Spatial Distribution of γ-Crystals in Metallocene-Made Isotactic Polypropylene Crystallized under Combined Thermal and Flow Fields

Abstract

The present Article reports the relationships between molecular orientation, formation, and spatial distribution of gamma-crystals in metallocene-made isotactic polypropylene (m-iPP) samples prepared by two industrial processes: conventional injection molding (CIM) and oscillatory shear injection molding (OSIM), in which combined thermal and flow fields typically exist. In particular, spatial distributions of crystallinity, fraction of gamma-crystal (f(gamma)) with respect to alpha-crystal, and lamella-branched shish-kebab structure in the shaped samples were characterized by synchrotron two-dimensional (2D) wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The results showed that the crystallinity in any given region of OSIM samples was always higher than that of CIM samples. The value of f(gamma) increased monotonously from skin to core in CIM samples, whereas the corresponding f(gamma) increased nonmonotonically in OSIM samples. The spatial distribution of gamma-crystal in OSIM samples can be explained by the epitaxial arrangement between gamma- and alpha-crystal in a lamella-branched shish-kebab structure. In the proposed model, the parent lamellae of alpha-crystal provide secondary nucleation sites for daughter lamellae of alpha-crystal and gamma-crystal, and the different content of parent lamellae results in varying amounts of gamma-crystal. In OSIM samples, the smallest parent-daughter ratio ([R]) = 1.38) in the core region led to the lowest fraction of gamma-crystal (0.57), but relatively higher gamma-crystal content (0.69) at 600 and 1200 mum depth of the samples (corresponding to [R] of 4.5 and 5.8, respectively). This is consistent with the proposed model where more parent lamellae provide more nucleation sites for crystallization, thus resulting in higher content of gamma-crystal. The melting behavior of CIM and OSIM samples was studied by differential scanning calorimetery (DSC). The observed double-melting peaks could be explained by the melting of gamma- and alpha-crystal of the shaped samples. The f(gamma) distribution calculated from the relative areas of the peaks in the DSC scans was also consistent with the WAXD results.