Process to prepare fluoropolymer dispersions and membranes

Abstract

Described is a process to prepare fluoropolymer organic-liquid dispersions containing a homogeneous mixture of reacted and unreacted sulfonyl halide groups. The dispersions are useful in the preparation of crosslinked membranes.

1 . A method to prepare a membrane comprising the steps of:
 a) providing a solution comprising a solvent for a polymer and said polymer containing pendant SO 2 X groups, wherein the polymer comprises a fluorinated backbone containing pendant groups described by the formula —(O—CF 2 CFR f ) a —(O—CF 2 ) b —(CFR′ f ) c SO 2 X, where X is a halogen, R f  and R′ f  are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0 to 2, b=0 to 1, and c=0 to 6;   b) combining the solution of step a) with a nucleophilic compound Y and a polar liquid, to form a reaction mixture;   c) removing by distillation substantially all of the solvent used in step (a) for dissolving the polymer containing pendant SO 2 X groups from the reaction mixture of step b) to form a dispersion wherein about 5% to about 95% of the pendant SO 2 X groups have reacted with the nucleophilic compound Y and about 95% to about 5% of the pendant SO 2 X groups remain unreacted; and   d) preparing a membrane from the dispersion of step c).   
     
     
         2 . The method of  claim 1  additionally comprising the step of mixing the reaction mixture of step b) or the dispersion of step c) with a crosslinkable compound. 
     
     
         3 . The method of  claim 1  wherein in step c) about 25% to about 75% of the pendant SO 2 X groups have reacted with the nucleophilic compound Y and about 75% to about 25% of the pendant SO 2 X groups remain unreacted. 
     
     
         4 . The method of  claim 2  wherein the crosslinkable compound is of the formula HNR 1 R 2 , and about 1% to about 100% of the remaining pendant SO 2 X groups in step c) are converted to pendant SO 2 NR 1 R 2  groups, wherein R 1  and R 2  are independently hydrogen or optionally substituted alkyl groups. 
     
     
         5 . The method of  claim 1  additionally comprising the step of contacting the membrane of step d) with a crosslinking promoter so that crosslinks are formed between pendant groups. 
     
     
         6 . The method of  claim 5  wherein the crosslinks comprise one or more sulfonimide moieties. 
     
     
         7 . The method of  claim 6  wherein the sulfonimide moieties comprise SO 2 NR 7 SO 2 R 8 SO 2 NR 9 SO 2 , wherein R 7 and R 9  are independently hydrogen or optionally substituted alkyl groups, and R 8  is a substituted or unsubstituted alkylene, a substituted or unsubstituted arylene, a substituted sulfonimide polymer, an ionene polymer, or a substituted or unsubstituted divalent heteroatomic function. 
     
     
         8 . The method of  claim 1  wherein step d) includes the step of incorporating a reinforcement material into the dispersion of step c) as the membrane of step d) is prepared, and additionally comprising the step of contacting the membrane of step d) with a crosslinking promoter so that crosslinks are formed between pendant groups. 
     
     
         9 . The method of  claim 1  wherein the polar liquid is selected from DMF, DMAC, NMP, DMSO, acetonitrile, propylene carbonate, methanol, ethanol, water, or a combination thereof. 
     
     
         10 . The method of  claim 1  wherein X is F. 
     
     
         11 . The method of  claim 1  wherein the solvent for the polymer is fluorinated. 
     
     
         12 . The method of  claim 11  wherein the solvent for the polymer is a fluorocarbon, fluorocarbon ether, hydrofluorocarbon, hydrofluorocarbon ether, chlorofluorocarbon, chlorofluorocarbon ether, 2H-perfluoro(5-methyl-3,6-dioxanonane, or combination thereof. 
     
     
         13 . The method of  claim 1  wherein the nucleophilic compound Y is water admixed with a non-nucleophilic base selected from LiH, NaH, and NR 4 R 5 R 6 , wherein R 4 , R 5  and R 6  are optionally substituted alkyl groups. 
     
     
         14 . The method of  claim 1  wherein the nucleophilic compound Y is selected from LiOH, NaOH, KOH, CsOH, or combinations thereof. 
     
     
         15 . The method of  claim 1  where in step c) the pendant SO 2 X groups have been converted to SO 3 M, wherein M is a univalent cation. 
     
     
         16 . The method of  claim 1  wherein the polymer of step a) comprises a fluorinated or perfluorinated backbone and pendant groups of the formula O—CF 2 CF(CF 3 )—O—CF 2 CF 2 SO 2 F or —OCF 2 CF 2 SO 2 F, or any combination thereof. 
     
     
         17 . The method of  claim 2  wherein the crosslinkable compound is of the formula HNR 1 R 2  wherein R 1  and R 2  are independently hydrogen or optionally substituted alkyl groups. 
     
     
         18 . A membrane prepared by the method of  claim 1 . 
     
     
         19 . A membrane prepared by the method of  claim 4 . 
     
     
         20 . A membrane prepared by the method of  claim 17 .