Microstructure and Gas Diffusivity of Poly(dimethylsiloxane) Dense Membrane Using Molecular Dynamics (MD) Simulation

The gas diffusion coefficients of H2, O2, N2, CO2, and CH4 at 308 K of a polydimethylsiloxane (PDMS) dense membrane with a focus on the free volume morphology were investigated based on molecular dynamics simulation. Using the constructed amorphous cell of PDMS, the density and glass transition temperature were determined. The fractional accessible volume (FAV) which can be determined by given radius prove size decreased dramatically from 0.46 to 0.17 with increasing probe radius. FAV at a probe radius of 1.74 Å, which was the same as the oxygen molecule radius (1.73 Å) based on kinetic diameter, was found to correspond to the fractional free volume (FFV) calculated using a group contribution method of Van Krevelen. The simulated values of the gas diffusion coefficients were consistent with the experimental values, and differed by 36% from the experimental ones. These simulated values decreased with increasing molecular size, such as the gas critical volume (H2 > O2 > N2 > CO2 > CH4). This trend was the same as that in the experiment. We found that the interesting trend between FAV and gas diffusivity in PDMS. As the calculated values of FAV at each radius size of the gas molecules increased, the gas diffusion coefficients increased lineally, except for that of CO2. Because CO2 is higher value of critical temperature compared with the other gases, we considered that the reason for this deviation for CO2 could be related to the gas condensability. Hence, FAV can be proved to be a more effective parameter than FFV in investigation for gas diffusion in polymer membranes.