Structural and kinetic factors governing the formation of the gamma polymorph of isotactic polypropylene

The molecular, thermodynamic, and kinetic factors that govern the formation and concentration of the alpha and gamma polymorphs in metallocene-catalyz ed isotactic poly(propylenes) have been studied with a set of polymers that have a wide range in molecular weight and defect contents. With these poly mers it was possible to investigate the influence of molecular weight on ga mma formation at a tired defect concentration, as well as the role of the d efect concentration at constant molecular weight, The major experimental te chniques used were wide-angle X-ray scattering and differential scanning ca lorimetry complemented by microscopy. From these studies the role of chain microstructure, the crystallization temperature, and the thermodynamic and kinetic requirement for the formation of the gamma form could be establishe d in more quantitative detail than heretofore. A particular important findi ng was the fact that at fixed defect concentration the fractional content o f the gamma polymorph goes through a maximum with crystallization temperatu re. The results that were obtained establish a quantitative framework withi n which the underlying bases that lead to formation of the gamma form, and its unique crystalline structure, are discussed.