Copolymerization-Regulated Hydrogen Bonds: A New Routine for High-Strength Copolyamide 6/66 Fibers

Hydrogen bond interactions are important for nylon fibers, which improve its mechanical properties and crystallization behavior, while hindering the movement and orientation of the molecular chain during the drawn process. In this study, hexamethylene adipamide was used as the second monomer in copolymerization with ε-caprolactam to obtain copolyamide 6/66 (CoPA), and high-tenacity fibers with a maximum value up to 8.0 cN/dtex were achieved by a multi-step drawn and thermal setting process. Results show that the hexamethylene–adipamide ratio affected the draw ratio (DR) of the as-spun fiber, on the tenacity of final high-performance fiber, and on crystalline. Both DR and tenacity showed evident increases with the hexamethylene–adipamide ratio up to 6% in CoPA and then changed smoothly. However, XRD and DSC results illustrate a decreased tendency with regard to crystallinity. The attenuated in-site total reflection Fourier transform infrared (ATR-FTIR) spectra were used to study the hydrogen bond interaction between the C=O group and N–H group and the crystal form of the fiber. Results show that the copolymerization destroyed the regularity of the main chain of CoPA and reduces the interaction of interstrand hydrogen bonds, facilitating the formation of the γ-crystalline form in as-spun fibers, fulfilling the transition from the γ to α crystalline form during the fiber-drawing step because of the release of the C=O group and N–H group from the hydrogen bond interaction at an elevated temperature close to the molten temperature of CoPA, and then reforming during the thermal-setting step which soiled the crystalline and improved the tenacity of the fiber. The copolymerization with a homologous monomer regulates the hydrogen bond interaction, fulfills the high drawn ratio and high tenacity fiber, and provides a new route for high-performance fiber preparation using traditional fiber formation of polymers.