Comparison of Structure-Permeability Relationships in PDMS and PTMSP

Natural gas is a vital energy source and its purification is by far the largest industrial gas separation applications. Poly(dimethylsiloxane) (PDMS), the most permeable rubbery polymer, is used commercially in natural gas separation processes. Poly(1-trimethylsilyl-1-propyne) (PTMSP), the most permeable polymer known, has a potential in nature gas separation processes. Permeability of most gases in PTMSP decrease with increasing temperature, which is in contrast to PDMS. Free volume properties of PDMS and PTMSP are simulated at different temperatures using Cavity Energetic Sizing Algorithm (CESA). We found the distributions of PDMS are shifted to larger cavities at higher temperature, and those of PTMSP are shifted to smaller cavities at higher temperature. Larger cavities of PDMS at higher temperature result in higher permeability of gases in PDMS. In contrast, smaller cavities of PTMSP at higher temperature may contribute to lower permeability of gases in PTMSP at higher temperature. The shape of cavity size distribution of PDMS is very like those of liquid structures, and those of PTMSP are very rough, which may suggest the non-thermodynamic equilibrium structure of glassy polymers.