Polycarbonate membranes and production thereof

Abstract

Polycarbonate membranes useful for hemodialysis are fabricated from polyether-polycarbonate block copolymers by a water gelation process. The process comprises casting onto a substrate surface a layer of a multicomponent casting solution comprising the copolymer dissolved in a water-miscible organic solvent together with a cosolvent which acts as a swelling agent for the copolymer, drying the layer to partially evaporate the solvents therefrom, immersing the partially dried layer in water to form a gelled membrane, and stripping the gelled membrane from the substrate surface. The membrane has improved strength and improved permeability to solutes in the "middle molecule" range while maintaining clinically acceptable ultrafiltration rates and clearance of low molecular weight solutes.

What is claimed is: 
     
       1. A process for producing a polycarbonate membrane useful for hemodialysis which comprises casting on to a substrate surface having a smooth finish a layer of casting solution comprising a polyether-polycarbonate block copolymer having a molecular weight within the range of from about 50,000 to about 750,000 as determined by the intrinsic viscosity measurement and containing from about 5 to about 35% by weight of repeating alkylene ether units and correspondingly from about 95 to about 65% by weight of repeating bisphenol A-carbonate units and a water-miscible organic solvent together with a cosolvent which acts as a swelling agent for said copolymer, drying said layer to partially evaporate the solvents therefrom, immersing said partially dried layer in water to form a gelled membrane, and stripping said gelled membrane from said substrate surface. 
     
     
       2. The process of claim 1 wherein said polyetherpolycarbonate block copolymer has a molecular weight within the range of from about 200,000 to about 500,000 as determined by the intrinsic viscosity measurement. 
     
     
       3. The process of claim 1 wherein said polyetherpolycarbonate block copolymer comprises the polymerization reaction product of phosgene with a mixture of from about 95 to about 65% by weight of bisphenol A and correspondingly from about 5 to about 35% by weight of a polyethylene glycol, having a molecular weight in the range of 600 to 6,000. 
     
     
       4. The process of claim 1 wherein said solvent has a boiling point in the range of from about 50° to about 85° C. 
     
     
       5. The process of claim 1 wherein said solvent comprises 1,3-dioxolane. 
     
     
       6. The process of claim 1 wherein said casting solution contains from about 1 to about 20 weight % of total solids and has a viscosity within the range of from about 5,000 to about 30,000 cps. 
     
     
       7. The process of claim 1 wherein said casting solution contains from about 10 to about 20 weight % of total solids and has a viscosity within the range of from about 7,000 to about 25,000 cps. 
     
     
       8. the process of claim 1 wherein said cosolvent-swelling agent is present in said casting solution in amounts ranging from about 10 to about 75% by weight based on the weight of said copolymer. 
     
     
       9. The process of claim 8 wherein said cosolvent-swelling agent is present in amounts ranging from about 15 to about 25% by weight based on the weight of said copolymer. 
     
     
       10. The process of claim 8 wherein said cosolvent-swelling agent is selected from the group consisting of dimethyl sulfoxide, dimethyl formamide and pyridine. 
     
     
       11. The process of claim 8 wherein said cosolvent-swelling agent comprises dimethyl sulfoxide. 
     
     
       12. The process of claim 1 wherein the layer of casting solution is air-dried at temperatures ranging from about 20° to about 30° C. for a period ranging from about 1.0 to about 5.0 minutes prior to being immersed in said water. 
     
     
       13. The process of claim 1 wherein said water is maintained at a temperature ranging from about 20° to about 30° C. 
     
     
       14. The polycarbonate membrane produced in accordance with the process of claim 1. 
     
     
       15. The polycarbonate membrane of claim 14 wherein said polyether-polycarbonate block copolymer has a molecular weight within the range of from about 200,000 to about 500,000 as determined by the intrinsic viscosity measurement. 
     
     
       16. A process for producing a polycarbonate membrane useful for hemodialysis which comprises casting onto a substrate surface having a smooth finish a layer of casting solution comprising a polyether-polycarbonate block copolymer having a molecular weight within the range of from about 50,000 to about 750,000 as determined by the intrinsic viscosity measurement and containing from about 5 to about 35% by weight of repeating alkylene ether units and correspondingly from about 95 to about 65% by weight of repeating bisphenol A-carbonate units and a water-miscible organic solvent together with a cosolvent which acts as a swelling agent for said copolymer, drying said layer to partially evaporate the solvents therefrom, immersing said partially dried layer in water to form a gelled membrane, and stripping said gelled membrane from said substrate surface. 
     
     
       17. A process for producing a polycarbonate membrane useful for hemodialysis which comprises producing a polyether-polycarbonate block copolymer having a molecular weight within the range of from about 50,000 to about 750,000 as determined by the intrinsic viscosity measurement and containing from about 5 to about 35% by weight of repeating alkylene ether units and correspondingly from about 95 to about 65% by weight of repeating bisphenol A-carbonate units, the method for producing said polyether-polycarbonate block copolymer comprising dissolving a polyether glycol compound, bisphenol A, and pyridine in a solvent to give a total solids content of about 5 to about 16%, by weight, reacting the dissolved bisphenol A, polyether glycol compound, and pyridine with phosgene by adding phosgene, at an initial feed rate, to said solution with vigorous stirring while maintaining the solution in the temperature range of about 20° C. to about 43° C. until crystals of pyridine hydrochloride begin to form, thereafter adding a chain terminator to the reacted solution and reducing the phosgene feed rate to about one-fifth of initial phosgene rate until the reaction solution undergoes a permanent color change,   forming a casting solution comprising said polyether-polycarbonate block copolymer dissolved in a solution comprising a water-miscible organic solvent together with a cosolvent, said cosolvent acting as a swelling agent for said copolymer,   casting a layer of said casting solution onto a substrate surface having a smooth finish,   drying said layer to partially evaporate the solvents therefrom,   immersing said partially dried layer in water to form a gelled membrane, and   stripping said gelled membrane from said substrate surface.   
     
     
       18. The process of claim 17 wherein the polyether-polycarbonate block copolymer has a molecular weight of from about 200,000 to about 500,000 as determined by intrinsic viscosity measurement and the polyether glycol compound is chosen from the group consisting of polythylene glycol and polypropylene oxide-polyethylene oxide block copolymers. 
     
     
       19. The process of claim 17 wherein the bisphenol A, polyether glycol and pyridine are dissolved in a halogenated alkane. 
     
     
       20. The process of claim 19 wherein the halogenated alkane is dichloromethane. 
     
     
       21. The process of claim 17 wherein about 3 moles of pyridine are present for each mole of bisphenol A and polyether glycol. 
     
     
       22. The process of claim 17 wherein the chain terminator is phenol. 
     
     
       23. The process of claim 17 wherein the precipitated polymer is additionally ground to a hard crumb, washed with hot water and dried. 
     
     
       24. The process of claim 17 wherein the bisphenol A is of a grade chosen from the group consisting of epoxy grade bisphenol A which has been further recrystallized from toluene and polycarbonate grade bisphenol A. 
     
     
       25. The process of claim 17 wherein the initial feed rate of phosgene is from about 500 ml/min to about 2000 ml/min.