Transient crystallization during drawing from ultra-high molecular weight polyethylene melts having different entanglement characteristics

The relationship between entanglement characteristics and transient crystallization during drawing from ultra-high molecular weight polyethylene (UHMW-PE) melts is discussed, based on a combination of in situ X-ray measurement and stress profile analysis. Films having different entanglement characteristics were prepared by solution blending of higher and lower MW samples having a viscosity average MW of 1.07 × 107 (higher) and 1.73 × 106 (lower), followed by compression molding at 180 °C. Stress profiles recorded at 155 °C above the melting temperature of 135 °C exhibited a plateau stress region, whose stress level was lower for the film prepared with more of the lower MW component. With drawing, an amorphous scattering gradually concentrated on the equator. Beyond the beginning point of the plateau stress region, such amorphous scattering abruptly disappeared and crystallization into a transient hexagonal phase occurred simultaneously. As soon as this hexagonal phase appeared, it rapidly transformed into an orthorhombic phase for the film exhibiting higher plateau stress. In contrast, the film exhibiting lower plateau stress exhibited a gentle transformation and a resurgence of the hexagonal phase in the later stage of drawing. These results demonstrate that “entanglement phase separation” proceeds during melt-drawing of UHMW-PE.