DIELECTRIC BEHAVIOR OF POLYMERS AND COMPOSITES

The chapter provides a survey of dielectric relaxation processes in heterogeneous dielectrics, i.e. polymers and composites. The particular role of the DC conductivity in high electric field is pointed out. Two kind of empirical relaxation functions, the exponential and power law ones, the most frequently used in description of experimental data are discussed. The exponential-type functions can be used to describe the dielectric response in the vicinity of the maximum of dielectric losses whereas, the power law is applicable in the range outside the relaxation region. Moreover, two approaches to the physical interpretation of the above mentioned response function, the relaxation time distribution of Debye-type elementary relaxors and the many-body interaction processes, are discussed. Examples of dielectric relaxation processes in polymers originating from heterogeneities in both micro and macro scales are presented and discussed with respect to the results of NMR, specific heat and mechanical losses studies. Examples of a broad distribution of relaxation times at glass transition in dipolar glasses as well as the dielectric behaviour of ferroelectric relaxors are presented. Moreover, results of dielectric studies of relaxation in many-phase systems i.e. ceramic-polymer composites, microemulsions and polymer dispersed liquid crystals are shown.