Melting and crystallization of ultra-high molecular weight polyethylene with appearance of hexagonal phase II. Strain-induced crystallization of thermal-contracted sample

Abstract

Strain-induced crystallization of a thermal-contracted sample, prepared from highly-drawn ultra-high molecular weight polyethylene(UHMW-PE), was examined by the wide angle X-ray diffraction method using a high temperature elongating apparatus. X-Ray diffraction measurements were isothermally carried out at three characteristic temperatures 148, 152, and 156°C, above the melting point. At each temperature, the molten sample was elongated in steps up to the break ratio, and change of the X-ray equatorial diffraction pattern was recorded with a position sensitive proportional counter. At 148°C below the orthorhombic to hexagonal transition point determined by thermal analysis of a constrained sample, the strain-induced crystallization of the only orthorhombic crystals occurred in the elongation. On the other hand, the hexagonal crystals grew together with the orthorhombic ones at 152°C that corresponds to the transition point, and at a yet higher temperature of 156°C. the only hexagonal ones grew. Such temperature dependence of the structure of growing crystals was discussed on the basis of the free energy diagram proposed in Part I to explain the melting process of the constrained UHMW-PE.