High-Density Polyethylene/Cycloolefin Copolymer Blends, Part 2: Nonlinear Tensile Creep

Viscoelastic behavior of most polymeric materials is nonlinear over a major portion of the interval of their response to external forces. The phenomenological theory of viscoelasticity, based on the assumption that molecular (segmental) motions are controlled by available fractional free volume f, was found adequate for description of the nonlinear tensile creep, implementation of the time–strain superposition and prediction of the nonlinear creep of studied blends. As f of thermoplastics with Poisson’s ratio smaller than 0.5 rises proportionally to tensile strain, advancing creep accounts for shortening of retardation times. Consequently, the shift factor along the internal time scale in the time–strain superposition is not constant for a creep curve, but monotonically rises with the elapsed creep time. Compliance curves for various stresses obey fairly well the internal time–strain superposition forming a generalized compliance curve related to an iso-free volume reference state. The predictive format for the blend compliance is based on the parameters characterizing the creep of parent polymers, data on the phase structure of blends obtained from the two-parameter equivalent box model and modified equations of the percolation theory. Applicability of the proposed format is demonstrated on a series of blends of high-density polyethylene with creep-resistant cycloolefin copolymer.