MELTING AND CRYSTALLIZATION OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE WITH APPEARANCE OF HEXAGONAL PHASE-II - STRAIN-INDUCED CRYSTALLIZATION OF THERMAL-CONTRACTED SAMPLE

Strain-induced crystallization of a thermal-contracted sample, prepare d 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 al three characteristic temperatures 148, 15 2, and 156 degrees C, above the melting point. At each temperature, th e molten sample was elongated in steps up to the break ratio, and chan ge of the X-ray equatorial diffraction pattern was recorded with a pos ition sensitive proportional counter. At 148 degrees C below the ortho rhombic to hexagonal transition point determined by thermal analysis o f 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 degrees C that corresponds to the transition point, and at a yet high er temperature of 156 degrees C, the only hexagonal ones grew. Such te mperature dependence of the structure of growing crystals was discusse d on the basis of the free energy diagram proposed in Part I to explai n the melting process of the constrained UHMW-PE.