MODELLING OF ASYMMETRIC MEMBRANE FORMATION

Since most studies of asymmetric membrane formation by the phase inversion method have emphasized the development of specific preparation recipes relatively little of the information has been quantified and systematized in terms of fundamental models for the structure formation. This study presents a discussion of mathematical modelling efforts addressing the thermodynamic and kinetic aspects of the processes which occur during the main structure forming step (i.e. quenching) of the membrane preparation. The thermodynamics involves construction of complete ternary phase diagrams for the nonsolvent-solvent-polymer system based on modified Flory-Huggins equations and evaluation of the effects of the diagram characteristics on the formation of a given membrane structure. One very important conclusion of the thermodynamic calculations is the demonstration of the importance of the concentration dependency of the solvent-polymer interaction parameter on phase diagram characteristics. A pseudobinary diffusion equation formalism for the mass transfer process which enables calculation of fluxes and concentration profiles and their superposition on the phase diagram was presented. In the pseudobinary approach the mixing rule is decoupled from the diffusion equation allowing their separate specification and the independent modification of thermodynamic and mass transfer models. Ternary concentration plots are used to predict formation conditions for completely dense structures and differentiate these conditions from those for either asymmetric or porous structures.; Component concentrations as a function of time and distance are presented for constant diffusivity and for two different surface boundary conditions (Constant flux and mass transfer boundary layer type). These profiles lead to a wide range of predictions concerning the formation of the overall and skin structures which are consistent with existing experimental observations or can be readily investigated by future experimentation.