Melting and crystallization of ultra-high molecular weight polyethylene with appearance of hexagonal phase I. Melting processes of fibers under constrained state

Abstract

Melting behavior under constrained state was studied for two kinds of commercial ultra-high molecular weight polyethylene (PE) fibers with different molecular weights by means of differential scanning calorimetry (DSC) and X-ray measurement. DSC melting curves of constrained samples were compared with those of unconstrained ones. It was shown that the constrained sample mainly melts through two endothermic peaks 2 and 3, while the unconstrained sample mainly melts through single endothermic peak 1 whose peak temperature is lower than those of peaks 2 and 3. These peak temperatures were very reproducible for each sample. Peak 1 of the unconstrained sample was attributed to the melting of orthorhombic phase composed of so-called extended-chain crystals. On the other hand, based on the X-ray data, peaks 2 and 3 of the constrained sample were assigned to a transition orthorhombic to hexagonal phase and 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 on the basis of a free energy diagram that can explain the experimental results successfully.