Preparation of High-Performance Copolyamide 6 via a Side-Methyl Topological Structure Design Strategy

Abstract To address the insufficient toughness and transparency of nylon-6 (PA6) caused by high crystallinity, this study proposes a copolymer strategy based on side-methyl structural regulation. A series of PA6/D6 copolyamides were prepared via in situ melt copolymerization of ε-caprolactam, 2-methyl-1,5-pentanediamine, and adipic acid, enabling high-value utilization of the MPMD byproduct. Structural analyses confirmed incorporation of D6 units, whose steric hindrance weakened hydrogen bonding, promoting transformation from the ordered α-form to the γ-form and reducing lamellar and spherulitic ordering. This resulted in lower melting/crystallization temperatures with good thermal stability, expanding the processing window, while restricted chain slippage increased melt viscosity and elasticity. Increasing D6 content enhanced ductility and transparency: PA6/D60.2 achieved elongation at break exceeding 1000% with high tensile strength, while PA6/D60.3 showed the highest transmittance. This work elucidates the mechanism of side-methyl steric hindrance on PA6′s structure, offering a strategy for tough, transparent, processable copolyamide 6.