Hydrogen Bonding and Dynamic Crossover in Polyamide-66: A Molecular Dynamics Simulation Study

Using molecular dynamics simulations, we study the hydrogen bond dynamics and thermodynamics of the bulk of polyamide-66 over a broad temperature range (300−600 K). We show that different dynamic properties (the structure relaxation time, the orientational time correlation function of the amide groups, and the self-diffusion coefficient) of unentangled polyamide-66 undertake a crossover transition in the same small temperature range (∼ 413 K) above the experimental glass transition temperature (350 K). The data can be fitted to a Vogel−Fulcher−Tammann law when T > 413 K and to an Arrhenius equation when T < 413 K. Our results show that the global dynamics of polyamide-66 is intimately related to the relaxation of the hydrogen bond network formed among the amide groups. The presence of a dynamic crossover at a temperature slightly higher than the glass transition one is in agreement with the more recent experimental data and glass theories.