Mechanical Relaxations in Single Crystals of Polyethylene

Abstract

The dynamic mechanical properties of single crystals of linear polyethylene have been measured at temperatures between −190° and 133°C. The measurements were made with a free-oscillation torsion pendulum at a nominal frequency of 1 cps on mats of single crystals which had been annealed at various temperatures up to 133°C. Broad-line NMR and small-angle x-ray measurements were also made on samples of single crystals which had undergone the same annealing treatment. The alpha and gamma relaxations previously observed in melt-crystallized polyethylene were found to occur also in the single crystals. Some evidence was also found for two other relaxations, one occurring below the temperature of the gamma relaxation and the other above that of the alpha relaxation. These relaxations are not discussed in detail. In the original, unannealed crystals, the gamma relaxation is about one-third the magnitude of that in a slowly cooled, melt-crystallized sample and also occurs at a lower temperature: −142°C as opposed to −120°C. The alpha relaxation is more pronounced in the single crystals, having about twice the magnitude of that in the melt-crystallized sample, and also occurs at a lower temperature: 30°C as opposed to 45°C. Annealing at temperatures below 100°C has no effect on the relaxations. Upon annealing at progressively higher temperatures above 100°C, the gamma relaxation increases and the alpha relaxation decreases in magnitude and at the same time the relaxations move to higher temperatures. After annealing at 133°C the relaxations occur at the same temperature as in the melt-crystallized sample, but are smaller in magnitude. These changes in the relaxations occur at the same annealing temperatures as the increase in the lamella thickness and NMR mobile fraction. The hypothesis is advanced that the gamma relaxation is due to the stress-induced reorientation of defects within the lamellae, and the alpha relaxation to the reorientation of the folds at the surfaces of the lamellae. This hypothesis is consistent with the variation of the magnitude of the relaxations with annealing. The increase in the magnitude of the gamma relaxation is attributed to the generation of defects within the lamellae, either thermally or through the formation of dislocations and vacancies as the lamellae thicken. The magnitude of the alpha relaxation is inversely proportional to the lamella thickness, and its decrease upon annealing can be attributed directly to the reduction in the number of folds as the lamellae thicken. This hypothesis is substantiated by the effect of electron irradiation of a mat of single crystals, which reduces the magnitude of the alpha relaxation but leaves the gamma relaxation unchanged. The gamma relaxation in the single crystals of polyethylene is shown to be identical to a low-temperature relaxation previously reported in crystals of n-alkanes. Evidence is also presented that the gamma relaxation in melt-crystallized polyethylene is due to the same mechanism as that in the single crystals. This suggests that the gamma relaxation in the bulk polymer does not arise from a discrete amorphous phase, but from within the lamellar structure. It is also suggested that the alpha relaxation in the melt-crystallized polyethylene is due to the same mechanism as that in the single crystals.