Non-linear tensile creep of polypropylene: Time-strain superposition and creep prediction

In most practical applications, isothermal compliance of polymeric materials depends on both time and stress so that their non-linear viscoelastic behavior is of primary importance. A concept is adopted that the non-linearity of tensile creep is mainly brought about by the strain-induced increment of the free volume (in materials with Poisson ratio smaller than 0.5). Consequently, the traditional stress-strain linearity limit can be viewed as an artificial limit related to limited accuracy of the measurements at low stresses and strains. The internal time-tensile compliance superposition of non-linear creep data is applied to construct a generalized compliance curve, which corresponds to a pseudo iso-free-volume state. The superposition of compliance curves obtained at different stresses requires shift factors along the time axis calculated a priori for individual data points. As the generalized curve can be generated by means of short-term creeps, the proposed procedure offers essential savings of experimental time. A most practical outcome of the outlined format is that the generalized dependence can be employed for predicting the real time-dependent compliance for any stress in the range of reversible strains. The results indicate that the compliance of PPs decreases with their crystallinity, while their creep rates are almost identical. Only rubber-toughened PP does show a slightly higher creep rate, which is attributed to the 'softening' effect of rubber particles in the PP matrix.