Variables for Tubes and Sheets

The overall porosity of an anisotropic membrane is determined by the polymer content of the casting solution and by the relative rates at which nonsolvent enters and nonsolvent leaves the casting solution (i.e., the precipitation path in the phase diagram). 

Obviously, the simplest way to adjust the overall porosity is to adjust the polymer content of the casting solution unless the resultant viscosity is out of 

Various methods have been used to adjust the rate of water entry and solvent removal. They include the use of additives in the casting dope or in the precipitant (water) and temperature adjustments to both. 

In addition to polymer content additives, and temperature, the nature of the solvent can have a marked influence on the transport properties of the resulting membrane. Eirich et al9 discovered that regardless of the nature of the polymer, the highest porosity membranes were made from casting solutions utilizing solvents with high solubility parameters. Eirich investigated cellulose acetate, polystyrene, polyvinyl chloride, polyvinylidene difluoride, polycarbonate, polymethyl methacrylate, polyacrylonitrile, and polyacrylonitrile/polyvinyl 

Since water (the precipitant) has a solubility parameter (23.4) higher than any of the solvents investigated, it would appear experimentally that the disparity between the solubility parameters of the solvent and the precipitant is a key factor in understanding these results. 

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