Real-Time X-ray Diffraction Study on Two-Stage Drawing of Ultra-High Molecular Weight Polyethylene Reactor Powder above the Static Melting Temperature

The polyethylene for the study was ultra-high molecular weight polyethylene (UHMW-PE) reactor powder extrusion drawn to a low draw ratio (DR) of ∼6 (first-stage draw). The structural change during the second-stage tensile draw of the extrudate has been studied above the static melting point by real-time wide-angle X-ray diffraction (WAXD). Information on the phase structure was also obtained by WAXD as a function of sample DR, temperature (T d), and applied tensile load and by retractive stress measurements. Upon heating the extrudate with the fixed ends in an X-ray high-temperature chamber, part of the initially orthorhombic crystals transformed into the hexagonal crystalline and amorphous phases at ≥150 °C, indicating that the retractive stress was not homogeneously distributed within a drawn sample. Such a transformation temperature increased with sample DR. When draw was initiated at a constant T d of 150−155 °C, the chain orientation in both the hexagonal and the amorphous phases rapidly increased, and these phases transformed into the orthorhombic crystals. Even when the T d was raised to 160 °C during drawing, the orthorhombic phase was predominant with no significant increase in the amorphous component. These results show that the major deformation proceeded in a highly crystalline state where the orthorhombic crystalline phase was predominant, giving an efficient draw in terms of molecular DR, at least up to 155 °C, and reflecting the characteristics of the UHMW-PE reactor powder used in this work. Such deformation behavior in the high-temperature range is markedly different from that previously reported for single-crystal mats and gel and melt-crystallized films of UHMW-PE, which produced poor mechanical properties.