Morphological Characteristics of β-Nucleating Agents Governing the Formation of the Crystalline Structure of Isotactic Polypropylene

The effect of the morphological characteristics of N,N′-dicyclohexylterephthalamide (DCHT), an aromatic bisamide β-nucleating agent (β-NA) showing a dual nucleation capability for the β- and α-crystals of isotactic polypropylene (iPP), on the polymorphic composition of iPP and the underlying mechanism were systematically investigated. DCHT crystals differing in their size and distribution state in the iPP melt and recrystallized morphology were obtained by modulating the final heating temperature (Tf), subsequent cooling rate (Vc), and content of DCHT. The crystalline structures formed on the different surfaces of needle-like DCHT crystals were clearly identified. For the iPP/0.1 wt % DCHT samples, the relative fraction of β-crystal (Kβ value) in the iPP sample nucleated with small-sized needle-like DCHT crystals is much higher than that in the iPP sample nucleated with large-sized needle-like DCHT crystals. This increase is primarily because there is less space for the crystallization of the iPP melt into α-spherulites arising from a more homogeneous distribution of small DCHT crystals in the iPP matrix. Upon cooling at the same Vc, dendritic DCHT crystals tend to induce more β-crystals than needle-like ones, especially at a high Vc, which is predominantly due to an increase in the relative density of β-nuclei to α-nuclei (Nβ/Nα). Interestingly, it was found that DCHT crystals crystallize into microcrystalline agglomerates in the subsequent cooling process when completely dissolved in the iPP/0.05 wt % DCHT melt. More interestingly, the Kβ value in the iPP sample reaches as high as ∼1.0 during isothermal crystallization at 135 °C, indicating that DCHT microcrystalline agglomerates unexpectedly exhibit a specific nucleation selectivity for β-crystal. The present investigation elucidates the significant impact of the morphological characteristics of DCHT on the polymorphic composition of iPP, providing a promising way to tune the microstructure and macroscopic performance of iPP materials.