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Procedure for Preparing Solution-Cast Perfluorosulfonate Ionomer Films and Membranes Sir: We recently described a procedure for dissolving perfluorosulfonate ionomers (PFSI's) ( I ) and solutions of Nafion PFSI have since become commercially available (C.G. Processing, Rockdale, MD). These solutions may be used to cast films of these polymers onto electrode (1-3) and other (4) surfaces. It is generally either tacitly or explicitly assumed that the morphologies, physical properties, and chemical characteristics (e.g., solubilities) of the as-received and filmcast PFSI's are the same ( I , 5,6). We have recently discovered that this is not true. When Nafion or Dow (7) PFSI solutions are used to cast thick (ca. 0.1 mm) films onto glass (or other) surfaces, the resulting films are hard and brittle and cannot be peeled intact from the surface. This may be contrasted to the as-received polymer which is a pliant, coherent, and elastic material. Furthermore, the solution-cast film is soluble at room temperature in a variety of polar organic solvents (see, e.g., Figure 1) and in warm water. Again, this may be contrasted to the as-received polymer which is totally insoluble in all solvents a t temperatures below ca. 200 OC ( I ) Preliminary X-ray scattering data show that these radical differences in mechanical properties and solubilities result from morphological differences between the as-received and solution-cast materials (8).
The poor mechanical properties and high solubilities of solution-cast PFSI's are, from a practical point of view, highly undesirable. For example, we are currently studying the possibility of using the Dow PFSI's to coat (and thus protect) the Pt catalyst in phosphoric acid fuel cells. We have found, however, that solution-cast PFSI films dissolve in warm, aqueous H3P04. Furthermore, the solubility in polar organic solvents precludes the use of PFSI-modified surfaces in such solvents. Finally, the poor mechanical properties mean that solution-cast PFSI's will be poor membrane materials. To circumvent these problems, we have developed a solution processing procedure that produces solution-cast PFSI films that have excellent mechanical properties (i.e., pliant, mechanically strong) and that are insoluble in all solvents tested at temperatures below ca. 200 OC. Because of the widespread interest in solution-cast PFSI films and membranes, we describe this new procedure in this correspondence.
EXPERIMENTAL SECTION We have shown that PFSI's can be dissolved in ethanol-water at 250 "C ( I ) . The gist of the solution processing procedure for producing insoluble, high-quality solution-cast PFSI is to replace the ethanol-water with a high boiling point solvent and then to remove this high boiling solvent at elevated temperatures. A typical procedure is as follows: A 0.445 (w/v) % solution of 1100 equivalent weight Nafion in ethanol-water was converted to the Na' salt form.by adding an equivalent quantity of concentrated NaOH. An equal volume of the desired high boiling point solvent (e.g., dimethyl sulfoxide Me,SO), ethylene glycol (EG), N,N'dimethylformamide (DMF)) was added to this solution. The majority of the ethanol and water was removed by heating on a hot plate at ca.80 "C. The solution was then placed in an oil bath, which had been preheated to the desired (see Figure 1) elevated temperature, and evaporated to dryness. The solid Nafion membrane obtained was then placed in a vacuum oven (at the same temperature used in the evaporation step) t o remove the last traces of the high boiling solvent. We call the membrane obtained from this high-temperature procedure "solution-processed"Nafion. The physical properties and solubilities of the solution-processedNafion varied with the solvent and the processing temperature employed (Figure 1). The 0003-2700/86/0358-2560$0 1.50/0
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Flgure 1. Solubiliiies of solution-processed Nafion in various solvents as a function of processing temperature: (A)DMF; (D) Me,SO; (0) ethylene glycol (see text for detalls).
following procedure was used to assay the solubility of the solution-processed material: 50 mg of the solution-processedpolymer was added to 10 mL of 50-50 ethanol-water and this mixture was agitated for 1h in an ultrasonic cleaner. The ethanol-water was then filtered through a Whatman No. 1 filter paper (to remove undissolved solids) and evaporated to dryness. The solid residue obtained was weighed and the percent of the material that had dissolved was calculated.
RESULTS AND DISCUSSION Figure 1 shows a plot of the percent of the solution-processed Ndion that dissolved vs. the temperature used during the processing procedure. For each solvent, solubility decreased as the processing temperature used increased. The mechanical properties of the membrane mirror the solubility; membranes that were highly soluble were brittle and hard and membranes that were highly insoluble were pliant, elastic, and coherent. Furthermore, we have found that membranes that are greater than ca. 5% soluble (i.e., above the dashed line in Figure 1)disintegrate when contacted with ethanol-water or even warm water. The bottom line is that highly insoluble membranes with good wet and dry mechanical properties can be obtained from DMF at temperatures above ca. 130 "C, from Me,SO at temperatures above ca. 170 "C and from EG a t temperatures above ca. 185 OC. We have exposed such solution-processed membranes to Me2S0, DMF, water, EG and other solvents a t temperatures as high as 200 "C and they neither dissolve nor disintegrate.
CONCLUSIONS We have developed a solution processing procedure that produces high-quality solution-cast PFSI films and membranes. We recommend that this procedure be used when preparing such films and membranes. We are currently using X-ray scattering methods to investigate the morphology of the solution-processed PFSI's. Preliminary results clearly show that the morphology of the PFSI obtained from room 0 1986 American Chemical Society
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temperature evaporation is quite different from the morphologies of as-received and solution-processed PFSI (8). Apparently, low-temperature evaporation does not impart sufficient thermal energy to the polymer chains to allow them to assume the morphology present in the as-received polymer. We will report the results of these studies soon.
Registry No. Nafion, 39464-59-0. LITERATURE CITED Martin, C. R.; Rhoades. T. A.; Ferguson, J. A. Anal. Chem. 1982, 54, 1639. Martin, C. R.; DoHard, K. A. J. Electroanal. Chem. 1983, 759, 127. Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984. 56, 1898. Szentirmay. M. N.; Campbell, L. F.; Martin, C. R. Anal. Chem. 1988, 58,661. Buttry, D. A.; Anson, F. C.J. Am. Chem. SOC. 1983, 105. 685.
(6) Rubinstein, I.; Rishpon, J.; Gottesfeld, S.J . Nectrochem. SOC.1986, 733,729. (7) U S . Patent 4417969. (8) Moore. R. B.; Martin, C. R., unpublished results, Texas A&M University, February 1986
Robert B. Moore, I11 Charles R. Martin* Department of Chemistry Texas A&M University College Station, Texas 77843
RECEIVED for review May 9,1986. Accepted July 1,1986. This work was supported by the Dow Chemical Co. and the Office of Naval Research.
AIDS FOR ANALYTICAL CHEMISTS Variable Path Length Flow-Through Cell for Spectrophotometry Thomas Choat, Johannes J. Cruywagen,* and J. Bernard B. Heyns Department of Chemistry, University of Stellenbosch, Stellenbosch, 7600 South Africa A convenient experimental setup for spectrophotometric titrations is to circulate the reaction mixture through a flowthrough cell by means of a peristaltic pump. A similar titration procedure can be followed when absorbance data for equilibrium studies are desired. Because the change in absorption of the reaction mixture with the change in concentrations of the components is monitored continuously, conditions can be controlled to obtain the most suitable set of data points. This method of data collection is advantageous because it affords greater accuracy and is less tedious than a procedure whereby a series of solutions is prepared and the absorbance of each solution is measured separately. Flow-through cells of fixed path lengths in the range 1-10 mm are commercially available. However, when a cell of fixed path length is used, it is often not possible to vary the concentration of a strongly absorbing reagent over a wide range without exceeding the optimum absorption limits of the spectrophotometer. For instance, cells with path length varying from 0.1 to 10 mm are commonly used when condensation reactions (1-3) are investigated and in such cases a titration procedure cannot be followed since switching of cells is impractical. The above considerations have led us to design an inexpensive flow-through cell of which the path length can be varied from 0 to 10 mm at precisely graduated increments of 0.01 mm.
CELL CONSTRUCTION The cylindrical outer casing of the cell was constructed from nickel-plated brass, and fitted with a PTFE inlet and outlet (see Figure 1). A fine pitch thread cut along part of the casing inner wall allows for the use of a graduated brass ring (henceforth, the drum) for adjustment of the optical path length. The quartz windows are mounted in two separate cylindrical inner Perspex housings, one of which is let into the drum, the other being fixed. Perspex to quartz joints are sealed with an epoxy adhesive. To maintain a liquid seal during use, the outer brass casing has an inner Perspex sleeve
along which can slide a tight-fitting O-ring, recessed into the movable inner housing. The drum is graduated into 100 divisions, and since a full rotation adjusts the optical path by 1 mm, the path length can be very precisely set. When set at its designed maximum optical path of 10 mm, our cell has a total length of 75 mm; the brass casing has an outer diameter of 37 mm. The cell is mounted in the spectrophotometer beam by means of a clamp bracket that can slide over the front end of the cell. The clamp bracket has a base plate that can be screwed down onto the floor of the cell compartment.
APPLICATION In typical equilibrium study experiments in this laboratory a thermostated reaction vessel is connected to the cell and charged first with a solution of the titrand at constant ionic medium. Circulation is then started, and the Tygon lines and the cell are freed of air bubbles and the path length set at a suitable value. After temperature equilibration the titration is carried out, using titrant(s) of the same ionic medium and spectra are measured against air at appropriate stages of the titration. The path length can be adjusted in the course of the titration if required. The reaction vessel and cell are afterward cleaned by flushing the whole system beveral times with deionized water and blowing dry with air. To obtain the correct blank absorbances, a solution of the ionic medium is then circulated and its spectrum recorded (again with air as reference) at the appropriate path length(s). If all the measurements for a titration can be made at a single path length, the following alternative procedure is perhaps more convenient: a known volume of the blank (ionic medium in this case) is first circulated and its spectrum recorded. A suitable volume of the titrand (in the same medium) is then added to the blank in the reaction vessel after which the spectrophotometric titration can be conducted in the usual way. CALIBRATION Precision machining methods allow the path length graduations to be given to very fine tolerances. However, the cell
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