Atomistically Modeling the Chemical Potential of Small Molecules in Dense Polymer Microstructures. 2. Water Sorption by Polyamides

The solution thermodynamics of water in dense amorphous polyamide structures are assessed. Amorphous microstructures of polyamide(6) (PA-6), polyamide(12) (PA-12), and a copolyamide of hexamethylenediamine and isophthalic acid (PA-6I) are generated according to the amorphous-cell methodology on an atomistically detailed level. Applying a combination of thermodynamic integration and Widom's particle-insertion method to systems containing different numbers of water molecules yields estimates of the excess chemical potential of water dissolved in the polymer systems. The degree to which the polymers favor the absorption of water from their environment can be assessed and it is in reasonable agreement with experimental values for the equilibrium water contents.