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.

Crystallization and Melting of Oriented Parent - Daughter Lamellae in Sheared Isotactic Poly(propylene)

Alpha isotactic poly(propylene) (α-iPP) exhibits a form of lamellar branching that is unique among semicrystalline polymers, where the branches have a distinct orientation relationship with the original crystalline lamellae. This is termed a parent–daughter relationship (PD). By allowing the structure to crystallize in an oriented form, a bimodal orientation of lamellae is developed and the individual contributions of PD lamellae can be observed using wide-angle X-ray scattering (WAXS). The present study investigated oriented PD lamellae during flow-induced crystallization and subsequent melting using time resolved rheo-WAXS. During crystallization the planes of the daughter lamellae were observed to curve towards the flow direction as they grew from their parent lamellae. This was explained by the influence of neighbouring daughter lamellae confining their growth direction. Oriented daughter lamellae were found to melt ~5°C lower than oriented parent lamellae, which provides a new explanation for the multiple melting behaviour observed in the melting thermograms of sheared α-iPP.