Solution cast membranes

Solution-cast membranes (100—200 /urn in thickness) from sulfonated polymers with an ion exchange... 

Williams (2002), Chairman of E I Du Pont de Nemours, describes its new process (solution cast membrane technology) for manufacturing Nafion membranes aimed at PEFC/MEAs. With large production the membrane NR112 can fall to about 80 per m, and NRlll to about 45 per m. Details of the two types are given. 

These solution-cast membranes are registered as NRE membranes. The effects of the manufacturing processes on membrane properties are compared in Table 10.1 [24, 32],... 

As a substimte to Nafion impregnation into a fluoropolymer matrix, solution cast films have been prepared from blends of Nafion and vinylidene fluoride-hex-afluoropropylene copolymer [16]. Upon addition of 20 60% vinylidene copolymer, a steep decrease in proton conductivity was observed, from 10 " S cm to 10 -10 S cm , which was accompanied by a significant decrease in methanol permeability. In a more recent paper [27], Si et al. solution cast membranes composed of Nafion and poly(vinylidene fluoride), henceforth denoted as PVDF, where characterized where the PVDF content was varied from 0 to 65 wt%. As expected, the conductivity and methanol permeability decreased with inaeasing PVDF content, with a much more dramatic drop in transport parameters when the PVDF content was >50%. From proton conductivity and methanol permeability data, the relative selectivity can be determined ... 

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. ... 

In a purely application driven paper, PTFEP, which is known for it excellent membrane forming properties was characterized as a membrane for the removal of thiophene from gasoline. PTFEP in this work was synthesized using the common ring opening polymerization procedure for poly[bis-chlorophosphazene], followed by substitution with sodium 2,2,2-trifluoroethoxide in THF solvent. Solution cast membranes were formed and characterized for transmembrane flux and selectivity using pervaporation as the separation technique. An inverse relationship between selectivity and flux was observed. 

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

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. 

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). 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

Pervaporation Membranes Pervaporation has a long history, and many materials have found use in pervaporation experiments. Cellulosic-based materials have given way to polyvinyl alcohol and blends of polyvinyl alcohol and acrylics in commercial water-removing membranes. These membranes are typically solution cast (from... 

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. 

The effect of annealing temperatures (65 - 250 °C) and blend composition of Nafion 117, solution-cast Nafion , poly(vinyl alcohol) (PVA) and Nafion /PVAblend membranes for application to the direct methanol fuel cell is reported in [148], These authors have found that a Nafion /PVAblend membrane at 5 wt% PVA (annealed at 230 °C) show a similar proton conductivity of that found to Nafion 117, but with a three times lower methanol permeability compared to Nafion 117. They also found that for Nafion /PVA (50 wt% PVA) blend membranes, the methanol permeability decreases by approximately one order of magnitude, whilst the proton conductivity remained relatively constant, with increasing annealing temperature. The Nafion /PVA blend membrane at 5 wt% PVA and 230 °C annealing temperature had a similar proton conductivity, but three times lower methanol permeability compared to unannealed Nafion 117 (benchmark in PEM fuel cells). 

Liu, C. and Martin, G. R. 1990. Ion-transporting composite membranes III. Selectivity and rate of ion transport in Nafion-impregnated Gore-Tex membranes by a high-temperature solution casting method. Journal of the Electrochemical Society 137 3114-3120. 

The concepts and techniques discussed by Cirkel and Okada are relevant with a view toward modifying the structure of solution cast Nafion membranes by manipulating counterion type, solvent, temperature, and other variables. 

Effect of Casting Solution Composition on the Casting Solution and Membrane Properties... 

The gradual increase in the formamlde fraction to solution increased the elapsed time before schlieren patterns appeared. For the solution composition formamlde/acetone (40/60), slow convection flow appeared suddenly after 650 seconds, but the formation of the pellicle at the nascent membrane Interface could be clearly seen 15 seconds after submersion in the water bath. Such pellicles could not be discerned for solutions cast from THF or pure acetone. 

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