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J. Phys. Chem. B 2003, 107, 2480-2484
Partitioning and Polymerization of Pyrrole into Perfluorosulfonic Acid (Nafion) Membranes Brandi L. Langsdorf, Brian J. MacLean, John E. Halfyard, Jeremy A. Hughes, and Peter G. Pickup* Department of Chemistry, Memorial UniVersity of Newfoundland, St. John's, Newfoundland, A1B 3X7 Canada ReceiVed: August 26, 2002; In Final Form: December 3, 2002
Details of Nafion/polypyrrole composite formation have been obtained using electronic absorption spectroscopy. Three distinct processsmonomer loading, polymerization, and removal of unreacted monomershave been studied. Pyrrole loading displays a square root dependence on time, indicative of a diffusion controlled partitioning process. Partitioning of pyrrole into Nafion, however, is complicated by protonation of pyrrole and acid-catalyzed oxidation to give oligomeric and polymeric species. These processes are affected by the presence of oxygen and are photosensitive. A variety of oxidizing agents have been used to effect polymerization including Fe3+, H2O2, ammonium persulfate, and UV irradiation.
Introduction The polymerization of aromatic monomers within polymer membranes is an attractive method to produce hybrid materials or composites that take advantage of desirable properties of two widely different classes of polymer. For example, polypyrrole and polyaniline have been polymerized within poly(vinyl chloride)1,2 and poly(vinyl alcohol)3 membranes to produce flexible conducting materials and within cation exchange membranes to produce membranes whose ion exchange characteristics can be controlled electrochemically.4-6 Recently, 1-methylpyrrole has been polymerized in situ in commercially available perfluorosulfonate membranes (Nafion) in order to produce proton exchange membrane (PEM) fuel cell electrolytes that are less permeable to methanol and other organic fuels.7,8 The resulting poly(1-methylpyrrole) Nafion membrane composite displayed a 40% reduction in methanol crossover when evaluated as a proton exchange membrane for use in direct methanol fuel cells. In addition, the composite system displayed an increase in ionic resistance of less than 20%. The optimization of these preliminary results will be greatly aided by a more complete understanding of the factors that influence production of this type of composite material. Perfluorosulfonate membranes are composed of perfluoroethylene chains with pendant sulfonate groups. The fluorinated backbone of the polymer imparts a high degree of chemical resistance and thermal stability making these materials attractive for a wide variety of industrial separation applications. The heterogeneity of the membranes is evident from the distinct regions of the polymer microstructure: (i) the hydrophillic anionic clusters, (ii) the hydrophobic terfluoroethylene backbone, and (iii) an interfacial region between the two.9,10 The overall microstructure depends greatly on hydration level, with a fully hydrated material composed of ionic clusters consisting of ∼70 anion exchange sites and 1000 water molecules, whereas in a completely dried perfluorosulfonate membrane, the ionic clusters are composed of only ∼25 anionic sites. In the completely protonated form, the ionic clusters present an extremely acidic environment that has been exploited as “superacid” nanoreactors.11 * To whom correspondence should be addressed. E-mail: ppickup@ mun.ca.
Although the polymerization of an aromatic monomer, such as pyrrole, within ion-exchange membranes has been exploited in many studies and applications,6 limited information is available concerning mechanistic details and the factors that influence the rate, extent, and location of the polymerization.12 Previous studies have focused on the generation of composites with high conjugated polymer loadings, to produce systems with high electronic conductivity and electrochemical activity. In contrast, conjugated polymer|Nafion composites for use as PEMs in fuel cells should have relatively low conjugated polymer loadings (1-5% by mass), so that high proton conductivity is maintained and the cell is not shorted by electronic conduction through the membrane.7 In this paper, the steps that comprise the production of polypyrrole Nafion membrane composites with low polypyrrole loadings are examined in detail. The partitioning of pyrrole into Nafion from aqueous solutions, its spontaneous reactions within the Nafion structure, and its polymerization with added oxidizing agents have been followed by electronic absorption spectroscopy. Experimental Section Materials. Nafion 115 (DuPont; donated by H Power Corp) was cleaned following a literature protocol.13 Membranes were in the acid form for all experiments reported here. Pyrrole (Aldrich) was purified by filtration through a plug of silica immediately prior to use. Nanopure water was used to prepare all aqueous solutions and for washing membranes. All other chemicals were used as received. Instrumentation. Absorbance spectra were collected using a Varian Cary 5E spectrophotometer. Nafion membranes were supporting between two quartz disks during measurements. Partition Experiments. In a typical partitioning experiment, Nafion 115 (ca. 100 mg) was immersed in 3.0 mL of aqueous pyrrole solution in a quartz cuvette. The change in concentration of the soaking solution was measured at regular time intervals with the film raised out of the beam. Between measurements, the film was submerged in the soaking solution and the cuvette was covered with a Teflon cap to prevent potential evaporation of pyrrole. The membrane was not rinsed at any time during these experiments.
10.1021/jp026840c CCC: $25.00 © 2003 American Chemical Society Published on Web 02/26/2003
Partitioning and Polymerization of Pyrrole
Figure 1. Loading of pyrrole into Nafion 115 membranes plotted as the decrease in the concentration of pyrrole in the soaking solution as a function of time1/2. Initial pyrrole concentrations were 200 (9), 100 ([), 50 µM (0).
Polymerization of Pyrrole in Nafion 115. In a typical polymerization experiment, Nafion 115 (ca. 100 mg) was immersed in 20 mM pyrrole for 120 s. The initial loading of the film was then measured by absorption spectrometry and the loaded film was then immersed in the oxidizing medium. Its absorption spectrum was measured at various times following removal from the oxidizing medium and rinsing with water. In some cases, the film was returned to the oxidizing medium between measurements.
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Figure 2. Electronic absorption spectra of Nafion 115 membranes (ca. 100 mg) following (a) 1, (b) 2, and (c) 22 h of immersion in 3 mL of an air saturated 0.93 mM aqueous pyrrole solution. A separate membrane and aliquot of solution was used for each experiment. Membranes were not rinsed before recording of their spectra.
Results Partitioning of Pyrrole into Nafion Membranes. The uptake of pyrrole into Nafion membranes was monitored by electronic absorption spectroscopy in various concentrations of aqueous pyrrole. Pieces of Nafion were immersed in aqueous pyrrole solutions, and the absorbance of the solution at λmax for pyrrole (206 nm) was monitored as a function of time. No oxidizing agent was used in these experiments, but air was not excluded unless specified. Figure 1 shows representative results, plotted against the square root of time, for pyrrole concentrations ranging from 50 to 200 µM. The concentration of pyrrole was found to decrease linearly with t1/2, as expected for a diffusion controlled process, and this decrease continued over the entire duration of these experiments. In separate experiments, the decrease in the pyrrole concentration in the solution was found to continue for at least 4 days. There is no indication that the uptake of pyrrole by the Nafion membranes approaches saturation over this time scale. The uptake of pyrrole can also be monitored by observing changes in the absorption spectrum of the Nafion membrane as a function of time, as illustrated in Figure 2. Over relatively short time scales ( 4 corresponds to a pyrrole diffusion coefficient in the membrane of
