ORIENTATION DISTRIBUTIONS AND MELTING BEHAVIOR OF EXTENDED AND FOLDED-CHAIN CRYSTALS IN GEL-DRAWN, ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE

X-ray diffraction (XRD) studies on the orientation distributions of the 1 1 0 and 1 0 0 planes in gel-drawn samples of ultra-high-molecular-weight polyethylene (UHMWPE reveal the complex character of the crystal-orientation distribution in the samples. The orientation distribution is composed of two contributions differing in width. The dependence of the contents of both components on the draw ratio, as well as their behaviour during melting of the sample, are studied. The character of the dependencies observed indicate that each component of the orientation distribution can be associated with a separate assembly of crystals occurring in the sample, both having the same orthorhombic crystal structure. The broad component corresponds to only slightly oriented folded-chain crystals, while the narrow component corresponds to almost perfectly oriented extended-chain crystals being composed of stretched macromolecules. The content of the folded-chain crystals decreases with an increase in the draw ratio, whereas the content of the extended-chain crystals increases. During melting, folded-chain crystals melt directly to an amorphous phase, while the extended-chain crystals undergo crystal-crystal transition forming a hexagonal phase.