TIT# Nanostructure evolution of isotropic high--pressure injection--molded
     UHMWPE during heating
AUT# Wang, Zhigang; Hsiao, Benjamin S.; Stribeck, Norbert; Gehrke, Rainer;
SOU# Macromolecules (2002), 35(6), 2200-2206
LOC# xv068
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ABS# Ultra high molecular weight polyethylene (UHMWPE) is injection molded
     under high pressure and studied by ultra small-angle X-ray scattering
     (USAXS) during melting in a time-resolved synchrotron radiation
     experiment. Results concerning melting and recrystallization of
     crystalline lamellae are compared to data obtained by differential
     scanning calorimetry (DSC). USAXS analysis reveals a coupled process of
     melting and crystallization which is not accompanied by external heat
     flow. 9 isotropic samples differing in molecular weight and molding
     pressure are heated at a rate of 5°C/min. 2D USAXS images integrate
     over temperature intervals between 3°C and 7°C. The materials are
     considered two-phase semicrystalline polymers. Scattering curves obtained
     by azimuthal averaging are transformed to interface distribution
     functions (IDF) which are perfectly fitted by a nanostructural model
     comprising an ensemble of thick, uncorrelated layers (50 nm thickness)
     and stacks of short-range correlated crystalline lamellae (20 nm).
     Crystalline layers are identified from their narrower layer thickness
     distribution and their melting behavior. After eliminating the scattering
     effect of amorphous layers, a composite crystallite thickness
     distribution is obtained. Its variation is studied as a function of
     temperature, molecular mass and molding pressure. In DSC thermograms
     samples prepared at high pressure exhibit a single strong melting peak,
     whereas the other samples show an additional melting peak at lower
     temperature. This might lead to the conclusion that the high-pressure
     samples predominantly contain extended chain crystals. USAXS shows that
     high-pressure materials contain considerable amounts of imperfect thin
     crystal lamellae that melt at lower temperature, while thick lamellae are
     formed. With low-pressure samples this coupled process of nanostructure
     transformation during annealing is found to be negligible.