Composite membranes, synthetic

Table II displays several thousand hours data on various samples of Quantro II tested against synthetic seawater at 15-20°C. Routine samples exhibit a flux in excess of 1 gfd at rejection of 98-99%. It is possible with composite membrane preparation to trade rejection for flux within certain limits. The objective of present work is to maintain a rejection of 99+ against seawater while modifying formulation in order to achieve a higher flux.

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

Davis, R.B. Schiffer, D.K. and Kramer C.E. "Hollow Fiber Reverse Osmosis Composite Membranes Process and Properties", ACS Symposium Series 153, Synthetic Membranes, Vol 1 ACS, Washington D.C. 367, 1981. 

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

Tubular (membrane, sponge) Composite CoUagen/synthetic polymer CoUagen/biological polymer CoUagen/ceramic... 

The in situ construction of the inorganic component within a cast polymer solution is not limited to metal oxides and in practice a range of other inorganic materials can be formed depending on the choice of precursor(s) incorporated in the polymer solution, and the nature of post-treatment following solvent removal. Roziere and Jones and co-workers have developed nano composite membranes in which zirconium phosphate is formed from zirconyl propionate introduced into a DMAc solution of sPEEK, by immersion of the cast film, after solvent removal, into phosphoric acid. This approach provides a robust synthetic route that can be generalised to other ionomers, and allows the amount of ZrP to be readily varied, even up to ca. 40-50 wt. %. 

Most commercial membrane separations use natural or synthetic, glassy or rubbery polymers. To achieve high permeability and selectivity, nonporous materials are preferred, with thicknesses ranging from 0.1 to 1.0 micron, either as a surface layer or film onto or as part of much thicker asymmetric or composite membrane materials, which are fabricated primarily into spiral-wound and hollow-fiber-type modules to achieve a high ratio of membrane surface area to module volume. 

The overall activity of a transporter is influenced by numerous parameters, which include buffer and membrane composition, membrane polarization, and osmotic stress, to name only a few. The comparison of the intrinsic activity of different transporters on an absolute scale is nearly impossible for this reason. This is not further problematic because absolute activities are probably the least interesting aspect of synthetic transport systems and arguably deserve little priority. What really matters is responsiveness to specific chemical or physical stimuh. This includes sensitivity toward membrane composition, membrane potential, pH, anions, cations, molecular recognition, molecular transformation (catalysis), or light. These stimuli-responsive, multifunctional, or smart transport systems are attractive for use in biological, medicinal, and materials sciences. Standard techniques to identify such unique characteristics rather than absolute activities or mechanistic details are outlined in this section. 

Figure 7.2 Scanning electron microscopy images of polysulfone composite membrane for carbon nanotubes active surface (a) and porous surface (b) and for the polysulfone membrane synthetized with Tween 80 surfactant active surface (c) and porous surface (d). All microscopies are effectuated at xlOO.

In order to solve the problems that occurred with unmodified cellulosic membranes, synthetic membranes were developed. The first synthetic polymeric membrane was produced in the early 1970s. Since that time, various synthetic polymers such as poly-sulfone, polyamide, poly(methyl methacrylate), polyethersulfone, polyethersulfone/ polyamide have been used in the production of synthetic hemodialysis membranes [20,21]. Synthetic membranes have large mean pore size and thick wall structure. These properties provide high ultrafiltration rate, which is necessary for hemodialysis to be achieved with relatively low transmembrane pressures [20]. The main difference in synthetic and cellulosic membranes is the chemical composition of the membrane. Synthetic membranes are made from manufactured thermoplastics, while both modified and unmodified cellulosic membranes are prepared from natural polymers [20]. 

Watanabe et al. [26] and Antonucci et al. [27] investigated the reduction of water above 100°C to maintain proton conductivity by incorporating hydrophilic micronsized metal oxide particles such as SiO and TiO into Nafion with limited success. Peak power densities of 250 and 150 mW-cm" in oxygen and air, respectively, were reported. Mauritz et al. used an in situ sol-gel technique to intfoduce a polymeric form of SiO into Nafion to form composite membranes [28]. Recently, Adjemian et al. utilized Mauritz synthetic procedure to fabricate Nafion/SiOj membranes and demonstrated that water management within Nafion improved at elevated temperatures in a 3-atm pressurized H /O PEMFC [29]. Various PFSAs, including Nafion and Aciplex, were studied as pure and in the SiO composite membranes for operation in H /O PEMFCs from 80 to 140°C. These cells demonstrated acceptable current densities, for instance, Nafion-IH/SiO and Nafion-112/Si02 achieved 850 and 1280 mA cm", respectively, at 0.4 V up to at least 130°C, 3 atm pressure, and 100% relative humidity. 

Although this article is limited to synthetic membranes, excluding all biological stmctures, the topic is stiU large enough to include a wide variety of membranes that differ in chemical and physical composition and in the way they operate. In essence, a membrane is a discrete, thin interface that moderates the permeation of chemical species in contact with it. This interface may be moleculady homogeneous, that is, completely uniform in... 

The lipid content of the membranes can be varied, allowing systematic examination of the effects of varying lipid composition on certain functions. For instance, vesicles can be made that are composed solely of phosphatidylchohne or, alternatively, of known mixtures of different phospholipids, glycohpids, and cholesterol. The fatty acid moieties of the lipids used can also be varied by employing synthetic lipids of known... 

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