Integral-asymmetric membranes

Further approaches to meet the requirement of high selectivity may Include the blending of glassy and rubbery polymers, the chemical alteration of the dense skin-layer of Integral-asymmetric membranes and morphological variations of dense polymer films by proper post-treatment—as exemplified In this paper for CA blend membranes. 

In integral asymmetric membranes, both toplayer and the sublayer consist of the same material. These membranes are prepared by phase inversion techniques. For this reason it is essential that the polymeric material firom which the membrane it to be prepared is soluble in a solvent or a solvent mixture. Because most polymers are soluble in one or more solvents, asymmetric membranes can be prepared from almost any material. 

Khulbe KC, Gagne S, Tabe-Mohammadi A, Matsuura T, Lamarche A-M. Investigation of polymer morphology of integral-asymmetric membranes by ESR and Raman spectroscopy and its comparison with homogeneous films. J. Membr. Sci. 1995 98 201-208 Khulbe KC, Matsuura T, Kim HJ. Raman scattering of PPO membranes. J. Appl. Polym. Sci. 2000 77 2558-2560... 

The thickness of the separative membrane layer for asymmetric membrane structures represents a trade-off between the physical integrity requirement, on the one hand, and the high flux requirement, on the other Current... 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Because the mechanisms are based on pore flow and size exclusion (cf. Section 2.2), the polymer material itself does not have direct influence on flux and selectivity in U F. The U F membranes usually have an integrally asymmetric structure, obtained via the NIPS technique, and the porous selective barrier (pore size and thickness ranges are 2-50 nm and 0.1-1 (im, respectively) is located at the top (skin) surface supported by a macroporous sublayer (cf. Section 2.4.2). However, the pore-size distribution in that porous barrier is typically rather broad (Figure 2.6), resulting in limited size selectivity. 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

One setback for the production of thin-film composite membranes and integrally skinned asymmetric membranes with separating layer thickness of less than 0.2 pm is the defects. A thin coating of a highly permeable polymer can help eliminate the defects. Surface coatings are also applicable in improving the fouling resistance of membranes for UF or NF applications (44). 

An integrally skinned asymmetric membrane with a porous skin layer (hereafter called substrate membrane) is prepared from a polymer solution by applying the dry-wet phase inversion method and dried according to the method described later, before being dipped into a bath containing a dilute solution of another polymer. When the membrane is taken out of the bath, a thin layer of coating solution is deposited on top of the substrate membrane. The solvent is then removed by evaporation, leaving a thin layer of the latter polymer on top of the substrate membrane. 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

The era of economically viable membrane development which began In the late 1950 s and continues to this date, may be divided into two time periods the first generation of integral-asymmetric, cellulosic membranes (1959 to 1970) and the second generation of asymmetric, non-celluloslc membranes (1971 to 1984). 

Loeb and integral-asymmetric CA membranes 2 increased water flux (400 L/m d) and salt 3... 

Dense phase polymer membranes with no supportive substructure, so-called symmetric membranes, must have a minimum thickness of about 1 mil (1 x 10 in. or 2.54 X 10" cm) to ensure mechanical integrity and freedom from imperfections such as holes in the membrane. Possible disadvantages of these membranes are low fluxes and limited selectivity. Asymmetric membranes overcome these limitations, being very thin dense polymers with a thickness in the order of 1 x 10 m, supported on a porous layer that is about 1 x 10 m thick. 

Fig. 3.6-1 SEM-cross-sections of (a) a typical symmetric membrane, (b) an integral asymmetric and (c) a composite membrane.

Integrally skinned asymmetric asymmetric membranes used for gas and liquid separations consist of a thin skin layer supported by a porous substructure. The skin layer determines the permeability and selectivity of the membrane, whereas the porous substructure functions primarily as a physical support for the skin. Both layers are composed of the same material and are integrally bonded. The skin layer usually has a thickness on the order of several hundred to several thousand angstroms. 

H.-L. Wang, J. Gao, J.-M. Sansinena, and P. McCarthy, Fabrication and characterization of polyaniline monolithic actuators based on a novel configuration integrally skinned asymmetric membrane, Chem Mater., 14 (6), 2546-2552, (2002). 

P. Zschocke and D. Quelhnalz. Integral asymmetric, solvent-resistant ultrafiltration membrane made ol partiahy sulphonated, aromatic polyether ether ketone. DE Patent 3 321860, assigned to Berghol Eorschungsinst (DE),... 

Integrally Skinned Asymmetric Membrane and Thin-Film-Composite Membrane.36... 

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