Charged hollow fiber membrane having hexagonal voids

1. A charged hollow fiber membrane comprising:

(i) an inner surface;

(ii) an outer surface;

(iii) a porous bulk disposed between the inner surface and the outer surface,

wherein the porous bulk comprises at least a first region including: a) a first set of pores having a controlled pore size and having outer rims; b) a second set of pores connecting the outer rims of the first set of pores, wherein the pore size of the first set of pores is greater than the pore size of the second set of pores; and c) a first polymer matrix supporting the first and the second sets of pores; wherein the porous bulk further comprises at least a second region including: d) a third set of pores having a controlled pore size and having outer rims; e) a fourth set of pores connecting the outer rims of the third set of pores, wherein the pore size of the third set of pores is greater than the pore size of the fourth set of pores; and (f) a second polymer matrix supporting the third and fourth sets of pores; and

(iv) at least one positively charged zone or at least one negatively charged zone disposed on the inner surface, on the outer surface, and/or in the porous bulk;

wherein each of the first and the second polymer matrices comprises a mixture of a polymer (a) and a negatively charged polymer (b1) or a positively charged polymer (b2);

wherein polymer (a) is a polymer independently selected from polyethersulfone (PES), polyether ether sulfone, bisphenol A polysulfone, polyphenylsulfone, a polyvinylidene halide, a polyolefin, polystyrene, polycarbonate, polyacrylonitrile, a cellulosic polymer, a fluoropolymer, and polyetherether ketone (PEEK) and wherein the negatively charged polymer (b1) is selected from maleic acid:methyl vinyl ether copolymer, polystyrene sulfonic acid, sulfonated polysufone, sulfonated polyether sulfone, polyacrylic acid, polymethacrylic acid, and polyvinylphenol and the positively charged polymer (b2) is selected from polyethylenimine, polydiallyldimethylammonium chloride, polyvinylamine, amine terminated polyethylene oxide or polyethylene glycol, poly(2-vinylpyridine), poly(4-vinylpyridine), and poly(1-vinylpyrrolidone-co-2-dimethylaminoethylmethacrylate).

2. The charged hollow fiber membrane of claim 1 , which includes a neutral zone between the at least one negatively charged zone and the at least one positively charged zone.

3. The charged hollow fiber membrane of claim 1 , wherein the pore size of the first set of pores is in the range of from about 50 nm to about 1000 nm.

4. The charged hollow fiber membrane of claim 1 , wherein the pore size of the second set of pores is in the range of about 0.2 to about 0.4 times the pore size of the first set of pores.

5. The charged hollow fiber membrane of claim 1 , wherein the pore size of the fourth set of pores is in the range of about 0.2 to about 0.4 times the pore size of the third set of pores.

6. The charged hollow fiber membrane of claim 1 , wherein the pore size of the third set of pores is at least 10% different from the pore size of the first set of pores.

7. A method of making a charged hollow fiber membrane of claim 1 , the method comprising:

(a) forming at least one coating composition comprising a solvent, dissolvable silica nanoparticles, a membrane-forming polymer, and a charged polymer;

(b) coating the at least one coating composition onto a filament;

(c) carrying out phase inversion of the at least one coating composition, or the coating compositions, coated onto the filament;

(d) dissolving the dissolvable silica nanoparticles and removing the filament from the coated filament to obtain a charged hollow fiber membrane; and optionally

(e) washing the charged hollow fiber membrane obtained in (d).

8. The method of claim 7 , which includes:

forming a first coating composition and a second coating composition, each comprising a solvent, a set of dissolvable silica nanoparticles, a membrane-forming polymer, and a charged polymer in (a), and

coating the coating the first and the second coating compositions onto the filament in (b).

9. The method of claim 8 , wherein the particle size of the dissolvable silica nanoparticles in the first coating composition is different from the particle size of the dissolvable silica nanoparticles in the second coating composition.

10. The method of claim 8 , wherein the membrane-forming polymer in the first coating composition is different from the membrane-forming polymer in the second coating composition.

11. The method of claim 7 , wherein the solvent in the first coating composition is different from the solvent in the second coating composition.

12. The method of claim 7 , wherein the at least one coating composition is coated by dip coating.

13. The method of claim 7 , wherein the dissolvable silica nanoparticles are dissolved by contacting the filament after (c) with an acid and an alkali.

14. A method of filtering a fluid, the method comprising passing the fluid through the charged hollow fiber membrane of claim 1 .

15. The charged hollow fiber membrane of claim 1 , wherein polymer (a) is polyethersulfone.

16. The charged hollow fiber membrane of claim 15 , wherein the negatively charged polymer (b1) is maleic acid:methyl vinyl ether copolymer.

17. The charged hollow fiber membrane of claim 15 , wherein the positively charged polymer (b2) is polyethyleneimine.