High-Pressure Crystallization of iPP Nucleated with 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol

1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) is highly effective in nucleation of the α- form of isotactic polypropylene (iPP). However, its role in high-pressure crystallization of iPP, facilitating the formation of the γ- polymorph, has not been explored. The present paper focuses on the influence of DMDBS on nucleation of high-pressure crystallization of iPP. iPP with 0.2-1.0 wt.% of the DMDBS was crystallized under elevated pressure, up to 300 MPa, in various thermal conditions, and then analyzed by PLM, WAXD, SEM, and DSC. During cooling, crystallization temperatures (T_c) were determined. It was found that under high-pressure DMDBS nucleated crystallization of iPP in the orthorhombic γ- form. As a consequence, T_c and the γ- form content increased for the nucleated iPP, while the size of polycrystalline aggregates decreased, although the effects depended on DMDBS content. The significant increase of T_c and the decrease of grain size under high pressure of 200-300 MPa required higher content of DMDBS than the nucleation of the α-form under lower pressure, possibly due to the effect of pressure on crystallization of DMDBS itself, which is a prerequisite for its nucleating activity.