Membrane structure polymer concentration

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Conversely, in most observed cases where solidification occurs as a result of continued depletion of solvent (as described in Case B), the highly concentrated polymer layer solidifies as a relatively dense, amorphous, plasticized film. Water diffusion into this highly plasticized layer becomes prevalent (Case A) at a stage where the contraction has gone "too far" to yield even a microporous membrane structure. 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

Because of the flat concentration profiles, the solution precipitates at virtually the same time over the entire film cross section, and no macroscopic gradients of activity or concentration of the polymer are obtained over the film cross-section. On a microscopic scale, however, because of thermal molecular motions, there are areas of higher and lower polymer concentration, which act as nucleation centers for polymer precipitation. These microscopic areas of higher polymer concentration are randomly distributed throughout the cast polymer film. Therefore, a randomly distributed polymer structure is obtained.during precipitation. This structure is also shown in Figure 13 in the form of a scanning electron microscope picture of the cross section of a symmetric membrane obtained with a vapor phase precipitant. 

Membrane Structure. A low polymer concentration in the casting solution... 

Flat stock mlcroporous membranes can be made using a variety of polymer types by dissolving the polymer In a solvent at an elevated temperature, followed by casting this solution on a temperature controlled flat surface, provided that the system, polymer/solvent, shows a miscibility gap. Extraction of the solvent, called "pore former". Is done with low molecular weight alcohols. Decidedly different pore structures can result depending upon, among other factors, polymer concentration and rate of solution cooling. 

Depending upon the concentration of the polymer In solution and upon how quickly the solution Is cooled, one of two membrane structures may be formed, each being clearly distinct from the Type I and Type II structures found for polypropylene (14). 

Type III (Figures 6 and 7) membrane consists of a relatively tight skin supporting spheres of polymer, whereas Type IV (Figures 8 and 9) contains "leaves" of polymer stacked upon a more open skin. Depending upon the cooling conditions and the polymer concentration, the same membrane can show a transition between Type III and Type IV structures within its cross-section (Figures 10 and 11). Typical membrane properties are found in Table III. 

Figure 6. Type III Structure of Polyvinylidene Fluoride Membrane Formed at Low Cooling Rates and Higher Polymer Concentrations (503X)...

The permeabilities of different components in a membrane depend on the mechanism by which the components are transported. For example, in homogeneous polymer membranes, the various chemical species are transported under a concentration or pressure gradient by diffusion. The permeability of these membranes is determined by the diffusivities and concentrations of the various components in the membrane matrix and the transport rates are, in general, relatively slow. In porous membrane structures, however, mass is transported under the driving force of a hydrostatic pressure difference via viscous flow and, in gen-... 

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