Influence of annealing on the microstructural, tensile and fracture properties of polypropylene films

The influence of annealing temperature on the fracture properties of iPP films (one homopolymer and two propylene–ethylene block copolymers) is presented. The fracture behaviour is studied by means of the Essential Work of Fracture (EWF) procedure, and is complemented by the study of the effect of thermal treatment on tensile properties and microstructure, using differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). It is shown that the initial metastable phase of quenched iPP films, widely known as smectic, transforms gradually into the monoclinic form as the annealing temperature is increased, resulting in an important improvement of the tensile properties, whereas the fracture parameters have different evolutions depending on the ethylene content. The reasons for a decrease in the essential work term and an increase in the plastic term as the crystal perfection grows are discussed on the basis of the microstructural changes of the crystalline phase and the smectic–monoclinic strain-induced phase transformation.