Calculation of alcohol-acetone-cellulose acetate ternary phase diagram and their relevance to membrane formation

Alcohol-acetone-cellulose acetate phase diagrams incorporated with methanol, ethanol, and isopropanol as nonsolvents are calculated according to a new form of the Flory-Huggins equation. Nonsolvent-cellulose acetate interaction parameters are measured by swelling experiments. Concentration-dependent nonsolvent-solvent interaction parameters are obtained by vapor-liquid equilibrium and the Wilson equation. It is shown that alcohol is a week coagulant compared with water, and water > methanol > ethanol > isopropanol for cellulose acetate. The phase diagrams characteristic of acetone-cellulose acetate combined with water, methanol, ethanol, and isopropanol as nonsolvents is different, which leads to the different morphological structure of a cellulose acetate membrane. The structure of a water coagulated membrane has large macrovoids from liquid-liquid phase separation. A methanol coagulated membrane has a honeycomb-like structure from spinodal microphase separation. An ethanol or isopropanol coagulated membrane has a thicker, dense top layer from the delay time phase separation. (C) 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1650-1657, 2001.