Bimodal crystalline orientation in polypropylene: Mechanism of formation

Melt-spun polypropylene fibers have two sets of crystallites, each with a different preferred orientation. A probable kinetic mechanism of nucleation and growth of the two sets of crystallites, based on an analysis of crystallization in oriented polymers, is proposed. Experimental data on the fractions of these two sets of crystallites in as-spun fibers and the effect of subsequent drawing or thermal annealing on the relative amounts of these two sets are presented.