The thermal and electrical properties of carbon black/liquid crystalline polymer composites

Abstract

Carbon black/liquid crystalline polymer (CB/LCP) composites were mixed by blending the carbon black powder with Vectra A950 by extrusion. Film samples of 1% to 7% and 10% CB were obtained from hot pressing the mixture in a mould with thin gaps. Wide-Angle X-ray diffraction was employed to determine the molecular orientation of the LCP matrix. The results reveal that at lower CB volume fraction, the LCP in the composite has a higher orientation whereas the LCP become randomly aligned at high CB volume fraction composite. The surface morphology of the film samples were studied by optical microscope and scanning electron microscope. From the optical photomicrographs, an increase in the CB volume fraction would increase the probability of forming large clusters. There is an obvious change from 3% to 4% CB of which the clusters grow and make contact. The CB aggregates are discernible from SEM of which the average size is about a few microns. The J-E characteristics of the CB samples reveal a linear relationship between the current and the applied voltage. It may be a result of highly conductive of the composites. Resistivities of the samples were also measured in the flow, transverse and thickness directions. The percolation threshold is found at between 3% and 4%. It is also found that the volume fraction dependence of resistivity satisfies the scaling law. The resistivity as a function of temperature was measured in the range from room temperature to 200 C. The resistivities of samples below percolation threshold exhibit negative temperature coefficient and the drop in resistivity is about three orders of magnitude. The resistivities against temperature of samples above the percolation threshold exhibit a negative slope at low temperature range. They reach a minimum at around 150 C then change to a positive slope up to 200 C. Thermal conductivity and thermal expansivity of the composites were determined by the laser flash radiometry method and the thermal mechanical analyzer respectively. The results can be correlated to the molecular orientation of the LCP matrix. Low CB volume fraction composites correspond to high molecular order, thus heat diffuse more easy relative to the high CB volume fraction composites which have lower molecular order. The results of sheet moduli of the composites determined by the laser induced ultrasonic method revealed the elasticity of the composites is also dominated by the molecular orientation of the LCP matrix.