# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## Results and Discussion
### Single Gas Permeability  
The single gas permeability of CO\({}_{2}\) and CH\({}_{4}\) through the pure 6 FDA-TMPDA, PIM-1 and Teflon AF1600 membranes are provided in Table 3 at 35 \({}^{\circ}\)C. Additionally included in the table is permeability of H\({}_{2}\)O under mixed gas conditions, where the carrier gas is N\({}_{2}\) at 4 bar, 35 \({}^{\circ}\)C, and 0.3 water activity. The 6 FDA-TMPDA membrane permeabilities are comparable to literature [22; 25], given the differences in annealing temperature, and clearly demonstrate the high water permeance and corresponding large water selectivity relative to CO\({}_{2}\). PIM-1 permeabilities are higher than the majority of the literature [21; 26; 27]. This difference is attributed to the low feed pressure, meaning the micro-voids are not saturated and hence the CO\({}_{2}\) solubility is greater than that observed for other studies undertaken \begin{table}
\begin{tabular}{c c c} \hline
**Selective Layer** & **Total Composite Thickness (\(\mu\)m)** & **Advancing Contact Angle (degs)** \\ \hline Pure Teflon AF1600 & \(8\pm 0.5\) & 128.3 \(\pm\) 0.2 \\
6 FDA-TMPDA & \(48\pm 1\) & 124.3 \(\pm\) 0.8 \\
6 FDA-TMPDA & \(50\pm 1\) & 124.2 \(\pm\) 0.8 \\
6 FDA-TMPDA & \(54\pm 1\) & 124.7 \(\pm\) 0.7 \\ PIM-1 & \(40\pm 1\) & 126.3 \(\pm\) 0.5 \\ PIM-1 & \(43\pm 1\) & 126.2 \(\pm\) 0.4 \\ PIM-1 & \(45\pm 1\) & 126.5 \(\pm\) 0.6 \\ \hline \end{tabular}
\end{table} 
Table 2: Advancing water contact angle (degree) on the composite PIM-1 and 6 FDA-TMPDA membranes with various thickness of the Teflon AF1600 protective layer, as well as that of pure Teflon AF1600. at higher pressures [28]. The water permeability through PIM-1 is comparable to the author's previous study [7], and demonstrates that PIM-1 has selectivity for water over CO\({}_{2}\). The Teflon AF1600 permeabilities are comparable with literature [15] and demonstrate the unique selectivity for CO\({}_{2}\) over H\({}_{2}\)O. The low permeability of water in Teflon AF1600 is associated with the super-hydrophobicity of the polymer and the observation of very low water sorption into the Teflon AF1600 membrane structure. It has been hypothesized that water vapor transports through the perfluoropolymer in the form of clusters, because of the super-hydrophobocity of the polymer chains. The formation and size of the water clusters reduces the diffusivity of water vapor, which when coupled with the very low water solubility results in low water permeability. Modelling of the CO\({}_{2}\) permeability through Equations (5) and (7) enables the interconnection between the micro-voids within the membranes to be determined. This is achieved by calculating the mobility factor (F\({}_{A}\)), and indication of the proportion of gas diffusing through the micro-voids that can transverse through the entire membrane. The other fundamental parameters of the three pure membranes have already been reported in the literature and are provided in Table 4. The calculated mobility factor for Teflon AF1600 is comparable to literature, but that of 6 FDA-TMPDA and PIM-1 are lower than that reported in the literature [7; 15; 25]. This implies that the 6 FDA-TMPDA and PIM-1 membranes studied here have reduced interconnectivity between the micro-voids, which is attributed to the annealing temperature of the membranes. The lower temperature limits chain rearrangement during the annealing process and has restricted interconnectivity between the micro-voids. The single gas permeabilities of CO\({}_{2}\) and CH\({}_{4}\) through the composite membranes are provided in Table 5 at 35 \({}^{\circ}\)C; arranged by polymer type and thickness of the perfluorinated layer. It is clear that all six composite membranes have CO\({}_{2}\) permeability greater than CH\({}_{4}\), and therefore have potential for the separation of CO\({}_{2}\) from natural gas.