Thermal stability of polypyrrole prepared from a ternary eutectic melt

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Abstract

Polypyrrole is prepared electrochemically from a room temperature ternary eutectic melt consisting of acetamide, urea and ammonium nitrate. The temperature effects of conductivity of polypyrrole are analyzed by Arrhenius and Mott equations to understand its conduction mechanism. The thermal degradation of both doped and dedoped samples of the polypyrrole in air and N2 atmosphere has been followed using thermogravimetric (TG) and differential thermal analysis (DTA). The kinetic analysis of the TG data has been carried out to obtain information on the energy of activation for the polymer decomposition.

Introduction

Excellent reviews on polypyrrole and its chemistry are available in literature [1], [2], [3], [4]. During the last two decades, investigations have centered mainly on improvement of the physical properties of polypyrrole such as processibility and stability. Serious research on functionalized polypyrroles, blends, composites and on the preparation of soluble dispersions of polypyrrole resulted in many useful publications [5], [6], [7], [8]. Polypyrrole has been identified for certain specific applications such as rechargeable batteries, EMI shielding, conducting textiles, electrochemomechanical devices, electrocatalysis and as membranes for gas and liquid separation processes [3], [9].

Solvents have a strong influence on the synthesis and the resulting properties of polypyrrole. In most cases, polypyrrole is prepared from electrolytic solutions based on the aprotic organic solvents, although some work has been done in aqueous solutions [10], [11], [12]. High-quality polypyrrole films having conductivities higher than 500 S cm−1 have been obtained in aqueous solutions of p-toluenesulfonate salts [13]. Among organic solvents, acetonitrile has been the most commonly used [14], [15]. However, films grown from dry acetonitrile are non-uniform and adhere poorly to the electrode surface. Better physical properties are obtained by increasing the water content of the acetonitrile solution [14], [16]. Pickup and Osteryoung [17] synthesized polypyrrole electrochemically in an ambient-temperature molten salt consisting of a mixture of aluminum chloride and 1-methyl-(l,3-ethyl)-imidazolium chloride. The acid–base nature of the melt depended on its composition. Polypyrrole could be prepared only from the neutral melt.

This paper is aimed at presenting a systematic report on the charge transport mechanism and the thermal degradation properties of polypyrrole prepared from a room temperature acetamide–urea–ammonium nitrate ternary eutectic melt. In an earlier report, we discussed the effect of this molten solvent on the stability, structural and redox properties of polypyrrole [18]. The redox activity of the nitrate doped polypyrrole in the melt was higher than that in aqueous solutions. This was attributed to a favourable polymer film morphology facilitating the anion insertion–elimination. Another advantage is that the conductivity of the film prepared from the melt is about 10−3 S cm−1 suitable for certain applications requiring semiconducting films. The Schottky diodes fabricated with the polypyrrole prepared from this eutectic melt resulted in improved junction properties [19].

Several reports exist on the conductivity versus temperature data for polypyrrole doped with different counter ions [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. But only very few reports [26], [28], [31] have analyzed the applicability of Mott model for polypyrrole. The thermal stability of polypyrrole has been reviewed [32]. Despite considerable research [33], [34], [35], [36] thermal degradation of polypyrrole is still not clearly understood as the presence of counter anion and their interaction with the polymer affects the degradation process. Recently a number of papers have appeared in literature on the thermal stability of polypyrrole composites [37], [38], [39], [40].

Section snippets

Experimental

The ternary melt was prepared as follows [18], [19]. Fifteen grams vacuum dried acetamide (Merck) was melted at 358 K and 10 g of urea (SD fine) was added to it and stirred. When the mixture became homogeneous, 8.3 g of ammonium nitrate (Merck) was added and stirred till the solution was clear.

Pyrrole (SRL) was purified by vacuum distillation. Polypyrrole was obtained on a platinum (1 cm2) substrate by applying a constant potential of 0.55 V versus Ag, AgCl/Cl(saturated) from the ternary melt

Temperature dependence of conductivity

The room temperature conductivity of the dry polymer sample was measured to be 5.7 × 10−2 S cm−1. The increase in conductivity with rise in temperature from 78 to 257 K is typical of semiconductors (Fig. 1a). The conductivity reaches a maximum at 257 K and thereafter there is a gradual decrease in conductivity.

The temperature dependent conductivity data can be fitted to Arrhenius equation of conductivity [3]:σ=σ0expEakTwhere Ea is the activation energy. The ln(σ) versus T−1 plot shows (Fig. 1b) two

Conclusions

The Arrhenius and Mott parameters obtained from the analysis of the temperature conductivity data for polypyrrole indicates 3D variable range hopping (VRH) conduction mechanism instead of nearest neighbour hopping. Detailed thermal characterization shows that the polymer is stable in air up to 673 K in the dedoped form and up to 553 K in the doped form. In N2 atmosphere, the polymer is thermally stable up to 750 K. From the kinetic analysis, the thermal activation energy for the decomposition of

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