INTRODUCTION:

Polyacrylonitrile is an industrially important material and its most promising use is the production of wool like fibre (Orlon) (1,2,3) and carbon fibre (black Orlon).Now there is a considerable interest in the synthesis of polyacrylonitrile based polymer electrolytes for the development of lithium batteries for hybrid electric vehicle, fuel cells, supercapacitors, electro chromic device and chemical sensors (4,5). Polyacrylonitrile can be prepared chemically, photochemically and electrochemically (6,7,8). There are several reports on the solution polymerization of acrylonitrile initiated electrochemically (9,10). Here we describe its electrochemical bulk polymerization which does not involve any solvent. Electrolytes that dissolve and dissociate in pure AN to furnish conducting medium are limited. The electrochemical process is a simple approach to initiate bulk polymerization because the polymer formation is inhibited by merely switching the passage of electric current so that the polymer mass does not turn into a hard compact mass.

MATERIALS AND METHODS:

Materials:

Acrylonitile was properly dried and then fractionally distilled .Electrolytes [quaternary salts] were dried prior to use.

Polymerization:

The polymerization was carried out in a simple H-shaped cell which contained 25 ml of a solution and housed two platinum electrodes of 0.7×2.5cm². The cell was divided into two compartments by a fine fritted glass disk of 1cm diameter. A known amount of the salt was dissolved in 25ml of AN. The volume of the catholyte was 12.5 ml at 30⁰C.The reaction mixture was stirred with a magnetic stirrer. The polymerization was carried out under constant current electrolysis condition. After a desired time the electrolysis was terminated. The whole catholyte was poured into cold methanol and polymers formed around the cathode were removed and collected in methanol .The polymers were filtered ,washed and dried to a constant weight. The electrode surface was cleaned before each experiment by rinsing with water and acetone ,then drying in air. The average molecular weights of the polymers were measured from the solution viscosity measurements in dimethylformamide (DMF) at 30⁰C by Ubbelhode viscometer. The limiting viscosity numbers [η] were converted into average molecular weights by the following correlation (10).

[η] in decilitre/g = 2.303x 10^-4 x M^0.75

RESULTS AND DISCUSSION:

Blank Experiments:

(C₄H₉)₄I was dissolved in pure AN and the resulting solution was poured into the electrolytic cell in which electrodes were placed. When the solution was kept overnight without electrolysis, the polymerization did not occur, suggesting the absence of chemical or thermal polymerization.

Polymerization during the Passage of Impressed Current:

When the solution was subjected to electrolysis, the catholyte began to turn yellow in one set of electrolysis and the cathode became heavily coated with yellow orange insoluble mass during the course of electrolysis. The locus of polymer formation was on the cathode and not in the body of the solution. Polyacrylonitrile does not dissolve its monomer. It is pertinent to mention that electrochemical initiation of an addition polymerization in polymer insoluble media does not necessarily lead to polymer coating on the electrodes. It is expected that polymer will not coat the electrodes in processes which involve an indirect and slow initiation reaction and slow polymer growth reaction, since in either case the participants in the polymerization may diffuse away from the electrode before they form insoluble polymer. In such a situation, polymer formation occurs primarily in the body of the solution and the electrode surface remains free of solid deposits. When the initiation and subsequent polymer growth reaction are fast, the polymer is more likely to coat the electrode rather than disperse throughout the electrolytic solution.

The prolonged electrolysis caused excessive polymer deposit onto the cathode which increased some cell resistance and did interfere much to pass a constant current. Whether or not electrode - deposits interfere with passage of current depends on (a) the mobility of electro active species which undergoes electron transfer with the electrode surface (b)the open porosity of the deposits, (c) the electrical conductivity of the deposits, (d) the electrical conductivity of the electrolyte solution, (e) the current density, (f) the rates of the initiation, growth and termination reactions and (g) the adherence of the deposit to the electrode surface.

Effect of Current:

The % conversion of AN to polyAN with duration of electrolysis at different currents in pure AN containing tetrabutyl ammonium iodide (TBAI) at30⁰C are presented in figure-1.which illustrates that the rate is initially slow for a few minutes after which abruptly increased. The initial slow rate is due to some induction periods with each impressed current. Such induction period decreases with increasing current. The induction period is possibly because of some residual impurities present in the polymerizing mixture that was not completely purged and consumed some initial amount of electricity.

The polymer conversion (%) increases with increasing current levels. This is owing to an increase in the concentration of initiating species with the rise of current levels. However the molecular weights of polymers obtained at different currents decease with the increase of impressed currents, as is seen from the table-1.

Table 1 Polymer yields and average molecular weights for the cathodic bulk polymerization of AN at different current at 30⁰C

Sl. No	Current (mA)	Limiting viscosity No. (decilitre/ gram)	Av. Mol. Wt. x 10^-5
1	5	1.50	1.2164
2	15	1.20	0.9033
3	20	0.98	0.6896
4	25	0.90	0.6156

Here molecular weights are higher compared to those observed in case of cathodic polymerization of acrylonitrile carried out in DMF (10). In bulk polymerization when polymer is insoluble in its monomer, initiating species are entrapped in the coiled chains and the termination process becomes slow resulting in formation of long polymeric kinetic chains. The molecular weights of polymers decrease with increasing current levels. This is due to the formation of more initiating species leading to the formation of more growing polymer chains.

Effects of Supporting Electrolytes:

Table 2 Average molecular weights for the cathodic bulk polymerization of AN with different supporting electrolytes at 15 mA at 30⁰C

Sl. No	Supporting electrolytes	Limiting viscosity No. (decilitre/ gram)	Av. Mol. Wt. x 10^-5
1	(C₄H₉)₄NI	1.15	0.8535
2	(C₄H₉)₄NBr	1.12	0.8239
3	(C₄H₉)₄NNO₃	1.10	0.8044
4	(C₄H₉)₄PBr	1.08	0.7849

Table -2 shows that the average molecular weights of polymers formed in the presence of different electrolytes are minimally different. This suggests that the electrolytes do not play important role in the bulk polymerization of AN, and it seems to furnish only conducting medium. We infer from this result that polymer formation takes place by the direct cathodic reduction of AN at the cathode.

Effect of Electrode Metals:

The nature of electrode materials may have significant effects on the yield and properties of polymers prepared electrochemically. The electrochemically initiated bulk polymerization of AN forms adherent, homogeneous and pore free film on commodity metals such as Fe and Al. Coating of this kind is interesting as possible corrosion inhibiting polymer layers. We previously reported electrochemical polymerization of pyrrole and aniline on Fe for corrosion protection (11). In order to investigate the effect of electrode metals on polymer yields and molecular weight the polymerization was carried out using different metal electrodes and the results is presented in table-3 below.

Table 3-Polymer yields and average molecular weights for the cathodic bulk polymerization of AN with different electrode metal at 15mA current at 30⁰C

Sl. No	Electrode metal	Polymer yields (%)	Limiting viscosity No. (decilitre/ gram)	Av. Mol. Wt. x 10^-5
1	Platinum	46	0.98	0.6896
2	Stainless steel	45	0.96	0.6709
3	Nickel	40	0.91	0.6247
4	Iron	42	0.85	0.5704
5	Copper	35	0.80	0.5261

The table-3 shows that the bulk polymerization of AN successfully occurs at commodity metals. Therefore costly Pt metal is not essential to use in electrochemical polymerization. In each case the cathode was coated with insoluble polymers.

Effect of Temperature

Table-4 Polymer yields and average molecular weights for the cathodic bulk polymerization of AN at different temperatures at a constant current-15mA and electrolysis time - 65 min

Sl. No	Temperature in ⁰C	Polymer yields (%)	Limiting viscosity No. (decilitre/ gram)	Av. Mol. Wt. x 10^-5
1	0	20	1.50	1.2164
2	10	27	1.40	1.1095
3	15	30	1.40	1.1095
4	25	37	1.30	1.0051
5	30	40	1.20	0.9033

Table-4 shows the variation in the yields of polymers and molecular weights at different reaction temperatures at a constant 15mA current and electrolysis time - 65minutes. It is evident from the data that the yield decreases with decreasing reaction temperature while molecular weight increases. Indeed, the result corresponds to the expectation because the rate of polymerization becomes slow at a low temperature because of the availability of less activation energy. The termination rate was lower owing to low temperature causing the molecular weight to increase.

Inhibition of Polymerization:

A free radical inhibitor 2, 2 – diphenylpicrylhydrazyl could not inhibit the polymer formation indicating the absence of free radical polymerization. On the other hand in the presence of proton donating materials such as water or acid, polymerization did not occur suggesting possibly an anionic mechanism.

Mechanism:

(i) On the basis of experimental data obtained the following mechanism is proposed.

The monomer is directly reduced at the cathode to generate radical anion.

(i) Monomeric radical anion dimerizes

(ii) Monoanion and / or dianion growth occurs leading to formation of high molecular weight polymers.

(iii) It is supposed that the growing polymer chain is terminated either by residual impurities present in the polymerizing mixture or by products generated during electrolysis. A similar mechanism is suggested in the literature ( 10 ).

CONCLUSION:

The bulk polymerization of acrylonitrile without a diluent via electrochemical process yields polyacrylonitrile of high molecular weights. The polymerization successfully takes place at the cathode of some commodity metals such as Fe, Ni and Cu.

ACKNOWLEDGEMENT:

The financial support of the University Grant Commission, New Delhi to carry out this work is gratefully acknowledged.

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Received on 19.09.2013 Modified on 05.10.2013

Asian J. Research Chem. 6(11): November 2013; Page 1068-1071