| Summary: | Solubility-selective polymer membranes are promising materials for C<sub>3+</sub> hydrocarbons removal from methane and other permanent gas streams. To this end, a dense solubility-selective membrane based on crosslinked poly(tetradecyl methyl siloxane) was synthesized. Sorption of methane, ethane, and <i>n</i>-butane in the polymer was measured in the temperature range of 5–35 °C. An abnormal temperature dependence of sorption was detected, contradicting the generally accepted view of sorption as an exothermic process. In particular, methane shows minimal sorption at 5 °C. The abnormal temperature behavior was found to be related to crystallization of the alkyl side chains at temperatures below ~10 °C. Gas permeability determined by sorption and permeation methods are in reasonable agreement with each other and decrease in the order <i>n</i>-C<sub>4</sub>H<sub>10</sub> > C<sub>2</sub>H<sub>6</sub> > CH<sub>4</sub>. The solubility of these alkanes changes in the same order indicating that poly(tetradecyl methyl siloxane) is indeed the sorption-selective membrane. The diffusivities and permeabilities of studied alkanes declined with decreasing temperature, whereas the <i>n</i>-C<sub>4</sub>H<sub>10</sub>/CH<sub>4</sub> permselectivity increases with decreasing temperature, reaching a value of 23 at 5 °C.
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