Structure development upon melt drawing of ultrahigh molecular weight polyethylene: Effect of prior thermal history

The effect of thermal history on the melt drawing of ultrahigh molecular we ight polyethylene (UHMWPE) reactor powder was studied. The samples for draw ing were prepared by compression-molding of reactor powder at various tempe ratures above the melting point (T-m). The drawing temperature (T-d) was 15 0 degrees C. It was found that the maximum achievable draw ratio at the opt imum T-d decreased from 60 to 23 when the prior-melt temperature increased from 160 to 230 degrees C. The highly drawn films exhibited tensile moduli less than or equal to 58 GPa and strength less than or equal to 0.95 GPa at room temperature. Scanning electron microscopy (SEM) observations of the d rawn films, etched by fuming nitric acid, revealed a characteristic "shish kebab" structure, as reported. Consistent with such morphology, differentia l scanning calorimetry (DSC) showed double melting endotherms at 134 and 14 3 degrees C, corresponding to the "kebob" and "shish" components, respectiv ely. The crystallinity evaluated by the total heat of fusion from the doubl e peaks increased steadily with the draw ratio, and the increase was more r apid 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 thei r crystal sizes were also significantly influenced by the prior-melting tem perature and the elongation ratio. The efficiency of the draw, evaluated fr om the fraction of the "shish" component and the tensile properties vs draw ratio, was also interpreted from the differences of the prior-melt prepara tion conditions. The results suggest that the different level of entangleme nt formation, which was associated with the scale of segmental diffusion, a ffected significantly the resultant structure and properties.