# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## 2 Experimental Section  
Amorphous Teflon AF1600 (copolymer of 65 mol% 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxode and 35 mol% tetrafluoroethylene) was purchased from DuPont (Houston, TX, USA). The polyimide 6 FDA-TMPDA was synthesized by the reaction between 4,4\({}^{\prime}\)-(hexafluoroisopropylidene) diphthalic anhydride and 2,3,5,6-tetramethyl-1,4-phenylenediamine in n-methylpyrolidone (AR grade) under Ar to give the polyamic acid, which was subsequently imidized with trimethylamine and acetic anhydride to yield 6 FDA-TMPDA. Details of the synthesis can be found elsewhere [20]. PIM-1 was the polycondensation product of monomers 5,5\({}^{\prime}\),6,6\({}^{\prime}\)-tetrahydroxy-3,3\({}^{\prime}\),3\({}^{\prime}\)-tetramethyl-1,1\({}^{\prime}\)-spirobisindane (TTSBI) and 2,3,5,6-tetrafluoroterephthalalonitrile (TFTPN) and synthesized following established procedures by [PERSON] et al. [21]. The dense 6 FDA-TMPDA and PIM-1 layers were prepared by controlled evaporation of a 2.5 \(w/v\) % solution from dichloromethane. Evaporation was achieved by partially sealing the solution from the atmosphere so that the dichloromethane partial pressure above the film surface was high, preventing rapid solvent loss. This procedure produced membranes of 6 FDA-TMPDA with a thickness of 42 \(\upmu\)m and membranes of PIM-1 with a thickness of 35 \(\upmu\)m. The perfluorinated polymer layer was spin coated from a 2 \(w/v\) % solution (fluorinated solvent PF5060 (3M, Melbourne, Australia)) on top of the 6 FDA-TMPDA and PIM-1 films for various thicknesses (Table 1). The different solvation properties of 6 FDA-TMPDA and PIM-1 compared to Teflon AF1600 means that there was no redissolving of the underlying polymer layer when the perfluoropolymer film was applied. All films were annealed under vacuum at 40 \({}^{\circ}\)C for 3 days. To minimize any age dependent behavior, all membranes were used within 4 days of annealing. This annealing temperature is lower than that which is common for the polyimide 6 FDA-TMPDA and PIM-1, and was used to ensure that the perfluorinated polymer did not undergo thermal degradation during the annealing process. The unique surface properties of Teflon AF1600 meant that it formed a discrete layer on top of the 6 FDA-TMPDA and PIM-1 membranes which could be peeled off the underlying membrane under the correct shear conditions. Hence, there was a distinct boundary between the two layers. Furthermore, the thickness of the Teflon AF1600 layer was dictated by the needed to form a uniformed film on the membrane, as lower amount of perfluorinated polymer do not form a consistent membrane. \begin{table}
\begin{tabular}{c c c c} \hline
**Selective Layer** & **Selective Layer Thickness (\(\upmu\)m)** & **Teflon AF1600 Thickness** & **Total Composite Thickness (\(\upmu\)m)** \\ \hline
6 FDA-TMPDA & \(42\pm 0.5\) & \(6\pm 0.5\) & \(48\pm 1\) \\
6 FDA-TMPDA & \(42\pm 0.5\) & \(8\pm 0.5\) & \(50\pm 1\) \\
6 FDA-TMPDA & \(42\pm 0.5\) & \(12\pm 0.5\) & \(54\pm 1\) \\ PIM-1 & \(35\pm 0.5\) & \(5\pm 0.5\) & \(40\pm 1\) \\ PIM-1 & \(35\pm 0.5\) & \(8\pm 0.5\) & \(43\pm 1\) \\ PIM-1 & \(35\pm 0.5\) & \(10\pm 0.5\) & \(45\pm 1\) \\ \hline \end{tabular}
\end{table} 
Table 1: Total thickness (\(\upmu\)m) of composite PIM-1 and 6 FDA-TMPDA membranes with various thickness of the Teflon AF1600 protective layer. Single gas permeabilities were undertaken on a variable pressure constant volume apparatus that has previously been reported [22]. Single gas measurements were undertaken with a feed pressure of 0.987 atm at 35 \({}^{\circ}\)C. Mixed gas permeability measurements were undertaken on a constant pressure variable volume instrument, which has also been previously reported and the protocol for exposure to water vapor has also been previously reported [6]. The feed gas was 10% CO\({}_{2}\) in CH\({}_{4}\) mixture at 9.87 atm and 35 \({}^{\circ}\)C. The choice in single gas and mixed gas feed pressures were to ensure the membranes experience the same partial pressure of CO\({}_{2}\) under both single and mixed gas conditions.