Molecular Origins of Dynamic Coupling between Water and Hydrated Polyacrylate Gels

Energy efficient separation of dilute alcohol–water mixtures is a critical consideration in commercialization of biofuels; pervaporation is an attractive separation technique for this purpose. Knowledge of the mechanism of solvent mobility inside polymeric membranes is of great interest for designing pervaporation-based separation processes. Recently, we employed molecular simulations to study water structure in three polyacrylate gels composed of homopolymers and copolymers of <i>n</i>-butyl acrylate (P­(BA)) and 2-hydroxyethyl acrylate (P­(HEA)). In this work, water and ethanol dynamics were studied using simulations in two systems: polyacrylate gels swollen to equilibrium and gels with low water content. Solvent dynamics show a concentration-dependent behavior in the gels. For gels swollen to equilibrium, both water and ethanol exhibit the highest mobility in the P­(HEA) gel due to the larger degree of swelling of the system, while for gels with a low solvent content, they show the lowest mobility in the P­(HEA) gel due to hydrogen bonding between solvent and polymer. Solvent dynamics in gels with low solvent content was characterized by determining solvent diffusivity, rotational relaxation time, and Van Hove autocorrelation function. The dynamics of water molecules is strongly coupled with polymer dynamics due to hydrogen-bonding interactions, while ethanol does not show such strong coupling due to a smaller degree of interaction with the polymer. Ethanol mobility instead follows the trend in the density and glass transition temperature of the polymer. Our results suggest that dynamic coupling between solvent and polymer can be exploited as a mechanism for separating dilute alcohol–water mixtures