Low frequency dielectric behaviour of polyamides

Abstract

At low audio frequencies and at temperatures above the glass transition a large relaxation occurs in polyamides. Both this low frequency relaxation and the relatively high d.c. conductivity of polyamides have been attributed by several workers to the movement of amide protons. A comparison of the dielectric behaviour of a normal polyamide (6,10 and 6,6) with poly(sebacyl piperazine), prepared from the secondary diamine piperazine, in which no NH groups are present in the repeat unit is described. D.C. step response measurements were used (as well as normal a.c. measurements) in order to separate the components of loss due to relaxation and conduction. The data showed that amide protons are not the main cause of this low frequency relaxation and on the basis of all existing information they do not make any significant contribution until temperatures over 110°C are reached. The main relaxation observed arises from interfacial polarization throughout the bulk of the polymer, almost certainly due to trapping of electrons at the boundaries between crystalline and amorphous regions. Possible mechanisms are discussed. The d.c. step response measurements showed that the dielectric behaviour at low frequencies is quite complicated and that almost certainly a second mode or relaxation occurs at still lower frequencies. Since the retardation times involved in this second mode are extremely long this behaviour requires further study with very long charging times.