Poly(imide siloxane) and carbon nanotube mixed matrix membranes for gas separation

Recent theoretical work by Sholl and coworkers has predicted that carbon nanotubes, if used as membranes, have the flux/selectivity properties that far exceed those of any other known inorganic or organic material. To verify this prediction, we have fabricated nano-composite membranes consisting of single-walled carbon nanotubes embedded in a poly(imide siloxane) copolymer and evaluated their transport properties. While the siloxane segment enhanced the interfacial contact, the polyimide component imparted mechanical integrity. A poly(imide siloxane) was synthesized using an aromatic dianhydride, an aromatic diamine and amine-terminated PDMS for the siloxane block. The weight percent of PDMS was determined to be 41 using 1H-NMR. Permeability measurements of He showed drops in permeability with the addition on close-ended CNTs. This large drop in permeability of He suggests that the copolymer adhered well to the CNTs and that the prepared CNT MMMs were defect free. However, the permeability of O2, N2 and CH4 increased in proportion to the amount of open-ended CNTs in the polymer matrix. This suggests that CNTs offer an attractive additive for universally enhancing the gas permeability of polymers.

A poly(imide siloxane) was synthesized using an aromatic dianhydride, an aromatic diamine and amine-terminated PDMS for the siloxane block. The weight percent of PDMS was determined to be 41 using 1H-NMR. The prepared copolymer was mixed with carbon nanotubes for the purpose of fabricating mixed matrix membranes suitable for gas separations. SEM images of CNTs showed that the nanotubes were well purified and cut into few hundred nanometer scale. The SEM for the cross sectional area images of MMM films indicated that CNTs were well dispersed in polymer matrix at 2 wt% of CNT loading, while at 10 wt% of CNTs loading, the tubes agglomerated and formed domains in the copolymer matrix. Permeability of He dropped with the addition of close-ended CNTs. This large drop in He permeability suggests that the copolymer adhered well to the CNTs and that the prepared CNT MMMs were defect free. The permeability of O2, N2 and CH4 increased in proportion to the amount of open-ended CNTs in the polymer matrix. The permeability of He, H2 and CO2 increased after the addition of 2 wt% CNTs. However, there was no difference in permeability between 2 and 10 wt% of CNTs loading in copolymer matrix. The increase in the diffusion coefficients for O2, N2 and CH4 in MMMs based on open-ended CNTs indicate the presence of high diffusivity CNT tunnels within the poly(imide siloxane) matrix. Therefore, it may be concluded that CNTs offer an attractive additive for universally enhancing the gas permeability of polymer.