Identification of structural processes in deformation of oriented polyethylene

Abstract

The investigation is concerned with the relation between changes in the submicroscopic structure, as revealed by low angle X-ray scattering in combination with the usual wide angle X-ray diffraction, and changes in the macroscopic sample dimensions during the deformation of oriented low density polyethylene. The samples examined are mainly drawn and rolled sheets possessing a double crystal texture, with a limited additional study on a drawn sample with fibre symmetry and on recently discovered single texture specimens. The deformations include tension and compression along selected sample directions applied mostly at room temperature, but also at various elevated temperatures. The salient feature of most of these experiments is the identity of the macroscopic strain and the changes in the submicroscopic periodicity along the direction in which the sample has been initially oriented. Even when this identity is not obeyed, as for deformation at the highest temperatures, a proportionality between the quantities concerned is always maintained. It is demonstrated how the changes in the structural periodicity can be subdivided into a rotation of unaltered crystallites, interpreted as interlamellar slip, into a change in chain inclination within the crystallites, interpreted as intralamellar slip, and into a change in the separation of the crystallites which includes the extension or compression of interlamellar amorphous material. It is shown that the relative contributions of each of these three effects is a function of the temperature of the deformation, the sample type and the type of stress applied. The results are evaluated and discussed in terms of existing conceptions of an oriented polymer and are related to earlier findings on this subject. It is pointed out in particular that the samples in question represent a very simple mechanical system: a series coupling of the individual structural processes involved suffices to describe the response of the sample to externally imposed stress. The identity relation between changes in structure and macroscopic sample dimensions is also revealed by swelling experiments. This, in addition to equating changes in lamellar separation with changes in sample dimension, also provides some definitive information on the location of the swelling agent.