Pure and mixed gas CH4 and n-C4H10 sorption and dilation in poly(1-trimethylsilyl-1-propyne)

Pure and mixed gas n-C4H10 and CH4 sorption and dilation in poly(1-trimethylsilyl-1-propyne) (PTMSP) are reported at temperatures ranging from −20 to 35 °C. The presence of n-C4H10 in the mixture considerably reduces CH4 solubility. For example, CH4 solubility (in the limit of zero CH4 fugacity) at 25°C decreases from 4.0 (pure gas) to 0.78 cm3(STP)/(cm3 polymer atm) in the presence of n-C4H10 at an activity of 0.60. At −20 °C, CH4 solubility decreases by almost an order of magnitude, from 10.2 (pure gas) to 1.22 cm3(STP)/(cm3 polymer atm) in the presence of n-C4H10 at an activity of 0.61. In contrast, n-C4H10 mixture sorption properties are not measurably affected by the presence of CH4. The dual mode sorption model parameters for CH4 and n-C4H10 in PTMSP were determined from pure and mixed gas sorption measurements, and this model can adequately describe the sorption data. The n-C4H10/CH4 mixed gas solubility selectivity in PTMSP decreases as temperature increases and as n-C4H10 activity increases. For example, at 25 °C, the n-C4H10/CH4 solubility selectivity decreases from 250 to 120 as n-C4H10 activity increases from 0.02 to 0.25. At −20 °C and an n-C4H10 activity of 0.24, the n-C4H10/CH4 solubility selectivity is 590. Penetrant-induced volume dilation of PTMSP can be adequately modeled by assuming that all swelling is caused by penetrant molecules sorbed in the polymer's dense equilibrium region (i.e., the Henry's law region) during sorption. However, the best fit partial molar volumes in the Henry's law region for the dilation data are considerably lower than the penetrant partial molar volumes in liquids, suggesting that further theoretical efforts are needed to develop predictive models of volume dilation in high free volume glassy polymers.