Deformation of polyethylene

• Main Author: Butler, M. F.
• Published: University of Cambridge 1996
• Subjects: 668.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597170

In order to characterise deformation as completely and unambiguously as possible it is desirable to eliminate the possibility of sample relaxation by performing measurements <I>in-situ</I>, during deformation. The bulk of the thesis describes a comprehensive series of investigations into the mechanical behaviour of a range of oriented and unoriented commercial grade polyethylenes deformed in tension and compression over a range of temperatures. High intensity synchrotron X-rays and fast electronic area detectors were used to enable the two-dimensional wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) patterns, which reveal information about the molecular and lamellar deformation respectively, to be measured simultaneously during deformation. In addition, the load-extension curve was measured at the same time as the X-ray scattering patterns. Sample relaxation was completely eliminated and for the first time the micro- and macroscopic deformation was precisely and unambiguously correlated. The use of a range of samples, with varying molecular weights, branch densities, branch types, lamellar thicknesses and percentage crystallinities, enabled a study to be made of the influence of microstructural variables (as well as experimental conditions such as deformation temperature) on the mechanical properties. A study was also performed on the mechanical properties of thin solution-cast PE films made from the same commercial grades used for the bulk samples. Crazing mechanisms were observed using optical and transmission electron microscopy. It was found that that results could be rationalised in terms of the degree of entanglement of the chains and the ease by which these entanglements were overcome during deformation. Increasing the entanglement density by increasing the molecular weight or by the incorporation of short chain branches increased the toughness of the films. As for the bulk samples, branch length was unimportant.