Solvent-nonsolvent exchange

Kawakami et al. prepared dense and asymmetric membranes from 2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and bis[4-(4-aminophen-oxy)phenyl]sulfone (APPS) by solvent evaporation (dense) and by the dry-wet phase inversion technique [47]. The surface morphology was studied by AFM. They reported that the solvent evaporation method adopted for the preparation of the dense membrane influenced the formation of nodules, while the dry-wet process in which solvent/nonsolvent exchange was involved determined the roughness of the skin layer. 

The cast polymer solution is immersed into a gelation bath after the partial evaporation of the solvent. From this moment the exchange between the solvent (which is in the cast film) and the gelation medium (which is usually a nonsolvent for the membrane polymer) starts to occur. The solvent-nonsolvent exchange is controlled by the counteidiffusion of solvent and nonsolvent through... 

More discussions on the solvent-nonsolvent exchange during the gelation process can he found in the literature [49]> [50],... 

In most polymers, the impact of solvent/nonsolvent pair selection on the resulting membrane structure and performances can be explained based on their mutual affinity (miscibility) (Mulder 1996). In general, if their miscibility increases, more solvent is required in the nonsolvent bath to affect demixing. In other words, higher solvent/ nonsolvent mutual affinity leads to delayed demixing and vice versa. However, such phenomenon is not clearly observed in PVDF polymers. Fortunately, an extensive study by Bottino et al. (1991) can be used as a basis to understand the effect of solvent selection on the structure and overall performance of PVDF membranes. The structure and performance of PVDF membranes prepared from PVDF/various solvents/ water system can be correlated with the solvent/nonsolvent diffusivity (Figure 8.11). At the very first moment after immersion, but before top skin formation occurs, solvent/nonsolvent exchange starts before PVDF diffusion becomes important. In this condition, the polymer can be considered practically stable. Therefore, the top... 

The entire phase inversion process of a polymeric solution is represented by the path from A to D. The original polymeric solution is at point A, where no precipitation agent (nonsolvent) is present in the solution. After the immersion of the polymeric solution into a nonsolvent coagulation bath, the solvent diffuses out of the polymer solution, whereas the nonsolvent diffuses into the solution. In the case when the solvent flux is higher than the nonsolvent flux, the polymer concentration at the interface would increase, and at some point, the polymer starts to precipitate (as represented by point B). The continuous replacement of the solvent by the nonsolvent would result in the solidification of the polymer-rich phase (point C). Further solvent/nonsolvent exchange would cause shrinkage of the polymer-rich phase and finally reach point D, where the two phases (solid and liquid) are in equilibrium. A solid (polymer-rich) phase that forms the membrane structure is represented by point S and a liquid (polymer-poor) phase that constitutes the membrane pores filled with nonsolvent is represented by point L. 

It should be noted that the type of solvent, polymer binder or nonsolvent, could affect the precipitation mechanism in the phase inversion process. As an example, the (V-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO) systems possess dissimilar precipitation lines (Figure 11.4), where the precipitation point for the polyethersulfone (PESf)/DMSO/water system is much closer to the original casting solution (0% water) line than that of the PESf/NMP/water system. Because the rates of solvent/nonsolvent exchange for both systems are not very much different, the time to reach the precipitation point for PESf/DMSO/water system is much shorter than that for the PESf/NMP/water system. As the macrostructure of the membrane is largely determined at the precipitation point, PESf/DMSO/water and PESf/NMP/ water systems would yield different macrostructures. 

Hydrodynamically unstable viscous fingering is a well-known phenomenon that occurs at the interface between fluids with different viscosities in the first moments of mixing and has been applied here to explain the formation of finger-like voids in ceramic membrane precursors [16]. When the suspension is in contact with the nonsolvent, a steep concentration gradient results in solvent/nonsolvent exchange, a rapid increase in local viscosity, and finally precipitation of the polymer phase. However, due to instabilities at the interface between the suspension and the precipitant. 

NIPS of a homogenous polymer solution can be induced by (1) immersion precipitation, immersion of the polymer solution into a nonsolvent bath such that the solvent-nonsolvent exchange occurs (2) vapor absorption, the absorption of a nonsolvent by the polymer solution when it is subject to a vapor containing nonsolvent until precipitation and (3) solvent evaporation, the evaporation of a volatile solvent from a polymer solution [2],... 

In a thermodynamic context, instantaneous demixing is likely to take place when the initial composition of the homogeneous polymer dope is relatively close to its metastable or unstable state. This is because a little amount of nonsolvent is sufficient to induce phase separation. In addition to thermodynamics, the kinetics of membrane formation, which often refers to the rate of solvent-nonsolvent exchange during the phase inversion, is observed to govern the final membrane structure in many cases [14,15]. If the solvent-nonsolvent exchange rate is slow, delayed danix-ing will occur even when a homogeneous system is thermodynamically close to its metastable or unstable state and vice versa. 

The solvent-nonsolvent exchanges that accompany this process are influenced by the surface phenomena and interfacial dynamics, which ultimately establish the skin structure of the membrane. It is apparent that surface tension data on polymer solutions is valuable to determine the suitability of nonsolvent additives, polymers and/or their solutions in organic solvents for casting asymmetric phase inversion membranes. 

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