Crystal morphology of strained ultra high molecular weight polyethylenes

Simultaneous tensile testing and wide and small angle X-ray diffraction (WAX and SAX) have been used to compare three different ultra high molecular weight polyethylenes (UHMWPE), two linear and one crosslinked, with low density polyethylene (LDPE) and high density polyethylene (HDPE), with the intention of identifying any behavioural characteristics displayed uniquely by the ultra high molecular weight materials. Two strain rates were used, 0.025/s and 0.125/s. The results demonstrate that the original morphology of the polymer has a controlling influence on the tensile behaviour. With spherulitic HDPE, increasing strain causes the development of a monoclinic crystal population and cavitation occurs at a strain of around 0.1, whereas LDPE shows little morphological change up to fracture. In comparison, the two linear UHMWPEs and the cross-linked UHMWPE, which are not spherulitic, show the development of a monoclinic structure with strain, which is similar to HDPE, but with no cavitation, which is a notable difference with HDPE. Cross-linking appears to have no significant effect on the stress-strain response of these materials. With UHMWPEs, a crystal perfection process, rather than cavitation, is associated with yielding. The WAX data for all of the linear polymers, HDPE and UHMWPE, shows that the loss of the orthorhombic phase and the creation of the monoclinic phase are approximately linear with strain. Lamellae and lattice alignment, judged by the degree of ovality of the SAX plot, is shown to be a useful discriminator of morphological change at low strain.