REAL-TIME IN-SITU X-RAY-DIFFRACTION STUDY OF POLYETHYLENE DEFORMATION

A range of polyethylenes has been studied by X-ray diffraction during deformation. Changes in the crystalline orientation have been related to the load-extension curve. This was altered by changing the molecula r weight, concentration of branches and strain rate. Crystalline orien tation was first observed just after the maximum load, before any visi ble signs of yield. At about the same point, the monoclinic phase was observed in specimens that were subjected to a higher load and it disa ppeared when stress softening occurred. Initially the (110) orthorhomb ic reflection oriented to give four maxima, which implies tilted polym er chains and lamellae. Subsequently the (110) concentrated on the equ ator showing chains oriented parallel to the draw direction. The trans ition from tilted to fibre orientation occurred at different points on a nominal stress-strain curve but at approximately the same local str ain, as estimated from the change in sample dimensions.