Non-linear long-term tensile creep of poly(propylene)/cycloolefin copolymer blends with fibrous structure

The tensile deformation of materials with Poisson's ratio smaller than 0.5 generates an additional free volume, which means that tensile creep under constant stress and temperature is a non-iso-free volume process. Fractional free volume rising proportionally to the creep strain accounts for a continuous shortening of retardation times. To, account for this effect, internal time has been introduced which is related to a hypothetical, pseudo iso-free-volume state. The shift factor along the time scale in the time-strain' superposition is not constant for an isothermal creep curve, but rises monotonically from point to point with the elapsed creep time. The reconstructed compliance dependencies obtained for various stresses approximately obey the time-strain superposition thus forming a generalised creep curve. A routinely used empirical equation. has been found suitable to describe the effects of time and stress on compliance of parent polymers and their blends. The previously proposed predictive format for the time-dependent compliance of polymer blends has been;found applicable also to poly(propylene) (PP)/cycloolefin copolymer (COC) blends with fibrous morphology. As COC shows a tendency to form fibres in a PP matrix, the mixing rule customarily used for fibre composites has been found more appropriate for injection moulded specimens than the equivalent box model for isotropic blends. The predicted compliance, curve for a pseudo iso-free-volume state can be transformed into a real curve for a selected stress sigma (in the interval up to the yield stress).