Asymmetric membrane macrovoids

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. 

Numerous asymmetric membranes were prepared under various conditions and their cross-section was examined by SEM. Typical of the results are those shown in Figure 5 for the membrane cast from 70 30 THF-formamide and gelled in IPA. Close inspection of Figure 5 reveals a thin, relatively dense skin supported by a microporous layer. The support layer contains macrovoids, the cause of which is presently under investigation. 

S.A. McKelvey, W. Koros, Phase separation, vitrification and the manifestation of macrovoids in polymeric asymmetric membranes. Journal of Membrane Science 112 (1996) 29. 

Figure 1.2 Electron micrograph ofa UF asymmetric membrane showing the skin layer on the top (feed side) and macrovoids in the interior.

Asymmetric membranes consist of a thin top layer supported by a porous sublayer and quite often macrovoids can be observed in the porous sublayer. Figure EH - 54 illustrates two ultrafiltration membranes from polysulfone and polyacrylonitrile, where the existence of these macrovoids can be clearly observed. 

B) there are much larger pores (about 50 nm). SEM images of a fractured, critical point dried membrane (Fig. 5.30) show robust, macrovoid structures. The top dense surface layer is clearly composed of a monolayer of densely packed, deformed particles, about 80 nm in diameter, packed so closely as to limit surface porosity. Less well packed particles form the more open bulk membrane texture. The structure shown confirms those hypothesized from earlier TEM replica studies of wet poly(amide-hydra-zine) and dry polyimide asymmetric membranes [157,158]. 

Polysulfone composite membranes provide a different chemical composition and structure compared with some of the examples shown. A polysulfone composite membrane is shown by SEM of cross sections (Fig. 5.42A, B) and of the top surface (Fig. 5.42C). A porous texture is seen (Fig. 5.42A) with larger macrovoids near the bottom surface. There is an open porous structure with a pore gradient, with smaller pores nearer the dense top surface (Fig. 5.42B). The asymmetric membrane has very fine surface pores, about 0.05-0.2/im across (Fig. 5.42C) with an underlying open network composed of string of polymer. The surfaces of composite membranes are generally dense, and SEM micrographs may not reveal any resolvable surface pores. Chemical etching of this dense surface layer is useful to observe the porous substructure (Fig. 5.42D). 

Microfiltration and UF membranes can be asymmetric, with a denser side and a more open side, or uniform without macrovoids (See Figure 16.3). The open area behind the denser surface in an asymmetric design means there is less resistance to water permeating the membrane. Operating pressure can be lower and the membrane systems can be more productive. The limitation of the asymmetric design is that the material, predominately used in the hollow fiber configuration, is not as strong as the uniform cross section. 

In addition, the high affinity between solvent and nonsolvent (low /12) causes the instantaneous demixing. In the case of an instantaneous process, the governing factor is the concentration gradient, which diverges in the direction in which diffusion occurs and causing asymmetric structures and macrovoids. Macrovoids consist in poms that could have sizes similar to the thickness of the membrane. 

The formation of macrovoids in these asymmetric polyaniline hollow fibers is undesirable as they weaken the mechanical strength and may lead to defects in the selective layer, which make them nonviable for many separations applications. The formation of macrovoids in hollow fiber membranes is generally caused by fast precipitation kinetics of the polymer solution in the coagulation bath. The formation of macrovoids when spinning polyaniline hollow fibers was foimd to be highly dependent on... 

Fig. 11. Cross-sectional scanning electron micrograph of an asymmetric Loeb-Sourirajan ultrafiltration membrane. The large macrovoids under the membrane skin (top surface) are common in this type of ultrafiltration membrane.

Wang, K. Y., Li, D. R, Chung, T. S., and Chen, S. B. (2004a). The observation of elongation dependent macrovoid evolution in single- and dual-layer asymmetric hollow fiber membranes. Chem. Eng. Sci. 59, 4657. 

Widjojo, N., and Chung, T. S. (2006). The thickness and air-gap dependence of macrovoid evolution in phase-inversion asymmetric hollow fiber membranes. Ind. Eng. Chem. Res. 45, 7618. 

FIGURE 15.7 Schematic diagram for macrovoid growth. (Adapted from D. Li, Duallayer asymmetric hollow-fiber membranes for gas separation, PhD Thesis, Chemical Biomolecular Engineering, National University of Singapore, Singapore, 2005.)... 

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