The effect of density on the properties of high molecular weight polyethylenes

Ultrahigh molecular weight polyethylene has been crystallized at various pressures (0–330 MN m−2) to material of a wide range of specific gravities (0.928–0.967). Tensile measurements illustrate that the properties of these materials are dependent on specific gravity (sg), in that the yield stress increases linearly with sg. Low sg (0.928–0.944) materials exhibit higher yields stress than low molecular weight materials of the same sg but the opposite is true for higher sg material. Low sg material also extends uniformly after yielding while above a sg of 0.944 necking becomes more marked. Orientation hardening is observed with all these materials above 200% extension and is more marked than with low molecular weight polymers. Differential scanning calorimetry (DSC) analysis of the pressure crystallized materials indicate that in the range 200–400 MN m−2 two phases are present, i.e., chain‐folded and chain‐extended lamellas. Increasing or decreasing the pressure changes the crystalline component to chain‐extended or chain‐folded morphologies, respectively. The reorganization of the crystalline components on drawing is discussed.