Time-resolved X-ray scattering studies of creep in oriented UHMWPE films

The change in the crystalline structure of oriented melt-crystallised and gel-cast UHMPWPE (Stamylan) films of various draw ratios during creep upon constant load has been followed in real time using synchrotron X-ray diffraction techniques at the synchrotron radiation source ANKA (Angstroemquelle Karlsruhe) at Forschungszentrum Karlsruhe (Germany). The shift of (002) Bragg reflections with time of loading and change in their halfwidth are observed for all the samples investigated. The analysis of the data shows a difference in the behaviour of loaded gel-cast and melt-crystallised films. The longitudinal crystallite sizes in the former grow with time, while in the latter they become smaller. Besides, the splitting of a (002) profile in WAXS patterns of melt-crystallised samples (including unstressed samples) was revealed. Several possible causes of this splitting are suggested. One of them is the co-existence of strained crystallites in monoclinic and orthorhombic crystalline modifications. served phenomena is discussed in terms of coherent disposition of crystallites in microfibrils, one-dimensional diffraction on taut-tie molecules, probable pulling-out of the molecules from crystallites upon creep and inhomogeneity of stress distributions.