Strain dependent evolution of structure and stress in propylene-based elastomer during stress relaxation

Abstract The stress relaxation behaviors of propylene-based elastomer with pure α form were investigated at 30 °C at various pre-stretched strains by means of in situ wide angle X-ray diffraction techniques. The stress decreased rapidly at the beginning of relaxation process and then showed a slow decay independent of pre-stretched strains. This decrease in stress during relaxation was accompanied by an obvious increase in the orientational degree and the crystallinity. However, the processes of increasing the crystallinity depended strongly on the imposed strain. At small strain ( e H 0.17), besides the transition of some oriented amorphous phase into oriented crystals, a fraction of unoriented amorphous phase transformed into unoriented crystals. At medium strains ( e H 0.76 and 1.28), a fraction of unoriented amorphous and unoriented crystals transformed to oriented crystals. A very small fraction of unoriented amorphous phase transformed into unoriented mesomorphic phase. At large strains ( e H 1.63 and 1.85), a fraction of oriented amorphous phase transformed to oriented crystals. The different network with varied fraction of amorphous and crystalline phases accounted for the distinct transformation processes at different pre-stretched strains. Especially at medium strains, the “appropriately stretched” high elastic network was formed so that higher portion of stress remained after the relaxation. Furthermore, it was found that crystallization during stress relaxation provides an alternative route for lowering the macroscopic stress by reducing the effective number of network chain segments in unit volume.