Anisotropic membrane membrane

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

Membranes made by the Loeb-Sourirajan process consist of a single membrane material, but the porosity and pore size change in different layers of the membrane. Anisotropic membranes made by other techniques and used on a large scale often consist of layers of different materials which serve different functions. Important examples are membranes made by the interfacial polymerization process discovered by Cadotte [15] and the solution-coating processes developed by Ward [16], Francis [17] and Riley [18], The following sections cover four types of anisotropic membranes ... 

Most of the membranes listed in Table 5.20 are formed through phase separation processes, i.e., melt extrusion or coagulation of a polymer solution by a nonsolvent. In melt extrusion, a polymer melt is extruded into a cooler atmosphere which induces phase transition. The melt extrusion of a single polymer usually gives a dense, isotropic membrane. However, the presence of a compound (latent solvent) that is miscible with the polymer at the extrusion temperature but not at the ambient temperature, may lead to a secondary phase separation upon cooling. Removal of the solvent then yields a porous isotropic membrane. Anisotropic membranes may result from melt extrusion of a dope mixture of polymers containing plasticizers. 

This interpretation is consistent with that of Sladek et.ai. who found both that the primary determinant of fecal coliform growth on a membrane filter was the size of the pores adjacent to the feed and that their optimum size was approximately 2.4 pM (8). Sladek s study resulted in the development of a commercial moderately anisotropic membrane with optimum feed surface pores, but with fine surface pores of 0.7 uM The ability of such a large pored membrane to retain all bacteria... 

The above set of rules - though accurately descriptive of earlier casting procedures - has led to serious misconceptions pertaining to the formation of anisotropic membranes, and therefore, misconceptions in the formulation of new polymeric casting solutions. It is evident that the polymer solution concentration progressively increases at the surface layer during the evaporation period, and... 

Phase separation controlled by diffusion exchange often results in a skin which is composed of a micellar assembly of nodules, as will be discussed below. When extremely hydrophobic polymers (e.g., modifled-PPO) are cast from dioxane into water (pg = p = p ) a dense polymer layer is formed at the solution s interface that somewhat resembles the type of layer formed by Interfacial polymerization. There is almost no inward contraction of the interfacial skin, and the coagulation process is controlled by diffusion through the dense, interfacial thin film. These result in an anisotropic membrane with a very fine "coral" structure (Figures 9 and 10). 

The development of anisotropic membranes based on a hydrophobic polymer matrix (e.g., polysulfone derivatives or phosphonylated-PPO) which does not collapse upon drying, made possible a more thorough investigation into the origin and role of the nodular layer. It is now clear that if the nodular layer extends to the interface without fusion, the membrane is open to solute permeation. Solute separation would then be dependent upon the serriedness of the nodules... 

Figure 10. Torn surface of a modified-PPO anisotropic membrane (also shown in Figure 9) after exposure to lateral stress...

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. 

Jem, K.M., Aris Rutherford, and Cussler, E.L., Anisotropic Membrane Transport, Chem. Engr. Comm, (in press). 

Besides the work with Carr and Schmidt, I had a very pleasant interaction with Ed Cussler over some problems in mass transfer through membranes with barriers [238], through anisotropic membranes [232], and by facilitated diffusion [247]. A severely mathematical problem of diffusion through a slot could be solved by a Schwartz-Christoffel transformation,41 and provided an asymptotic formula that proved useful in the barrier problem [223]. Cussler was delightful to work with because he refused to accept any mathematical solution that could not be unpacked and shown to be physically sensible (cf. [244]). Next to Bosanquet, he is the most intuitive problem solver with whom I have worked. Chapter 19 [232] is reprinted here. 

A general theory of anisotropic membranes (with E.L. Cussler) Chem. Eng. Commun. 58, 3-16 (1987). (Reprint N). 

Loeb-Sourirajan anisotropic membrane Thin-film composite anisotropic membrane... 

The transition between reverse osmosis membranes with a salt rejection of more than 95 % and molecular weight cutoffs below 50 and ultrafiltration membranes with a salt rejection of less than 10% and a molecular weight cutoff of more than 1000 is shown in Figure 2.42 [74], The very large change in the pressure-normalized flux of water that occurs as the membranes become more retentive is noteworthy. Because these are anisotropic membranes, the thickness of the separating layer is difficult to measure, but clearly the permeability of... 

The membrane classification scheme described above works fairly well. However, a major membrane preparation technique, phase separation, also known as phase inversion, is used to make both isotropic and anisotropic membranes. This technique is covered under anisotropic membranes. 

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