Crystallization and melting isotactic polypropylenes

The measurement of crystallization and melting behaviors under constant rates of cooling and heating and the Avrami's analysis of isothermal crystallization were carried out for various metallocene-(MET-PP) and Ziegler-Natta-catalyzed (ZNPP) random polypropylene copolymers with various ethylene contents. Both the melting point T-m and the crystallization temperature T-c decrease with increasing ethylene content. Compared at the same ethylene content, both are lower for MET-PP because of the higher uniformities of stereoirregular bonds and copolymerization bonds. T-m and T-c. show a linear correlation, and compared at the same T-c, T-m of MET-PP is lower than that of ZN-PP. This is because MET-PP has narrower distributions of isotactic sequence length and hence of lamellar thickness than ZN-PP. The heat of fusion and the latent heat of crystallization decrease with ethylene content. At the same ethylene content, both are lower for MET-PP, owing also to the higher uniformities of stereoirregular bonds and copolymerization bonds. From the Avrami's analysis, it is assumed that there is little difference in crystallization modes of both catalyst PPs. (C) 2002 Wiley Periodicals, Inc.