Membrane phase inversion

Phase inversion is a process whereby a polymer is transformed in a controlled manner from a liquid to a solid state. The process of solidification is very often initiated by the transition from one liquid state into two liquids (liquid-liquid demixing). At a certain stage during demixing, one of the liquid phases (the high polymer concentration phase) will 

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Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

Chemical Phase Inversion Svmrnetrical phase-inversion membranes (Fig, 22-71) remain the most important commercial MF membranes produced. The process produces tortiioiis-Bow membranes. It involves preparing a concentrated solution of a polvrner in a solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, iisiiallv w ater, sometimes from the vapor phase. The technique is irnpressivelv v ersatile, capable of producing fairlv uniform membranes wFose pore size rnav be varied within broad limits. 

The present study should be seen as a step in the evolution of the colloidal morphology of phase inversion membranes, which conceptually began with dense polymer films and diverged into the two principal branches skinned and skinless membranes (Figure 1). 

Figure I. Evolution of the colloidal morphology of phase inversion membranes...

H. Strathmann and K. Kock, The Formation Mechanism of Phase Inversion Membranes, Desalination 21, 241 (1977). 

The first, and currently only, successful solvent-permeable hyperfiltration membrane is the Starmem series of solvent-resistant membranes developed by W.R. Grace [40]. These are asymmetric polyimide phase-inversion membranes prepared from Matrimid (Ciba-Geigy) and related materials. The Matrimid polyimide structure is extremely rigid with a Tg of 305 °C and the polymer remains glassy and unswollen even in aggressive solvents. These membranes found their first large-scale commercial use in Mobil Oil s processes to separate lube oil from methyl ethyl ketone-toluene solvent mixtures [41-43], Scarpello et al. [44] have also achieved rejections of >99 % when using these membranes to separate dissolved phase transfer catalysts (MW 600) from tetrahydrofuran and ethyl acetate solutions. 

The difficulty in analyzing the phase inversion process is due to the interactions between the three components involved, and to the complex diffusion and convective processes that play an important role during the membrane formation. However, many researchers agree that there are two dominating factors controlling the formation of phase inversion membranes thermodynamics and kinetics, correlated to each other during the solidification of casting solutions. 

Young T-H and Chen L-W. Pore formation mechanism of membranes from phase inversion process. Desalination 1995 103(3) 233-247. Han M-J and Nam S-T. Thermodynamic and rheological variation in polysulfone solution by PVP and its effect in the preparation of phase inversion membrane. J. Membr. Sci. 2002 202(l-2) 55-61. 

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

Iversen et al. [6] found that for a polymer strucmre similar to the interstices between closely packed spheres (phase inversion membrane), Equation 38.4 is able to well describe the tortuosity-porosity relationship whereas for a polymer structure similar to random spheres or clusters (stretched membrane), Equation 38.5 has to be used. 

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. 

Equation (30) gives a good description of transport through membranes consisting of a number of parallel pores. However, very few membranes possess such a structure in reality. Membranes consists a system of closed spheres, which can be found in organic and inorganic sintered membranes or in phase-inversion membranes with a nodular top layer structure. Such membranes can best be described by the Kozeny-Carman relationship ... 

Phase-inversion membranes frequently show a sponge-like structure. The volume flux through these membranes is described by the Hagen-Poiseulle or the Kozeny-Carman relation, although the morphology is completely different. 

Phase inversion refers to the process by which a polymer solution (In which the solvent system Is the continuous phase) inverts Into a swollen three-dimensional macromolecular network or gel (where the polymer Is the continuous phase). As a thin film designed for use as a barrier, such a gel constitutes a phase inversion membrane. 

Casting-solution and environmental variables permit far greater control over the ultimate structure and performance of phase inversion membranes than does the modification of a primary gel into a secondary gel by postformation treatments. Because the properties of the primary gel determine to a large extent those of its secondary counterpart, the former should be considered as more fundamental and important in discussing the effects of fabrication parameters such as casting-solution composition, upon performance. Once a primary gel has been formed, it may be utilized as such (particularly for low-pressure applications) or it may be subjected to various physical and/or chemical treatments for conversion into a more pressure-resistant secondary gel. 

Spherical cells exist in the final gel matrix. Although all phase inversion membranes possess spherical micelles in their nascent Sol 2 condition, only the thermal process retains the spherical mlcellular shape into the final open-cell gel... 

The permeability constant of the composite membrane is therefore represented by the harmonic average of the permeability constants of the individual layers, the respective weights being x /Ji., the ratio of layer thickness to the total. Although composite membranes Include layers of dense films or even liquid layers in series with films, in this discussion the term is being limited to those series in which at least one of the members is a phase inversion membrane of either the integrally-skinned or skinless variety. 

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