ALPHA-RELAXATION RETARDATION MODE IN SEMICRYSTALLINE POLYMERS WITH FLEXIBLE CHAINS

The molecular mobility of two semicrystalline polymers with flexible c hains-chemically crosslinked low density polyethylene (XLPE) and poly( vinylidene fluoride) (PVDF)-has been investigated in the temperature r ange between room temperature (above their glass transition temperatur e) and their melting point. D.s.c. was used to characterize the confor mational evolutions related to crystalline domains. Thermostimulated c urrents and creep were used to study the dynamics of these movements. For both polymers, the alpha-relaxation/retardation mode is located in the temperature range of a small endothermic phenomenon. These low te mperature d.s.c. peaks are not related to any melting process but rath er to conformational disorder in the crystal-amorphous interphase. The fractional polarization/stress technique was applied to obtain the di stribution of relaxation/retardation times. A compensation phenomenon was found above the alpha-relaxation mode of beta-PVDF and XLPE. It ha s been ascribed to conformational movements associated with an order-d isorder phase transition in the vicinity of the melting point. These h igh temperature phases would correspond to the paraelectric phase of b eta-PVDF and to the hexagonal phase of XLPE. Insofar as the alpha-mode is liberating molecular mobility at the crystal-amorphous interphase, it appears as a precursor of the order-disorder transition.