Microporous polymeric membrane

HUal N, NigmatuUin R, Alpatova A (2004) Immobilization of cross-linked lipase aggregates within microporous polymeric membranes. J Membrane Sci 238(1-2) 131-141 Hiol A, Jonzo MD, Rugani N et al. (2000) Purification and characterization of an extracellular lipase from a thermophilic Rhizopus oryzae strain isolated from palm fruit. Enzyme Microb Technol 26 421 30... 

Microporous polymeric membranes are used widely for filtration and purification processes, such as filtration of wastewater, preparation of ultra-pure water, and in medical, pharmaceutical or food applications, including removal of microorganisms, dialysis and protein filtration. 

Microporous polymeric membrane separators are characterized by pore sizes in the micrometer scale. Microporous polymeric membrane separators are mainly made of polyethylene (PE), polypropylene (PP), and the combinations of them (PE/PP and PP/PE/PP) because of their high chemical and mechanical stabilities. According to the number of layers, they can be classified into monolayer and multilayer polymeric microporous membranes. 

The dry and wet processes are two main manufacturing methods to prepare microporous polymeric membranes. Both methods are conducted through an extruder and a stretching process to increase the porosity and improve the tensile strength. Generally, separators made by dry process exhibit distinct slit-pore and straight microstructures, whereas those made by wet process show intercormected spherical or elliptical pores. Both methods use cheap polyolefin materials, so the microporous polymeric membranes are not expensive. 

Hilal N, Nigmatullin R, Alpatova A (1995), Immobilization of cross-linked lipase aggregates within microporous polymeric membranes , J. Memb. Sci., 238,131-141. 

Thermally-induced phase separation (TIPS) has been shown to be an excellent way to make microporous polymeric membranes. Microporous membranes are generally prepared by TIPS process, which is based on the phenomenon that the solvent quality decreases when the temperature is decreased. On removing the thermal energy by cooling or quenching, a polymer-diluent solution phase separation occurs. After the phase separation, the diluent is removed, typically by solvent extraction, and the extractant is evaporated to yield a microporous structure. Typically, the TIPS process has been used to produce isotropic structures that is, the pore size does not vary with direction in the membrane. A few studies have been reported on the formation of... 

The separation of the two metals will he primarily determined by the ratio of the two distribution coefficients, kmio/k o. exactly as in solvent extraction. Separation of copper from nickel using L1X65N in the liquid membrane using these principles has been studied by Lee et al. (1978). A general review of metal extraction using liquid membranes supported in microporous polymeric membranes is provided by Danesi (1984-85). 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

By electropolymerization of pyrrole in solvents containing polyelectrolytes such as potassium polyvinylsulfate, it is possible to prepare films of polypyrrole with polymeric counterions which have good conductivity (1-10 S cm-1) and strength (49 MPa) 303 304,305). Such a material could be used reversibly to absorb cations in an ion exchange system. Pyrrole has also been electrochemically polymerized in microporous polytetrafluoroethylene membranes (Gore-tex), impregnated with a perfluorosulphonate ionomer 3061. 

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

Figure 2.41 The change in nitrogen flux through a PTMSP membrane caused by the presence of a condensable vapor in the feed gas [71]. This behavior is characteristic of extremely finely porous microporous ceramic or ultrahigh-free-volume polymeric membranes such as PTMSP. The condensable vapor adsorbs in the 5- to 15-A-diameter pores of the membrane, blocking the flow of the noncondensable nitrogen gas...

Interfacial polymerization membranes. This type of anisotropic membrane is made by polymerizing an extremely thin layer of polymer at the surface of a microporous support polymer. 

The most extensive studies of plasma-polymerized membranes were performed in the 1970s and early 1980s by Yasuda, who tried to develop high-performance reverse osmosis membranes by depositing plasma films onto microporous poly-sulfone films [60,61]. More recently other workers have studied the gas permeability of plasma-polymerized films. For example, Stancell and Spencer [62] were able to obtain a gas separation plasma membrane with a hydrogen/methane selectivity of almost 300, and Kawakami et al. [63] have reported plasma membranes... 

An interesting group of composite membranes with very good properties is produced by condensation of furfuryl alcohol with sulfuric acid. The first membrane of this type was made by Cadotte at North Star Research and was known as the NS200 membrane [32], These membranes are not made by the interfacial composite process rather a polysulfone microporous support membrane is contacted first with an aqueous solution of furfuryl alcohol and then with sulfuric acid. The coated support is then heated to 140 °C. The furfuryl alcohol forms a polymerized, crosslinked layer on the polysulfone support the membrane is completely black. The chemistry of condensation and reaction is complex, but a possible polymerization scheme is shown in Figure 5.10. 

Evaporative mass transfer of volatile solvents through microporous hydrophobic membranes is employed in order to concentrate feed solutions above their saturation limit, thus obtaining a supersaturated environment where crystals may nucleate and grow. In addition, the presence of a polymeric membrane increases the probability of nucleation with respect to other locations in the system (heterogeneous nudeation)... 

The experimental results reported in this chapter are related to membrane contactors manufactured by GVS S.P.A by using microporous polymeric flat-sheet... 

Considerable effort is being devoted to developing new polymeric membrane materials. A special type of oxygen-enrichment membrane has also been explored, which consists of a solvent immobilized within a microporous solid support (Fig. 7D). Dissolved in the liquid is a carrier... 

In some cases, the rate-controlling polymeric membrane is not compact but porous. Microporous membranes can be prepared by making hydrophobic polymer membranes in the presence of water-soluble materials such as polyethylene glycol), which can be subsequently removed from the polymer matrix by dissolving in aqueous solution. Cellulose esters, loosely cross-linked hydrogels and other polymers given in Table 4.2 also give rise to porous membranes. 

Membrane Techniques The interest in membrane techniques for sample preparation arose in the 1980s. Extraction selectivity makes membrane techniques an alternative to the typical sample enrichment methods of the 1990s. Different membrane systems were designed and introduced into analytical practice some more prominent examples are polymeric membrane extraction (PME), microporous membrane liquid-liquid extraction (MMLLE), and supported liquid membrane extraction (SEME) [106, 107]. Membrane-assisted solvent extraction (MASE) coupled with GC-MS is another example of a system that allows analysis of organic pollutants in environmental samples [108-111] ... 

The u% of synthetic polymeric membranes for water purification is now an established technoli. Historically, this developn nt dates to the beginning of this century, when Zsigmondy and Bachmann prepared the first microporous membrane from cellulose esters. SimOar microfiltration membranes are now widely used in applications ranging fiom sterile filtration to fine particle removal. 

More recently, composite membranes have been made by interfacial polymerization or by in situ polymerization A representative case is illustrated in F. 8. Here, a microporous polysulfone membrane is used as a substrate. This membrane is soaked in a dilute aqueous solution of a low molecular weight polyethylenimine (PEI). Without drying, this membrane is then contacted with a crosslinking agent such as toluene diisocyanate (TDI) or isophthaloyl chloride dissolved in hexane, after which the membrane is cured in an oven. A highly crosslinked, salt-rejecting interfacial layer is formed in this way. A summary of the properties of three of the more important composite membranes is presented in Table 10. 

J.M. Duval, B. Folkers, M.H.V Mulder, G. Desgrandchamps, and C.A. Smolders, Adsorbent filled membranes for gas separation. Pan 1. Improvement of the gas separation properties of polymeric membranes by incorporation of microporous adsorbents, J. Membrane Sci. 50 189 (1983). 

Knudsen diffusion may take place in a microporous inorganic membrane or through pinholes in dense polymeric membranes. It may also take place in a mixed matrix membrane with insufficient adhesion between the phases. 

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