Structural Changes and Stress Build-Up during Uniaxial Deformation of Propylene Based Thermoplastic Elastomer by In-Situ Synchrotron X-ray Studies

摘要

In-situ structural changes at the crystalline and lamellar levels during uniaxial extension and subsequent retraction of thermoplastic elastomers (TPE) based on ethylene-propylene (EP) copolymers with propylene crystalline structures were studied by time-resolved synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques at room temperature. Before extension, WAXD showed a single diffraction peak and an amorphous halo with no preferred orientation, indicating the presence of the random-oriented PP crystallites with a-form structure. SAXS showed a circular pattern, indicating a random oriented lamellar structure. During the first cycle extension, TPE exhibited a maximum stress around 100% strain, corresponding WAXD/SAXS patterns showed that partially melting and re-alignment of crystal were occurred. Above 200% strain, WAXD/SAXS patterns indicated that a completely oriented lamellar crystalline structure of a-form was formed and increased with strain. This suggested that the amorphous part of molecules were oriented with strain. In retraction, the orientation of the strain-induced crystallites decreased, but a fraction of the crystalline phase remained oriented at zero stress. On the other hand, the fraction of orientated amorphous molecules decreased and diminished at stress zero. During the second cycle, the oriented crystal increased slightly with strain in extension. The fraction of oriented amorphous molecules increased in extension and decreases in retraction with strain. The results suggested that PP crystallites contribute to its modulus as the network points and the amorphous molecules behave as flexible chains.