Mechanism of Membrane Formation. VI. Convective Flows and Large Void Formation during Membrane Precipitation

The factors governing the formation of voids and large cavities in a wide variety of membranes made from different polymers cast from different solvents and precipitated in various nonsolvents have been studied. It has been shown that the formation of large voids in membranes can be eliminated by (1) lowering the tendency of the nonsolvent to penetrate into the casting solution or (2) increasing the viscosity of the cast solution or creating a thick gel layer on top of this cast solution. It is suggested that the formation of large finger-like cavities in membranes originates from convective flows formed within the cast (fluid) polymer solution upon its immersion in the bath of nonsolvent for final precipitation. It is also shown that the driving forces leading to the formation of these convective flows are not density gradients.