SYNOPSISBoth flat-sheet and tubular composite reverse osmosis (RO) membranes were prepared by depositing aqueous solutions of poly (vinyl alcohol) (PVA) and a dehydration catalyst on asymmetric poly (arylether sulfone) (PES) substrate membranes. The PVA coatings were insolubilized by heat treatment to create stable hydrophilic gel-layer membranes. The influence of variables such as PVA concentration, catalyst concentration, curing time, and curing temperature was investigated. It was shown that a simple manipulation of one or two variables could lead to membranes with widely differing salt retention and water permeability characteristics. The insolubilized PVA coatings were intended to serve as hydrophilic gel sublayers on which ultrathin salt-retention barriers could ultimately be formed by interfacial polycondensation. For this purpose, high-flux gel layers were required, whereas salt-retention capabilities were not regarded as important. However, the promising salt retentions obtained at 2 MPa (up to 85% NaCl retention and 92% MgS04 retention) showed that some of these PES-PVA composite membranes could function as mediumretention, medium-flux RO membranes, even in the absence of an interfacially formed salt-retention barrier. Several methods have been developed to make very thin synthetic membranes. Only two of these have become technologically important, namely, ( i ) the original approach of Loeb and Sourirajanl for the preparation of asymmetric membranes by the solvent-nonsolvent phase-inversion process, and ( ii ) the method of interfacial polymerization.' When carried out at one surface of a porous support material, this polymerization technique is widely used commercially as a means of producing thin-film composite reverse osmosis (RO) membranes.During the last two decades, primary emphasis in nonpolysaccharide membrane research and development has focused on aromatic polyamides and other related rigid nitrogen-containing hydrophilic heterochain polymers, e.g., aromatic polyhydrazides and p~lyureas.~ Although a wide variety of materials exist to choose from for RO applications, the various aromatic polyamides in integrally skinned (asymmetric ) and interfacially formed composite membranes have, for many years, been the most attractive candidates on the basis of their performance and cost. These membranes exhibit water permeability and salt-rejection characteristics, as well as thermal, biological, and chemical resistance properties, superior to those of conventional asymmetric cellulose acetate membranes.As a result of the inherent chain rigidity and a low extent of water-induced swelling, these aromatic backbone polymers exhibit low water permeabilities and, consequently, the membranes have to be operated at high transmembrane pressures (6-10 MPa) to obtain useful water fluxes. This makes them unsuitable for desalination of aqueous salt so-
