Free volume and alcohol transport properties of PDMS membranes: Insights of nano-structure and interfacial affinity from molecular modeling

Abstract

A molecular dynamics (MD) simulation technique was adopted to study the structural properties and the feed transport behaviors of a polydimethylsiloxane (PDMS) membrane during the pervaporation process. The fractional free volume (FFV), fractional accessible volume (FAV), cohesive energy density, radial distribution function, and mean-squared displacement (MSD) of the PDMS membrane were analyzed to understand the effect of the swelling and operational temperatures on the membrane structure. In the feed transport behavior analyses, the ethanol/water travelling trajectories in the feed channel were observed and quantified to analyze the feed-membrane surface affinity. The MSD, squared-moving distance, and diffusivity of the ethanol/water molecules at different temperatures were estimated to elucidate the diffusion mechanism of the feed species in the PDMS membrane matrix. The swelling of the PDMS membrane resulted in an expanded membrane structure because of the higher ethanol–membrane interaction, which led to a larger cavity. The increased operational temperature induced polymeric chain mobility and an enlarged membrane free volume. In the ethanol/water–membrane surface affinity analysis, the ethanol molecules exhibited a higher affinity to the membrane than did the water molecules, which suggested that an ethanol molecule would transfer to the membrane surface prior to the transfer of water. During the diffusion mechanism analysis, the water molecule exhibited a higher mobility than did the ethanol molecule because it has a smaller size and lower affinity to the PDMS, which resulted in a lower transfer resistance. When the results of the feed-membrane affinity on the membrane surface were coupled with the feed mobility in the membrane matrix in this theoretical work, the PDMS membrane exhibited a preference for the ethanol molecules in the ethanol–water mixture, which concentrated the ethanol solution. In this work, the results of the MD simulation agreed well with the experimental data. The MD technique proved itself as a potential approach to characterize the PDMS membrane performance during the pervaporaton separation process. ► Ethanol swelled the membrane and enlarged cavities due to the stronger interaction to PDMS. ► Elevated temperature prompted chain fluctuation, which leads to the expansion of free volume. ► Ethanol showed higher aggregation rate to membrane surface as compared to water. ► Water revealed higher mobility in the membrane matrix due to its smaller size and less affinity. ► The feed-membrane affinity dominated transport mechanism in PDMS membrane during the PV process.