Molecular Weight Dependency of Surface Free Energy of Native and Stabilized Crystallites in Isotactic Polypropylene

Two isotactic polypropylene samples were investigated to study the influence of molecular weight on the crystallization and meting behaviors via temperature- dependent small-angle X-ray scattering techniques. In a phase diagram of inverse lamellar thickness and temperature, the crystallization and melting behaviors can be described by two linear dependencies of different slopes and different limiting temperatures at infinite crystalline lamellar thickness. The slope of the crystallization line depends on the surface free energy of the just formed native crystallites, whereas that of the melting line is linked to the surface free energy of stabilized ones. The two polypropylene samples showed different crystallization lines and melting lines, indicating strong changes in surface free energies of the native as well as stabilized crystallites. Such changes are consequences of changes in molecular conformation during crystallization for samples with different molecular weights. Indeed, the low molecular weight sample crystallizes extensively into an extended-chain conformation, whereas the high molecular weight one ends up with normal folded-chain crystallites.