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

Abstract

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 (TC) 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 TC 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 TC and not on the nature of the nucleating agents.