Characterizing and modeling the tensile deformation of polyethylene: The temperature and crystallinity dependences

Abstract

For various polyethylenes at ambient and elevated temperatures, tensile deformation was characterized by measurements of true stress-true strain curves at constant strain rates, the determination of the elastic and plastic part of strains, and registrations of the stress relaxation at fixed strains. Some peculiar features show up: (i) The yield point is associated with a drop in the stiffness rather than an onset of plastic flow. (ii) The elasticity reaches a plateau at a temperature and crystallinity invariant critical strain (epsilon(H) approximate to 0.6). (iii) Moduli as derived from the stretching curve can be strongly modified by viscous forces. A recently introduced model treats the stress as arising from three contributions, rubberlike forces originating from the stretched network of entangled amorphous chains, forces transmitted by the skeleton of crystallites, and viscous forces described by Eyring's equation. Adjustment of the measured data to the model provides a decomposition of the stress in the three parts and thus allows an analysis of the effects of temperature and crystallinity.