Strategic control of polyamide 6 crystals for enhanced mechanical performance in carbon-fiber-reinforced polyamide 6 using a nucleating agent

The influence of nucleating agents on the mechanical properties of polyamide 6 (PA6)/carbon fiber (CF) composites was investigated to enhance composite performance by reducing the spherulite size of PA6. The incorporation of a sorbitol derivative (sor) or an aromatic phosphate ester sodium salt (pho) into PA6 significantly decreased the spherulite size and accelerated the crystallization rate. In quasi-isotropic composites, the flexural modulus and strength increased by ∼15% upon sor or pho addition. By contrast, the transverse tensile strength of unidirectional (UD) composites remained essentially unchanged, and the fracture surface observations suggested that the nucleating agents had negligible effects on the PA6–CF interfacial adhesion. However, in terms of the mechanical properties along the CF axis in the UD composites, the nucleating agents improved the tensile strength by ∼10% and compressive strength by ∼25%, with pho exhibiting the greatest enhancement effect. Even in the absence of nucleating agents, the compounding process contributed to PA6 refinement and improved the mechanical performance of the composite. Characterization of the crystalline structure of PA6 between CFs and within the interphase region—using polarized optical microscopy, X-ray diffraction, and polarized micro-Raman spectroscopy—revealed that the affinity of sor and pho with the CF surface promotes the formation of thermodynamically stable, oriented α phase PA6 crystal aligned along the CF axis at the interphase. These oriented crystals mainly contribute to the improved compressive strength, highlighting the critical role of material design and processing conditions during composite fabrication for the control of crystal structure in CFRTP.