MELTING AND CRYSTALLIZATION OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE WITH APPEARANCE OF HEXAGONAL PHASE-I - MELTING PROCESSES OF FIBERS UNDER CONSTRAINED STATE

Melting behavior under constrained state was studied for two kinds of commercial ultra-high molecular weight polyethylene (PE) fibers with d ifferent molecular weights by means of differential scanning calorimet ry (DSC) and X-ray measurement. DSC melting curves of constrained samp les were compared with those of unconstrained ones. It was shown that the constrained sample mainly melts through two endothermic peaks 2 an d 3, while the unconstrained sample mainly melts through single endoth ermic peak 1 whose peak temperature is lower than those of peaks 2 and 3. These peak temperatures were very reproducible for each sample. Pe ak 1 of the unconstrained sample was attributed to the melting of orth orhombic phase composed of so-called extended-chain crystals. On the o ther hand, based on the X-ray data, peaks 2 and 3 of the constrained s ample were assigned to a transition orthorhombic to hexagonal phase an d a melting of the hexagonal phase, respectively. Based on the visual observation, peak 3 was also shown to be accompanied by the break and rapid thermal-contraction of the sample. These peculiar transition and melting mechanisms of PE under the constrained state were discussed o n the basis of a free energy diagram that can explain the experimental results successfully.