Polymer membrane microporous

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

Various theories have been proposed to describe the transport in all of these types of polymer membranes. Theories for macroporous and microporous membranes have been based on hydrodynamic and frictional considerations while those for nonporous gels have been based on Eyring s theory and use a free volume approach to describe the movement of solute through the mesh of the polymer. 

Other modifications to the theory of Anderson and Quinn [142] have been reviewed by Deen [146]. Malone and Quinn [147] modified the above theory to include the effect of electrostatic interactions on transport in microporous membranes. Smith and Deen [148] have also looked at these electrostatic or double layer interactions. More recently, Kim and Anderson [149] investigated the hindrance of solute transport in polymer lined micropores. Also, as briefly mentioned above, an excellent review of the theories presented for transport in microporous membranes has been given by Deen [146]. 

Although the literature of gas separation with microporous membranes is dominated by inorganic materials, polymer membranes have also been tried with some success. The polymers used are substituted polyacetylenes, which can have an extraordinarily high free volume, on the order of 25 vol %. The free volume is so high that the free volume elements in these polymers are probably interconnected. Membranes made from these polymers appear to function as finely microporous materials with pores in the 5 to 15 A diameter range [71,72], The two most... 

Although these microporous membranes are topics of considerable research interest, all current commercial gas separations are based on the dense polymer membrane shown in Figure 8.2. Separation through dense polymer films occurs by a solution-diffusion mechanism. 

Clearly, in such a reaction the removal of the produced water will lead to an enhanced conversion. Commercially available polymer membranes cannot withstand the severe operation and cleaning conditions for this process (150-300°C) and microporous silica membranes again come into the picture. 

Ion channels play an essential role in medical diagnostics and drug development. Such applications require the integration of ion channels together with a lipid bilayer into an artificial microstructured polymer membrane (Fig. 4). The polymer membrane is attached to a metal coated optical prism. The membrane contains micropores of approximately one micrometer in diameter. Lipid bilayers are stretched across the pores. The bilayers host the receptor molecules. After activation of an ion channel, thousands of ions stream into the cavity below the ion channel. The change of ion concentration can easily be detected by SPR measurements. 

In some cases, the rate-controlling polymeric membrane is not compact but porous. Microporous membranes can be prepared by making hydrophobic polymer membranes in the presence of water-soluble materials such as polyethylene glycol), which can be subsequently removed from the polymer matrix by dissolving in aqueous solution. Cellulose esters, loosely cross-linked hydrogels and other polymers given in Table 4.2 also give rise to porous membranes. 

Application of polymer membranes to separation of aqueous and organic phases in liquid-liquid extraction processes is called microporous membrane liquid-liquid extraction (MMLLE). An organic acceptor solvent, filling the pores of the hydro-phobic membrane, stays in direct contact with the aqueous phase near the membrane surface, where mass transfer takes place. This kind of extraction is similar to SEME, but takes place in a two-phase system and is slower and less selective because of the absence of carrier agent. Because the polymer membranes are insoluble, an arbitrary combination of aqueous and organic phase is possible and the extraction efficiency mainly depends on the partition coefficient. 

Note SLM, supported liquid membrane (aq/org/aq) MMLLE, microporous membrane liquid-liquid extraction (aq/org) PME, polymer membrane extraction (aq/polymer/org) MESI, membrane extraction with sorbent interface (aq (or gas)/polymer/gas/sorbent) CFLME, continuous flow liquid membrane extraction (aq/org (in flow)/aq) LPME2, two-phase liquid phase microextraction in hoUow fibers (aq/org) LPME3, three-phase liquid phase microextraction in hollow fibers (aq/org/aq). 

We have used apoenzymes as molecular-recognition transporters for binding and selective transportation of molecules without production of unwanted chemical reactions on reactants. Figure 24.10 shows the design of membrane consisting of microporous polymer membrane sandwiched between two thin films of polypyrrole [3]. The apoenzymes are physically trapped within the pores of the membrane and polypyrrole films. More details regarding the fabrication of the membranes, materials, and experimental setup can be found in the Ref. [3]. 

The symmetric, microporous polymer membranes made by phase inversion are widely used for separations on a laboratory and industrial scale.22 Typical applications range from the clarification of turbid solutions to the removal of bacteria or enzymes, the detection of pathological components, and the detoxification of blood in an artificial kidney. The separation mechanism is that of a typical depth filter which traps the particles somewhere within the structure. In addition to the simple "sieving" effect, microporous phase inversion membranes often show a high tendency of adsorption because of their extremely large internal surface. They are, therefore, particularly well suited when a complete re-... 

Most technically utilized homogeneous polymer membranes consist of a composite structure where a very thin homogeneous selective polymer film is supported by a thicker microporous structure providing the mechanical strength. 

It was not equally obvious that dense ceramic hydrogen-permeable membranes would be of similar interest. There are clearly needs for hydrogen purification membranes, but polymers and microporous materials as well as metals such as palladium and its alloys appeared to fill these needs. In addition, possible candidates for dense ceramic hydrogen-permeable materials were not as appealing as the oxygen-permeable ones in terms of performance and stability. 

The majority of polymer membranes used for microfiltration and ultrafiltration of liquids are prepared by the wet phase inversion process. Such membranes exhibit a typical asymmetric structure characterized by a thin dense surface layer and a thick microporous bulk. Poly(phthalazinone ether sulfone ketone) (PPESK) copolymers, c.f. Figure 7.10, show glass transition temperatures in the range of 263-305°C. The polymers show an outstanding chemical stability. They are soluble only in 98% H2SO4. Concentrated aqueous solutions of sodium chlorate, hydrogen peroxide, acetic acid, and nitric acid show no effect. ... 

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