Structure, mechanical properties and modelling of polypropylene for different degrees of crystallinity

Abstract

The objective of this study, dealing with the physico-chemical and mechanical characterisation of isotactic polypropylene, is to develop at room-temperature, a pre-necking micromechanical behaviour model. Spherolites in the injected samples are of 40 μm mean diameter and do not have preferential orientation. This is also true for the monoclinical-type crystal structure which induce an isotropic mechanical behaviour on the macroscopic scale. The WAXD measurements coupled with tensile tests show that the crystallinity does not evolve in the considered strain range. Polypropylene mechanical behaviour is of elastoviscoplastic type with a viscoelastic characteristic even for small strain. The study of several specimens having undergone annealing has shown that polypropylene mechanical behaviour, as well as the appearance of damage, are greatly dependent on the crystallinity and the thickness of the crystalline lamellae. Tensile tests using the dip test technique and loading-unloading were also carried out. They highlighted the effect of internal stress on the viscoplastic behaviour and indicate that the yield stress range decreases with tension. At the end of this study, we propose a pattern to represent the specific morphology of the polypropylene. In distinguishing the behaviour associated to amorphous and crystalline phases in a generalised self-consistent scheme, we simulate the elastic properties of the polypropylene, as functions of its crystallinity.