α-Relaxation/retardation mode in semicrystalline polymers with flexible chains

The molecular mobility of two semicrystalline polymers with flexible chains—chemically crosslinked low density polyethylene (XLPE) and poly(vinylidene fluoride) (PVDF)—has been investigated in the temperature range between room temperature (above their glass transition temperature) and their melting point. D.s.c. was used to characterize the conformational evolutions related to crystalline domains. Thermostimulated currents and creep were used to study the dynamics of these movements. For both polymers, the α-relaxation/retardation mode is located in the temperature range of a small endothermic phenomenon. These low temperature d.s.c. peaks are not related to any melting process but rather to conformational disorder in the crystal-amorphous interphase. The fractional polarization/stress technique was applied to obtain the distribution of relaxation/retardation times. A compensation phenomenon was found above the α-relaxation mode of β-PVDF and XLPE. It has been ascribed to conformational movements associated with an order-disorder phase transition in the vicinity of the melting point. These high temperature phases would correspond to the paraelectric phase of β-PVDF and to the hexagonal phase of XLPE. Insofar as the α-mode is liberating molecular mobility at the crystal-amorphous interphase, it appears as a precursor of the order-disorder transition.