GB835137A - Film-forming compositions, ion selective permeable membranes made therefrom and method of preparation thereof - Google Patents

Abstract

Ion selective permeable membranes of cast homogeneous polymer mixtures of water-in-soluble, acid and alkali-resistant, thermoplastic, film-forming polymers soluble in organic solvents and substantially linear water-soluble polyelectrolytes, the proportion of polyelectrolyte being at least 10% by weight of the polymer mixture and the membrane being cast in the form of a finely porous film at least 0.001 inch thick, may be prepared by casting a film of the polymer mixture from an organic solvent solution thereof and contacting the film with a polar liquid. The film-forming polymer may be a homopolymer of vinyl chloride or vinylidene chloride, a polyvinyl acetal, e.g. polyvinyl butyral, or a copolymer of vinyl chloride and acrylonitrile or vinylidene chloride. The polyelectrolyte may be polystyrene sulphonic acid, polyacrylic acid, metal chelating polymers, poly N-methyl, 2-vinyl-pyridinium iodide, polystyrene o- and p-trimethyl ammonium iodide, poly (vinylbenzyl trimethyl ammonium chloride), quaternized poly-N-vinyl and poly-N-allyl amines and salts of polyamines containing primary, secondary or tertiary amino groups. The polymer mixture may be dissolved in a solvent for both, e.g. dimethyl formamide, but preferably in a mixture comprising a solvent for the film-forming polymer, e.g. acetone, nitromethane, nitroethane, dimethylformamide, cyclopentanone, cyclohexanone, dimethyl acetamide, N,N - dimethyl acetamide and N,N,N1,N1 - tetramethyloxamide, and a solvent for the polyelectrolyte, e.g. an aliphatic alcohol containing up to 6 carbon atoms such as methanol, ethanol and isopropanol. Films may be cast on glass plates or chromium-plated drums, and multi-layer films may be cast on rotating cylinders by allowing previously cast film to set and applying another coating. The casting solutions may be sprayed upon porous bases, supports or frameworks, e.g. plastic screens, porous ceramic materials and insulated metal screens. After drying the films may be treated with water, methanol or isopropanol. To facilitate the preparation of the casting solution the polyelectrolyte may be rendered more hydrophobic by using a small amount of a cross-linking comonomer, e.g. divinyl benzene, or by forming an ester, e.g. the methyl ester, or amine salt, e.g. with tetrabutyl ammonium hydroxide, tetramethyl ammonium hydroxide, aniline or dimethyl aniline, of an acid resin such as polystyrene sulphonic acid, and hydrolysing after the formation of the membrane. Polystyrene sulphonic acid may be prepared by heating polystyrene with concentrated sulphuric acid in the presence of silver sulphate and poly (vinylbenzyltrimethylammonium chloride) by chloromethylating polystyrene and aminating the product with trimethylamine. The membranes may be used on deionizing water, concentrating dilute salt solutions, separating ionic mixtures, the preparation and subsequent purification of organic acids and amines from their salts and in physical and biological research investigations.