Cross-sectional structures membranes

The cross-sectional structure of the buccal mucosa is shown in Fig. 2.7. The buccal mucosa consists of the epithelium and the underlying connective tissue, the lamina propria, separated by a basement membrane. 

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]. 

Figure 2.1 Polymeric membrane shapes and cross-sectional structures. Tubular membranes are similar to flat sheet membranes because they are cast on a macroporous tube as support. Capillary membranes are hollow fibers with larger diameter, that is, >0.5 mm.

The processes used to prepare cellulosic membranes generally lead to homogenous cross-sectional structures. Cellulose prepared from xanthate derivatives may exhibit a cuticle or skin structure however, this asymmetry does not produce significant resistance to mass transfer. Most membranes currently used for hemodialysis are prepared via the cuprammonlum process. These membranes do not form a skinned structure during coagulatlon/regeneratlon. 

Figure 1. Typically asymmetric membrane (a) and composite membrane (b) cross-sectional structures.

Fujii et al. [13] studied morphological structures of the cross section of various hollow fibers and fiat sheet membranes by high-resolution field emission scanning electron microscopy. Figure 6.8 shows a cross-sectional structure of a flat sheet cellulose acetate RO membrane. The layer near the top surface is composed of a densely packed monolayer of polymeric spheres, which is supported by a layer formed with completely packed spheres. The contours of the spheres in the top layer can be observed. The middle layer is also composed of loosely packed and partly fused spheres, which are larger than the spheres in the surface layer. In the middle layer, there are many microvoids, the sizes of which are the same as the spheres. The layer near the bottom is denser than the middle layer, and the spheres are deformed and fused. Interstitial void spaces between the spheres, which may be called microvoids, are clearly observed. This structure seems common for the flat sheet as well as the hollow fiber membranes. For example. Fig. 6.9 shows a cross section of a hollow fiber made of PMMA B-2 (a copolymer containing methyl methacrylate and a small amount of sulfonate groups). The inside surface layer is composed of the dense structure of compactly packed fine polymeric particles. The particle structure of the middle layer... 

Fig. 6.9. Cross-sectional structure of PMMA B-2 hollow fiber membrane. Reprinted from [13]. Copyright 1992, with kind permission from the Society of Polymer Science, Japan...

As mentioned earlier, the study of the cross-sectional structure of membranes by AFM was hampered by difficulties involved in making a smooth cross-sectional surface. Wood [14] made some earlier studies on the cross section of poly(phenylene oxide) (PPO) hollow fibers. But her attempt did not provide clear information on the nodular structure. 

As discussed, the cross-sectional view of the membranes observed by AFM has similar characteristics to those observed by high-resolution FE-SEM, confirming that AFM can be used to study the cross-sectional structure of polymeric membranes, particularly in terms of their nodular structures. The void spaces between the nodules may form water channels in reverse osmosis and ultraflltration. They may also become defects when they appear at the densely packed monolayer of nodules or nodular aggregates. Information on the nodular structure will therefore help to eliminate the unwanted defects in the skin layer of the asymmetric membranes. 

One of the important features of AFM that cannot be easily utilized in the AFM study of cross-sectional structures is the roughness parameters, since the surface roughness of the sample depends on the method of slicing or fracturing the membrane. As well, nanoscale or submicroscale void spaces have not yet been observed in the cross section. The AFM applications in the study of the cross-sectional morphology have only begun. More efforts in this research held are called for. 

An example of a PE hollow fiber membrane will be described briefly. A hollow, microporous PE fiber is shown in the SEM micrographs in Fig. 5.37. The fibers were prepared by fracturing in liquid nitrogen to show the bulk cross sectional structure (Fig. 5.37A), and a cold razor blade was used to fracture the fiber for the longitudinal view (Fig. 5.37B). Combination of both views shows the dimensions and the porous structure. 

MiUlei Klieser, Kreutz, W. On the Cross-Section Structure of the Mitochondrial Cristae-Membrane as Revealed by X-Ray Diffraction. Z. f. Naturforschung iic, 612 (1976)... 

The gas separation membrane was first commercialized in 1980 in order to recover hydrogen gas produced in the oil refining process. This membrane was a hollow-fiber membrane comprised of polysulfone. The shell of the hollow fiber exhibited an asymmetric cross-sectional structure, which showed the porous structure with pore size varying from one surface to the other surface of the membrane. The asymmetric structure is most preferred for the gas separation membranes due to several reasons described later. A certain number of the hollow-fiber membranes were integrated into a bundle, and the bundle of the hollow-fiber membrane was installed into the piping unit (module) and shipped to the user. 

FIGURE 8.13 Cross-sectional structure of PVDF membranes prepared using water as the nonsolvent and TEP, NMP, DMF, and DMA as solvent (from left to right). (Data from Yeow, M.L. et al. Journal of Applied Polymer Science, 92,1782-1789,2004. doi 10.1002/app.20141.)... 

IP3 Receptors. Figure 2 Key structural features of IP3 receptors. The key domains are shown in the central block. The upper structures show the suppressor domain (PDB accession code, 1XZZ) and the IBC (1N4K) with its (red) and p (blue) domains. A proposed structure for the pore region is shown below, with the selectivity filter shown in red only two of the four subunits are shown. The lowest panel shows reconstructed 3D structures of IP3R1 viewed (left to right) from ER lumen, the cytosol and in cross-section across the ER membrane (reproduced with permission from [4]). 

The latter type of compounds should preferably carry either one type I unit or at most two units (positioned as far apart as possible), and have an elongated structure (which does not fold as verapamil, for example) with a small cross-sectional area, Ad- The first type of compounds is expected to be transported slowly, whereas the second type may not be transported. Table 20.2 summarizes the drug properties relevant for transporter binding and lipid partitioning of a substrate (modulator or inhibitor) of P-gp. Inspection of the information contained in Table 20.2 shows that the synthesis and membrane incorporation of inhibitors with a low number of H-bond acceptor patterns should be simpler and more efficient than that of inhibitors with a large number of patterns. 

Circular Bragg nanolasers (CBNLs) of several geometries and Bragg reflector orders were fabricated within a thin membrane of InGaAsP semiconductor material21. A cross-section of the semiconductor epitaxial structure used is illustrated... 

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