Percolation threshold of conductive polycarbonate/carbon composites as revealed by electron microscopy

Abstract

Several reports have appeared recently on conductive polymeric composites based on mixtures of nonconductive polymers with conductive solid microadditives [1-4]. Among the various combinations, polymer-carbon-black composites are of special interest because they combine the good application properties of commerical polymers (processability, low cost) with relatively high conductivity, thus offering an interesting type of light material. The low concentration of the additive required to form a conductive network is specially attractive. In a previous letter we have reported [5] on the electrical conductivity (a) variation ofpolycarbonate (PC)/carbon composites as a function of the concentration of the microadditive. The influence of film thickness and temperature dependence on the conductivity level reached was also examined [5]. In these systems the supermolecular structure of the polymer matrix has only a minor influence on the carrier mobility [6, 7]. The conductivity mainly depends on the percolation threshold for the conducting particles. In this system, carriers are transported above the percolation threshold through the carbon network predominantly involving a tunnelling conduction mechanism [6, 8]. The specific type of initial chain-like carbon-black structure, defined through the aggregation of primary particles (few nanometres in size) in strongly bonded aggregates of several hundreds of nanometres and weakly bonded agglomerates of these aggregates of several micrometres, presumably plays an important part in defining the onset of the conductivity threshold [9]. The purpose of the present letter is twofold: first, to report on the influence of the structure of carbon-black particles when transferred over into the polymer matrix on the conductivity level; secondly, to attempt to explain the change observed in the percolation threshold. We have used two types of carbon-black materials: XE2 from Philips Petroleum (A) having a welldeveloped chain-like structure (dibutyl phthalate absorption (DBP) - 400 cc/100 g) and acetylene carbon black from S.E.A. Tudor (B) with a lessdeveloped structure (DBP-~ 140 cc/100 g). Several mixtures of polycarbonate (bisphenol A) from Bayer with different carbon-black concentrations were prepared using a plastograph. Polycarbonate was melted at the working temperature of 230 ° C. Thereafter the filler was added and finally the mixture was mechanically stirred at 50 r.p.m, for 10 m until homogeneous materials were obtained. Compressionmoulded films (T = 230°C, p = 50bar), 500#m thick and 2 x 2cm of lateral dimensions, were