Swelling of a model polymer network by a one-site solvent: computer simulation and Flory-Huggins-like theory

A molecular-dynamics-Widom test particle-simulation was used to investigate the swelling of a model polymer network in contact with a one-site solvent under subcritical and supercritical conditions. Particle motion is computed via molecular dynamics. Simultaneously, the solvent particle concentration is controlled by direct comparison of the chemical potentials in two reference systems (pure solvent and network including solvent), which are calculated using Widom's test particle method. The simulated swelling isotherms exhibit complex behavior: at the subcritical conditions considered here, the swelling ratio decreases with increasing pressure. At the intermediate supercritical temperatures the isotherms exhibit a peak, which disappears with the elevation of temperature. At high temperatures, the swelling ratio of the network increases monotonically with increasing pressure. The corresponding isobars also exhibit a maximum, which broadens and shifts to higher temperatures with increasing supercritical pressure. These results are in qualitative agreement with the prediction of a modified Flory-Huggins theory and with the results of known experiments. Furthermore, the self-diffusion coefficients of the solvent in the network and in its pure state are simulated. The solvent mobility in the network is significantly decreased because of the hindrance of network beads, but exhibits different behavior at subcritical in comparison to supercritical temperatures.