Homogeneous casting solution

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. 

Thermally Induced Phase Separation In the TIPS process, an initially homogeneous solution consisting of a polymer and solvent(s) phase separates due to a decrease in the solvent quality when the temperature of the casting solution is decreased. After demixing is induced, the solvent is removed by extraction, evaporation, or freeze drying. 

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. 

Vapor-Induced Phase Separation During the VIPS process, phase separation is induced by penetration of nonsolvent vapor, into the homogeneous polymer solution consisting of polymer and solvent(s). Mass transfer is usually much slower than that in the wet casting process thus, the VIPS process has been used to obtain membranes with symmetric, cellular, and interconnected pores [86,87],... 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

Two different techniques have been employed for the precipitation of membranes from a polymer casting solution. In the first method, the precipitant is introduced from the vapor phase. In this case the precipitation is slow, and a more or less homogeneous structure is obtained without a dense skin on the top or bottom side of the polymer film. This structure can be understood when the concentration profiles of the polymer, the precipitant and the solvent during the precipitation process are considered. The significant feature in the vapor-phase precipitation process is the fact that the rate-limiting step for precipitant transport into the cast polymer solution is the slow diffusion in the vapor phase adjacent to the film surface. This leads to uniform and flat concentration profiles in the film. The concentration profiles of the precipitant at various times in the polymer film are shown schematically in Figure 13. 

In order to align the molecules in one direction fi om the solution, we devised a zone-casting technique whose principle is based on observations performed during the solvent evaporation of a drop-cast solution [22]. One can observe that the deposited material is mainly oriented in the direction of the solvent evaporation. Theoretically, at adequate conditions it should be possible to obtain from simple drop-casting a homogeneous film with a radial alignment of the superstructure. However, this is quite difficult, since the concentration... 

A particular versatile method to prepare stable templates and casts is the sol-gel process. In this process an inorganic ester is hydrolyzed to form the free acid. Afterwards the acid condenses to form a three-dimensional network or gel (Scheme 1). Usually it is desirable to have a fast hydrolysis which has a large reaction constant ki followed by a slower polycondensation with a smaller constant A 2- This allows one to homogenize the solution and cast it into the desired form before it solidifies. The values of ki and /C2 can in many cases be adjusted by changing pH and temperature of the solution. 

During the quenching process the homogeneous polymer solution separates into two phases a polymer-rich solid phase, which forms the membrane structure, and a solvent-rich liquid phase, which forms the liquid-filled membrane pores. Generally, the pores at the film surface, where precipitation occurs first and most rapidly, are much smaller than those in the interior or the bottom side of the film, which leads to the asymmetric membrane structure. There are different variations to this general preparation procedure described in the literature e.g., Loeb and Sourirajan used an evaporation step to increase the polymer concentration in the surface of the cast polymer solution and an annealing step during which the precipitated polymer film is exposed for a certain time period to hot water of 70° to 80°C.28... 

When the carbon content is greater than about 2 wt% and less than about 5 wt% carbon, the material cannot be heated to give a homogeneous solid solution. At all temperatures below the eutectic temperature of 1148 °C the solid is a mixture of austenite and cementite or ferrite and cementite (FeaC). The effect of this is that the materials are hard, brittle and resist deformation. The material can be cast into the desired shape, and is referred to as cast iron. Commercial cast irons rarely contain much more than about 4.5 wt% carbon. 

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]. 

Fig. 1.4.32 Cross-sectional diagram of the casting cell used in phase separating a homogeneous polymer solution. The aluminum foil not only provides a seal for the material inside the cell but also acts as a cooling surface. The vapor space in the cell is assumed to act as a thermal insulator (Redrawn with permission from Guo et al., 1995)...

Phase inversion is known to be an effective way to create porous structures in membranes, where a competitive mutual diffusion between solvent and nonsolvent occurs to yield the porous structure. Phase inversion can be described as a demixing process whereby the initially homogeneous polymer solution is transformed in a controlled manner from a liquid to a solid state [24]. Apart from immersion in a nonsolvent bath, or immersion precipitation (IP), a variety of related techniques, such as precipitation by solvent evaporation, precipitation by absorption of water Irom the vapor phase, and precipitation by air cooling, corresponding to thermally induced phase separation (TIPS), vapor-induced phase separation (VIPS), and air-casting phase separation... 

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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