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 for only brief periods (5 min), followed by cooling to room temperature. These films were then quickly heated and drawn at a constant temperature of 140−155 °C, above the static melting temperature ( T m ≈ 135 °C). The stress/strain behavior from this state was found to be significantly influenced by both the prior melt temperature and the time needed for preparation of the initial morphologies. X-ray diffraction showed that the strain-induced crystallization from the melt had occurred during draw above a critical strain, corresponding to the draw ratio of ∼10. Differential scanning calorimetric data clarified that the rate of this crystallization is accelerated by the higher draw stress for the samples prepared at the higher prior-melt temperature. Combination of X-ray and calorimetric results also indicated that the samples drawn from the melt contain two kinds of crystals, i.e., highly chain-extended and oriented crystals having 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, gradually 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 could be successfully drawn from melt up to a maximum draw ratio of 45−50 at the optimum temperature of 150 °C. Such highly drawn films exhibited a tensile modulus of 55 and a strength of 0.95 GPa, respectively. For the solid-state drawing below T m, the stress/strain behavior of the samples molded at different temperatures were all identical and had a low draw ratio of ∼6. This suggests a difference in entanglements as they affect draw above and below the static melting temperature.