# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## Results and Discussion
### Single Gas Permeability
The composite membranes with a PIM-1 layer have CO\({}_{2}\) permeabilities on the order of 1803-2547 barrer, which are over three times that observed for those composite membranes with a 6 FDA-TMPDA layer, 607-640 barrer. This is attributed to the different permeability through the polyimide and PIM-1 layers respectively, rather than the perfluorinated layer, as this layer is expected to have similar CO\({}_{2}\) permeability irrespective of the composite membrane. The CO\({}_{2}\) permeability decreases through the composite membranes as the thickness of the membrane increases, which corresponds to a thicker Teflon AF1600 layer. This reveals that the Teflon AF1600 layer is hindering gas permeance relative to the PIM-1 or 6 FDA-TMPDA layers; which is anticipated from the lower single gas permeabilities through the individual Teflon AF1600 membrane (Table 3). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Membrane** & **D (cm2/s)** & **k\({}_{\text{D}}\) (cm3/cm3 atm)** & **C\({}_{\text{H}}\) (cm3/cm3)** & **b (atm-1)** & **F** \\ \hline
6 FDA-TMPDA [25] & \(6.5\times 10^{-7}\) & 3.2 & 59 & 0.55 & 0.18 (0.57) \\ PIM-1 [7] & \(5.5\times 10^{-6}\) & 1.3 & 57 & 0.52 & 0.33 (0.9) \\ Teflon AF1600 [15] & \(2.9\times 10^{-6}\) & 1.22 & 15.4 & 0.10 & 0.12 (0.17) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: CO\({}_{2}\) permeability and competitive sorption parameters for 6 FDA-TMPDA, PIM-1 and Teflon AF1600, taken from the literature. The F values are calculated from the data, with those values in brackets corresponding to literature. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Membrane** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **H\({}_{2}\)O** & **CO\({}_{2}\)/CH\({}_{4}\)** & **H\({}_{2}\)O/CO\({}_{2}\)** \\ \hline
6 FDA-TMPDA & \(605\pm 11\) & \(46\pm 2\) & \(22600\pm 550\) & 13.2 & 37.2 \\ PIM-1 & \(5613\pm 23\) & \(355\pm 6\) & \(25200\pm 700\) & 15.8 & 4.5 \\ Teflon AF1600 & \(527\pm 18\) & \(40.5\pm 1.8\) & \(425\pm 40\) & 13.0 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Single gas permeability (barrer) in pure 6 FDA-TMPDA, PIM-1 and Teflon AF1600 membranes, at 35 \({}^{\circ}\)C.  
There is little change in the CO\({}_{2}\)/CH\({}_{4}\) selectivities of the two composite membranes as a function of thickness, as the results are within error of each other for the two composite membrane series. Assuming the perfluorinated polymer layers in the composite membranes have the same CO\({}_{2}\) and CH\({}_{4}\) permeabilities as pure membranes of the polymers (Table 3), then the respective average permeabilities of the 6 FDA-TMPDA and PIM-1 layers can be determined through Equation (3), and are provided in Table 6. This assumption is valid because of the poor solvation properties of the Teflon AF1600 polymer, meaning it does not integrate, blend or be modified by the underlying 6 FDA-TMPDA or PIM-1 layer structure. The composite membranes all have higher CO\({}_{2}\) and CH\({}_{4}\) permeability for the 6 FDA-TMPDA and PIM-1 layers than the pure membranes of these polymers. For 6 FDA-TMPDA there is an increase in 7%, which suggests a more open morphology in the composite membrane compared to the pure membrane, but given the similarly with the error of the measurement this cannot be definitively concluded. The corresponding CO\({}_{2}\)/CH\({}_{4}\) selectivity of the 6 FDA-TMPDA and PIM-1 layers in the composite membranes are comparable to the individual membranes of these polymers, revealing that the difference between the composite structure and individual membranes impacts CO\({}_{2}\) and CH\({}_{4}\) equally. The water permeability through the composite membranes is provided in Table 7, at a water activity of 0.3, and clearly demonstrates a dramatic decrease relative to that observed for the pure 6 FDA-TMPDA and PIM-1 membranes (Table 3). Indeed, all six composite membranes have comparable water permeability indicative that the Teflon AF1600 layer is acting as a protective film and reducing the water transport through the composite membrane structures.