Membrane immersion precipitation

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 ... 

Exposure time of proto-membrane before precipitation. The effect of exposure to atmosphere before immersion is dependent on the solvent property (e.g., volatility, water absorption) and atmosphere property (e.g., temperature, humidity). This step (i.e., combination of EIPS or VIPS with NIPS cf. above) has significant effects on the characteristics of the skin layer and the degree of anisotropy of the resulting membrane [14]. 

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. 

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). 

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 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. 

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. 

In this section the basic principles of membrane formation by phase inversion will be described in greater detail. All phase inversion processes are based on the same thermodynamic principles, since the starting point in all cases is a thermodynamically stable solution which is subjected to demixing. Special attention will be paid to the immersion precipitation process with the basic charaaeristic that at least three components are used a polymer, a solvent and a nonsolvent where the solvent and nonsolvent must be miscible with each other. In fact, most of the commercial phase inversion membranes are prepared from multi-component mixtures, but in order to understand the basic principles only three component systems will be considered. An introduction to the thermodynamics of. polymer solutions is first given, a qualitatively useful approach for describing polymer solubility or polymer-penetrant interaction is the solubility parameter theory. A more quantitative description is provided by the Flory-Huggins theory. Other more sophisticated theories have been developed but they will not be considered here. 

Membrane formalion by phase inversion techniques, e.g. immersion precipitation, is a non-equilibrium process which cannot be described by thermodynamics alone since kinetics have also to be considered. The composition of any point in the ca.st film is a function of place and time. In order to know what type of demixing process occurs and how it occurs, it is necessary to know the exact local composition at a given instant. However, this composition cannot be determined very accurately experimentally because the change in composition occurs extremely quickly (in often less than 1 second) and the film is very tto (less than 200 itm). However it can b e described theoretically. 

One of the main variables in the immersion precipitation process is the choice of the solvent/nonsolvent system. In order to prepare a membrane from a polymer by phase inversion the polymer must be soluble. Although one or more solvents may be suitable for the chosen polymer, the solvent and nonsolvent must be completely miscible. Water is frequently used as a nonsolvent but other nonsolvents can also be used. Some solvents for cellulose acetate and polysulfone which are miscible with water are listed in table III.6. The solubility of these organic solvents with water must be considered further. As described in the previous section, the miscibility of components of all kind is determined by the free enthalpy of mixing... 

For porous membrane.s obtained by instantaneous demixing, the separation properties are mainly determined by the choice of solvent/nonsolvent. Indeed this type of structure can almost be considered to be independent of the choice of polymer. Table IE -9 gives a list of polymers from which ultrafiltration membranes have been made using DMAc or DMF as the solvent and water as the nonsolvent. The polymer concentration varied from 10-20% and immersion precipitation occurred at room temperature. 

Membranes are frequently prepared by an immersion precipitation process in which three components are used polymer (P), solvent (S), en nonsolvent (NS). Consider a system that demixes at 30% by weight of nonsolvent independently on the polymer concentration. The critical point is located at 5% by weight polymer. 

Immersion precipitation is one of the most important techniques to prepare phase inversion membranes. During this process demixing can occur instantaneously or delayed. 

Membrane formation frequently takes place by immersion precipitation in which three components are involved solvent (S), nonsolvent (NS) and polymer (P). 

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