Time-resolved small angle X-ray scattering and wide angle X-ray diffraction studies on premelting during annealing of gelation crystallized ultrahigh molecular weight polyethylene films

Abstract

The phenomenon of melting-recrystallization during annealing below an equilibrium melting temperature (141.6°C) of gelation crystallized ultrahigh molecular weight polyethylene (UHMWPE) films has been investigated by means of time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). Isothermal annealing below 130°C shows that the SAXS long period (periodic distance of successive lamellae) increases, then subsequently levels off with increasing annealing time. At 140°C, the long period increases from 118 to 690 Å without any sign of levelling off. This type of increase in long period has been customarily attributed to lamellar thickening associated with solid state diffusion of intracrystalline chains. Recent synchrotron radiation studies on annealing of single crystal mats of conventional linear polyethylene demonstrated that melting-recrystallization is responsible for the increase in long period. The present time-dependent WAXD studies show a gradual decrease in crystallinity for some initial periods of annealing at 137°C, which slows down with elapsed time. At 140°C, the terminal melting occurs after 15 min. We propose that the increase in long period during annealing of this gelation crystallized UHMWPE near its melting temperature is due to the melting of successive lamellae rather than lamellar thickening.