Homogeneous polymer membranes

Most technically utilized homogeneous polymer membranes consist of a composite structure where a very thin homogeneous selective polymer film is supported by a thicker microporous structure providing the mechanical strength. 

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Yasuda s free volume theory [57] has been proposed to explain the mechanism of permeation of solutes through hydrated homogeneous polymer membranes. The free volume theory relates the permeability coefficients in water-swollen homogeneous membranes to the degree of hydration and molecular size of the permeant by the following mathematical expression ... 

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-... 

Dense homogeneous polymer membranes are usually prepared (i) from solution by solvent evaporation only or (ii) by extrusion of the melted polymer. However, dense homogeneous membranes only have a practical meaning when made of highly permeable polymers such as silicone. Usually the permeant flow across the membrane is quite low, since a minimal thickness is required to give the membrane mechanical stability. Most of the presently available membranes are porous or consist of a dense top layer on a porous structure. The preparation of membrane structures with controlled pore size involves several techniques with relatively simple principles, but which are quite tricky. 

Ion-exchanger membranes with fixed ion-exchanger sites contain ion conductive polymers (ionomers) the properties of which have already been described on p. 128. These membranes are either homogeneous, consisting only of a polyelectrolyte that may be chemically bonded to an un-ionized polymer matrix, and heterogeneous, where the grains of polyelectrolyte are incorporated into an un-ionized polymer membrane. The electrochemical behaviour of these two groups does not differ substantially. 

H Yasuda, DE Lamaze. Permselectivity of solutes in homogeneous water-swollen polymer membranes. J Macromol Sci Phys 5 111-134, 1971. 

Wang, H., Holmberg, B.A., and Yan, Y. (2002) Homogeneous polymer-zeolite nanocomposite membranes by incorporating dispersible template-removed zeolite nanocrystals. /. Mater. Chem.,... 

Strathmann et al.20) examined the water and salt sorption and the homogeneity of water distribution in various polymers and indicated that the uniformity of water distribution in the polymer is an important parameter controlling reverse osmosis desalination efficiency. As summarized in Table 2, the average number of water molecules included in a cluster is 1.4 to 2.9 for the superior barrier materials such as aromatic polyamides, polyamide-hydrazide, and polybenzimidazopyrrolone, while the number for the other polymer membranes is larger than 5. 

It has been established that other ammine ligand-based Ru complexes including mononuclear and dinuclear ones are all active catalysts for water oxidation (vide infra) in both homogeneous aqueous solution and in heterogeneous phase such as a polymer membrane and clay.,8)... 

The high mineralization activity of the PVDF-W10 membrane in comparison to the homogeneous catalyst can be ascribed to the selective absorption of the organic substrate from water on the hydrophobic PVDF polymer membrane that increases the effective phenol concentration around the catalytic sites. Moreover, the polymeric hydrophobic environment protects the decatungstate from the conversion over longer time to a less-reactive isomer that has a maximum absorption at a wavelength of 280 nm. 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

Vapor-Induced Phase Separation During the VIPS process, phase separation is induced by penetration of nonsolvent vapor, into the homogeneous polymer solution consisting of polymer and solvent(s). Mass transfer is usually much slower than that in the wet casting process thus, the VIPS process has been used to obtain membranes with symmetric, cellular, and interconnected pores [86,87],... 

The organic polymer membranes have an upper operating limit of approximately 200 C due to their thermal stability. Below this temperature, there exist organic polymer membranes suitable for homogeneous catalytic reactions in some compatible solvents. This temperature also represents the lower limit for most heterogeneously catalyzed reactions [Armor, 1992]. 

Other complex molecules in a homogeneous solution and the decomposition products would be solvated and stabilL by water molecules. This kind of degradative oxidation is probably prevented by the microheterogeneous environment imposed by the polymer membrane on the isolated metal complex entities. This work not only demonstrates realization of an efficient four-electron water oxidation system utilizing a polymer membrane, but also shows remarkable stabilization of the water oxidation catalyst against decomposition in a membrane. 

Most of the available commercial microporous membranes such as polysulfone, polyethersulfone, polyamide, cellulose, polyethylene, polypropylene, and polyvinylidene difluoride are prepared by phase inversion processes. The concept of phase inversion in membrane formation was introduced by Resting [75] and can be defined as follows a homogeneous polymer solution is transformed into a two-phase system in which a solidified polymer-rich phase forms the continuous membrane matrix and the polymer lean phase fills the pores. A detailed description of the phase inversion process is beyond the scope of this section as it was widely discussed in Chapters 1 and 2 nevertheless a short introduction of this process will be presented. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

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