Time-domain dielectric relaxation of poly(methyl methacrylate) and nitrile rubber

The time-domain dielectric response (after-effect) of poly(methyl methacrylate) (PMMA) and nitrile butadiene rubber (NBR) to a voltage step (100 V) was measured at varying temperatures. From the variation of the sample capacitanceC with time, we determined the ratioF_d/C, withF_d=(dC/dlnt)_max and C denoting the difference between the initial and the extrapolated equilibrium capacitance values. For PMMA around room temperature (RT) this ratio assumed values similar to those reported for mechanical stress relaxation. With NBR, such values were observed only at temperatures significantly below RT. A modified Kohlrausch-Williams-Watts (stretched exponential) function provided a good fit to the measuredC(t) data.