Membrane preparation solution cast membranes

Figure 6.36 Piezodialysis of a potassium chloride solution using a mosaic ion exchange membrane (membrane thickness about 200 A, ion exchange capacity of cation and anion exchange parts are almost the same). Salt enrichment Es = [(CujCi) — 1] X 100 (C, concentration of feed solution Cu concentration of permeate). ( ) Membrane prepared by casting dioxane solution of the polymer. ( ) Membrane prepared by casting benzene solution of the polymer. (A) Membrane prepared by casting cyclohexane solution of the polymer.

The structures of PFSA membranes have been analyzed and discussed by many researchers, and the cluster-network model for hydrated membranes proposed by Gierke [22] has been a basic model symbolic of the PFSA characteristics up to now. As for the structure of the diluted aqueous solution of PFSA, it is important to understand the structure of ionomer dispersion and catalyst ink, comprising catalyst particles, ionomer, and solvent, for the preparation of cast membrane and catalyst layer, respectively. Aldebert et al. 

High-molecular-weight polymers are obtained with an outstanding solubility in polar aprotic solvents. This polymer is suitable for the preparation of anion exchange membranes by solution casting using dimethylacetamide as solvent [12]. 

Membrane Preparation Details Casting Solution Composition and Preparation Conditions... 

Contact angle measurements Contact angle measurements were made on a Shimazu Surface Tensometer Model ST-1 on block copolymer membranes prepared by casting from solution. A series of liquids having different surface tensions were used. 

Moore, R. B., and Martin, C. R., 1986, Procedure for preparing solution-cast perfluorosulfonate ionomer films and membranes. Anal. Chem. 58 2569-2570. 

Membranes were prepared using dry Nafion powder [31] that was obtained by evaporating the solvent from a commercial Nafion solution. The powder was equilibrated in an aqneous NaCl/HCl solution with a preset NaVH " concentration ratio to adjust the protonation degree of the Nafion polymer. The snbstitnted Nafion was dried again and then mixed with an appropriate amount of PBI in DMAc solvent. Membranes were solution cast into a glass dish. After solvent evaporation at 80°C, the films were annealed at 150°C for 3 h. The resultant membranes (50-100 pm in dry thickness) were conditioned by boiling in IM H SO followed by extensive washings with de-ionized water. Membranes were stored at room temperatnre in water for later use. 

The general procedure to prepare solution-cast PFSA membrane is as follows. First, since the PFSF cannot dissolve in common solvents, it should be converted to Na+ form ionomer by hydrolysis with aqueous NaOH. Afterward, the Na+ form ionomer is immersed into FI2SO4 or HNO3 solution to convert to H+ form ionomer. Then, the H+ form ionomer dissolves in solvent. The reported solvents include dimethylfor-mamide (DMF), dimethyl sulfoxide (DMSO), ethylene glycol (EG), and mixture of water with alcohols. Finally, the membranes were fabricated by solution cast of the obtained H+ form ionomer solution. 

Recently, we develop a new method to prepare PFSA membranes from their precursor in hexafluoropropene trimer (HFPT) solvent. Dissolution of the PFSF has been achieved by swelling it in HFPT (2 wt%) and heating for 2 h at 250°C under pressure. Then, the PFSF membranes were fabricated at 80°C or 120°C. After hydrolysis, these membranes are converted into PFSA membranes. Compared with those of traditional solution-casted membranes, the obtained membranes exhibit higher dimensional stability, proton conductivity, and chemical stability and remarkably decreased methanol crossover. ... 

A PBI solution with an inherent viscosity of 1.2 dL/g in cone. H2SO4 was used to prepare the membranes through solution-casting in a vacuum oven at 80-90 °C. The film formed was subsequently peeled off and treated with water at 60 °C for one week to remove the residual solvent completely. The film was then dried at 100 °C under vacuum for two days and the doping was carried out by keeping the membranes immersed in 88% H3PO4 solution for 72 h and subsequently drying them at 100 °C for two days. The phosphoric acid uptake was about 13 moles per repetitive unit. 

Dense Symmetrical Membranes. These membranes are used on a large scale ia packagiag appHcations (see Eilms and sheeting Packaging materials). They are also used widely ia the laboratory to characterize membrane separation properties. However, it is difficult to make mechanically strong and defect-free symmetrical membranes thinner than 20 p.m, so the flux is low, and these membranes are rarely used in separation processes. Eor laboratory work, the membranes are prepared by solution casting or by melt pressing. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Fig. 13. Phase diagram showing the composition pathway traveled by a casting solution during the preparation of porous membranes by solvent evaporation. A, initial casting solution B, point of precipitation and C, point of soHdification. See text.

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Water Permeation and Solute Separation through the Membrane. The measurements of water permeability of the 67 membranes prepared under different conditions were carried out by using an Amicon Diaflo Cell (effective membrane area, 13.9 cm2) under a pressure of 3 kg/cm2 at 25 °C. Some results are listed in Table 1067. It is apparent that much higher water absorption and permeability than the cellulosic membrane are characteristic of the 67 membranes prepared by both the casting polymerization and conventional casting. 

The permeability tests for alkali metal ions in the aqueous solution were also conducted. When an aqueous salt solution moves to cell 2 through the membrane from cell 1, the apparent diffusion coefficient of the salt D can be deduced from a relationship among the cell volumes Vj and V2, the solution concentration cx and c2, the thickness of membrane, and time t6 . In Table 12, permeabilities of potassium chloride and sodium chloride through the 67 membrane prepared by the casting polymerization technique from the monomer solution in THF or DMSO are compared with each other and with that the permeability through Visking dialyzer tubing. The... 

Enzyme containing Nation membranes prepared according to the proposed protocol have shown high specific activity and stability of immobilized glucose oxidase. As expected, the simplicity of preparation provided high reproducibility. When the same casting solution is used, the maximum deviation in membrane activity is <2%. This, however, is also the precision limit for kinetic investigations. 

A membrane prepared by PVA blending with PAcr.Ac. in aqueous solution, casting, solvent evaporation and then crosslinking by heat treatment (at 150 °C), has been used. 

Membrane Preparation. Dried cellulose diacetate is dissolved in acetone in the weight ratio of 1 to 3 or 4. Gaseous ammonia is directed at room temperature over the solution surface in a rotary evaporator, the ammonia being readily absorbed by the polymer solution. Optimal ammonia concentration is 5 to 6 wt-%, a typical casting solution composition is cellulose diacetate/acetone/ ammonia 18.8/75.2/6.0 (solvent-to-polymer ratio 4). Casting is at room temperature. The precipitation bath is maintained at pH 4 through controlled addition of hydrochloric acid to compensate for the alkaline intake. 

Asymmetric Membrane Preparation. The preparation of the as5Tnmetric membranes was done in a fashion similar to the "classical" technique referred to below, although the casting solutions often deviated from the "classical" formulations. In all cases, a solution of polymer plus at least two other components was cast on a glass plate with a doctor s knife set at a thickness of 15 mils (0.381 mm). After a brief evaporation period the membrane was gelled in a non-solvent bath. Finally, the membrane was thoroughly washed in distilled, deionized water. 

Performance of Membranes Prepared from Classical Casting Solutions ... 

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