Rubbery polymer membranes polar

The solubility selectivity of a membrane for a specific gas pair could be increased (in principle) by inducing specific interactions between the polymer and the more soluble component of the gas pair. For example, the substitution of certain polar groups in some rubbery polymers has been found to increase their solubility selectivity for CO2 relative to CH4 (Story and Koros, 1991 Koros, 1985). Unfortunately, the increase in the polarity of a polymer also tends to increase its chain packing density, and as a result, decreases the gas diffusivity in membranes made from that polymer. 

Therefore the species i concentration in the liquid imposed on the membrane surface is drastically reduced, leading to a substantial reduction in the species i flux. Consequently, concentration polarization can be severe in pervaporation processes where the membrane enriches the species substantially in the permeate. An example is the selective removal of VOCs from an aqueous solution through a rubbery pervaporation membrane (e.g. silicone polymer based). For a description of such concentration polarization effects in VOC removal from water, consult Wijmans et al. (1996). 

The subscripts 1 and 2 refer to downstream and upstream conditions, respectively. The ratio of the two diffiision coefficients is commonly called the diffusivity selectivity of a membrane and the ratio of the two solubilities, the solubility selectivity. A typical range of permeabilities and selectivities achievable with rubbery materials is indicated in Table 3 for several gas pairs of commercial interest. More extensive tabulations are available (41). The presence of polar groups in the polymer molecule generally leads to low diffusivity for penetrants (15) eg, the diffusivity decreases strongly on increasing the nitrile content in a series of... 

Since temperatures are moderate at the outlet of the FGD unit (compare Table 7.1), polymeric membranes appear to be the most suitable ones for post-combusion CO2 capture. In the recent past, particularly, PEO-based block copolymers received considerable attention. The high polarity of PEO leads to a high CO2/N2 gas separation factor in combination with a high CO2 permeance. Due to the rubbery nature of PEO-based block copolymers, the presence of water hardly influences the transport of CO2 and other gases. The chemical and mechanical resistance of the polymer to flue gas conditions are currently evaluated in the NanoGLOWA project. Table 7.4 summarizes the rather favorable separation characteristics of PEO, which will be further used to investigate the economics of membrane gas separation processes of post-combusion CO2 capture. 

The importance of regularity, polarity, and chain stiffness becomes apparent in the uses of polyethers. Of the common polyethers, only polyformaldehyde, poly(ethylene oxide), and the polymer based on 2,2-dichloromethylpropylene-1,3-oxide are crystalline. Since poly(ethylene oxide) is water soluble, only the remaining two polyethers are useful structural materials. Phenoxy, phenylene oxide, and sulfone polymers are useful thermoplastics because the chain stiffness contributed by aromatic rings in the backbone raises 7 well above room temperature. Polysulfones are used in the manufacture of membranes for gas separations (Section 13.6). The aliphatic ether backbone is more conducive to low 7 s that are suitable for rubbery applications. Fully cured epoxy resins are dimensionally stable network polymers. Low-molecular-weight polymers of ethylene oxide [often abbreviated as PEG for poly(ethylene glycol)] are often listed as ingredients of... 

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