Introduction of ether bonds as a design strategy for sustainable nylon

Nylons (PAs) exhibit high mechanical strength and high melting temperatures, due to strong hydrogen bonding between amide groups promotes chain alignment and the formation of robust crystalline structures. However, the high melting temperatures also hinder mechanical recycling. Moderately suppressing crystallinity without substantially compromising mechanical properties is therefore a promising strategy for the development of sustainable nylons. We addressed this by incorporating ether bonds into the nylon backbone to increase chain flexibility and disrupt efficient chain packing to suppress crystallinity, while maintaining high amide concentration to preserve mechanical strength. Based on this design strategy, we synthesized a novel nylon PA-I via interfacial polymerization and compared its properties with conventional nylons.
Thermal analysis revealed that the introduction of ether bonds significantly lowered the melting temperature (T_m) and glass transition temperature (T_g). The Tm of PA-I was lower than its structural analogue PA-II by ca. 70 °C, suggesting suppressed crystallinity. Notably, the Tm of PA-I was even lower than that of PA12, whose T_m is the lowest among commercial nylons, indicating improved thermal processability. The T_g of PA-I was lower than that of PA-II by 55 °C, confirming enhanced backbone flexibility. WAXS analysis showed that the increased backbone flexibility suppressed lamella to grow thick, but did not decrease the degree of crystallinity. Despite the increased backbone flexibility and reduced lamella thickness, PA-I showed a little higher yield stress than PA12, owing to its higher amide concentration. Because of the low T_m, PA-I showed better resistance to repeated processing than PA6,6. During the repeated processing, the molecular weight of PA6,6 decreased much faster than PA-I, indicating that lowering the T_m effectively improves resistance to repeated processing.