Structural Evolution In Propylene-Based Elastomer With Gamma 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 gamma form (iPPcoE12-gamma), which was pre-stretched to various strains at 30 degrees 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-gamma sample became oriented in the process of the relaxation. Due to the unique "cross-beta" arrangement in the gamma form, the fraction of elastic stress of iPPcoE12-gamma was higher than that of iPPcoE12-alpha with pure a form at a small pre-stretched strain (epsilon(H) = 0.27) and the increased fraction of oriented parts was much less than that of iPPcoE12-alpha at medium strains (epsilon(H) = 0.71 and 1.27). A comparative analysis of the relaxation behavior of iPPcoE12-gamma and iPPcoE12-alpha revealed that the elasticity and stability of iPPcoE12-gamma 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-alpha had its advantages in elasticity. In addition, it was found that the gamma to a transition occurred during the relaxation, and the unoriented crystallites with. form were more difficult to transform than the oriented one.