Synthetic polymer membranes applications

Most membranes are manufactured from synthetic polymers The application of such membranes is generally... 

Douglas R, Lloyd is Assistant Professor of Chemical Engineering at the University of Texas at Austin which he joined in 1981. Prior to his current position, he was (since 1978) in the faculty in the Chemical Engineering Department at VPI SU. He received his Ph. D. degree in Chemical Engineering from the University of Waterloo, and his research Interests Include the preparation, characterization and application of synthetic polymer membranes. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

For example, Novasina S.A. (www.novasina.com), a Swiss company specializing in the manufacture of devices to measure humidity in air, has developed a new sensor based on the non-synthetic application of an ionic liquid. The new concept makes simple use of the close correlation between the water uptake of an ionic liquid and its conductivity increase. In comparison with existing sensors based on polymer membranes, the new type of ionic liquid sensor shows significantly faster response times (up to a factor of 2.5) and less sensitivity to cross contamination (with alcohols, for example). Each sensor device contains about 50 pi of ionic liquid, and the new sensor system became available as a commercial product in 2002. Figure 9-1 shows a picture of the sensor device containing the ionic liquid, and Figure 9-2 displays the whole humidity analyzer as commercialized by Novasina S.A.. 

We have tried to relate the performance of a deteriorated membrane to its structure by classical methods. Recent advancement in the techniques of morphological and physicochemical analyses is remarkable, and is much contributing to better understanding of the membrane behaviour. We have now various types of RO membranes made of synthetic polymers available, and most these analytical procedures are applicable for the analysis of these membranes. Investigations on the membrane structures are much more required, and they will reveal the relations between materials and structure, and structure and performance. We believe these Investigations will contribute to development not only in the membrane Itself, but in the application of the membrane. We hope the progress of membrane science will expand RO marke t. 

To open up new application fields, the structure and properties of BC membranes/composites can be modified by low-molecular organic and inorganic compounds, including monomers and metals/metal oxides, via carbohydrates and polysaccharides, up to different types of other natural and synthetic polymers. 

Polymers for membrane preparation can be classified into natural and synthetic ones. Polysaccharides and rubbers are important examples of natural membrane materials, but only cellulose derivatives are still used in large scale for technical membranes. By far the majority of current membranes are made from synthetic polymers (which, however, originally had been developed for many other engineering applications). Macromolecular structure is crucial for membrane barrier and other properties main factors include the chemical structure of the chain segments, molar mass (chain length), chain flexibility as well as intra- and intermolecular interactions. 

One long-term objective of this research is to utilize the finest attributes associated with the worlds of both biological and synthetic materials to create nanomechanical systems powered by biological motors. Important fields of application include miniaturized (nanofluidic) analytical systems,131 molecular sorting,132 controlled adaptation of materials on a molecular to mesoscopic scale,133 and engineering lipid and polymer membrane systems with cellular processes.134... 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

The range of available membrane materials used in water and wastewater treatment is quite broad, but most of them are synthetic membranes. Synthetic membranes can be organic or inorganic however, the most important class of membrane materials is organic or polymer membrane. The choice of a given polymer as a membrane material is not arbitrary (13). Inorganic materials generally possess superior chemical and thermal stability relative to polymeric materials. However, both types of membranes have different applications. A list of common membranes is shown in Table 2. 

Other applications being developed are composites with synthetic polymers for use as semipermeable membranes. 

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