Structure Development upon Melt Drawing of Ultrahigh Molecular Weight Polyethylene: Effect of Prior Thermal History

Abstract

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 (Tm). 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 ≤58 GPa and strength ≤0.95 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 prior-melted 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.