Temperature and molecular weight dependence of fracture behaviour of polypropylene films

The effect of temperature on fracture behaviour of isotactic polypropylene films has been studied on two PP samples of molecular weights M_W=270 kg mol⁻¹ and M_W=150 kg mol⁻¹, using the Essential Work of Fracture method. Two ductile-brittle transitions as a function of temperature are in evidence at respectively 10 and 60 °C. The former transition occurs for the highest molecular weight and the latter one for the lowest molecular weight.

Three processes are involved in the temperature effect on PP toughness: (1) The decrease of yield stress with temperature according to Eyring’s law; (2) The role of cooperative motions in the amorphous phase: the ductile-brittle transition of the sample of highest M_W corresponds to the glass transition; and (3) The role of the mobility of the crystalline phase: the ductile-brittle transition of the sample of lowest M_W corresponds to the C transition.