Crystallinity-Regulated Hydration, Molecular Mobility, and Mechanical Softening of Poly(ether ether ketone): A Reactive Molecular Dynamics Study

Abstract

Poly(ether ether ketone) (PEEK) is a high-performance semicrystalline thermoplastic widely used in aerospace, biomedical, and energy applications because of its excellent mechanical properties and resistance to harsh environments. However, the molecular mechanisms governing its short-time response to moisture exposure remain incompletely understood. In this work, reactive molecular dynamics simulations were performed to investigate the short-time hydration response of amorphous and semicrystalline PEEK in the dry state and at 1 and 3 wt % water. Water distribution, local hydration structure, molecular mobility, density-derived glass-transition behavior, and constrained directional moduli were analyzed to establish relationships among crystallinity, hydration, and thermomechanical behavior. Water showed greater local association with carbonyl oxygen than with ether oxygen, while increasing water content increased local coordination and molecular fluctuations. In semicrystalline PEEK, crystalline domains spatially confined these hydration-induced mobility enhancements, whereas the density-derived apparent glass-transition temperature showed a modest downward trend at 3 wt % water that was not resolved within the observed simulation variability. Increasing water content nevertheless progressively reduced the constrained directional moduli through plasticization, particularly within the amorphous and crystal–amorphous interfacial regions, whereas the crystalline phase retained substantial chain-direction stiffness. These results indicate that within the constructed models and sampled time scale, crystalline morphology restricted hydration-induced plasticization by limiting water accessibility and molecular mobility. No persistent hydration-induced bond-cleavage events were observed over the simulated time scale.