Concentration Effect of a Bis-amide Nucleating Agent on the Shear-Induced Crystallization Behavior of Isotactic Polypropylene

The concentration of nucleating agent (NA) and flow fields are both important factors affecting the crystalline behavior and final properties of semicrystalline polymers. In this work, the concentration effect of N,N'-dicyclohexylterephthalamide (DCHT), an effective beta-NA for isotactic polypropylene (iPP), on the shear-induced crystallization of iPP was investigated by comparing the crystallization temperatures (T(c)s) of iPP/DCHT under unsheared, shear-unrelaxed, and relaxed conditions via rheological measurement. For iPP with an extremely low concentration of 0.01 wt %, T-c greatly improved after shearing and distinctly reduced after relaxation of 30 min, which was similar to that of pure iPP. For iPP with moderate concentrations of 0.05-0.3 wt %, the T-c could still increase after shearing but remained nearly constant after relaxation. In this concentration range, the increment in T-c induced by shear gradually decreased with increasing concentration. Interestingly, for iPP with high concentrations of 0.5 and 1 wt %, the T-c hardly changed regardless of applying shear or relaxation. The results of Fourier transform infrared suggested that DCHT existed in different forms including free molecules only and the coexistence of free DCHT and the associated assembly in various iPP/DCHT samples, and polarized optical microscopy observation showed that the DCHT assembly exhibited diverse morphologies at different concentrations. Combining the roles of free DCHT molecules and the associated assembly with diverse morphology in the shear-induced crystallization of iPP, a possible mechanism explaining the effect of DCHT concentration was proposed and experimentally verified.

Automated summary

The concentration of nucleating agent DCHT significantly influences the crystallization behavior of iPP, with lower concentrations leading to increased Tc after shearing and decreased Tc after relaxation, while higher concentrations show minimal changes in Tc. Various forms of DCHT and their assembly play a crucial role in the shear-induced crystallization process of iPP.