Phase inversion membranes, synthetic

Kestig RE. Phase-Inversion Membranes in Synthetic Polymeric Membranes A Structural Perspective. New York John Wiley Sons, 1985, pp. 237-286. 

Resting, R.E., Phase inversion membranes. In Material Science of Synthetic Membranes, Lloyd, D.R., Ed., American Chemical Society, ACS Symposium Series 269, Washington, 1985, p. 131. 

Resting, R. E., Synthetic Polymeric Membranes A Structural Perspective , 2nd edn., Wiley-Inter-science. New York, 1985. Membrane types are emphasized—polymer membranes, the polymeric films, phase inversion membranes, liquid and dynamically formed membranes, biological membranes. 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

J.G. Wijmans and C.A. Smolders, Preparation of Anisotropic Membranes by the Phase Inversion Process, in Synthetic Membranes Science, Engineering, and Applications, P.M. Bungay, H.K. Lonsdale and M.N. de Pinho (eds), D. Reidel, Dordrecht, pp. 39-56 (1986). 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

New cellulose manbranes were recently prepared by the phase inversion method using a green solvent, the ionic liquid [BMIM][C1] [32], After functionalization with a synthetic ligand 2-(3-aminophenol)-6-(4-amino-l-naphthol)-4-chloro-5-triazine, these adsorptive membranes were evaluated for human immunoglobulin G (IgG) adsorption. The authors envisage that a change in the conditions and chemistry for membrane activation with the biomimetic ligand may improve the performance of the affinity cellulose membranes. 

The formation of membranes from synthetic polymers is typically achieved by the phase inversion (PI) process, which starts with a stable solution of the polymer which is then subjected to controlled demixing [14]. As a result, a porous structure is obtained where the polymer-rich phase forms the matrix of the membrane. The demixing of a previously formed liquid film (either flat or hollow-fiber cf Scheme 1) can be achieved by two main processes ... 

Molecular modification of ozone-pretreated PVDF via thermally induced graft copolymerization with V-isopropylacrylamide (NIPAM) was carried out in NMP solution to produce the PVDF-g-PNIPAM copolymers [63]. The synthetic route is shown in Scheme 8.1. Microporous membranes were fabricated from the PVDF-g-PNIPAM graft copolymers by phase inversion in aqueous media. The surface composition and morphology of the resulting PVDF-g-PNIPAM membranes can be adjusted by the casting temperature, while the flux through the membrane exhibits temperature-responsive behavior. In addition to pure water flux, the copolymer membranes also exhibited reversible temperature-dependent permeability to calcein and... 

Microfiltration and ultrafiltration membranes can be made from organic polymers or inorganic materials such as ceramic, glass, or metal or organic polymers. Materials used in MF and UF membrane fabrication are shown in Table 6.1. A number of different techniques are employed to prepare synthetic MF/UF membranes the most important are phase inversion, coating, sintering, and track etching. 

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