Atomistic Modeling of the Hygromechanical Properties of Amorphous Polyamide 6,6

Polyamide 6,6 (PA66) is a key engineering polymer, whose unique mechanical properties arise from strong interchain hydrogen bonding. However, its hygroscopic nature makes it highly sensitive to water uptake, which markedly alters its thermomechanical behavior. Contrary to traditional experimental approaches, this study uses atomistic molecular dynamics (MD) simulations to investigate the role of water in modifying the glass transition temperature (T_g) and the viscoelastic response of amorphous PA66. Simulations capture a nonmonotonic dependence of T_g on water content. At low water concentrations, isolated water molecules bind to amide groups and restrict chain mobility, while beyond ∼2.5 wt %, water clustering disrupts the hydrogen-bond network and causes a pronounced T_g depression. Analysis of amide-group fluctuations reveals a master correlation between local segmental dynamics and bulk density, verifying the known temperature-humidity equivalence in terms of density variation. The computed Young's modulus exhibits systematic softening with increasing temperature and water content, consistent with experimental trends, albeit a more pronounced impact of water at low temperatures. Time-temperature superposition behavior is observed for both dry and hydrated systems. This work provides molecular-scale information on the hygromechanical coupling in PA66 and demonstrates the ability of MD simulations to predict water-induced transitions that govern the macroscopic behavior of polyamides.