Immersion precipitation

Most commercially available membranes are prepared by immersion precipitation a polymer solution (polymer plus solvent) is cast on a suitable support and immersed in a coagulation bath containing a nonsolvent. Precipitation occurs because of the exchange of solvent and nonsolvent. The membrane structurfe ultimately obtained results from a combination of mass transfer and phase separation. 

All phase inversion processes are based on the same thermodynamic principles as will be described in section HI - 6. 

This is one of the phase inversion processes, in which the cast film is immersed in a nonsolvent (gelation media). Polymeric membrane soUdifies as the nonsolvent defuses in and solvent defuses out of the film. This method is used to prepare asymmetric membranes. 

In a wet or dry-wet process of phase inversion, the thermodynamic properties of the polymer solution and gelation medium give us some information on the overall porosity of a final membrane but not on the pore size and its distribution. The pore size and its distribution are mainly controlled by kinetic effects. This means that upon the immersion of polymer solution into a coagulation bath, mass transfer mainly determines the asymmetric structure of the membrane. The mass transfer is normally expressed by the exchange rate of solvent/nonsolvent at the interface between the polymer solution and the gelation medium. This exchange rate depends upon the nonsolvent tolerance of the polymer solution, the solvent viscosity and so on [14]. 

The dry-cast process is marked by complete evaporation of solvent and nonsolvent from an initially homogenous polymer solution. This process is more amenable to experimental as well as modeling studies than other phase-transition processes for membrane formation, since it avoids the complications associated with the use of a coagulation bath, as in the immersion precipitation process. 

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Recently, an in-depth review on molecular imprinted membranes has been published by Piletsky et al. [4]. Four preparation strategies for MIP membranes can be distinguished (i) in-situ polymerization by bulk crosslinking (ii) preparation by dry phase inversion with a casting/solvent evaporation process [45-51] (iii) preparation by wet phase inversion with a casting/immersion precipitation [52-54] and (iv) surface imprinting. 

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 ... 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

Immersion Precipitation (Wet Casting) A homogeneous polymer solution consisting of a polymer and solvent(s) is cast on a support and is immersed in a nonsolvent bath. During the immersion, casting solvent diffuses into the nonsolvent bath and, countercurrently, nonsolvent in the bath penetrates into the solution. The nonsolvent has a limited solubility in the polymer, and when it reaches its critical concentration in the solution, precipitation takes place. Then, the solvent and nonsolvent in the solution are extracted and film is annealed. 

Young, T-H., Lin, D.-J., Gau, J.-J., Chuang, W.-Y., and Cheng, L.-P. (1999), Morphology of crystalline Nylon-610 membranes prepared by the immersion-precipitation process Competition between crystallization and liquid-liquid phase separation, Polymer, 40, 5011-5021. 

Reuvers AJ and Smolders CA. Formation of membranes by means of immersion precipitation The mechanism of formation of membranes prepared from the system cellulose acetate-acetone-water. J. Membr. Sci. 1987 34 67-86. 

Cheng LP, Soh YS, Dwan AH, and Gryte CC. An improved model for mass transfer during the formation of polymeric membranes by the immersion-precipitation process. J. Polym. Sci. Polym. Phys. B 1994 32 1413-1425. 

There are a number of different techniques belonging to the category of phase inversion solvent evaporation, precipitation by controlled evaporation, precipitation from the vapor phase, thermal precipitation, and immersion precipitation (13,34—36). The most commercially available membranes are prepared by the last method. 

J. G. Wijmans, J. P. B. Baaij, and C. A. Smolders, The mechanism of formation of micro-porous or skinned membranes produced by immersion precipitation. Journal of Membrane Science 14, 263-274 (1983). 

A. J. Reuvers, J. W. A. Van den Berg, and C. A. Smolders, Formation of membranes by means of immersion precipitation part 1. a model to describe mass transfer during immersion precipitation. Journal of Membrane Science 34, 45-65 (1987). 

Phase inversion refers to the controlled transformation of a cast polymeric solution from a Hquid into a soHd state. During the phase-inversion process, a thermodynamically stable polymer solution is usually subjected to controlled Hquid-H-quid derabdng. This phase separation of the cast polymer solution into a polymer-rich and a polymer-lean phase can be induced by immersion in a non-solvent bath ( immersion precipitation ), by evaporating the volatile solvent from a polymer that was dissolved in a solvent/non-solvent mixture ( controlled evaporation ), by lowering the temperature ( thermal precipitation ) or by placing the cast film in a vapor phase that consists of a non-solvent saturated with a solvent ( precipitation from vapor phase ) [1]. 

An example of the methodologies we are presenting is the case of producing polysulfone microcapsules by phase inversion by immersion precipitation. 

In this experimental section, the production of polysulfone microcapsules containing vanillin is described. Our method is based on the phase inversion by immersion precipitation technique. The herein described process has been successfully employed for the encapsulation of vanillin into polysulfone microcapsules and, in addition, vanillin release from those capsules has been characterized. Polysulfone/vanillin microcapsules could have an application in laundry industry and also in medical applications. ... 

J. Kong and K. Li, Preparation of PVDF hollow-fiber membranes via immersion precipitation, J. Appl. Polym. Sci. 81 (2001) 1643-1653. 

By far the most used technique for membrane preparation is the immersion precipitation method (nonsolvent-induced phase separation). A homogenous polymer solution is cast as thin him and subsequently immersed into a nonsolvent bath, typically water or mixtures of water and solvent. The dif-fusional exchange of solvent and nonsolvent brings the him solution into an instable state resulting in phase separation, either by liquid-liquid (l-l) and/or solid-liquid (s-l) demixing, depending on the type of polymer and the precipitation conditions employed [92,93]. 

R.M. Boom, T. van den Boomgaard, C.A. Smolders, Mass transfer and thermodynamics during immersion precipitation for a two-polymer system. Evaluation with the system PES-PVP-NMP-water. Journal of Membrane Science 90 (1994) 231. 

J.G. WijMANS, J.P.B. Baaij, C.A. Smolders, The Mechanism of Formation of Microporous or Skinned Membranes Produced by Immersion Precipitation. Journal (f Membrane Science 14 (1983) 263. 

Cheng L. P.,YoungT.H.,Fang L., Gau, J. J. (1999), Formation of particulate microporous poly(vinylidene fluoride) membranes by isothermal immersion precipitation from the l-octanol/dimethylformamide/poly(vinyhdene fluoride) system. Polymer, 40(9), 2395-2403. 

Lin D.J., Chang H.H., Chen T.C., Lee Y.C., Cheng L.P. (2006), Formation of porous poly(vinylidenefiuoride) membranes with symmetric or asyimnetric morphology by immersion precipitation in the water/TEP/PVDF system. Ear. Polym. /,42,1581-1594. 

Lti L.Z., Yang Z.S., Wang Z.Y, Yang Y.C.,Tian S.N. (2011), Preparation and performances of PVDF hydrophobic microporous membrane via immersion precipitation assisted with template,/oMrna/ of Tianjin Polytechnic University, 50 4), 6-10... 

Young T.H, Cheng L.P, Lin D.J., Fane L., Chuang W.Y. (1999), Mechanisms of PVDF membrane formation by immersion-precipitation in soft (1-octanol) and harsh (water) nonsolvents, Polymer, 40,5315-5323. 

PVDF-based ultrafiltration nanohybrid membranes prepared via an immersion-precipitation method using PHEMA-b/oc -PMMA-grafted silica nanoparticles as additives were shown to increase the pure water flux, improve the bovine serum albumin (BSA) rejection to a high level (>90%), and reduce membrane fouling at... 

Most of the membranes in use today are phase inversion membranes obtained by immersion precipitation. Phase inversion membranes can be prepared ftom a wide variety of polymers. The only requirement is that the polymer must be soluble in a solvent or a solvent mixture. In general the choice of polymer does not limit the preparation technique. 

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