Strength and fracture of crystalline isotactic polypropylene and the effect of molecular and morphological parameters

Abstract

In semicrystalline isotactic Polypropylene the sensitivity to crack propagation can be lowered by the right choice of the molecular composition (atactic content, molecualr weight) and by a controlled production of a certain morphology. Coarse spherulitic morphologies produced by slow undercooling from the melt exhibit the lowest values in respect to yield or fracture stress and fracture toughness. This is due to the fact that plastic deformation is mainly concentrated on the small volume of the weak spherulite boundaries. An improvement of the properties can be reached in this type of morphology when a Polypropylene with a higher content of atactic polymer chains is used.

For temperatures under T _g the strength of a material was defined in this work as the product of fracture stress and fracture toughness. Optimum values of strength are measured for fine spherulitic Polypropylene produced by quenching from the melt, especially when the polymer has a relatively high moecular weight. In this type of morphology a dense craze formation in front of the crack tip provides high energy dissipation. Thus crack propagation under static load is rendered more difficult even at low temperatures.