Investigating glass transition in a PA6T/66 copolymer through molecular dynamics simulations

Tailoring copolymer composition is a key strategy for enhancing the thermal and mechanical performance of semi-aromatic polyamides. In this work, we investigate the thermal behavior of poly(hexamethylene terephthalamide-<i>co</i>-hexamethylene adipamide) (PA6T/66) copolymers by probing their glass transition temperature (<i>T</i>_g), a critical parameter governing material stability. Classical molecular dynamics simulations reveal <i>T</i>_g trends across PA6T/66 systems with varying molar ratios of poly(terephthaloyl hexylenediamine) (PA6T), in alignment with experimental data obtained from temperature-dependent density analysis. Increasing PA6T content promotes interchain hydrogen bond (HB) formation, which enhances thermal stability by restricting segmental mobility. However, beyond 55% PA6T content, <i>T</i>_g decreases due to steric hindrance from stacked benzene rings and a shift in the interchain/intrachain HB equilibrium, which disrupts cohesive interactions. These findings reveal the thermal properties at the atomic scale by which PA6T content modulates <i>T</i>_g, providing a molecular-level understanding that offers valuable guidelines for designing PA6T/66 copolymers with enhanced thermal performance.