Nucleation Retardation of Polyamide 6,6 by Tuning Hydrogen Bonding Reorganization via Nigrosine for High-Speed Injection Molding Applications

The mechanism of regulating hydrogen bond organization for polyamide 6,6 (PA 6,6) by nigrosine was investigated by temperature-dependent FTIR and in situ synchrotron X-ray diffraction during cooling/heating processes. The intermolecular interaction between amide groups and nigrosine was verified by the FTIR spectroscopy method. Upon melt cooling, the formation of H-bonds is promoted by nigrosine at the initial stage (regime I) with increased enthalpy change (ΔH₁), which results from denser molecular coils as the interaction with nigrosine increases the ability to form H-bonds between adjacent amide groups, while at a lower temperature (regime II), the ΔH₁ decreases with increasing nigrosine as nigrosine restrains the mobility of PA 6,6 chains that retards the formation of H-bonds between nonadjacent amide groups. Moreover, the ordering arrangement of H-bonds into 2-D hydrogen bond sheets (H-sheets) is hindered for both the regime I′/II′ stages with decreased ΔH₂ due to the decreased mobility of chain segments, which retards the nucleation of PA 6,6 crystallization. The crystal perfection index (CPI) was investigated by WAXD, and it was found to decrease with the increase of nigrosine. Combined with microstructure analysis, we proposed that partial nigrosine can insert between H-sheets, which therefore tunes the Brill transition behavior of PA 6,6. Overall, the regulation mechanism of nigrosine on PA 6,6 crystallization has been established via interaction with amide groups to tune the organization of H-sheets. The retardation effects on crystallization can improve the flowability of PA 6,6 for high-speed injection molding applications.