# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## Results and Discussion
### Single Gas Permeability
For both 6 FDA-TMPDA and PIM-1 composite membranes, increasing the thickness of the Teflon AF1600 layer reduces the corresponding water permeability, supporting the conclusion that the perfluoropolymer layer restricts water transport through the membrane. The corresponding H\({}_{2}\)O/CO\({}_{2}\) selectivity of the composite membranes are reduced relative to the pure membranes (Table 3), and for the PIM-1 composite membranes the H\({}_{2}\)O/CO\({}_{2}\) selectivity is essentially unity. Hence, there is no selectivity for water. This is an important finding and demonstrates that the perfluoropolymer in a composite structure achieves water resistant polymeric membranes with reasonable permselectivity for gas separation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Membrane** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)/CH\({}_{4}\)** \\ \hline
6 FDA-TMPDA layer & 645 \(\pm\) 12 & 49.6 \(\pm\) 1 & 13.0 \\ PIM-1 layer & 5697 \(\pm\) 60 & 354 \(\pm\) 9 & 16.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Calculated permeabilities (barrer) through composite membranes’ 6 FDA-TMPDA and PIM-1 layers. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Membrane** & **Thickness (\(\mathbf{\mu}\))** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)/CH\({}_{4}\)** \\ \hline
6 FDA-TMPDA & 48 & 640 \(\pm\) 10 & 47.6 \(\pm\) 2 & 13.5 \\
6 FDA-TMPDA & 50 & 617 \(\pm\) 14 & 48.3 \(\pm\) 4 & 12.8 \\
6 FDA-TMPDA & 54 & 607 \(\pm\) 13 & 47.4 \(\pm\) 4 & 12.8 \\ PIM-1 & 40 & 2547 \(\pm\) 38 & 181 \(\pm\) 6 & 14.1 \\ PIM-1 & 43 & 2007 \(\pm\) 41 & 144 \(\pm\) 6 & 14.0 \\ PIM-1 & 45 & 1803 \(\pm\) 36 & 131 \(\pm\) 6 & 13.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Single 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.  
The water permeability through the 6 FDA-TMPDA and PIM-1 layers of the composite membranes can be calculated from Equation (3), if it is assumed that the water permeability through the Teflon AF1600 layer is equal to that of the individual membrane (Table 3). The average calculated water permeability is provided in Table 8. These permeabilities are clearly reduced compared to that of the pure polymeric membranes. For 6 FDA-TMPDA, H\({}_{2}\)O permeability has been reduced by 35% and for PIM-1, H\({}_{2}\)O permeability has been reduced by 33%. These decreases are significant and beyond error associated with experimentation. A likely explanation for this behavior is a change in the mode in which water vapor is transporting through the 6 FDA-TMPDA and PIM-1 layers, due to the composite structure. The water cluster that is hypothesized to transverse the Teflon AF1600 layer is likely being partly retained within the 6 FDA-TMPDA and PIM-1 layers, lowering diffusivity and hence reducing the corresponding water permeability. This can be demonstrated through [PERSON]'s Law of Diffusion, which relates diffusivity ratio to molecular weight ratio [29]:  
\[\frac{\text{D}_{1}}{\text{D}_{2}}=\sqrt{\frac{\text{M}_{2}}{\text{M}_{1}}} \tag{8}\]  
Accounting for competitive sorption effects in the permeability, the diffusivity of water in the pure and composite membranes of 6 FDA-TMPDA and PIM-1 can be calculated. The diffusivity ratio for composite and pure PIM-1 membranes correspond to a molecular weight ratio of 2.2:1 (composite to pure membrane), and for 6 FDA-TMPA the molecular weight ratio is 2.4:1. This reveals that within the composite membrane water is diffusing through the PIM-1 and 6 FDA-TMPDA as clusters of twice the size or more of water diffusing through pure membranes of these polymers. Hence, the perfluoropolymer layer acts as a barrier to water vapor transport and also reduces water transport through the selective layer. This mechanism for water permeation through the membranes is graphically depicted in Figure 1.