MELTING AND CRYSTALLIZATION OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE WITH APPEARANCE OF HEXAGONAL PHASE-I - MELTING PROCESSES OF FIBERS UNDER CONSTRAINED STATE
S. Tsubakihara et al., MELTING AND CRYSTALLIZATION OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE WITH APPEARANCE OF HEXAGONAL PHASE-I - MELTING PROCESSES OF FIBERS UNDER CONSTRAINED STATE, Polymer Journal, 28(6), 1996, pp. 489-495
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.