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

The effect of pressure on solubility and the influence of temperature on solubility, permeability, and diffusivity of C ₃F ₈ and its hydrocarbon analogue, C ₃F ₈, are reported in rubbery poly(dimethylsiloxane) (PDMS) and glassy poly(l-trimethylsilyl-1-propyne) (PTMSP). The solubility of C ₃F ₈ is lower than that of C ₃H ₈ in both polymers at all temperatures and pressures investigated. The isosteric enthalpy of mixing C ₃F ₈ with PDMS and PTMSP is higher than that of C ₃H ₈ due to less favorable polymerfluorocarbon interactions in the case of C ₃F ₈, and it decreases with increasing C ₃F ₈ concentration. Assuming a coordination number of 10, the energy associated with mixing C ₃F ₈ molecules and PDMS segments is 4.5 kJ/mol more than that required to mix C ₃H ₈ molecules with PDMS segments, in the limit of infinite dilution. The isobaric activation energy of permeation (E _P) for C ₃F ₈ is positive for both polymers, and that for C ₃H ₈ 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 C ₃H ₈ and C ₃F ₈. For PTMSP, the difference in E _P values for C ₃F ₈ and C ₃H ₈ stems mainly from a substantially larger E _D value for C ₃F ₈ than for C ₃H ₈.