Preparation of Porous Membranes

A variety of techniques have been developed to produce various porous membranes - such as particle dispersion and dip-coating, sol-gel processing, chemical vapor deposition, phase separation and leaching, pyrolysis, anodic oxidation, and so on [6, 7]. A good method should be of simple operation, good control of membrane quality (no cracks and pinholes, homogeneous thickness, narrow pore size distribution, and sufficient mechanical strength), as well as low cost. 

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Fig. 13. Phase diagram showing the composition pathway traveled by a casting solution during the preparation of porous membranes by solvent evaporation. A, initial casting solution B, point of precipitation and C, point of soHdification. See text.

The mere preparation of porous membranes is accompanied with a noticeable decrease of permselectivity 11, which is undesirable for reverse osmosis and ultrafiltration, A thin dense layer should be adopted to attain a high permeability with — out the decrease of permselectivity, but this necessarily decreases the mechanical strength. This conflict is largely resolved by the construction of asymmetric or composite membranes as described also in the present review. 

Chung DJ, Ito Y, Imanishi Y (1994) Preparation of porous membranes grafted with poly (spiropyran-containing methacrylate) and photocontrol of permeability. J Appl Polym Sci 51 2027-2033... 

In the preparation of porous membranes, if one makes use of phase separation followed with crystalline solidification in polymer solutions to control the pore sizes, whether does the longer time for phase separation make larger sizes of pores ... 

Titania membranes show excellent chemical resistance, and can be used in both acidic and basic pH and moreover, they show interesting photocatalytic activity. Titania UF membranes have been commercialized by several companies. At present, extensive efforts have focused on the preparation of porous membranes having small pore sizes in the NF range (1-2 nm). Since pore sizes are believed to be controlled by the sizes of the packed particles, controlling the sol size is a crucial process for membrane processing. Anderson et al. [23] prepared nanosized TiOi and Zr02 particles (3-5 nm) by carrying out hydrolysis and condensation reactions of metal tcrt-amyloxide with a small amount of water (molar ratio of H20/Ti = 3—5) in tcrt-amyl alcohol solutions. Hydrolysis and condensation reactions of metal alkoxides are written as follows ... 

Yan, F. and Goedel, WA. (2004) A simple and effective method for the preparation of porous membranes with three-dimensionally arranged pores. Adv. Mater., 16 (11), 911-915. 

Liang Z, Susha AS, Yu A, Caruso F (2002). Nanotubes prepared by layer-by-layer coating of porous membrane templates. Adv. Mater. 14 1849-1853. 

Bolwin, K., Giilzow, E., Bevers, D., and Schnurnberger, W. Preparation of porous electrodes and laminated electrode-membrane structures for polymer electrolyte fuel cells (PEFCs). Solid State Ionics 1995 77 324-330. 

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... 

Composite membranes combine two or more different materials with different characteristics to obtain optimal membrane performance. Basically, the preparation involves (i) preparation of porous support that is usually made by a phase-separation process (cf. Section 2.4.2), and (ii) deposition of a selective barrier layer on this porous... 

Microporous and, particularly, ultramicropous membranes are more difficult to characterize. Different procedures based on the low-pressure part of the N2 adsorption isotherm have been proposed [36], but they often require knowledge of the shape of the pores and of gas-surface interaction parameters which are not always available. Small angle X-ray scattering (SAXS) is another technique which is well suited to micro-porous powders, but difficult to execute in the case of composite layers, as in microporous membranes. Xenon-129 NMR has recently been proposed [37] for the characterization of amorphous silica used in the preparation of microporous membranes, but the method requires further improvement. Methods based on permeability measurements appear to be limited by the lack of understanding of the mass transport mechanisms in (ultra)microporous systems. 

Unlike the RDE technique, which is quite popular for characterizing catalyst activities, the gas diffusion electrode (GDE) technique is not commonly used by fuel cell researchers in an electrochemical half-cell configuration. The fabrication of a house-made GDE is similar to the preparation of a membrane electrode assembly (MEA). In this fabrication, Nation membrane disks are first hot-washed successively in nitric acid, sulphuric acid, hydrogen peroxide, and ultra-pure water. The membranes are then coated with a very thin active layer and hot-pressed onto the gas diffusion layer (GDL) to obtain a Nation membrane assembly. The GDL (e.g., Toray paper) is very thin and porous, and thus the associated diffusion limitation is small enough to be ignored, which makes it possible to study the specific kinetic behaviour of the active layer [6],... 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

The microstnictuie of a porous membrane can vary according to the schematic in Figure 1.2. The shape of the pores is strongly dictated by the method of preparation which will be reviewed in Chapter 3. Those membranes that show essentially straight pores across the membrane thickness are referred to as straight pore or nearly straight pore membranes. The majority of porous membranes, however, have interconnected pores with tortuous paths and are called tortuous pore membranes. 

Preparation of Porous Hollow Fiber Membranes by Extrusion... 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

Soon after the first reports on the layer-by-layer adsorption appeared, the method was also used for surface modification of polymers [60, 61], and for the preparation of composite membranes [62-65]. Composite membranes were obtained by alternate dipping of porous supports into solutions of cationic and anionic polyelectrolytes so that an ultrathin separation layer was... 

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