Membrane-casting techniques

The classic evaporation technique (9,78) is nonetheless considered relatively slow, and only minimal control over the thickness and uniformity of the final membrane product is achieved (79). 

A photocuring technique based on an acrylate matrix has thus been employed to fabricate calcium and potassium electrodes which dispenses with the volatile casting solvent, e.g. tetrahydrofuran (64,79). Thus, the calcium salt of bis-[4-(l, l,3,3-tetramethylbutyl)phenyl]phosphoric acid (6 mass %) DOPP (18 mass %) Ebecryl 150-bisphenol A type epoxyacrylate (42 mass %) 1,6-hexanedioldiacrylate (26 mass %) and Uvecryl P36-copolymerizable benzophenone photoinitiator (8 mass %) were photolysed with a mercury lamp. Calcium ISEs with Nernstian slopes of 29.7 mV decade were fabricated from the resultant clear, flexible, thin (0.1 mm) films (79). This procedure, which is unsuitable for a sensor or mediator absorbing in the ultraviolet, could find significant application in the realm of ISFET fabrication (64,79). 

Pungor and K. Toth, Acta Chim. Acad. Sci. Hung. 41 (1964) 239. 

Ammann, W.E. Morf, P. Anker, P.C. Meier, E. Pretsch and W. Simon, Ion-selective Electrode Revs. 5 (1983) 3. 

Garbett and K. Torrance, lUPAC Int. Symp. on Selective Ion-sensitive Electrodes, UWIST, Cardiff, April 1973, Paper No. 27. 

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Membrane-casting Techniques. Uhtil recently, PVC membranes have been exclusively formed by solvent casting techniques but which are not well-suited to the fabrication of ISFET devices. Membrane components in tetrahydrofuran are difficult to manipulate on a micro scale and are prone to absorb atmospheric moisture, thus weakening the adhesion at the sensor-ISFET interface. Che innovation which dispenses with the tetrahydrofuran casting stage is based on an in situ photolysis of the model calciun sensor cocktail admixed with monobutyl methacrylate + benzoyl peroxide + benzoin methyl ether at 340 nm (3). Hie resultant matrix adhered well to the ISFET gate and its potentiate trie response compared favourably with the analogous P and P2-MPMA ISE (Table II). 

Shortly after the concept of an asymmetric membrane was established, composite membrane research was initiated. A composite membrane is also asymmetric but it consists of two polymer layers which are the membrane barrier layer and the porous support layer (Figure 4.6). The porous support is formed separately, by conventional membrane casting techniques, from one polymer. The porous support has a thickness of between 75 and 100 micrometers and its porosity is due to numerous small perforations through the support. The membrane barrier layer is a dense thin film of another polymer that is formed or deposited in a subsequent operation on the porous support. The membrane barrier layer varies in thickness from 400 to 1,000 angstroms. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

To investigate the mechanism of membrane formation as a function of the casting techniques listed in point (4) above. 

Perhaps the simplest solution-precipitation membrane preparation technique is thermal gelation, in which a film is cast from a hot, one-phase polymer/solvent solution. As the cast him cools, the polymer precipitates, and the solution separates into a polymer matrix phase containing dispersed pores filled with solvent. Because cooling is usually uniform throughout the cast film, the resulting membranes are relatively isotropic microporous structures with pores that can be controlled within 0.1-10 i m. 

Figure 3.24 Method developed by Ward, Browall and others at General Electric to make multilayer composite membranes by the water casting technique [55]...

Support tortuosity should be minimized to reduce the diffusional path length. However, many membrane preparation techniques, such as casting, produce support materials with tortuous pores. Operating Pressure Considerations... 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

Second, insoluble crosslinked barrier layer compositions are possible, and, in fact, are almost universal in the composite membrane approach. Optimum reverse osmosis performance and chemical stability can be achieved, in part, due to preparation of crosslinked compositions. This is readily possible by the composite membrane approach, but not so simple by the asymmetric membrane approach. The PA-300, FT-30, and PEC-1000 barrier layer compositions, for example, are simply not feasible to prepare by asymmetric film casting techniques. The composite approach, therefore, is far more versatile. 

Film Casting Technique. After the membrane is cast on the glass plate, the acetone is allowed to evaporate from the films for a short time before immersion in water. If evaporation of the acetone were allowed to proceed to completion before the film was immersed in water, the desalinization capacity of the film would be seriously impaired. The proper time interval between casting and immersion depends on the rate of evaporation of acetone from the membrane and hence is a function of the temperature at which the membrane is cast and allowed to remain, prior to immersion. At room temperature, this time interval is relatively short, which made it difficult under these conditions to fabricate membranes which would give satisfactory, reproducible data. For this reason the membranes are cast in a freezer, with all components at about 0° C. It is also... 

With support from the OSW, Reid at the University of Florida pursued an alternative design in the mid 1950 s based on filtration equipment available at the time. His design used pressurized air to drive water across polymeric films. Of the commercially available films, cellulose acetate was the most attractive due to its high salt rejections. Unfortunately, product water permeation rates were low. Since permeation rate was inversely proportional to film thickness, solution casting techniques were developed to form films as thin as possible [10]. Around the time of Hassler s and Reid s work, the term reverse osmosis was adopted to describe membrane desalination [8]. 

Techniques for membrane casting from organic solutions and casting directly from acid solutions have been developed. 

The discovery and the characterization of ionically conducting polymeric membranes (see Chapters 1 and 2) have provided the interesting possibility of developing new types of lithium batteries having a thin-layer, laminated structure. Various academic and industrial laboratories [1-5] are presently engaged in the development of this revolutionary type of battery, i.e. the so-called Lithium Polymer Battery (LPB). The key component of the LPB is the polymeric ionic membrane which acts both as electrolyte and separator furthermore, the membrane can be easily fabricated in the form of a thin film (typically 50 jum thickness) by a number of convenient casting techniques. 

NR membranes were obtained by a casting technique 10 ml of latex was used, annealed at temperatures of 65, 80 and 120 °C and thermal... 

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