MORPHOLOGY AND MELTING BEHAVIOR OF NASCENT ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE

The nascent morphology of UHMW PE exhibits high melting point, high cr ystallinity, and increased WAXS line breadth relative to samples forme d by melt crystallization. Different empirical relationships between c rystal size and melting point are observed for nascent and molded samp les. This differentiation is removed following nitric acid treatment o f the nascent flake. Solid-state annealing behavior is differentiated by several regimes. Regime I is characterized by increasing crystallit e dimensions and crystallinity at low annealing temperatures. Regime I I[a] and II[b] is identified by double melting in DSC scans of molding s and nascent flake, respectively. The double melting is due to partia l melting with incomplete recrystallization. Regime II[a] of moldings is differentiated from Regime II[b] of flake by an increase in melting point of the higher melting endotherm. Within Regime II[b], the parti al melting of the nascent structure is sensitive to the distribution o f morphological stability. Regime LII is initiated at annealing temper atures approaching the zero heating rate melting point, and shows melt ing kinetics by DSC or time-resolved WAXS using synchrotron x-ray radi ation. The superheat, partially associated with Regime III behavior, i s sensitive to morphological heterogeneity and annealing history. Morp hological models are discussed which highlight the role of noncrystall ine regions and melting kinetics on the melting behavior of nascent fo rm crystallinity. (C) 1998 John Wiley & Sons, Inc.