Thermally stimulated currents of polypropylene and its composites

The theory of dielectric relaxation is briefly summarized in another chapter Dielectric relaxation and dielectric strength of polypropylene and its composites in this book. The thermally stimulated current (TSC) technique is a special subclass of dielectric measuring methods. Nevertheless, because of its versatility and specific features, it is worth treating separately. 

TSP current is the sum of two components one is of dipolar origin, the other is ascribed to conduction. The dipolar part is proportional to the time derivative of the polarization (dipole moment density), which can be described as follows, assuming a single relaxation time (t) and an exponential dielectric response function [1]  

It has been shown that the TSD curves of dipolar dielectrics can be well correlated with the temperature-dependent low frequency loss spectra. In the case of TSD the effective frequency is, however, in the order of 10 -10 Hz, which explains its great resolving power and its sensitivity to phase heterogeneity. 

The thermally stimulated relaxation in heterogeneous dielectrics, consisting of relaxing components and exhibiting interfacial relaxation, presents a special problem, which has been solved exactly only for bilayer systems [2]. Here, the accumulation of space charge at the interfaces may cause anomalous depolarization effects (currents of reversed polarity). The same is true for samples measured with air gap or with one-sided electrodes (this latter is frequently used for corona charged samples). 

Curves Wl and W2, taken from [3], represent very clean samples, the only difference is that W2 was measured after annealing for 16.5 h at 140°C in vacuum. This causes mainly recrystallization, which is reflected in a minor modification of the 3 peak, ascribed to the glass transition of 

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