Membrane preparation anisotropic membranes

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

The membrane classification scheme described above works fairly well. However, a major membrane preparation technique, phase separation, also known as phase inversion, is used to make both isotropic and anisotropic membranes. This technique is covered under anisotropic membranes. 

Other anisotropic membranes. This category covers membranes made by a variety of specialized processes, such as plasma deposition, in the laboratory or on a small industrial scale to prepare anisotropic membranes for specific applications. 

The production by Loeb and Sourirajan of the first successful anisotropic membranes spawned numerous other techniques in which a microporous membrane is used as a support for a thin, dense separating layer. One of the most important of these was interfacial polymerization, an entirely new method of making anisotropic membranes developed by John Cadotte, then at North Star Research. Reverse osmosis membranes produced by this technique had dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Souri-rajan process. Almost all reverse osmosis membranes are now made by the interfacial polymerization process, illustrated in Figure 3.20. In this method, an aqueous solution of a reactive prepolymer, such as a polyamine, is first deposited in the pores of a microporous support membrane, typically a polysul-fone ultrafiltration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, such as a diacid chloride in hexane. The amine and acid chloride react at the interface of the two immiscible... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

J.G. Wijmans and C.A. Smolders, Preparation of Anisotropic Membranes by the Phase Inversion Process, in Synthetic Membranes Science, Engineering, and Applications, P.M. Bungay, H.K. Lonsdale and M.N. de Pinho (eds), D. Reidel, Dordrecht, pp. 39-56 (1986). 

By far the majority of polymeric membranes, including UF membranes and porous supports for RO, NF or PV composite membranes, are produced via phase separation. The TIPS process is typically used to prepare membranes with a macroporous barrier, that is, for MF, or as support for liquid membranes and as gas-liquid contactors. In technical manufacturing, the NIPS process is most frequently applied, and membranes with anisotropic cross-section are obtained. Often,... 

Therefore further efforts were directed to prepare an anisotropic membrane using some weakly acidic cation exchange groups. 

T. Sata, Properties of ion-exchange membranes combined anisotropically with conducting polymers. 2. Relation of electrical potential generation to preparation conditions of composite membranes, Chem. Mater. 1991, 3, 838-843. 

Kimura Y, Chen J, Asano M, Maekawa Y, Katakai R, Yoshida M. Anisotropic protonconducting membranes prepared from swift heavy ion-beam irradiated ETFE films. Nucl... 

Membranes are prepared from polymer(s) dissolved in a solvent using either a dry process or a wet process. In the dry process, a volatile solvent is used for dissolving the polymer(s) and the extruded polymer solution is transferred into an evaporation chamber to yield a porous, isotropic or anisotropic membrane. In the wet process, on the other hand, the extruded mixture is coagulated by exposing the mixture to a nonsolvent in the form of vapor or liquid. The latter process is often referred to as the phase inversion process. 

Investigations of the structure and dynamics of membrane-associated peptides by MAS NMR have been reviewed by Huster/ Membrane-associated peptides typically consist of up to 40 amino acid residues and can bind to phospholipid membranes by either hydrophobic or electrostatic interactions or a combination of both. Once membrane bound, peptides undergo motions with restricted geometry that are not sufficient to average out the anisotropic NMR interactions. The review by Huster is devoted to structural and dynamical investigations of membrane-associated peptides by solid-state MAS NMR techniques. The literature covered by the article is restricted to MAS applications of unoriented samples only and omits all NMR studies on oriented membrane preparations including some specific techniques. 

Cellulose acetate was the first material used to make RO membranes. This kind of material was first used by Loeb and Sourirajan in 1963. Nowadays the use of these membranes is limited due to their lower performance than composite membranes. Cellulose acetate membranes are inexpensive, very easy to prepare, resistant against oxidants and mechanically tough in nature. The membrane has an asymmetric or an anisotropic structure and consists of a thin active layer on a coarse supportive layer. However, this kind of membrane is very sensitive towards the pH and temperature of the feed water. Thus, it is better to maintain the feed water pH between 4 and 6 because the membranes are slowly hydrolyzed with time and above 35°C, the properties of the membrane change (Vos et a/., 1966). Moreover, these types of membranes are very susceptible to biological attack. 

Another important group of anisotropic composite membranes is formed by solution-coating a thin (0.5-2.0 xm) selective layer on a suitable microporous support. Membranes of this type were first prepared by Ward, Browall, and others at General Electric [52] and by Forester and Francis at North Star Research [17,53] using a type of Langmuir trough system. In this system, a dilute polymer solution in a volatile water-insoluble solvent is spread over the surface of a water-filled trough. 

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