Time resolved WAXS/SAXS observations of crystallisation in oriented melts of ultra high molecular weight polyethylene

Abstract

Ultra high molecular weight polyethylene (UHMWPE) has been drawn in the melt state at 140, 145 and 150 °C at extension rates ∼1 s−1 while simultaneously recording two dimensional SAXS and WAXS with a time resolution of 0.1 s. The first observable crystallisation is mainly in the orthorhombic form at a level of about ∼1wt%. At higher draw ratios additional crystallisation is in the hexagonal form up to ∼10wt%. The crystallisation is accompanied by strong SAXS equatorial scatter with maxima at ∼25 nm period; in some cases meridional maxima are also visible at ∼120 nm. Substantial crystallisation occurs on subsequent cooling to 130 °C, accompanied by strong meridional maxima of narrow lateral width. The observed crystal forms are consistent with a temperature–strain phase diagram, favouring hexagonal at higher strains. There are indications that the thermodynamic orthorhombic to hexagonal transition Ttr is above 150 °C so that all the observable hexagonal structures are metastable. The initial orthorhombic crystals are associated with the high molecular weight tail and provide the strain hardening to enable the formation of subsequent hexagonal crystals. The equatorial SAXS lobes are interpreted in terms of lateral density fluctuations that are associated with an arrangement of columns of oriented chains comprising both orthorhombic and hexagonal structures. The columns are embryonic shish structures that on cooling nucleate kebab overgrowths.