Asymmetric nanotube containing membranes
This invention relates to heterogenous pore polymer nanotube membranes useful in filtration, such as reverse osmosis desalination, nanofiltration, ultrafiltration and gas separation.
1. A process for the fabrication of a membrane of selective permeability, said process comprises:
(a) coating a substrate with a film of polymer solution comprising a polymer dissolved in a solvent;
(b) forming over said film of polymer solution a nanotube dispersion layer comprising nanotubes dispersed in a liquid carrier that is partially miscible with, but forms a separate liquid phase from, said polymer solution;
(c) evaporating said liquid carrier from said nanotube dispersion layer to leave a residual nanotube dispersion in said polymer solution and nanotubes protruding from said nanotube dispersion; and
(d) contacting said residual nanotube dispersion layer with a liquid that is at least partially miscible with said solvent but in which said polymer is substantially insoluble, to cause precipitation of said polymer to form an asymmetric membrane terminating on one side in a skin that forms a substantially impermeable barrier around said nanotubes with said nanotubes protruding through said skin.
2. The process of claim 1 wherein the liquid carrier contains a nanotube dispersing agent.
3. The process of claim 2 wherein the dispersing agent is polystyrene-poly-3-hexylpolythiophene copolymer.
4. The process of claim 1 wherein the polymer is selected from the group consisting of poly(1,4-phenylene ether-ether-sulfone), poly(1-hexadecene-sulfone), poly(1-tetradecene-sulfone), poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), poly[1[4-(3-carboxy-4-hydroxyphenylazo)-benzenesulfonamido]-1,2-ethanediyl, polyphenylsulfone and polysulfone.
5. The process of claim 1 wherein the nanotubes are single walled carbon nanotubes.
6. The process of claim 1 wherein the coating of the polymer solution onto the substrate is carried out at −10° C. to 90° C.
7. The process of claim 1 wherein the coating of the polymer solution onto the substrate is carried out at ambient temperature.
8. The process of claim 1 wherein the polymer concentration of the polymer solution is 10% to 30% by weight.
9. The process of claim 1 wherein the polymer solution is coated onto the substrate to a thickness of 0.001 mm to 1 mm.
10. The process of claim 1 wherein the forming of the nanotube solution over the film of polymer solution is carried out at 10° C. to 70° C.
11. The process of claim 1 further comprising separating said membrane from said substrate.
12. The process of claim 1 wherein the polymer is polysulfone.
13. The process of claim 1 wherein the polymer concentration of the polymer solution is about 20% by weight.
14. The process of claim 1 wherein the polymer solution is coated onto the substrate to a thickness of about 0.25 mm.
15. The process of claim 1 wherein the dispersion solvent is evaporated at ambient temperature to 50° C. for 5 seconds to 15 minutes.
16. The process of claim 1 wherein the precipitation step is carried out at ambient temperature to −10° C. for 5 minutes to 15 minutes.
17. The process of claim 1 wherein the substrate is a porous non-woven fabric.
18. A membrane of selective permeability, said membrane comprising an asymmetric porous polymer and terminating on one side in a skin, with open-ended nanotubes embedded in said skin, said nanotubes having substantially random orientations relative to said skin and at least a portion of the nanotubes each having two open ends protruding through opposite surfaces respectively of said skin to provide fluid communication through each of said nanotubes through said membrane, and said skin foiming a substantially impermeable barrier around said nanotubes.
19. The membrane of claim 18 wherein said pores are of increasing diameter with increasing distance from said skin.