Viscoelasticity and viscoplasticity of semicrystalline polymers: Structure-property relations for high-density polyethylene

Observations are reported on two commercial grades of high-density polyethylene (HDPE) in uniaxial tensile tests, relaxation tests, creep tests and cyclic tests with a strain-controlled deformation program. Constitutive equations are derived for the viscoelastic and viscoplastic responses of semicrystalline polymers at three-dimensional deformation with small strains. A polymer is modeled as a two-phase continuum consisting of a crystalline skeleton and an amorphous phase treated as a transient network of chains. Its viscoelastic response is associated with thermally activated rearrangement of strands in the temporary network. The viscoplastic behavior reflects fine and coarse slip of lamellar stacks and sliding of junctions between chains in the network. Adjustable parameters in the stress-strain relations are found by fitting the experimental data. The study focuses on the effect of molecular weight of HDPE on its mechanical properties.