Cellulose acetate membranes scanning electron microscopy

The development of the Loeb-Sourlrajan asymmetric cellulose acetate membrane (1) has been followed by numerous attempts to obtain a similar membrane configuration from virtually any available polymer. The presumably simplistic structure of this cellulose acetate membrane - a dense, ultrathln skin resting on a porous structure - has been investigated by transmission and scanning electron microscopy since the 1960s (2,3). The discovery of macrovoids ( ), a nodular intermediate layer, and a bottom skin have contributed to the question of the mechanism by which a polymer solution is coagulated to yield an asymmetric 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... 

The mechanisms of formation and the structural characteristics of cellulose acetate membranes for reverse osmosis have been investigated. Scanning electron microscopy studies showed the membranes to be high molecular weight condensation structures of the cellular type. 

Scanning electron microscopy (SEM) offers higher magnification than optical microscopy, extending the resolution down to the nanometer level. The first electron micrographic picture of a cellulose acetate membrane was taken... 

Figure 5.43. 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 (F). Scanning electron microscopy 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 (arrowheads) reveals a dense layer, with a porous microstructure, shown more clearly at higher magnification (E).

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