In situ SAXS analysis of extended-chain crystallization during melt-drawing of ultra-high molecular weight polyethylene

Phase development during melt-drawing of ultra-high molecular weight polyethylene (UHMW-PE) was analyzed by in situ small-angle X-ray scattering measurements using synchrotron radiation at the SPring-8. Films with different entanglement characteristics were prepared by solution blending of higher and lower MW samples with a viscosity average MW of 1.07 × 107 (higher) and 1.73 × 106 (lower), followed by compression molding at 180 °C. Independent of blend ratio, the strong streak attributed to extended-chain crystals (ECCs) appeared on the equator at the beginning point of the plateau stress region in the stress profile, which was one of the characteristic features of melt-drawing. A series of detailed analyses of these streaks suggested that two components with different dimensions were formed during melt-drawing: one was a precursory ECC, and the other was a mature ECC. These two ECC components grew synchronously at the beginning point of the plateau stress region for both films, independent of the entanglement characteristics of the film. However, the time of this synchronized growth was longer for the film with the lower MW component than for the film containing only the higher MW component. The resultant morphologies of the melt-drawn samples observed by transmission electron microscopy also reflected these characteristics. These results demonstrate that molecular entanglement characteristics dominate the unique crystallization mechanism that forms ECCs with different dimensions during melt-drawing of UHMW-PE.