New Understanding on Regulating the Crystallization and Morphology of the β‑Polymorph of Isotactic Polypropylene Based on Carboxylate–Alumoxane Nucleating Agents

Carboxylate–alumoxane derived from p-n-alkylbenzoic acids, where n-alkyl group changes from 2 to 8 carbon atoms, exhibits dual nucleating ability and nucleates isotactic polypropylene (iPP) into predominantly in the β-phase under specific conditions. The selectivity of the β-phase nucleation depends on the concentration of the nucleating agent, end melting temperature and cooling rate. The β-phase obtained from p-n-alkylbenzoate–alumoxanes is compared with the β-phase obtained from calcium pimelate (CaP), an efficient β-phase selective nucleating agent, using the results from DSC, WAXS, and SAXS analysis. The lamellar morphology of iPP nucleated with different nucleating agents crystallized at different crystallization temperatures (T_C) under controlled nonisothermal conditions are evaluated using SAXS analysis. The long period increases with increasing crystallization temperature and the long period of the β-phase is always larger than that of the α-phase for a given crystallization temperature. Furthermore, the variation of long period with crystallization temperature clearly brings out two crystallization temperature ranges; the low temperature range and the high temperature range. However, the β-phase shows a lower changeover temperature compared to that of the α-phase. The one-dimensional correlation analysis of the β-phase shows that the thickness of the crystal lamellae (lc) increases with T_C and exhibits the low and high crystallization temperature ranges, while the thickness of the amorphous layer (la) more or less remains constant. In-situ high temperature WAXS studies capture the β-phase to the α-phase transition and the transformed material correlates well with the lamellar thickness of the β-phase. The morphological difference between the α- and the β-phases are discussed and attributed to the differences in the impact properties and the melting temperature. This study clearly demonstrates that the lamellar morphology mainly depends on the T_C and not on the nature of the nucleating agents.