PREPARATION OF COMPOSITE MEMBRANES BY A SPIN COATING PROCESS

The formation of composite membranes on a laboratory scale by spin coating a selective layer of poly (4-methyl-1-pentene) (PMP) onto a microporous polysulfone support is described. In most cases the support had been precoated with an intermediate layer of poly (dimethyl siloxane) (PDMS). The thickness of the PMP layer, determined indirectly from gas flux measurements on the composite membrane, can be controlled within certain limits by varying the spinning speed, the coating solution concentration or the number of selective layers coated onto the substrate. Coated skin thickness varies approximately with the inverse of the square root of spin speed and with the second power of the coating solution concentration. The model coefficients compare well with those of models proposed for the production of photoresists in which polymer films are spin coated onto non-porous substrates. Gas permeation characteristics of the composite membranes were adequately described by a series resistance model. Composite membranes made without a PDMS intermediate layer required more than one layer of the selective polymer to bridge the larger pores and ensure a defect-free skin. For thin selective layers, these membranes have selectivities somewhat higher than that of PMP, since the PSF matrix of the substrate participates in the separation. Thus, multilayer composite membranes formed directly on microporous PSF substrates can be superior to composites with a PDMS intermediate layer.