Spatial distribution of γ-crystals in metallocene-made isotactic polypropylene crystallized under combined thermal and flow fields

The present Article reports the relationships between molecular orientation, formation, and spatial distribution of γ-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 γ-crystal (f_γ) with respect to α-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_γ increased monotonously from skin to core in CIM samples, whereas the corresponding f_γ increased nonmonotonically in OSIM samples. The spatial distribution of γ-crystal in OSIM samples can be explained by the epitaxial arrangement between γ- and α-crystal in a lamella-branched shish-kebab structure. In the proposed model, the parent lamellae of α-crystal provide secondary nucleation sites for daughter lamellae of α-crystal and γ-crystal, and the different content of parent lamellae results in varying amounts of γ-crystal. In OSIM samples, the smallest parent-daughter ratio ([R]) = 1.38) in the core region led to the lowest fraction of γ-crystal (0.57), but relatively higher γ-crystal content (0.69) at 600 and 1200 m 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 γ-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 γ- and α-crystal of the shaped samples. The f _γ distribution calculated from the relative areas of the peaks in the DSC scans was also consistent with the WAXD results.