Physical and mechanical properties of polypropylene fractions

A large-scale fractionation of polypropylene according to molecular weight as well as to crystallinity has been carried out. The fractions obtained differed widely in properties. A fraction of low crystallinity, known as a 'stereoblock copolymer', possessed rubberlike properties. Evaluation of the fractions and of mixtures thereof showed that yield stress, dynamic modulus, drop in dynamic modulus at the transition point, mechanical damping and hardness depend only on the degree of crystallinity, whereas impact strength and elongation at break also depend on molecular weight and molecular weight distribution. A high impact strength requires a high molecular weight and a narrow molecular weight distribution. A correlation was found between tensile impact strength and loss factor at the glass transition point. The morphological structure of the various fractions is described.