The effect of polymer solution composition and film-forming procedure on aromatic polyamide membrane skin layer structure

Abstract

Linear polymer phase inversion membrane casting solutions are composed of suspensions of relatively spherical polymer macromolecules. Estimates of macromolecule dimensions can be calculated, dependent on the type and concentration of the components in each particular solution. Membrane skin layer formation may be considered as interpenetration or partial coalescence and subsequent crystallization of solution surface macromolecules. The surface morphology of asymmetric membranes can be characterized by assuming a porous structure. With knowledge of the surface porous structure, membrane performance can be predicted for most solute—solvent combinations and operating conditions. It is assumed that all permeate transport through a membrane skin layer occurs between surface paracrystalline macromolecules. Macromolecule to pore radius comparisons can then be used to determine the effect of the casting composition and/or membrane-making procedure on the disposition and subsequent degree of interpenetration of surface macromolecules. Relationships between the casting composition/membrane-making procedure and interstitial coagulation may be used for membrane design considerations. This method of analysis is shown to be useful in examining reverse osmosis (RO) and ultrafiltration (UF) aromatic polyamide films.