Influence of Nucleation Interface on the Nucleation and Crystallization of Isotactic Polypropylene: Aromatic Bisamide β-Nucleating Agents as an Example

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Epitaxial crystallization is a key mechanism by which efficient nucleating agents (NAs) promote the nucleation of semicrystalline polymers, relying on both lattice and topographic matches between polymer crystals and nucleating substrates at the nucleation interface. However, when these two factors are comparable, the effect of chemical structural variations at the nucleation interface on polymer epitaxial nucleation remains poorly understood. In this study, two nucleation interfaces between isotactic polypropylene ( i PP) β-crystal and aromatic bisamide β-NAs with comparable lattice and topographic matches were constructed, using a newly developed β-NA, N-cyclohexyl-N′-cyclopentylterephthalamide (CHCPT), and a commercial β-NA, N,N′-dicyclohexylterephthalamide (DCHT). The influence of the nucleation interface modification (adjustment in the β-NA chemical structure, e.g., the half replacement of cyclohexyl by cyclopentyl) on i PP crystallization and epitaxial nucleation was systematically investigated using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), fractionated crystallization, and molecular mechanics (MM) calculations. Compared to DCHT, CHCPT shows higher β-nucleation selectivity, greater nucleation efficiency, and faster overall crystallization rate toward i PP. MM simulations reveal that at the nucleation interface, CHCPT exhibits a lower average interfacial energy with i PP β-crystal than DCHT, corresponding to a reduced interfacial free energy difference (Δσ) toward i PP β-crystal as determined from fractionated crystallization studies. The lower Δσ reduces the nucleation energy barrier, thereby enhancing the β-nucleation rate in the i PP/CHCPT system. These findings reveal that chemical structural modification at the nucleation interface via the variation in the β-NA’s chemical structure influences the intermolecular interactions between β-NA and i PP and ultimately affects the epitaxial nucleation rate and crystallization behaviors of i PP. This work provides new insights into the mechanism by which the nucleation interface affects the epitaxial nucleation ability of nucleating agents and expands the understanding of rational molecular design for high-efficiency nucleating agents.

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