Stress relaxation and time/temperature superposition of polypropylene fibres

Abstract

The viscoelastic properties of polypropylene fibres are investigated in a single-step stress relaxation test for small strains at temperatures between −50 and + 30 °C, thus well encompassing the glass transition temperature. For the analysis of the curves a formal two-component (TC) model is applied, which comprises an elastic and a viscoelastic component, the time-dependence of which is described by a relaxation function on the basis of a cumulative log-normal distribution; the shape of this was found to be independent of temperature. The component moduli, as well as the characteristic relaxation time, show pronounced temperature-dependencies. Above the transition temperature, at ca. −15 °C, they all follow Arrhenius relationships, with activation energies which lie between −10 and −30 kJ mol−1. The consistency of the analytical model is checked against the results of dynamic extensional tests, which furthermore enable us to identify the process as the α-transition in isotactic polypropylene. Evaluation of the properties of the components of the TC model in relation to the polymer morphology suggests that the relaxation process resides in the restricted amorphous phase of the material.