PREPARATION OF POLYMER-CERAMIC COMPOSITE MEMBRANES WITH THIN DEFECT-FREE SEPARATING LAYERS

Polymer-ceramic composite membranes with essentially defect-free separating layers have been prepared by a solution deposition technique. Rigid polymers were used for the selective organic layer. These included high molecular weight samples of 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofuran-dione, isopropylidenedianiline (6FDA-IPDA), 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofuran-dione, methylenedianiline (6FDA-MDA), tetramethylhexafluorobisphenol-A polycarbonate (TMHFPC), and tetramethylhexafluoropolysulfone (TMHFPSF). Attempts to prepare composite membranes from lower molecular weight samples of bisphenol-A polycarbonate (PC) and tetrabromohexafluorobisphenol-A polycarbonate (TBHFPC) were unsuccessful. The PC and TBHFPC composites exhibited a decrease in the measured gas flux after polymer deposition; however, the selectivities of the composites were considerably less than that of a dense film of the same materials. A microporous ceramic membrane prepared by Anotec Separations was used as the support layer. This ceramic membrane provided minimal resistance to gas flow. The selective composite membranes were found to have high gas fluxes and gas separating abilities essentially equivalent to that of a dense isotropic film. The estimated, effective skin layer thicknesses for these membranes are on the order of 1500 angstrom to 1.0 mum. The formation of these composites is believed to occur through a sieving process in which large swollen polymer chains are sieved out of solution by the ceramic support. Polymer solutions that had swollen coil diameters that were smaller than that of the ceramic membrane did not produce selective composite membranes, while those solutions with swollen coil diameter that were larger than that of the ceramic membrane produced defect-free polymer layers.