Rubbery polymer membranes properties

In selective separation of hydrocarbons from their mixtures with air or from their aqueous solutions, it makes sense to use membranes based on rubbery polymers, whose permeability increases with the decrease in glass transition point. Permselectivity of rubbery polymers is dominated by the sorption component, which increases with condensability of the hydrocarbon penetrant. Higher activity of the component being separated in the feed mixture results in plasticization of the membrane and can make it swell. This can produce a non-monotonic dependance of selective properties of the membrane on activity of the component being separated. As a rule, permselectivity for mixtures of penetrants is significantly lower than their ideal values. Negative values of sorption heat of easily condensable hydrocarbons can result in existence of non-monotonic temperature dependencies of mass transfer coefficients. 

White [25] investigated the transport properties of a series of asymmetric poly-imide OSN membranes with normal and branched alkanes, and aromatic compounds. His experimental results were consistent with the solution-diffusion model presented in [35]. Since polyimides are reported to swell by less than 15%, and usually considerably less, in common solvents this simple solution-diffusion model is appropriate. However, the solution-diffusion model assumes a discontinuity in pressure profile at the downstream side of the separating layer. When the separating layer is not a rubbery polymer coated onto a support material, but is a dense top layer formed by phase inversion, as in the polyi-mide membranes reported by White, it is not clear where this discontinuity is located, or whether it wiU actually exist The fact that the model is based on an abstract representation of the membrane that may not correspond well to the physical reality should be borne in mind when using either modelling approach. 

The efficiency of the pervaporation process is controlled mainly by the intrinsic properties of the polymers used for membrane preparation. It has been demonstrated that permeability in glassy polymers (cellulose) is lower than that in rubbery polymers (polydimethylsiloxane). Moreover, as the contribution of the solubility to the permeability dominates in nonglassy polymers, the permeability increases with increasing molecular mass of permeants. 

There are, however, some exceptions, when glassy polymers demonstrate membrane properties more characteristic for rubbery ones, i.e. the permeation is defined by solubility of permeating gases. 

PolydimethylsUoxane (PDMS) embedded with zeolite particles displayed permeation improvements compared to the original polymer, but only when zeolite loadings of 40 wt% or larger are implemented.Certain target separations, such as n-pentane/ i-pentane, did not improve relative to neat PDMS permeation properties. Fundamental transport of gas molecules through solid-rubbery polymer systems has been studied using zeolite 5A-silicone mbber membranes. ... 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

Although metallic and ceramic materials are used as membranes, polymeric materials account for the vast majority of commercial products. Polymer selection depends on a number of factors including intrinsic transport properties, mechanical properties, thermal stability, chemical stability (e.g., chemical resistance and biocompatibility), membrane manufacturability, cost, and patentability. The two most common types of polymers are glassy engineering thermoplastics and rubbery polysiloxanes. 

Because the so-called ultrahigh free volume polymers aroused much interest during the last 10 years, they will be briefly described in this introductory chapter. The publication of the physical properties of poly(l-trimethylsilyl-l-propyne) (PTMSP) in 1983 [281] aroused much interest in the field of membrane research. Up to this time it had been believed that the rubbery poly(dimethyl si-loxane) has by far the highest gas permeability of aU known polymers. Very surprisingly, the glassy PTMSP showed gas permeabilities more than 10 times higher than PDMS. This could be attributed to its very high excess-free volume and the interconnectivity of the free volume elements. Since then a number of... 

The transmembrane flux is inversely proportional to the membrane thickness and is directly proportional to the membrane area and to the applied pressure differential across the membrane. Thus, a membrane manufacturer s primary objectives typically revolve around means for producing the thinnest possible membrane in a structural form that will accommodate the applied pressure while maximizing the membrane surface area. Thus, knowledge of the stractur-al and mechanical properties of the membrane material is of paramount importance. Unfortunately, many polymers of interest as gas-permeable membranes are rubbery materials with poor mechanical strength. Hence, many membranes require an underlying support material that can accommodate the appHed pressure load. 

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