Effect of chain entanglements on plastic deformation behavior of linear polyethylene

Samples of semicrystalline polyethylene in which the chain topology within the amorphous component was altered by using two-stage processing, including crystallization in a chain-extended fashion at high pressure in the first step, were produced, and their deformation behavior in the plane-strain compression was studied. The deformation and recovery experiments evidenced that the state of the molecular network governed by entanglements density is one of the key parameters controlling the response of the material on imposed strain, especially in the range of moderate and high strains. It influences markedly the shape of the true stress-true strain curve, changing the strain hardening modulus and the onset of strong strain hardening. The strain hardening modulus decreases while onset of strong strain hardening increases with a decrease of the entanglement density within amorphous component. Depending on the density of entanglements, PE samples demonstrated various amount of rubberlike recoverable deformation and permanent plastic flow. In the case of reduced concentration of entanglements the permanent flow was easier than in materials with higher entanglement density and could set in quite early, at relatively low strains, becoming a favorable deformation mechanism. As a result, the strong strain hardening was postponed to higher strains as compared to samples of equilibrium entanglement density. When the topology of amorphous component was modified to increase the network density, that network became stiffer with reduced ability of strain-induced disentangling of chains. Consequently, there was relatively less permanent flow and strain hardening begun earlier than in the reference material of unaltered chain topology.