Structural evolution in propylene-based elastomer with γ form during stress relaxation

The stress relaxation and the structural evolution of a random propylene-ethylene copolymer of about 12 mol% of ethylene co-units with pure γ form (iPPcoE12-γ), which was pre-stretched to various strains at 30 °C, were investigated by means of in situ wide angle X-ray diffraction techniques. The two-dimensional X-ray diffraction patterns were decomposed into oriented and unoriented parts. It turned out that the unoriented fraction of the iPPcoE12-γ sample became oriented in the process of the relaxation. Due to the unique “cross-β” arrangement in the γ form, the fraction of elastic stress of iPPcoE12-γ was higher than that of iPPcoE12-α with pure α form at a small pre-stretched strain (εH = 0.27) and the increased fraction of oriented parts was much less than that of iPPcoE12-α at medium strains (εH = 0.71 and 1.27). A comparative analysis of the relaxation behavior of iPPcoE12-γ and iPPcoE12-α revealed that the elasticity and stability of iPPcoE12-γ were more attractive at smaller strains (smaller than critical strain C—the strain of the starting of the fragmentation of crystalline blocks), while at larger strains (larger than 1.27) iPPcoE12-α had its advantages in elasticity. In addition, it was found that the γ to α transition occurred during the relaxation, and the unoriented crystallites with γ form were more difficult to transform than the oriented one.