Crystallization kinetics of metallocene type polypropylenes - Influence of molecular weight and comparison with Ziegler-Natta type systems

The crystallization kinetics from the melt of metallocene type isotactic poly(propylenes) having the same chain defect concentration and molecular weights ranging from 68480 to 288430 have been studied by differential scanning calorimetry. The crystallization rates and the variation of the rates with crystallization temperature follow a pattern that is basically independent of molecular weight. This result contrasts with the molecular weight dependence on the crystallization rate observed in linear polyethylene, random ethylene copolymers as well as other semicrystalline systems. Most significant is the fact that the metallocene poly(propylenes) show apparently significantly higher sigma(c) sigma(u) products than do the Ziegler type fractions of matched molecular weight and defect concentration. This difference can be interpreted as the metallocene type crystallites having higher effect on surface interfacial free energies than the Ziegler type, or can result from the two different chain types having different sequence propagation probabilities.