In situ WAXD study of structure changes during uniaxial deformation of ethylene-based semicrystalline ethylene-propylene copolymer

In situ strain-induced structure changes during uniaxial deformation of an ethylene-propylene copolymer, containing 78 wt% (or 85 mol%) of ethylene moiety, were studied by synchrotron wide-angle X-ray diffraction (WAXD). The chosen sample could crystallize into orthorhombic, pseudo hexagonal or a mixed form, depending on the annealing conditions. Crystallization at high temperatures (e.g. 50 °C) favored the formation of orthorhombic form, while crystallization at low temperatures (e.g. 20 °C) favored the formation of pseudo hexagonal form. Under deformation, the transition from orthorhombic to pseudo hexagonal form was observed at relatively low strains (e.g. 0.12). At higher strains, WAXD data indicated the occurrence of direction-dependent crystal destruction at strains <0.25 and subsequent re-crystallization with extended-chain conformation at high strains (>1.0) all of the pseudo hexagonal form. The drastic changes in the crystalline structures (orthorhombic to pseudo hexagonal) and phase transitions (crystal destruction and re-crystallization) at modest strains can be attributed to the high mobility of the amorphous ethylene-propylene segments at room temperature.