The polymer concentration in the solution is adjusted depending on the viscosity. Higher solution viscosities are required for the production of hollow-fiber membranes, as compared to flat sheet production, because the fiber fabrication is performed without a casting surface. The morphology of the membrane is strongly influenced by the concentration gradient of the permeating coagulant (nonsolvent) within the cast layer. If the concentration profile is flat, coagulation occurs virtually the same time over the entire layer, yielding an isotropic porous membrane. This happens, for example, when the membrane is precipitated by exposing the cast layer to the coagulant vapor phase or if solvents with low vapor pressure are used. On the contrary, if the concentration gradient of the permeating coagulant is steep, as when membranes are coagulated in a nonsolvent bath, an anisotropic porous membrane forms, which can be used for ultrafiltration, reverse osmosis, and gas separation. Thermally induced phase separation (TIPS) in polymer solutions is one of the most versatile and widely used methods for the production of microporous membranes [122]. In the TIPS process, a homogeneous solution is formed by the dissolution of a polymer in a diluent at a high temperature and phase separation is then induced by cooling the polymer solution. The compatibility between the polymer and the diluent is one of the key factors affecting the morphology of the membrane. In many cases, polyolefin has been used as the polymer material to prepare microporous membranes [123,124]. A high-density polyethylene hollow-fiber membrane has been prepared by polymer crystallization via the TIPS process [125]. Poly(ethylene-co-vinyl alcohol) hollow fiber membranes with 44 mol% ethylene content, showing better pore connectivity and rejection of w20 nm diameter solute, has been prepared by TIPS, using a mixture of 1,3-propanediol and glycerol (50:50) as diluent [126]. Polymers containing ethylene oxide units are of considerable interest since ether oxygen linkages lead to flexible polymer chains and specific interactions with metal ions, polar molecules such as H 2 O and H 2 S, and quadrupolar molecules such as CO 2 . Thus rubbery membrane materials have been made for the removal of acidic gases such as CO 2 and H 2 S from natural gas (mainly CH 4 ) using a highly branched, cross-linked PEO hydrogel (see below). Unlike conventional size-sieving membrane materials, which achieve high permeability selectivity mainly via high diffusivity selectivity, these polar rubbery membrane materials exhibit high CO 2 permeability and high CO 2 /CH 4 mixed-gas selectivity due to high gas diffusivity and high CO 2 /CH 4 solubility selectivity [127]. In a typical method of preparation of the aforesaid rubbery membrane material [127], a prepolymer solution is prepared by adding 0.1 wt% initiator (e.g., 1-hydroxy-cyclohexyl phenyl ketone) to poly(ethylene glycol) diacrylate (PEGDA, 743 g/mol) or mixtures of PEGDA and poly(ethylene glycol) methyl ether acrylate (460g/mol). After mixing and sonicating to eliminate bubbles, the solution is TABLE 5.20 Some Commercial Polymer Membranes and Their Applications Material Applications a Cellulose acetate (CA) MF, UF, RO, D, GS Cellulose triacetate (CTA) MF, UF, RO, GS CA-CTA blend RO, D, GS Cellulose esters, mixed MF, D Cellulose, regenerated MF, UF, D Polyamide, aromatic MF, UF, RO, D Polyimide UF, RO Polyacrylonitrile UF, D Polysulfone MF, UF, D, GS Polytetrafluoroethylene MF Poly(vinylidene fluoride) MF, UF Polypropylene MF Polydimethylsiloxane GS Source: Cabasso, I., 1987. Encyclopedia of Polymer Science and Engineering, Vol. 9, J. I. Kroschwitz, ed., Wiley-Interscience, New York. a MF, microfiltration; UF, ultrafiltration; RO, reverse osmosis; D, dialysis; GS, gas separation. Polymers in Special Uses 5-109 q 2006 by Taylor & Francis Group, LLC Coagulation bath Coagulation bath Internal coagulation External coagulation Outer skin Inner skin Solvent exchange Bore fluid Bore fluid Air-gap Moisture induced phase seperation Wet-spinning process Dry-jet wet-spinning process FIGURE 5.67 Comparison of precipitation in the wet-spinning and dry-jet wet-spinning processes. (After Chung, T. S. and Hu, X. 1997. J. Appl. Polym. Sci., 66, 1067. With permission.) Nascent fiber in a wet-spinning process with small or no air-gap distance. (long-range and random chain structure) Vigorous non-solvent immersed precipitation Solvent exchange Nascent fiber in a dry-jet wet-spinning process with a reasonable air-gap distance. (Short-range and random chain structure) Membrane with a compact and short-range inter-related nodular structure Membrane with a long-range inter-related nodular structure Solvent exchange Slow moisture induced precipitation FIGURE 5.68 Schematic of skin morphologies in the wet-spinning and dry-jet wet-spinning processes. (After Chung, T. S. and Hu, X. 1997. J. Appl. Polym. Sci., 66, 1067. With permission.) 5-112 Plastics Technology Handbook q 2006 by Taylor & Francis Group, LLC . (After Chung, T. S. and Hu, X. 1997. J. Appl. Polym. Sci., 66, 1067. With permission.) 5-112 Plastics Technology Handbook q 2006 by Taylor & Francis Group, LLC