Biobased Polyamide 5652 with α-Crystal Form: Characterization and Low Moisture Absorption

Biobased polyamide 56 (PA56) is recognized for its low-carbon footprint and environmental sustainability, yet its application in engineering is constrained by high moisture absorption. In response to this problem, our study presents a breakthrough with the development of PA5652 achieved through solid-state polymerization, featuring an α-crystal form morphology. Integrated analyses through molecular dynamics (MD) simulations, Fourier-transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy reveal strong intermolecular trans doubly hydrogen-bonded interactions in PA5652. These trans-double H-bonds induce the formation of the α-crystalline phase in PA5652, while variable-temperature infrared spectroscopy and X-ray diffraction (XRD) confirm the reversible Brill transition of its α-crystalline phase at elevated temperatures. Crystalline superstructures exhibit characteristic negative birefringence under polarized optical microscopy, indicative of a well-ordered lamellar packing. The melting point (Tm), crystallization temperature (Tc), degree of crystallinity (Xc), and saturation moisture absorption rate (Mt) of PA5652 are 270 °C, 241 °C, 48.3%, and 5.8%, respectively. Compared to PA56, the Tm, Tc, and Xc have increased by 8%, 14.7%, and 39.2%, while the saturation Mt has decreased by 79.4%. This study mitigates the moisture absorption of biobased PA by regulating the crystalline structure, providing a new pathway for research in this field.