Membranes Modification Methods

Membrane (top layer) structures are often able to achieve two specific objectives  

To obtain a further decrease of the effective pore size 

MgO- and Ag-modified membranes were obtained by homogeneous precipitation of the hydroxide from a typical solution consisting of 0.75 M urea and 0.2-0.5 M AgN03 or Mg(NOj)2 in water. The solution is introduced into the pores of the support and/or the y-alumina top layer by impregnation. An increase in temperature results in (I) evaporation of the solvent and concentration of the solution and (2) the decomposition of urea (at T 90°C) resulting in the formation of NH3 and a decrease in the pH followed by precipitation of the metal hydroxide. The hydroxide is next converted to the oxide form at 350-450°C. 

Recently the synthesis of zeolitic membranes was reported by Suzuki 

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Development of better membranes and membrane modification methods... 

In order to enhance the overall performance of the membrane, it is necessary to modify the membrane material or the structure (41). The objectives for modification of the existing membranes are to increase flux, selectivity, and chemical resistance (solvent resistance, swelling resistance, and fouling resistance). Some of the most commonly practiced membrane modification methods are listed in Table 3. 

The first reported membrane modification method involved annealing of porous membranes by heat-treatment. Zsigmondy and Bachmann in 1922 demonstrated that... 

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). 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Besides the synthesis methods for porous membranes and their modification methods discussed above, other synthesis methods have been reported. These are outlined below. Preparation of dense membranes is discussed in Section 2.2. The other types are the so-called dynamically formed membranes which... 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

Chemical surface modification methods of gas-separation membranes include treatment with fluorine, chlorine, bromine, or ozone. These treatments result in an increase in membrane selectivity with a decrease in flux. Cross-linking of polymers is often applied to improve the chemical stability and selectivity of membranes for reverse osmosis, pervaporation, and gas-separation applications (41). Mosqueda-Jimenez and co-workers studied the addition of surface modifying macromolecules, and the use of the additive... 

PPE is a versatile material for performing chemical modification on its backbone, either on the phenyl ring or on the methyl group. The backbone of PPE can be modified with various methods. Most common is the introduction of sulfonic acid groups, which can be done with chlorosulf-onic acid. Other backbone modification methods are shown in Table 4.4. The methods are used mainly in the preparation of membranes and will be discussed in this particular section. The modification by bromination and subsequent alkynylation leads to polymers that contain substituted alkynes on the aromatic ring ... 

This chapter deals with CS or its derivatives as a membrane material in the field of membrane technology. This covers the brief history of membranes, qualities of CS as a good membrane material, methods to prepare CS-based membranes, cross-linking agents and its effects, and modification of CS and its applications in the various fields like wastewater purification, pervaporation, fuel cells, and hemodialysis. 

Molecular layering (ML) technique is one of the most promising methods of membrane modification at the atomic level [76, 77]. The ML method is based on the chemisorption of reagents on a solid substrate surface and consists of the irreversible interaction of low-molecular reagents and functional groups of a solid substrate surface under the conditions of continuous reagent feed and the subsequent removal of the formed gaseous products. 

Radiation-indneed grafting is another modification method that utilizes ultraviolet or ionizing radiation to produce active sites on the manbrane snrface for attaching different kinds of groups (IAEA-TECDOC-1465 2005). The polymerization of the monomer grafted to these active sites results in membranes with different properties. The most common monomers used for radiation-induced grafting are vinyl acetate, A-vinyl pyrrolidone, acrylic acid, metacrylic acid, or V-vinyl pyridine. 

Membrane Modification via "Grafting-From" Method Without the Use of a Photoinitiator... 

As a rule, two methods are used for membrane modification via the grafting-from approach without a photoinitiator an immersion method when the membranes are irradiated while immersed in a monomer solution, and a dip method when the membranes are dipped in a monomer solution and then irradiated in nitrogen or another inert gas (Pieracci et al. 2000). 

The last method that should be discussed for controlling fouling, which is also the most attractive and widely investigated, is changing the manbrane properties. For instance, when the hydrophilicity of a manbrane is improved, its antifouling properties would be better in comparison with the hydrophobic membranes, depending on the type of application. The discussion in this chapter is mostly focused on membrane modification by incorporating nanoparticles in polymeric manbranes in order to modify the membrane properties. 

It is well known that the surface chemical and physical properties play a dominant role in the separation characteristics of a membrane. Most of the currently used membranes are made of polymers because they have excellent bulk physical and chemical properties, they are inexpensive, and are easy to process. However, the surface properties of polymers, their hydrophobicity, and their lack of functional groups stand in the way of many other applications (Chan et al. 1996). So far, various polymers have been used for membrane fabrication. However, due to the limited number of polymeric materials on the market, one cannot expect any significant increase in the variety of the membranes offered. What is more, large-scale production of brand-new polymers has not been commercialized during the last decade, nor is it expected to be launched in the near future. These observations have forced material scientists to search for alternative methods to increase the number and variety of membranes being prepared. There are two directions for new membrane manufacturing (i) to modify a polymer in bulk and then prepare the membrane from it or (ii) to prepare the membrane from a standard polymer and then modify its surface. The first method needs the optimization of the membrane formation for the particular derivative separately. The second seems to be less complicated and less expensive, and it can offer a wide variety of new membranes based on one starting matrix. The authors intention is to present the plasma methods for membrane modification and tailor them based on the end-user requests. 

Surface modification is a valuable tool for the design of appropriate membrane, as the interfacial characteristics required can rarely be achieved by bulk modification of the membrane-forming polymer without complications during membrane fabrication (He et al. 2009). Surface modification methods have been employed by the membrane manufacturers to produce hydrophilic, low-binding membranes (Peinemann and Nunes 2008) with improved membrane performances and properties (Pandey and Chauhan 2001 Robeson 1999). These methods include UV... 

Several efforts have been made to improve the performance of Nation, using various modification methods. One of the methods is to incorporate inorganic materials in the Nafion membrane. The incorporation of inorganic materials is believed to improve the proton conductivity and reduce the methanol permeability, because the barrier properties of the membrane are expected to increase as the concentration of rigid backscattering and the tortuous pathways that molecules encounter during the permeation increase due to the presence of inorganic particles (Silva et al. 2005). 

Plasma modification of membrane surfaces discussed in Chapter 7 is a useful technique that will continue to be an essential part of many membrane modification strategies. In a very short time, by using very small quantities of reactants, novel membranes can be prepared. By applying this method, it is possible to create a whole range of new membranes using only one type of porous substrate. The bulk properties of the membrane that infer advantage to the process remain... 

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