Symmetric Microporous Phase Inversion Membranes

The symmetric, microporous polymer membranes made by phase inversion are widely used for separations on a laboratory and industrial scale.22 Typical applications range from the clarification of turbid solutions to the removal of bacteria or enzymes, the detection of pathological components, and the detoxification of blood in an artificial kidney. The separation mechanism is that of a typical depth filter which traps the particles somewhere within the structure. In addition to the simple sieving effect, microporous phase inversion membranes often show a high tendency of adsorption because of their extremely large internal surface. They are, therefore, particularly well suited when a complete re- 

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Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

In addition to high filtration rates, asymmetric membranes are most fouling resistant. Conventional symmetric structures act as depth filters and retain particles within their internal structure. These trapped particles plug the membrane and the flux declines during use. Asymmetric membranes are surface filters and retain all rejected materials at the surface where they can be removed by shear forces applied by the feed solution moving parallel to the membrane surface. The difference in the filtration behavior between a symmetric and an asymmetric membrane is shown schematically in Figure 1.10. Two techniques are used to prepare asymmetric membranes one utilizes the phase inversion process and the other leads to a composite structure by depositing an extremely thin polymer film on a microporous substructure. 

A.2A Polyolephines Semicrystalline low-density polypropylene (PP) and polyethylene (PE) are used to produce microporous hoUow-fiber membranes by thermal phase inversion in the presence of plasticizers and solvents. Commercial PP and PE membranes feature a symmetric wall with pores whose maximal size is a fraction of a micron... 

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