PLASTIC-DEFORMATION IN POLYETHYLENE CRYSTALS STUDIED BY MICROINDENTATION HARDNESS

The microhardness H of films of oriented polyethylene (PE) prepared by gelation from semidilute solution using ultra-high-molecular-weight PE and of films of isotropic melt-crystallized linear PE in a wide range of molecular weights has been determined. Comparison of a thermodynamical approach, which takes into account the dependence of the experimental crystal hardness H(c) upon the crystal thickness l(c) with Young's dislocation model for yield, gives good agreement for the oriented films. In case of isotropic PE films, only thermodynamic predictions can explain the experimental H(c) data. The temperature dependence of H(c) is also discussed in the light of both models. It is found that the softening coefficient of melt-crystallized PE samples depends not only on the thermal lattice expansion but also on the thermal activation of dislocations arising from the finite thickness of the crystals.