# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## Results and Discussion
### Mixed Gas Permeability
The importance of these results is the difference in membrane performance under mixed gas conditions relative to that observed for single gas measurements. The impact of competitive sorption on both CO\({}_{2}\) and CH\({}_{4}\) permeability in the composite membranes clearly reduces the permselectivity, making the composite design less attractive as a potential natural gas sweetening membrane. Similar to the single gas composite membranes, the permeability of CO\({}_{2}\) and CH\({}_{4}\) through the 6 FDA-TMPDA and PIM-1 layers of the composite membranes can be calculated through Equation (3), and the results are provided in Table 11. This assumes that the Teflon AF1600 layer permeability is unchanged from the mixed gas individual membrane performance (Table 9). For 6 FDA-TMPDA the calculated CO\({}_{2}\) and CH\({}_{4}\) permeabilities are essentially the same as the individual polymer, while for PIM-1, the CO\({}_{2}\) permeability is higher than the individual membrane but CH\({}_{4}\) is essentially the same. The water permeability through the composite membranes under mixed CO\({}_{2}\)-CH\({}_{4}\) gas conditions as a function of water activity in the feed are provided in Figure 2 for composite PIM-1 membranes of overall thickness of 43 \(\upmu\)m and in Figure 3 for composite 6 FDA-TMPDA membranes of overall thickness of 50 \(\upmu\)m; also included in both figures is the corresponding CO\({}_{2}\) permeability of the composite membrane, to indicate under which water activity conditions the composite membranes have selectivity for CO\({}_{2}\) over water. For both composite membranes the water permeability increases as a function of increasing water activity. This behavior has been observed for other water studies and is associated with increased water solubility at higher activities [30; 31; 32]. However, for both PIM-1 and 6 FDA-TMPDA composite membranes the increase in permeability at very high water activities is substantial. High water activity may overload the ability of Teflon AF1600 to act as a barrier to water permeation, breaking down the water transport mechanism through the composite membrane and \begin{table}
\begin{tabular}{c c c c c} \hline
**Membrane** & **Thickness (\(\upmu\)m)** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)/CH\({}_{4}\)** \\ \hline
6 FDA-TMPDA & 48 & \(478\pm 42\) & 23.9 \(\pm\) 4 & 20.0 \\
6 FDA-TMPDA & 50 & \(490\pm 38\) & 25.8 \(\pm\) 2 & 19.0 \\
6 FDA-TMPDA & 54 & \(472\pm 41\) & 23.5 \(\pm\) 2 & 20.1 \\ PIM-1 & 40 & \(1927\pm 109\) & \(166\pm 11\) & 11.6 \\ PIM-1 & 43 & \(1640\pm 110\) & \(139\pm 18\) & 11.8 \\ PIM-1 & 45 & \(1482\pm 125\) & \(125\pm 17\) & 11.8 \\ \hline \end{tabular}
\end{table} 
Table 10: Mixed gas CO\({}_{2}\) and CH\({}_{4}\) permeabilities (barrer) through 6 FDA-TMPDA and PIM-1 composite membranes with Teflon AF1600, denoted by overall membrane thickness, at 35 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c} \hline
**Membrane** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)/CH\({}_{4}\)** \\ \hline
6 FDA-TMPDA layer & \(479\pm 11\) & \(22.5\pm 2\) & 21.3 \\ PIM-1 layer & \(3513\pm 153\) & \(290\pm 5\) & 12.1 \\ \hline \end{tabular}
\end{table} 
Table 11: Calculated permeabilities (barrer) through composite membranes’ 6 FDA-TMPDA and PIM-1 layers. enabling water to permeate as smaller clusters and individual molecules at high activities (Figure 1). Importantly, the PIM-1 composite membrane has water permeability lower than the CO\({}_{2}\) permeability for water activities below 0.2. This demonstrates that composite membranes based on PIM-1 and Teflon AF1600 can be CO\({}_{2}\) selective under certain water activity conditions while having higher CO\({}_{2}\) permselectivity than pure Teflon AF1600. The 6 FDA-TMPDA composite membrane does not display this selectivity behaviour, with the membrane always having selectivity for water over CO\({}_{2}\).