Structural and Dynamic Properties of Penetrant Molecules in Unsupported and Supported Hydrated Gels

Pervaporation, a membrane based separation process, is considered as an energy efficient technique for separating dilute alcohol solutions. The efficiency of the pervaporation process is governed by the complex interplay of transport and thermodynamic properties of the system components. A detailed knowledge of the molecular mechanisms underpinning these processes will help in designing efficient membranes. Molecular simulations can provide insights at atomistic level as they account for all the specific interactions governing these processes.

In this work, molecular simulations are used to investigate the dynamics of solvent molecules and polymer gels in unsupported and polysulfone supported polyacrylate gels. In unsupported gel systems, the structural arrangement of solvent molecules is determined by the equilibrium swelling concentration of the gels. At high concentrations, water molecules form large clusters and interact predominantly with the neighboring water molecules. The formation of large clusters in turn enhances their dynamics. However, at low concentrations, water molecules are well dispersed and interact preferentially with the polymer groups. These preferential interactions couple the dynamics of water and polymer gels. Unlike water, ethanol molecules do not exhibit such coupling.

Addition of polysulfone support alters the dynamics of the gels and the solvent molecules from their unsupported behavior. The polyacrylate gels and the solvents exhibit enhanced dynamics as the gels undergo volume expansion upon adding the support. In addition, each polymer layer exhibits different dynamics at the interface and in the bulk regions (.i.e. regions of the layers far from interface). These deviations in the interfacial and bulk dynamics are governed by the packing (or density) of each layer in the supported systems. This work shows that addition of support alters the volumetric and dynamic properties of the gels from the unsupported behavior. Thus, for designing pervaporation membranes, it is imperative to understand the behavior of membrane polymers in both unsupported and supported state. Molecular simulations provide a detailed understanding of the factors governing the behavior of the gels and can thus complement experiments in engineering efficient membranes.

Embargo status: Restricted until 01/2023. To request the author grant access, click on the PDF link to the left.