# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## Results and Discussion
### Mixed Gas Permeability  
The mixed gas permeabilities of the individual membranes of 6 FDA-TMPDA, PIM-1 and Teflon AF1600 are provided in Table 9 for a 10% CO\({}_{2}\) in CH\({}_{4}\) feed gas. For all three membranes the CO\({}_{2}\) and CH\({}_{4}\) permeabilities are reduced relative to the single gas feeds, expect for Teflon AF1600 CH\({}_{4}\) permeability which is within error of the single gas measurement. This reduction in permeability is associated with competitive sorption between the two gases present reducing the solubility of each gas within the membrane structure. The change in CO\({}_{2}\)/CH\({}_{4}\) selectivity varies, for PIM-1 and Teflon AF1600 the selectivity reduces and indicates that CO\({}_{2}\) sorption is more impacted by CH\({}_{4}\). Contrasting to this, 6 FDA-TMPDA membrane selectivity is increased under mixed gas conditions, implying that CH\({}_{4}\) sorption is more strongly impacted by CO\({}_{2}\), than the other way. This behavior is associated with both the membrane morphology and the affinity each gas has for the micro-voids of the three polymeric membranes [19]. Interestingly, under mixed gas conditions Teflon AF1600 has a greater CO\({}_{2}\) permeability than 6 FDA-TMPDA, which differs from the single gas result, and implies in the composite membranes the 6 FDA-TMPDA layer will hinder gas permeance more than the perfluoropolymer. The reduction in CO\({}_{2}\) permeability under mixed gas conditions can be modelled through competitive sorption theory (Equations (6) and (7)), with the Langmuir affinity constant of CH\({}_{4}\) the only variable, given the established parameters for the three polymers already supplied in Table 4. Hence, the determined Langmuir affinity constants are provided in Table 9. The affinity constant for Teflon AF1600 and 6 FDA-TMPDA closely align with those previously reported in the literature [15; 22], while to the best of the author's knowledge no CH\({}_{4}\) affinity constant has previously been reported for PIM-1. The CH\({}_{4}\) affinity constants for 6 FDA-TMPDA and PIM-1 are considerably less than that of CO\({}_{2}\), which is associated with the strong affinity CO\({}_{2}\) has for those two polymers; while for Teflon AF1600 the affinity constants for CO\({}_{2}\) and CH\({}_{4}\) are comparable, which is associated with the poor affinity Teflon AF1600 has for many gases and vapors. Figure 1: Schematic of water transport through the respective individual and composite membranes. \begin{table}
\begin{tabular}{c c c c c} \hline
**Membrane** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)/CH\({}_{4}\)** & **b\({}_{\text{CH}}\) (atm\({}^{-1}\))** \\ \hline
6 FDA-TMPDA & \(477\pm 32\) & \(20.9\pm 3\) & 22.4 & 0.11 \\ PIM-1 & \(3350\pm 220\) & \(288\pm 21\) & 12.5 & 0.16 \\ Teflon AF1600 & \(485\pm 36\) & \(42.1\pm 8\) & 11.5 & 0.07 \\ \hline \end{tabular}
\end{table} 
Table 9: Mixed gas permeability (barrer) in pure 6 FDA-TMPDA, PIM-1 and Teflon AF1600 membranes, at 35 \({}^{\circ}\)C.  
The mixed gas permeabilities of the composite membranes are provided in Table 10 for a 10% CO\({}_{2}\) in CH\({}_{4}\) feed gas. For the 6 FDA-TMPDA composite membranes, the CO\({}_{2}\) permeability is within error of the individual Teflon AF1600 and 6 FDA-TMPDA membranes, while the CH\({}_{4}\) permeability is similar to that of the 6 FDA-TMPDA membrane. From the pure membrane measurements both polymers have similar permselectivity and hence the composite structure achieves no enhancement over the individual polymers. In contrast, the PIM-1 composite membranes have CO\({}_{2}\) and CH\({}_{4}\) permeabilities that are reduced compared to the individual membrane, while the CO\({}_{2}\)/CH\({}_{4}\) selectivity is the same; given individual PIM-1 and Teflon AF1600 membranes have comparable selectivities. Similar to the single gas measurements, increasing thickness of the Teflon AF1600 layer reduces the permeability of both gases species, as the perfluoropolymer is a stronger barrier to gas transport for the PIM-1 composite membrane.