrographs of poly(m-toluidine) films taken at a magnification of (a) 50 x and (b) 1.03 K x . Monomer concentration = 0.8 M; current strength applied = 1 mA; sheet resistance of electrode = 100/cm2;

time of deposition = 20 min.

fibrillar to granular morphology in the case of poly(m- toluidine) can be explained by considering the steric contribution of the methyl group. The methyl group present at the meta position leads to distortion in the chain, which results in a breakdown into small frag- ments, which appear as granules in the micrographs.

Further, the influence of the substituent on the rate of the polymerization process can be clearly seen at lower concentrations of monomer ( 0 . 4 ~ ) and at higher mag- nification, 1-07 K x . Densely joined fine granules cover- ing the whole surface of the electrode are observed in poly(o-toluidine) (Fig. 4a). On the other hand, poly(m- toluidine) exhibits a few scattered granules of variable size together with some stellate-shaped trilobed struc- tures in between (Fig. 5a), suggesting that the growth of poly(o-toluidine) takes place at a higher rate than that of poly(m-toluidine).6 At a lower magnification (100 x ) poly(o-toluidine) shows an exclusively fibrillar nature.

POLYMER INTERNATIONAL VOL. 40. NO. 3, 1996

204 A. G. Bedekar, S. F . Patil, R. C. Patil

Fig. 4. Scanning electron micrographs of poly(o-toluidine) Fig. 5. Scanning electron micrographs of poly(rn-toluidine) films taken at a magnification of (a) 1.07K x and (b) 100 x . films taken at a magnification of (a) 1.01 K x and (b) 250 x . Monomer concentration = 0.4 M; current strength Monomer concentration = 0.4 M; current strength applied = 1 mA; sheet resistance of electrode = 10Q/cm2; applied = 1 mA; sheet resistance of electrode = 10Q/cm2;

time of deposition = 20 min. time of deposition = 20 min.

Fibres of length 500pm are seen especially at the edges of the electrode and are unidirectional in nature (Fig. 4b). In contrast, poly(m-toluidine) films at lower mag- nification (250 x ) appear to be composed of small fibrils interconnected with each other (Fig. 5b). Thus the above results demonstrate that the position of the sub- stituent determines the growth rate in the poly- merization reaction. Although the concentration of monomer used for depositing polymeric films was changed the morphology remained unaltered.

Weight loss curves for poly(o-toluidine) and poly(m- toluidine) films deposited at 0.4 and 0.8 M monomer concentrations, respectively, also show appreciable dif- ferences (Fig. 6). These curves show that poly(m-tolu- idine) decomposes completely at lower temperature than poly(o-toluidine). Both polymers show a three stage decomposition with the stages being clearer for poly(rn-toluidine).' ' The initial weight loss occurring in

the two polymers differs only by 2% (5% for poly(rn- toluidine) and 7-8% for poly(o-toluidine)). The differ- ence of 2-3% in weight loss for the first stage may be explained by the fact that the trapping of water mol- ecules in small chain fragments is lower in the case of poly(m-toluidine). However, the loss incurred in the second stage is significantly different in the two species. Poly(o-toluidine) shows a weight loss of about 13% at both monomer concentrations (0.4 and 0-8 M), this extending from 90°C to 210°C. Contrary to this, poly(rn- toluidine) weight losses of about 24% and 64% occur for the films deposited at 0.4 and 0 . 8 ~ monomer con- centration, respectively. The drastic increase in the weight loss in the second stage for poly(m-toluidine) films deposited at 0.8 M monomer concentration can be attributed to the formation of a larger fraction of smaller chain fragments which undergo complete break- down. Part of these fragments may undergo rearrange-

POLYMER INTERNATIONAL VOL. 40, NO. 3, 1996

Electrochemically deposited poly(o-toluidine) and poly(m-toluidine) jilms

Monomer concentration

205

Temperature C'C) - Fig. 6. Weight losses of (a) poly(o-toluidine) and (b) poly(rn-toluidine) films as a function of monomer concentration. Deposition

time = 20min; current strength applied = 1 mA.

ment to form crosslinked polymers resulting in the plateau in weight loss in the region' 200-300°C. The morphological characterization clearly gives evidence for the formation of smaller chain fragments during polymerization of m-toluidine monomer. The final stage of decomposition in the case of poly(m-toluidine) is complete at - 440°C for both monomer concentrations though the weight losses are about 71 and 31%. For the same stage ending at N 630"C, poly(o-toluidine) exhibits a loss of about 80%, which is accounted for by the decomposition of the polymer backbone as a whole.

The difference in the final decomposition temperature of about 190°C, along with the greater weight loss in the second stage, shows that, under similar conditions of deposition, polymerization is hindered for the meta- substituted monomer in comparison with the ortho- substituted analogue.

CONCLUSIONS

Under identical conditions, poly(o-toluidine) poly- merizes at a higher rate than poly(m-toluidine). A lower concentration of monomer ( 0 . 2 ~ ) is found to be appro- priate for poly(o-toluidine) for obtaining a selectively conducting phase in the polymer. Poly(m-toluidine) behaves similarly at higher concentrations of monomer (0.8 and 1.0 M. The overall morphology is better devel- oped at 0 . 8 ~ monomer concentration in both cases.

The higher thermal stability displayed by poly(o-tolu- idine) compared with poly(m-toluidine) indicates the predominant formation of long chains in the former case.

ACKNOWLEDGEMENT

One of the authors (RCP) is thankful to the CSIR, New Delhi, for award of a Senior Research Fellowship.

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