Molecular Dynamics Simulation On Silicon Rubber Pervaporation Membranes And Their Diffusion Properties

Understanding the physical-chemical properties and the diffusion properties of polymeric membranes, polymer-organic hybrid membranes, and polymer-inorgainc membranes was crucial to design pervaporation membranes with tailored structure and desired properties. In this study, the detailed atomistic structures of the membranes were built, including polymeric poly-dimethylsiloxane (PDMS) and poly(methylphenylsilane) (PMPhS), polymer-organic hybrid membranes (PDMS-calix[4]arene (CA), PDMS-cyclodextrin (CD)), polymer-inorganic hybrid membranes (PDMS-graphite (CG) and PDMS-silica (SiO₂)). The physical-chemical properties (including solubility parameters, segmental mobility, free volume) and the diffusion properties of benzene/water in these membranes were studied by molecular dynamics simulations.The solubility parameters of PMPhS membranes revealed that with the increase of phenyl group the membrane exhibited closer solubility parameter value to that of benzene. Although the FFV values of PMPhS membranes were decreased with the increase of phenyl group, the diffusion coefficients of benzene was enhanced. Thus the incorporation of phenyl group could improve the separation performance.For polymer-organic hybrid membranes, in our previous experimental investigation, the normalized permeation rate of benzene (NPRb) and separation factor (benzene/water) through PDMS-CA hybrid membranes did not follow the usual monotonous or single peak/valley change, but accompanied minimum and maximum values instead. In the present study, it was expected to explain the unusual phenomenon by MDs method, and the MD results agreed well with the experimental results. The simulation outcome revealed that MSD and fractional free volume (FFV) values were closely dependent on interaction energy. Diffusion coefficients of benzene and water at"infinite dilution"and saturated condition displayed the same changing tendency, although the values at saturated condition were a bit larger. Moreover, it was observed that diffusion coefficients were not only related to FFV, but also affected by the interaction between CA and the penetrants. The solubility parameters of CD-f-PDMS membranes revealed that with the increase of CD the membrane exhibited closer solubility parameter value to that of benzene. The MSD of polymer chains in CD-f-PDMS hybrid membranes was decreased with the increase of CD content, which was possibly attributed to the different interaction energy between PDMS and CD. The FFV was decreased with the increase of CD content. The diffusion coefficients of benzene/water were closely related to the FFV, i.e., larger FFV resulted in bigger diffusion coefficients.For polymer-inorganic hybrid membranes, incorporated CG and SiO₂ decreased the interaction between polymer chains of PDMS, had little influences on chain mobility, and decreased fraction of free volume. The diffusion coefficients of benzene and water in the membranes showed that the diffusion coefficients in hybrid membranes were smaller than those in the control membrane. These differences mainly came from the differences of free volume properties. It was also found that diffusion coefficient of water in PDMS-SiO₂ hybrid membranes was larger than that in PDMS-CG hybrid membranes, since water molecules could penetrate through the silica particles but be obstructed by graphite particles.