Counits Content and Stretching Temperature-Dependent Critical Stress for Destruction of γ Crystals in...

Tensile deformation behavior of three random propylene-ethylene copolymers with the same molecular weight and different contents of counit was investigated at different temperatures from room temperature to close to melting point via tensile tests, step-cycle tests, and in situ wide-angle X-ray diffraction techniques. Upon stretching, the original crystalline lamellae must be destroyed, generating new highly oriented ones. A critical stress has been suggested, under which the original crystallites can be destructed. The propylene-ethylene copolymer samples in pure γ-form transformed gradually into α-form during tensile stretching. This crystalline transition proceeded via a destruction (melting) of the original γ-form crystals followed by recrystallization of the freed polymeric chain segments into α-form along stretching direction. This result provides a marker for investigating the critical stress mentioned above. Such critical stresses triggering the destruction of γ-form crystals for the propylene-ethylene copolymers of different ethylene counit contents were successfully calculated. It turned out that this critical stress depended on the ethylene counit content and stretching temperature. Samples with less ethylene counits show higher critical stress because of a lower degree of insertion of the ethylene counits into the crystalline unit cell than samples with higher ethylene counit content. The critical stresses remained constant when the samples were stretched at low-temperature region, whereas they decreased significantly at high stretching temperatures close to the melting points due to strong thermal distortion of the crystalline lattices, making the crystallites less stable.