The influence of thermal expansion on thermally stimulated currents has been studied by means of model calculations based on the bistable model of Fröhlich, considering especially the case of materials characterized by expansion coefficients markedly different above and below the relaxation range. A good qualitative and quantitative agreement has been obtained between theory and experiment in elastomeric materials (styrene-isoprene-styrene and isoprene-styrene-isoprene block copolymers) showing that thermal expansion is the main factor responsible for the appearance of current reversals in thermally stimulated polarization processes.
The thermoluminescence (TL) of elastomeric materials modified by doping and chemical or ~-induced crosslinking was measured following irradiation with u.v. light at 77 K and compared with the thermally stimulated depolarization currents (TSDC) obtained during warmup of samples previously polarized from 298 to 77 K, stressing the comparison of luminescence and current peaks observed in the glass ~ransition range. Significant.shiftings of the TL peaks were found as a function of degree of crosslinking and nature of the dophnt, even though the glass transition temperature and the position of the TSDC peaks were not affected, showing that the kinetics of the light-emitting process is not only governed by the thermal motions of the polymer units but also critically depends on the nature of traps and luminescent species. Generally speaking, the TL method appears to be much more sensitive than the classical relaxation methods to any structural, morphological, or environmental change of the macromolecular chains and seems, thus, particularly attractive to obtain valuable information on new aspects of polymer relaxation.The thermoluminescence (TL), which involves measuring and analyzing light emitted during warmup of materials previously irradiated at low temperature, is generally considered as a useful means for studying molecular motion and structural transitions occurring in polymeric solids (1-5). On the basis of significant correlations observed between the TL glow curves and the relaxation peaks obtained by mechanical or dielectric loss measurements, it is usually assumed that the luminescence emission depends on the molecular mobility either because the macromolecular chains can act as effective carrier traps (e.g., Via formation of dielectric cavity traps defined by the chains themselves) or because the release of charge carriers from other traps (such as free radicals or neutral molecules with positive electron affinity) can only occur as a result of the onset of local motion involving molecules located in the immediate vicinity of the trap (1-5). As an alternative to the detrapping phenomenon, it has been sometimes postulated that charge recombination can occur by physical approach and reaction of a radical ion with the parent ion or any other neighboring positive ion (3). In principle, such a process should be also closely related to structural changes in the polymeric matrix because it is likely to arise only in a temperature range where large scale motion sets in, i.e., in the vicinity of the glass transition temperature Tg. Whatever the exact trapping and detrapping mechanisms involved and the nature of luminescence centers, the macromolecular motions are, thus, expected to contribute Significantly to the light-emitting processes in such a way that the TL glow curve could be considered as reflecting the relaxation itself and could be used to determine the transition temperatures.We have recently shown, however, that, at least in certain polymers, the type of impurity present in the matrix can atso play a pre...
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