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.

The effect of thermal history on the melt drawing of ultrahigh molecular weight polyethylene (UHMWPE) reactor powder was studied. The samples for drawing were prepared by compression-molding of reactor powder at various temperatures above the melting point (T m). The drawing temperature (Td) was 150°C. It was found that the maximum achievable draw ratio at the optimum Td decreased from 60 to 23 when the prior-melt temperature increased from 160 to 230°C. The highly drawn films exhibited tensile moduli e58 GPa and strength e0.95 GPa at room temperature. Scanning electron microscopy (SEM) observations of the drawn films, etched by fuming nitric acid, revealed a characteristic "shish kebab" structure, as reported. Consistent with such morphology, differential scanning calorimetry (DSC) showed double melting endotherms at 134 and 143°C, corresponding to the "kebob" and "shish" components, respectively. The crystallinity evaluated by the total heat of fusion from the double peaks increased steadily with the draw ratio, and the increase was more rapid for the samples that were priormelted at a higher temperature and/or for a longer time. The formation of "shish" and "kebob" components and their crystal sizes were also significantly influenced by the prior-melting temperature and the elongation ratio. The efficiency of the draw, evaluated from the fraction of the "shish" component and the tensile properties vs draw ratio, was also interpreted from the differences of the prior-melt preparation conditions. The results suggest that the different level of entanglement formation, which was associated with the scale of segmental diffusion, affected significantly the resultant structure and properties.

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Ultra-oriented high density pOlyethylene (HDPE)

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