Ultrasonic measurements of the mechanical relaxations and complex stiffnesses in oriented linear polyethylene

The five complex stiffnesses of highly oriented linear polyethylene produced by hydrostatic extrusion and die drawing have been determined by ultrasonic measurements. The results are compared with the elastic stiffnesses for crystalline polyethylene calculated theoretically. The development of anisotropic mechanical behaviour with draw ratio is discussed in terms of present structural understanding of highly oriented polyethylene. Although the very high stiffnesses obtained at the highest draw ratios are attributed to increasing crystal continuity, it is noted that the development of anisotropy in terms of low-temperature ultrasonic behaviour can be predicted to a good approximation by the reorienting unit aggregate model. This surprising result suggests that the overall orientation may still be the key parameter at low temperatures and high frequencies where there is no molecular mobility in the structure.