Cellulose acetate membranes cross section

Membrane polymeric materials for separation applications are made of polyamide, polypropylene, polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, cellulose diacetate, polystyrene resins cross-linked with divinylbenzene, and others (see Section 2.9) [59-61], The use of polyamide membrane filters is suggested for particle-removing filtration of water, aqueous solutions and solvents, as well as for the sterile filtration of liquids. The polysulfone and polyethersulfone membranes are widely applied in the biotechnological and pharmaceutical industries for the purification of enzymes and peptides. Cellulose acetate membrane filters are hydrophilic, and consequently, are suitable as a filtering membrane for aqueous and alcoholic media. 

FIGURE 20.4 Scanning electron micrographs (SEM) micrographs of the cross section of a cellulose acetate membrane of 0.45 pm pore size after being used for beer CMF experiments. A dense fouling layer is observed on the membrane surface. (From Moraru, C.I., Optimization and membrane processes with applications in the food industry Beer microfiltration. PhD thesis. University Dunarea de Jos Galati, Romania, 1999.)... 

Schulz and Asumnaa (48), based on their SEM observation, assumed that the selective layer of an asymmetric cellulose acetate membrane for reverse osmosis consists of closely packed spherical nodules with a diameter of 18.8 nm. Water flows through the void spaces between the nodules. Calculate the water flux by Eq. (30) assuming circular pores, the cross-sectional area of which is equal to the area of the triangular void surrounded by three circles with a diameter of 18.8 nm (as shown in Eig. 8). 

Figure 1.17 Scanning electron micrographs of membrane cross sections prepared from three different polymer-solvent systems by precipitation in water (a) 12% cellulose acetate in DMAc (b) 12% polyamide in DMSO (c) 12% polysulfone In DMF.

Nodules are defined as spherical cells with a diameter of a few hundred angstroms that are compacted irregularly at the membrane surface. They can also be observed underneath the membrane surface when a cross-sectional picture is taken. Each nodule contains several tens of thousands of macromolecules. Schultz and Asunmaa were the first to report the observation of nodules on the surface of an ultrathin cellulose acetate membrane by electron microscope [1]. Figure 4.1 shows the picture taken by them. The nodular structure of the membrane surface is clearly seen with an average nodular diameter of 188 3 A. The same authors also took a picture of an asymmetric cellulose acetate membrane and found that it, too, had a nodular structure. Panar et al. [2] then observed the close monolayer packing of micelles with diameters from 400 to 800 A when a cross-sectional picture of an asymmetric aromatic polyamide-hydrazide membrane was taken (Fig. 4.2). The top monolayer covers a support layer where the spherical micelles are irregularly packed with void spaces of 75-100 A. They attributed the formation of the nodules to the micellar structure that was initially present at the surface of the polyamidehydrazide solution. 

The studies of membrane morphology by SEM have produced a large number of cross-sectional pictures for polymeric membranes since the onset of asymmetric cellulose acetate membranes by Loeb and Sourirajan. The contribution of those pictures to the design of novel membranes with improved performance was truly phenomenal. SEM requires cumbersome sample preparation, which may hinder true images. AFM does not need such sample preparation, and the pictures taken by AFM are considered to reflect the true nature of membrane morphology. 

Fig. 5.28 Cellulose acetate membrane structures are shown by complementary techniques. The optical micrograph (A) shows an overview of the membrane, cast on a woven fabric support (bottom). A surface layer (arrows) is observed above large, rounded macrovoids (V). SEM cross sections reveal these macrovoids in more detail (B) and also show the nature of the fine pores (C). A TEM micrograph (D) of a section near the surface (arrows) reveals a dense layer, with a porous microstructure, shown more clearly at higher magnification (E).

Figure 13. Cross-section of the skin zone of a reverse osmosis membrane consisting of cellulose acetate-polyfbromophenylene oxide phosphonate)...

Figure 4.3 Cross section of a cellulose acetate RO membrane.

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... 

Optical, scanning and transmission electron micrographs of a commercial cellulose acetate asymmetric membrane are shown in Fig. 5.26. Each view provides a different perspective on the membrane structure while, together, they give the complete structural model. Specimen preparation for OM and TEM cross sections was by microtomy of embedded membrane strips using a method developed to limit structural collapse (Section 4.3.4). An optical micrograph (Fig. 5.26A)... 

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