GB723233A - Improvements in and relating to semi-permeable membranes - Google Patents

Abstract

A film of polyvinyl alcohol is subjected to a swelling treatment in presence of water to a stage beyond that at which the porosity begins to decrease considerably more rapidly than the decrease in the rate of transference of substances through the film, when the latter is used as a semi-permeable membrane, but not beyond the stage at which the rate of transference (i.e. the speed of the membrane) becomes impracticably low, and the film is then conditioned to a non-aqueous liquid in which it is to be used as a semi-permeable membrane. The water content of the film may be increased by the swelling treatment to a value between 6 and 30 per cent weight for weight. The swelling may be effected by exposing the film to an atmosphere of high relative humidity, e.g. for five days in an atmosphere of relative humidity 65 per cent or more at 15 DEG C. Alternatively the film may be soaked in an aqueous solution, e.g. a mixture of ethanol and water. The conditioning may be effected by soaking the film successively in dry ethanol, in a mixture of ethanol and the non-aqueous solvent in which the film is to be used, and finally in the latter solvent alone. In an example, polyvinyl alcohol, obtained from polyvinyl acetate by alkaline hydrolysis, is dissolved in boiling water to form a solution containing 1.5-2 grams of polyvinyl alcohol per 100 ml. of water. The solution is cooled, filtered, poured on to a glass plate, and allowed to evaporate at 25 DEG C. and 30-40 per cent relative humidity for 5 days. The film which is formed is stripped from the glass surface and exposed to an atmosphere of 70 per cent relative humidity for 5 days. It then has a water content of 0.071 grams of water per gram of dry polyvinyl alcohol. It is then conditioned to benzene by soaking first in dry ethanol, then in a 50-50 ethanol-benzene mixture, and then in benzene only. The membrane thus obtained is impermeable to polystyrenes having degrees of polymerization ranging from 18-2100. A similarly swollen film is conditioned to butanone by soaking first in ethanol, then in a 50-50 ethanol-butanone mixture, and then in butanone only. The membrane obtained is impermeable to a phenol-formaldehyde resin of degree of polymerization 20 but is permeable to one of degree of polymerization 7. In another example, a polyvinyl alcohol film is swollen by exposure to an atmosphere of relative humidity 85 per cent until its water content is 8.6 per cent. The film is conditioned to butanone and used as a semi-permeable membrane for completely separating a yellow dye of molecular weight 4000 from Victoria blue (molecular weight 500) in butanone solution. A similar film is conditioned to ethanol and used to separate a phenol-formaldehyde resin fraction of degree of polymerization 7 from one of 5. A polyvinyl alcohol film is swollen by exposure to an atmosphere of 93 per cent relative humidity for 5 days so that it contains 24 per cent of water. The film is conditioned to ethanol and then installed in an osmometer. It is permeable to phenol in ethanol but retains and may be used to measure the molecular weights of a phenol-formaldehyde resin fraction (degree of polymerization 5, molecular weight 524), dihydroxy-diphenylmethane and beta-naphthol. A similar film is conditioned to benzene. It is permeable to anthracene in benzene solution, but retains penta-iodobenzene and diacenaphthylidene.ALSO:A semi-permeable membrane for use in a non-aqueous solvent is obtained by increasing the water content of a film of polyvinyl alcohol by a swelling treatment beyond the stage in which the porosity begins to decrease considerably more rapidly than the speed (i.e. the rate of transference of substances across the membrane) decreases, but not beyond the stage in which the speed becomes impracticably low, and then conditioning the film to the non-aqueous solvent in which it is to be used. The water content of the film may be increased by the swelling process to a value between 6 and 30 per cent. weight for weight. The swelling may be effected by exposing the film to an atmosphere of high relative humidity, e.g. to air of relative humidity 65 per cent. or more at 15 DEG C. for five days, or by soaking the film in an aqueous solution e.g. a solution of ethanol and water. The conditioning may be effected by soaking the film successively in dry ethanol, in a mixture of ethanol and the non-aqueous solvent in which the film is to be used, and finally in the latter solvent alone. The membrane produced may be used in apparatus for dialysis, osmosis, ultrafiltration or the like, provided that it is not subjected to any substantial mechanical pressure over a large part of its area, for such pressure may make it impermeable. The membrane should not be used at temperatures as high as 80 DEG to 100 DEG C. In an example, an aqueous solution of polyvinyl alcohol is poured into a casting cell made by sealing a glass ring on to a plate glass surface and allowed to evaporate at 25 DEG C. in air of 30-40 per cent. relative humidity for five days. The film so obtained is stripped and exposed to air of relative humidity 70 per cent. for five days so that its water content rises to 7.1 per cent. as a result of the swelling. The film is then conditioned to benzene by successive soaking in ethanol, ethanol-benzene and benzene. The film is used as a membrane in an osmometer for determining the average molecular weights of samples of polystyrene dissolved in benzene. In other examples, polyvinyl alcohol films are swollen by exposure to air of various relative humidity values ranging from 70-93 per cent. so that the water content ranges from 7-24 per cent. and they are then conditioned to butanone, ethanol or benzene. They are rendered permeable to a phenol-formaldehyde resin of degree of polymerization 7 but impermeable to one of degree of polymerization (D.P.) 20, or permeable to one of D.P. 5 but impermeable to one of D.P. 7, or they are rendered capable of completing separating a yellow dye of molecular weight 4,000 from Victoria blue (molecular weight 500). They may be used in an osmometer to measure the molecular weights of phenol-formaldehyde resin fractions, dihydroxy diphenyl methane, beta-napthol, pentaiodobenzene and diacenaphthylidene.