Melting behavior of strained crystals of ultra-high molecular weight polyethylene with and without solvent

The origin of the gel formation in dilute solutions of ultra high molecular weight polyethylene (UHMWPE) is the subject of this thesis. At low heating rates v, 0.5 ;Fraction (III) is stable in the presence of a solvent. However, its concentration in the sample can be made to vary between 5 and 25% by the appropriate choice of the solvent and thermal history. The network is stabilized during the dissolution of fraction (II) in decalin while it is reduced if the solvent is trichlorobenzene. Other thermal histories minimize the effect of solvent on the partition of the crystals between fractions (II) and (III). Crystallization in a temperature gradient is effective in diminishing fraction (III) and increasing fraction (II). A hypothesis to explain the high crystallinity of gels obtained by quenching is proposed.;The metastable states obtained during dissolution do not revert to the equilibrium state when the solvent is evaporated. Mechanical properties such as drawability and also the melting trace of the dried gels are affected by the solvent-induced changes in the melting pattern. The amount of fraction (III) appears to be a good test of drawability.;The dissolution traces at low v provide a much-needed quantitative characterization of UHMWPE gels, the crystals formed on quenching having a dissolution trace distinguishable from those grown isothermally or under shear.;The crystallinity found including fraction (III) (0.84-0.94) upgrades the calorimetric crystallinity to the values found by techniques not involving melting. Arrested melting which occurs between fractions (II) and (III) reveals the existence of a network whose cohesive junctions are unmelted and stabilized by strain. The amount of fraction (III) which is about 20% in the nascent sample increases to about a third of all crystals in a sample submitted to cycles of slow melting and slow crystallization.;Since fraction (III) results from a general process of melting, its occurrence in other polymers is expected and indeed has been found in preliminary experiments on poly-4-methylpentenel (P4MP1) and polypropylene (PP). (Abstract shortened by UMI.).