Sorption and transport properties of propane and perfluoropropane in poly(dimethylsiloxane) and poly(1-trimethylsilyl-1-propyne)

The effect of pressure on solubility and the influence of temperature on solubility, permeability, and diffusivity of C3F8 and its hydrocarbon analogue, C3H8, are reported in rubbery poly(dimethylsiloxane) (PDMS) and glassy poly(l-trimethylsilyl-l-propyne) (PTMSP). The solubility of C3F8 is lower than that of C3H8 in both polymers at all temperatures and pressures investigated. The isosteric enthalpy of mixing C3F8 with PDMS and PTMSP is higher than that of C3H8 due to less favorable polymer-fluorocarbon interactions in the case of C3F8, and it decreases with increasing C3F8 concentration. Assuming a coordination number of 10, the energy associated with mixing C3F8 molecules and PDMS segments is 4.5 kJ/mol more than that required to Mix C3H8 Molecules with PDMS segments, in the limit of infinite dilution. The isobaric activation energy of permeation (E-P) for C3F8 is positive for both polymers, and that for C3H8 is negative in both polymers. This result is particularly interesting for PTMSP since all previous studies of activation energy of gas permeation in PTMSP report values that are near zero or negative; this study provides the first report of a positive E-P value in PTMSP. In PDMS, differences in both activation energy of diffusion (E-D) and enthalpy change on sorption contribute significantly to the difference in E-P values of C3H8 and C3F8. For PTMSP, the difference in EP values for C3F8 and C3H8 stems mainly from a substantially larger ED value for C3F8 than for C3H8.