Process for preparing membranes and membrane structures from a sulfonated block copolymer fluid composition
View Patent ↗The present invention relates to an improved method for making sulfonated block copolymers and to methods for making membranes from such block copolymers. In particular, the present invention relates to an improved method for making sulfonated block copolymers having at least two polymer end blocks that are resistant to sulfonation and at least one polymer interior block that is susceptible to sulfonation where the sulfonation agent is C 2 to C 8 acyl sulfate. In the improved process the residual carboxylic acid formed from the C 2 to C 8 acyl sulfate is converted to C 1 to C 4 alkyl esters by contacting the residual carboxylic acid with at least a 0.9:1 molar ratio of a C 1 to C 4 alcohol to residual carboxylic acid, resulting in an improved sulfonated block copolymer solution. The present invention further relates to the use of such sulfonated block copolymer solutions to prepare various membranes and other articles.
1. A process for preparing a membrane, comprising:
(a) applying a fluid composition of sulfonated block copolymer, ester of residual carboxylic acid and non-halogenated aliphatic solvents onto the surface of a substrate, said fluid composition prepared by the steps of:
i. providing a precursor block copolymer having at least two end blocks A and at least one interior block B, wherein each A block is a polymer block resistant to sulfonation and each B block is a polymer block susceptible to sulfonation;
ii. reacting the precursor block polymer with a C2 to C8 acyl sulfate in a reaction mixture further comprising at least one non-halogenated aliphatic solvent to form a sulfonated block polymer, therein resulting in the formation of a solution of a sulfonated block copolymer and a residual carboxylic acid; and
iii. subsequently reacting the residual carboxylic acid contacted with a C1 to C4 alcohol or mixture thereof in a molar ratio of alcohol to residual carboxylic acid of at least 0.9:1 to form the corresponding alkyl esters of the carboxylic acid and to reduce the residual level of the organic residual acid to less than 2.0 wt % based on the total weight of the solution, wherein the reaction mixture is free of any halogenated solvent,
(b) spreading the fluid composition to form a uniform thickness layer on the substrate, and
(c) allowing the solvent to evaporate from the fluid composition resulting in a solid membrane.
2. The process of claim 1 wherein step (b) includes spreading the fluid composition through a die or reverse roll coater.
3. The process of claim 2 wherein the thickness of the resulting layer is between 0.1 microns and 1 mm.
4. The process of claim 1 wherein said fluid composition is spread on a porous substrate.
5. The process of claim 4 wherein said porous substrate is selected from the group consisting of microporous membranes, expanded poly(tetrafluoro)ethylene and fibrous mats.
6. The process of claim 1 wherein step (c) includes heating the fluid composition.
7. The process of claim 1 wherein said fluid composition is spread onto the surface of a substrate in an atmosphere containing water vapor at a relative humidity greater than 10%.
8. A membrane formed by the process of claim 1 .
9. The membrane of claim 8 having a tensile strength of at least 100 psi when wet.
10. The membrane of claim 8 exhibiting ion exchange capacity of between about 0.4meq/g and about 2.5 meq/g.
11. The membrane of claim 8 wherein the membrane is in the shape of a flat sheet, a tube, or in the shape of a perform article.