Melt drawability of ultrahigh molecular weight polyethylene

Some ultrahigh molecular weight polyethylenes have been shown to be highly ductile but only under specific conditions. When the initial starting state is reactor powder, the ductility is presumed to be due to the low initial entanglement. Samples were prepared for such draw by compression molding of reactor powder that had been held at different temperatures in the melt fo r only brief periods (5 min), followed by cooling to room temperature. Thes e films were then quickly heated and drawn at a constant temperature of 140 -155 degrees C, above the static melting temperature (T-m approximate to 13 5 degrees C). The stress/strain behavior from this state was found to be si gnificantly influenced by both the prior melt temperature and the time need ed for preparation of the initial morphologies. X-ray diffraction showed th at the strain-induced crystallization from the melt had occurred during dra w above a critical strain, corresponding to the draw ratio of similar to 10 . Differential scanning calorimetric data clarified that the rate of this c rystallization is accelerated by the higher draw stress for the samples pre pared at the higher prior-melt temperature. Combination of X-ray and calori metric results also indicated that the samples drawn from the melt contain two kinds of crystals, i.e., highly chain-extended and oriented crystals ha ving a higher T-m and chain-folded ones having a lower T-m. With increasing draw ratio, the relative amount of the former crystals, formed on draw, gr adually increased. The efficiency of draw, evaluated from tensile tests and thermal shrinkage measurements, was also interpreted from the differences of the prior-melt conditions, which increase entanglement. The samples coul d be successfully drawn from melt up to a maximum draw ratio of 45-50 at th e optimum temperature of 150 degrees C. Such highly drawn films exhibited a tensile modulus of 55 and a strength of 0.95 GPa, respectively. For the so lid-state drawing below T-m, the stress/strain behavior of the samples mold ed at different temperatures were all identical and had a low draw ratio of similar to 6. This suggests a difference in entanglements as they affect d raw above and below the static melting temperature.