Orientation effects in polyethylene terephthalate caused by high pressure and their influence on dielectric relaxation properties

Abstract

IN STUDIES dealing with the effect of pressure on electrical relaxation processes in polymers one of the following two methods of producing pressure is used: hydrostatic pressure produced by liquids [1, 2], and the pressure produced by punches in cells, which are similar in design to moulds [3-6]. Both methods have shortcomings: with the first method there is a danger that the physical properties of the polymer change as a result of liquid penetrating the specimen and transmitting pressure and with the second method pressure is not uniform. Although there is a widespread view that at temperatures higher than the glass transition point of the polymer the pressure sustained by the polymer in the mould can be regarded as hydrostatic, the l~ossibility of orientation in the sample should be borne in mind since it may considerably alter the relaxation effects studied. This paper deals with orientation in polyethylene tereph%halate (PETP) samples subjected to pressure in a cylindrical mould and examines the effect of various types of orientation of samples on electrical relaxation properties.