Time-domain dielectric relaxation in PMMA and NBR

Time-domain dielectric relaxation (after-effect) was studied using samples of PMMA and NBR subjected to a voltage step of 100 V (rise time 3 μs). The response curves consisted of a transient dipole reorientation part followed by a conduction stage. After separation of the two contributions the transient response was fitted to a stretched exponential and the value of its exponent determined. The primary aim of the measurements was to compare the values of β in the stretched exponential exp(-kt^β) with those describing mechanical stress relaxation, where for a number of solids with varying structure and composition, including both polymers and metals, β≈0.27±10% has been reported. For PMMA agreement with the latter value was found within a temperature range varying from -5°C to +32°C, that is well below T_g. With NBR similar values of β were found only at relatively low temperatures (-25°C to -30°C). There was a distinct increase in β with temperature for both polymers. The time-domain spectrometry allowed the data to be converted into the corresponding frequency response