Polymeric membranes evaporative casting

F.G. Paulsen, S.S. Shojaie, and W.B. Krantz, Effect of evaporation step on macro- void formation in wet-cast polymeric membranes. Journal of Membrane Science 91 (1994) 265-282. 

Fig. 3. Fabrication of polymer membrane ISEs (a) dropwise addition of casting solution to glass ring resting on glass slide (b) ring loosely covered, THF evaporates (c) ion-selective polymeric membrane forms on the slide (d) membrane removed from slide and small disk cut out and (e) pasted onto a piece of PVC tubing (f) a Ag/AgCl wire and internal reference solution complete electrode.

The majority of polymeric membranes can be formed via a phase separation/inversion process by which a polymer solution (in which the solvent is the continuous phase) inverts into a swollen three-dimensional macromolecular network or gel (where the polymer is the continuous phase). Phase separation can be induced by solvent evaporation (the dry-cast process), nonsolvent/solvent exchange (the wet-cast process), cooling (the thermal-cast process), and polymer leaching (the polymer-assisted process). 

The dry-cast process for polymeric membrane formation involves dissolving the polymer in an appropriate volatile solvent containing a small amount of nonsolvent to form a single-phase solution. Subsequent evaporation of the solvent causes a phase separation to occur at a sufficiently low solvent concentration. The resulting nonsolvent-rich dispersed phase forms the pores, whereas the polymer-rich phase forms the matrix structure of the membrane. Thus, in the cellulose acetate/acetone/water system, acetone evaporates, cellulose acetate (CA) becomes the continuous phase, and water forms the pores. 

This patent deals with proton-conducting membranes having improved resistance to methanol crossover. The membranes are obtained by solution casting followed by solvent evaporation from a solution containing an organic solvent, a polymer (preferably a polyphosphazene), and an oxoacid. It is claimed that a particularly useful application for these polymeric membranes is in methanol fuel cells. 

Figure 33.10 Schematic of an apparatus for the evaporative casting of polymeric membranes from a casting solution consisting of cellulose acetate, acetone, and water the simultaneous downward movement of the phase separation fiont and hquid-gas interface was monitored with a 10-MHz UTDR transducer placed on the underside of the aluminum support surface.

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. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

The above set of rules - though accurately descriptive of earlier casting procedures - has led to serious misconceptions pertaining to the formation of anisotropic membranes, and therefore, misconceptions in the formulation of new polymeric casting solutions. It is evident that the polymer solution concentration progressively increases at the surface layer during the evaporation period, and... 

Usually, a flat sheet membrane is prepared by spreading a casting solution on a flat surface and evaporating the solvent. A thin polymeric layer that is formed between air and the bulk of the casting solution is called the active or the top layer of the membrane. The performance of the membrane depends largely on the physical or molecular structure of the active layer. 

Thicker membranes from polymers are usually cast as solution (drop, spin, and spray coating), and the membrane forms after evaporation of the solvent. Alternate routes are application of monomers, direct polymerization on the electrode surface, and mechanical attachment of the ready-to-use membranes. Usually, modifiers (mediators, catalysts, enzymes, etc.) are dissolved in the membraneforming solution. Membranes can be generated also by electropolymerization, most commonly from aniline, pyrrole, or thiophene [117-119] the resulting 2D structure can entrap active molecules (e.g., enzymes) or serve as anchors for the actual modifier. Attachment of active molecules to polymeric structures can... 

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