Molecular deformation mechanisms in polyethylene.

Abstract

This work is concerned with details of the molecular changes caused by deformation and also establishes any conformational differences between linear and branched polyethylene before, during and after deformation. Four blends of isotopically labelled polymers of different types, rapidly quenched from the melt, have been studied by Mixed Crystal Infra-red Spectroscopy and Small Angle Neutron Scattering (SANS), in order to clarify any differences in the molecular basis of drawing behaviour and in the initial labelled chains conformation. For all sample types, the neutron scattering results suggest that adjacent folding is not the major type of chain folding here. This point is confirmed by our infrared results where most of the crystal stems contributing to the doublet components are in groups of only 3 to 4 adjacent labelled stems. Differences in initial conformation between the linear and copolymer samples were highlighted by both SANS and FTIR techniques. The evolution of the radius of gyration as a function of molecular weight following the relationship Rg ∝ Mw ∝ determined from the SANS data, is different for linear and copolymer sample types, suggesting a more compact arrangement as the molecular weight of the copolymer DPE guest molecules increases. This was found consistent with the infrared results, where results from both curve fitting and the simulation of the infrared CD2 bending profiles show that the number of small groups of adjacent labelled stems is significantly larger when the DPE guest is a copolymer molecule. Our comparative studies on various types of polyethylene lead to the conclusion that their deformation behaviour under drawing has the same basis, with additional effects imputed to the presence of tie-molecules and branches. Three major points were identified in this thesis. The changes produced by drawing imply (1) the crystallisation of some of the amorphous polymer and the subsequent orientation of the newly formed crystals, (2) the re-orientation of the crystalline ribbons and (3) the beginning of crystallite break-up. However, additional effects were observed for the high molecular weight linear sample and the copolymer sample and were attributed, respectively, to the presence of tie- molecules and of branches. It was concluded that both the tie-molecules and the branches are restricting the molecular movement during deformation, and that the branches may be acting as "anchors".</p