ABSTRACT
A membrane is a device which allows separation of materials roughly on the basis of their molecular size, although other factors are also important.
1.2.1 Materials
A wide variety of materials have been used for membranes as shown in Table 1.1. The first membranes used commercially were uniform in structure and had very slow flow rates (or fluxes) through them. In the late 1950s Loeb and Sourirajan2 developed a method of phase inversion for the preparation of cellulose acetate membranes which involved dissolving the ester in a solvent to make a viscous solution which was poured in a thin film onto glass. The ester was then precipitated by contacting the upper surface of the film with cold water. The addition of various pore-formers and conditioning agents was later found to improve the product and
it was found possible to achieve a large variety of pore sizes. In the early 1960s Michaels3 made an asymmetric polyionic membrane and now a wide variety of polymers have been formed into membranes. Recently rather coarse membranes have been made from sintered ceramics, from stainless steel and from alumina using an anodising process. Other membranes are produced by biaxially stretching a polymeric sheet; in the first stretch the pores are created, and then they are opened by stretching at right angles to the first stretch. At least one type of membrane is made from polycarbonate by irradiating the membrane and then etching out along the tracks taken by the radiation.
