Better understanding of the fundamental electrical properties of polymeric materials is a key to their further progress in the practical application, not only as electrical insulating materials, but also as functional elements. In this paper, recent topics and their important results will be reviewed on electrical conduction and related problems including carrier traps and electrical breakdown in various polymers. At first, the electronic nature, e.g. photoconduction and electron transport are explained in insulating, semi-insulating, and conducting polymers. Also, interfacial phenomena at both metal-polymer and polymer-polymer interfaces, carrier traps (their physico-chemical nature) and impurity effects on electrical conduction are introduced with their results and experimental techniques. Finally, interesting topics on electrical breakdown related to polymer morphology will be given.

Electrical Conduction and Carrier Traps in Polymeric Materials

Field-induced thermally stimulated currents

In this review the physical properties, mechanisms and models are reviewed which explain the high piezoelectricity of the ferroelectric polymers of the polyvinylidene fluoride (PVDF) family, the odd nylons, the vinylidene cyanide (VDCN) copolymers and polyurea. Because PVDF and its copolymers with trifluoroethylene (TrFE) and tetrafluoroethylene (TFE) have been studied extensively in the last 25 years the main part of the review deals with these polymers. Additionally a short summary of the different piezoelectric applications of PVDF is presented. The odd nylons, the VDCN copolymers and the polyurea are compared with PVDF to show the different stabilization mechanisms of the remanent polarization and the different temperature behavior of the piezoelectricity. This review not only concentrates on the work done in the last few years but in order to describe the behavior of these polymers in detail, it is necessary to go back to the roots. Therefore many articles from the late seventies and early eighties are included.

Piezoelectric polymer electrets

Intrinsic polymer dielectrics for high energy density and low loss electric energy storage

The paper is directed towards the materials scientist who wishes to employ published values of diffusion coefficients in his investigations. A brief review of self-diffusion behaviour and the likely inaccuracies of the various techniques used in the measurement of diffusion is followed by a selected bibliography and tabulation of coefficients for oxides.

Self-diffusion in simple oxides (A bibliography)

The conductivity of dry poly(hexamethylene adipamide) (nylon 66) was measured as a function of time and temperature. Three temperature ranges were observed in which the time dependence of conductivity differed: (a) below 80°C. the conductivity decreased continuously with time; (b) between 80°C. and 110°C. the conductivity remained constant over long periods; (c) above 120°C. a continuous decrease in conductivity was again observed. In other experiments the volume of gas evolved from the nylon film was measured under continuous potential and compared with the total current passed through the sample. It was observed that above 120°C. the gas evolved corresponded to about one-half the volume calculated if the conduction process involved only protons. Below 120°C. the gas evolved corresponded to an increasingly small fraction of the total current until below 90°C. no evolution of gas was observed. This suggests that at temperatures above 120°C. conduction involves the transport of both protons and electrons, whereas at lower temperatures it is electronic. Mechanisms of conduction are discussed.

D. A. Seanor

Papers

Electronic and ionic conductivity in nylon 66