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 ca
Trang 2The 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 H2O and
H2S, and quadrupolar molecules such as CO2 Thus rubbery membrane materials have been made for the removal of acidic gases such as CO2and H2S from natural gas (mainly CH4) 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 CO2permeability and high CO2/CH4 mixed-gas selectivity due to high gas diffusivity and high CO2/CH4solubility 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
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.
Trang 3Coagulation bath Coagulation bath
Internal coagulation
External coagulation Outer skin Inner skin
Solvent exchange
Air-gap
Moisture induced phase seperation
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.)