Does more solvent in bore liquid create more open inner surface in hollow fiber membranes?

Abstract

AbstractThe solvent content in the bore liquid controls the inner structure of a hollow fiber membrane. Based on the theory of liquid–liquid phase separation, at a higher solvent concentration, the inner surface is more open and the transport resistance is lower. However, the theory does not describe clearly the membrane structure formation at a very high solvent concentration. A logic deduction is that more solvent in the bore liquid results in more open inner structure. In this report, a polymer solution, polysulfone (PSf)/N‐methylpyrrolidone (NMP)/diethylene glycol (DegOH) was used to investigate the relationship between the membrane resistance and solvent content in the bore liquid. A triple orifice spinneret was used to prepare an open outer surface, minimizing the transport resistance in the membrane outer layer. Bore liquid solutions with NMP concentration close to the critical solvent concentration (CSC) were used. The membranes were characterized by scanning electron microscopy (SEM), pore size and pore size distribution, nitrogen and pure water permeability. The results indicated that at a concentration far below the CSC, a higher NMP content corresponds to a more open inner surface, but the aggressive addition of solvent to a concentration close to CSC tends to form closed cells; thus water and gas permeability significantly decreases. This phenomenon was explained by nuclei formation and growth, followed by the formation of a dense skin and closed cell structure in a mild coagulant. Copyright © 2007 John Wiley & Sons, Ltd.