Composite membranes, ultrathin film

An ultrathin-film composite membrane selective for theophylline has been reported [48]. The theophylline-imprinted polymer was prepared inside pores of a microporous alumina support membrane with a thickness of 500 nm and a pore size of 20 nm, in which pores of the membrane were filled by the polymerization solution containing the template theophylline, methacrylic acid, and ethylene glycol dimethacrylate, and the membrane was illuminated with UV light for 1 h, followed by immersion in methanol containing 10 %(v/v) acetic acid to remove the template and any excess monomer. Because the membrane is extremely thin, the flux rate is high, being at least two orders of mag-... 

L.M. Moretto, P. Ugo, M. Zanata, P. Guerriero, and C.R. Martin, Nitrate biosensor based on the ultrathin-film composite membrane concept. Anal. Chem., 70(10), 2163-2166 (1998). 

Hong, J.M. Anderson, P.E. Qian, J. Martin, C.R. Selectively-permeable ultrathin film composite membranes based on molecularly imprinted pol mers. Chem. Mater. 1998, 10, 1029-1033. 

Biosensors Based on Ultrathin Film Composite Membranes... 

This approach is based on a concept borrowed from the membrane-based separations area - ultrathin film composite membranes. Ultrathin film composite membranes consist of an ultrathin (less than ca. 100 nm-thick) polymer dcin coated onto the surface of a microporous support membrane. These composite membranes have made a tremendous impact on the field of membrane-bas separations because they can offer high permeate flux without sacrificing chemical selectivity. These two qualities (high permeate flux and high chemical selectivity) are also required in polymeric barrier layers in chemical sensors. Therefore, the ultrathin film composite membrane concept ould be applicable to sensor design. In this paper we present proof of this concept by riiowing the response characteristics of a prototype glucose sensor based on an ultrathin frlm composite membrane. 

The development of ultrathin film composite membranes was one of the most... 

Figure 1 Schematic diagram of the prototype ultrathin film composite membrane-based glucose sensor.

A significant advance was made in the art of thin-film-composite membranes by Cadotte in 1970 with the advent of the NS-lOO membrane ( 5). This reverse osmosis membrane contained an ultrathin aryl-alkyl polyurea formed Insltu on a mlcroporous polysul-... 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

In general, barrier layers in sensors must provide some degree of chemical selectivity, yet must also allow for high rates of analyte flux (so as to minimize sensor response time). These membrane requirements (high selectivity and high flux) are identical to the requirements in the membrane separations area (1). Hence, if ultrathin film composites are ideal in this area, these composites should also be ideally-suited for sensor applications. 

Thin film composite (TFC) is an ultrathin barrier membrane on polysulfone support layer, of good chemical stability. It has a wide operating pH range of 2.0 to 12.0 at 0 to 40 °C, but cannot tolerate chlorine. TFC membranes are better at rejecting silica than CA membranes. 

Suzuki, H. (1987) Composite membrane having a surface layer of an ultrathin film of cage-shaped zeolite and processes for production thereof US Patent 4,699,892. 

The structure of the so-called "composite" membranes used in reverse osmosis is also much more complex than the conventional, simplistic description of the ultrathin semipermeable film deposited on and supported by a porous substrate. Most of these membranes which exhibit high flux and separation are composed of an anisotropic, porous substrate topped by an anisotropic, ultrathin permselective dense layer which is either highly crosslinked, or exhibits a progressively decreased hydrophilicity toward the surface. The basic difference between the conventional anisotropic (asymmetric) membrane and the thin film composite is that the latter might be... 

The origin of thin-film-composite reverse osmosis membranes began with a newly formed research institute and one of its first employees, Peter S. Francis. North Star Research and Development Institute was formed in Minneapolis during 1963 to fill a need for a nonprofit contract research institute in the Upper Midwest. Francis was given the mission of developing the chemistry division through support, in part, by federal research contracts. At this time the Initial discoveries by Reid and Breton ( ) on the desalination capability of dense cellulose acetate membranes and by Loeb and Sourlrajan (,2) on asymmetric cellulose acetate membranes had recently been published. Francis speculated that improved membrane performance could be achieved, if the ultrathin, dense barrier layer and the porous substructure of the asymmetric... 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

The apparatus used to make small sections of water-cast composite membranes is shown in Figure 3.23. The dilute polymer solution is cast on the surface between two Teflon rods. The rods are then moved apart to spread the film. The thin polymer film formed on the water surface is picked up on a microporous support. The main problem with this method is the transfer of the fragile, ultrathin film onto the microporous support. This is usually done by sliding the support... 

The methods for preparation of nonporous composite membrane catalyst are discussed in Ref. 10. The porous stainless steel sheets were covered with a dense palladium alloy film by magnetron sputtering [113] or by corolling of palladium alloy foil and porous steel sheet. The electroless plating of palladium or palladium alloy on stainless steel [114] or on porous alumina ceramic [115,116] gives the composite membranes with an ultrathin, dense palladium top layer. 

The first composite reverse osmosis membrane reported in the technical literature was developed by Peter Francis of North Star Research Institute in 1964 (4). This membrane was formed by float-casting an ultrathin film of cellulose acetate (CA) upon a water surface, removing the membrane from the water surface by lamination onto a pre-formed microporous support film and drying to bond the membrane to the support. This float-casting procedure has since been described in the technical literature for both flat sheet and tubular membranes ( 5, 6, T). 

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