MEMBRANE MORPHOLOGY - A MODEL CORRELATING THICKNESS OF MEMBRANE-SURFACE SKIN LAYER WITH SOLVENT EVAPORATION TIME

A simple model has been developed which relates membrane surface skin layer thickness to solvent evaporation time. This model can be used to determine other important membrane morphology parameters such as pore number (or pore density) in every pore size distribution on the membr ane surface. With pore number becoming available, membrane morphology can now be more completely, explicitly, and quantitatively characteriz ed. The changes in membrane morphology parameters during the solvent e vaporation step of making cellulose acetate butyrate (CAB) membrane (i n acetone) were investigated based on the Surface Force-Pore Flow (SFP F) model and a new model developed in this work. The analyses show tha t in the solvent evaporation step of making CAB membranes, there exist three distinct morphology stages or phases associated with solvent ev aporation time: (I) intrinsic pore-determining phase, (II) phase of fo rming dominantly massive small pores, and (III) phase of forming domin antly large pores. It is the combined effect of several morphology par ameters that determines the membrane performance, which in this case y ields a high solute separation and permeate rate when solvent evaporat ion time is set at 60 seconds.