Langmuir 1994,10,2188-2196
2188
Self-Assembling of Perfluorinated Polymeric Surfactants in Nonaqueous Solvents. Electron Spin Resonance Spectra of Nitroxide Spin Probes in Nafion Solutions and Swollen Membranes Ewa Szajdzinska-Pietekt and Shulamith Schlick* Department of Chemistry, University of Detroit Mercy, Detroit, Michigan 48219-0900
Andrzej Plonka Znstitute of Applied Radiation Chemistry, Technical University of Lodz, 93-590Lodz, Poland Received January 10,1994. Zn Final Form: April 18,1994@ Structural details on the self-assembling of perfluorinated ionomer (Nafion)chains in solutions and in swollen membranes have been obtained from ESR studies in systems containing doxylstearic acid spin probes. Results previously obtained for aqueous systems are extended in this study to formamide (FA), ethanol (EtOH), andN-methylformamide (NMF)as solvents. The slow-motional ESR component detected in swollen membranes and solutionsin FA has been assigned to spin probes bound to polymer aggregates. The additional, motionally averaged, component detected in FA solutions was assigned (at least in part) to spin probes associated with single chains. These assignments are similar to those in the aqueous systems. Comparison of the corresponding order parameters suggests that self-assemblingof polymer amphiphiles occurs at higher polymer concentrationand leads to less ordered aggregates in FA, compared to aqueous systems. The present ESR data do not indicate aggregation in NafioniEtOH and N a f i o m F solutions. The ESR spectra of the spin probes in membranes swollen by EtOH are consistent with a plasticizingeffect of the solvent, rather than with a phase separated morphology. These conclusions are in agreement with two types of studies of Nafion membranes and solutions: by ESR in systems containing paramagnetic V02+ as the counterion and by 19FNMR. Based on the results for aqueous and FA systems, we propose a mechanism for the transition between the micellar structure in solution and the reverse micellar structure in the swollen membranes, which we call the fringed rod model. The model assumes that at high polymer concentrationssome chains can be incorporated in more than one rod, thus effectively providing the cross-linkingnecessary for complete connectivity of the polymeric material.
I. Introduction Nafion, a perfluorinated ionomer made by DuPont, has the formula given in Chart 1and is widely used as an ion selective and separation membrane.'V2 Numerous studies have been directed toward elucidation of the structure and dynamics of Nafion membranes in the acid form or neutralized by various counterions, and swollen by ~ o l v e n t s . ~ -Spectroscopic l~ and scattering results for the membranes swollen by water suggest a phase separated
* Author to whom correspondence
should be addressed.
t O n leave from the Institute of Applied Radiation Chemistry,
Technical University of Lodz, Lodz, Poland. Abstract published in Advance A C S Abstracts, J u n e 1, 1994. (1)Perfluorinated Ionomer Membranes; Eisenberg, A., Yeager, H. D., Eds.; American Chemical Society: Washington, DC, 1982. (2) Structure and Properties of Ionomers; Pineri, M., Eisenberg, A., Eds.; NATO AS1 Series, Reidel: Dordrecht, 1987. (3)Barklie, R. C.; Girard, 0.;Braddel, 0. J . Phys. Chem. 1988,92, @
1371. (4) Martini, G.; Ottaviani, M. F.; Pedocchi, L.; Ristori, S. Macromolecules 1989,22,1743. ( 5 ) Martini, G.; Ottaviani, M. F.; Ristori, S.; Visca, M. J . Colloid Interface Sci. 1989,128, 76. (6)Alonso-Amigo, M . G.; Schlick, S. Macromolecules 1989,22,2628. (7) Schlick, S.;Alonso-Amigo, M. G. Macromolecules 1989,22,2634. (8) Bednarek, J.; Schlick, S. J . Am. Chem. SOC.1990,112,5019;J . Am. Chem. SOC.1991,113,3303. (9)Schlick, S.;Gebel, G.; Pineri, M.; Volino, F. Macromolecules 1991, 24,3517. (10)Lossia, S. A.; Flore, S. G.; Nimmala, S.; Li, H.; Schlick, S. J. Phys. Chem. 1992,96,6071. (11)Schlick, S.; Alonso-Amigo, M. G.; Bednarek, J. Colloid Surface A: Physicochem. Eng. Aspects 1993,72,1. (12)Avalos, J.; Gebel, G.; Pineri, M.; Schlick, S.; Volino, F. Polym. Prepr. (Am. Chem. SOC.Diu. Polym. Chem.) 1993,34,448. (13)Li, H.; Schlick, S. Polym. Prepr. (Am. Chem. SOC.Diu. Polym. Chem.) 1993,34,446.
Chart 1. Nafion Ionomer, Acid Form
morphology, which consists of water pools surrounded by the hydrophobic fluorocarbonchains; the ionic head groups and the counterions are located a t the interface.' Similarities and differences between this structure and the reverse micellar solutions formed from the ternary mixture surfactant-water-oil have been proposed.1° In contrast to scattering studies, however, multifrequency electron spin resonance (MESR) and 19F NMR results do not support the concept of phase separation in membranes swollen by nonaqueous solvents such as methanol, ethanol, dimethylformamide, or t e t r a h y d r o f ~ r a n . ~ ~ ~ J ~ J ~ A soluble Nafion powder has been recently prepared14J5 and the self-assembling of the ionomer chains in water, formamide (FA),N-methylformamide (NMF),and ethanol (EtOH) solutions has been studied by small angle neutron and X-ray scattering (SANS and SAXS,respective1y).l6-l8 The results for all solvents are similar and have been interpreted in terms of aggregation of Nafion unimers into rodlike micelles arranged in a planar hexagonal array. In the proposed model the perfluoro backbone constitutes the solvophobic core of the rod, the pendant chains are (14)Grot, W.G.; Chadds, F. U.S.Patent 0066369,1982. (15)Martin, C.R.; Rhoades, T. A,;Ferguson, J . A.Ana1. Chem. 1982, 54, 1639. (16)Aldebert, P.; Dreyfus, B.; Pineri, M. Macromolecules 1986,19, 2651. (17)Aldebert, P.; Dreyfus, B.; Nakamura, N.; Pineri, M.; Volino, F. J . Phys. (Paris) 1988,49,2101. (18)Gebel, G. Thesis, Universite Joseph Fourier, Grenoble, 1989.
0743-7463/94/2410-2188$04.50/00 1994 American Chemical Society
Self-Assembling Surfactants in Nonaqueous Solvents Chart 2. Doxy1 Stearic Acid Spin Probes = 5,10 X = Li (DSA) X = MEll-M. (DSE)
located at the periphery of the rods, and the ionic groups are at the rod-solvent interface. Recent 19FN M R results are in accord with this model for water and FA solutions; in EtOH, however, all 19Fnuclei (from the backbone and the pendant chains, identified by their chemical shifts) are represented by motionally averaged signals, suggesting the formation of a true solution.9J2 An ESR study of Nafion neutralized by paramagnetic V02+ cations and dissolved in water and alcohols has provided additional support for the conclusions derived from the 19FNMR experiment^.'^ Additional details on the self-assemblingof the polymer chains in solutions have been obtained from ESR studies, using doxylstearic acid spin probes, Chart 2.19920In the latter paper (referred to as I), results have been reported for aqueous solutions of Nafion neutralized by Li+,doped with 5- and 10-doxylstearic acids (5DSA and lODSA, respectively), and with 10-doxylstearic methyl ester (10DSE).20 The dominant, slow motional, ESR signal detected in solutions (concentration 0.5-9% (w/w) ionomer) and in membranes was assigned to spin probes bound to aggregated polymer chains. "he additional, motionally narrowed, signal detected in the solutions for 5DSA and lODSA has been assigned (at least in part) to spin probes associated with the unimers. The results in I have also suggested that the nitroxide group of lODSA and lODSE is located deeper inside the aggregates than that of 5DSA, but the 14N hypefine splittings for all probes reflect a polar environment. A more ordered structure at a given temperature in the membranes compared to solutions and an increase of the aggregate size a t higher ionomer concentrations have also been suggested in I. The present work extends these studies to nonaqueous solvents: FA, NMF, and EtOH. The primary objective was to verify the structural model for the ionomer solutions and swollen membranes, in an attempt to reconcile the scattering and spectroscopic results. An additional objective was to contribute to a better understanding of a fundamental problem: how self-assemblingof amphiphilic molecules depends on the properties of the solvent. FA has been often used as a nonaqueous solvent for low molecular weight (usually priotated) amphiphiles; formation of simple micelles, microemulsions, and liquid crystalline phases in this solvent has been demon~ t r a t e d . ~ The l-~~ microemulsions formed in FA have the potential to become important reaction media, especially for reagents such as perfluorinated olefins, which do not dissolve in water.23 Comparison with the respective aqueous systems has indicated higher critical micelle concentrations, higher Kram points, and smaller sizes (19)Lee, K. H.; Schlick, S. Polym. Prepr. (Am. Chem.SOC. Div. Polym. Chem.) 1989,30,302. (20)Szajdzinska-Pietek,E.; Schlick, S.; Plonka, A. Langmuir 1994, 10, 1101. (21)Rico, I.; Lattes, A. J. Phys. Chem.1986, 90, 5870. (22)Binana-Limbele, W.;Zana, R. Colloid Polym. Sci. 1989,267, 440. (23)Lattes, A.;Rico, I. Colloids Su$. 1989,35, 221. (24)Schubert..K. V.:. Busse,. G.:. Strey, - .R.: Kahlweit, M. J.Phys. Chem. 1993, 97, 248.
(25)Bergenstahl, B. A.;Stenius, P. J. Phys. Chem. 1987,91,5944. (26)McIntosh, T.J.;Magid, A. D.; Simon, S. A. Biochemistry 1989,
28. 7904. (27)Jonstromer, M.; Sjoberg,M.; Warnheim, T. J.Phys. Chem. 1990,
Langmuir, Vol. 10, No. 7, 1994 2189 Table 1. Physicochemical Parameters of Solvents parameters ViscositYQ dielectric constant6 surface tensionC dipole momentd H-bonding capability acceptor number
water 0.89 78.39 71.81 1.85 1.17 54.8
FA EtOH 3.30 1.08 111.00 24.55 57.91 21.90 1.69 3.73 0.71 0.83 39.8 37.9
NMF ref 1.65 32 182.40 32 38.70 32 3.83 42 33 32.1 33
and lower stability of the aggregates. It has been suggested that the difference between water and FA is not in the different interaction of the solvents with the head groups but with the hydrocarbon tails of the amphiphiles." "his aspect is important for our system, because of the higher solvophobicity of perfluorinated, compared to protiated, chains.28 Self-assembling processes have been also observed in NMF.25 To the best of our knowledge, however, there is no report in the literature on micelle formation in EtOH, except in the small-angle scattering studies of Nafion mentioned above.16-18
II. Experimental Section Materials. The Nafion 117 membranes, with an equivalent weight of 1100 g of polymer per mol of SOsH and a thickness of 0.178 mm, were obtained from DuPont. The soluble Naflon powder in the Li+form was a gift from G. Gebel (Grenoble,France) and was prepared by solubilization of Nafion/Li+in an autoclave at 250 "C in a 50/50 (v/v)water/EtOH mixture and evaporation of the solvent at %80 0C.16-18The spin probes, 5DSA and lODSA from Aldrich and lODSE from Molecular Probes, Eugene, OR, were used as received. The solvents, FA (purified) from Baker & Adamson, EtOH (200 proof dehydrated alcohol) from US. Industrial Chemical Co., and NMF (99%) from Aldrich, were kept over molecular sieves and used without additional purification. The perfluoropolyether oil (MW = 800) was produced by Montefluos, Milan, Italy. Other chemicals were reagent grade. Relevant physicochemical properties of the solvents are summarized in Table 1;data for water are also given, for comparison. Sample Preparation. All samples for ESR measurements were prepared in a glovebox, in an oxygen-free atmosphere. The NafionlLi powder was directly dissolved in FA, NMF, or EtOH (prebubbled with nitrogen) to the desired concentration in the range 1-25% (w/w). An appropriate amount ofLiOH was added to the solutions to ensure completeneutralization ofthe polymer. Ionomer solutions in EtOH and NMF are perfectly clear, while those in water and FA are opalescent. ESR Measurements. ESR spectra were measured in the temperature range 125-360 K with a Bruker X-band spectrometer, Model ECS 106,equipped with the ESP 3220 data system for aquisition and manipulation and with the EM111VT variable temperature unit. Additional experimental details have been published.20
HI. Results ESR spectra of the spin probes were recorded in the temperature range 125-360 K in Nafion solutions and swollen membranes, in the neat solvents, and in an EtOW HzO mixture (1:lby volume). The notation used is probe/ solventfor solutions in the neat solvent,pmWsolvent'S for ionomer solutions, and probdso1vent.M for swollen membranes. Neat Solvents. The spin probes 5DSA and lODSA can be dissolved in sub-millimolar concentrations in all solvents. For lODSE such concentrations can be obtained in EtOH and NMF; in FA the solubility is about 1order of magnitude lower. At 300K motionally narrowed ESR spectra are observed for all the solutions; the same hyperfine splittings Aim(14N)were measured for all probes, within f0.03G. The maximum anisotropic tensor component A,, determined
~
94,7549.
(28)Gebel, G.;Riatori, S.; Loppinet, B.; Martini, G. J.Phys. Chem. 1993,97, 8664.
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2190 Langmuir, Vol. 10, No. 7, 1994
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Chart 3. PerfluoropolyetherOil (PFPE)
a t 300 K of both probes in the ionomer solution and in the neat solvent are similar, in terms of line widths and Aiso values. At lower temperatures the lines broaden, and from the rigid limit spectra at 125 K, is identical for the below 200 K the spectra are typical of slow tumbling spin three probes in NMF and EtOH solutions, within f 0 . 2 G. probes, reaching a rigid limit at 125 K and an extreme In FA, phase separation occurs on freezing, as in water.20 separation ca. 1 G higher in the ionomer solutions, A rigid limit spectrum at 110 K was obtained only for compared to the solution in the neat solvent. Selected lODSA, and A,, was determined within f 0 . 5 G. The ESR spectra, of 10DSE/EtOH and lODSE/EtOWS (polyaverage Aisovs A,, values for the three probes are plotted mer concentration 23% (w/w)),are presented in Figure 2. in Figure 1. Previous data for aqueous solutions20are The line widths in the temperature range 220-280 K are also included. The linear correlation is in agreement with higher in the presence of Nafion. The differences with the earlier results for 5-, 12-, and 16DSAprobes and their respect to the neat solvent do not exceed 5%for 5DSA but methyl esters in solvents of different p ~ l a r i t i e s .Higher ~~ are larger for lODSE, up to 25% for the m = -1 (high values ofAisoand A,, in nitroxides are due to a higher spin field) signal a t 220 K. It is clear from Figure 2 that a t a density on the nitrogen nucleus in a more polar environgiven temperature lODSE spectra are more “rigid in the ment. The data shown in Figure 1indicate that there is presence of Nafion than in neat EtOH. In both cases, no correlation between the hyperfine splittings and the however, the spectra indicate higher mobility of lODSE dielectric constant E ofthe solvent; E increases in the order in comparison to 5DSA, which is opposite to the trend EtOH < H20 < FA < NMF, from 24.55 for EtOH to 182.40 observed for the spin probes bound to the aggregated for NMF (Table 1). Alinear dependence betweenAisoand polymer in water and FA (see below) systems. the acceptor number (AN)of the solvent is shown in Figure Figure 3 presents ESR spectra obtained at 300 K for 1(inset). The AN parameter was introduced by Mayer et 5DSA and lODSA spin probes in membranes swollen by al. and is calculated from the 31P chemical shifts of ethanol; two spectral components are detected. The line triethylphosphine oxide in solvents that lower the electron widths of the narrow triplet are ~ 3 0 larger % than in neat density at the phosphorus atom due to the inductive ethanol. These widths, and the relative intensities of the effect.30 Other parameters commonly used as a measure two components, are very sensitive to the ethanol content of solvent acidity, such as the Dimroth-Reichardt pain the membranes. Because of the high volatility of the rameter, ET,or the hydrogen bond donating ability, a,31,32 solvent, it is difficult to reproduce the spectra quantitado not parallel the observed variations of the hyperfine tively. In drier samples the line widths of the motionally splitting constants. narrowed component are broader, and the slower comThe lODSE probe in a perfluoropolyether oil (PFPE, ponent is more pronounced. Even for a clearly visible Chart 3)33was also examined; the results (Aiso= 14.15 G solvent excess, however, two components are detected and and A, = 34.0 f 0.5 G ) do not fit the linear dependence the line widths of the motionally narrowed component shown in Figure 1for the other solvents, suggesting that are broader compared with the neat solvent. The amount I4N splittings for doxy1 spin probes in perfluorinated and of lODSE retained by the membrane was less than l/10 protiated media with different polarities cannot be directly that of BDSA and lODSA probes, suggesting that lODSE compared. is more solvophilic with respect to EtOH, and is washedNafiodEtOH Systems. ESR spectra of 5DSA/EtOWS out during sample preparation.20 and lODSE/EtOW-S have been examined. ESR spectra N&on/N1VLF Systems. ESR spectra of spin probes 5DSA and lODSE have been examined in NafionNMF (29)Griffith, 0.H.; Jost, P. In Spin Labeling. Theory and Applicasolutions and in swollen membranes. The probes were tions;Berliner, L. J., Ed.; Academic Press: New York, 1976;p 501. not retained in the membranes, suggesting even higher (30)Mayer, U.;Gutman, V.; Gerger, W. Monatsh. Chem. 1976,106, 1235. solvophilicity in NMF than in EtOH. (31)Marcus, Y.Ion Soluation; J. Wiley: Chichester, England, 1985. Motionally narrowed signals are obtained in 25% (w/w) (32)Fawcett, W. R.J.Phys. Chem. 1993,97, 9540. Nafion solutions in NMF at 300 K, but the lines are broader (33)Sanguineti, A.; Chittofrati, A.; Lenti, D.; Visca, M. J. Colloid Interface Sci. 1993,155, 402. compared to the neat solvent (especially for lODSE), CF,(OCF&F),OCF, CF,
Self-Assembling Surfactants in Nonaqueous Solvents
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indicating lower mobility of the spin probes in the presence of Nafion. The effect is more pronounced at lower temperatures, as seen in Figure 4. For example, at 250 K a highly anisotropic spectrum is observed for the solution, while in the neat solvent the signal is isotropic. Spectral changes do not seem to be related to the melting point of the solvent (269.5 K). Nafion/FA Systems. Figure 5 presents ESR spectra recorded at 300 K in two Nafion/FA solutions and in swollen membranes. For 5DSA/FA/S and 10DSA/FA/S (parts A and C of Figure 5 )the motionally narrowed signal is dominant in the 1% (w/w)Nafion solution, while in the 22% (w/w) solution the slow motional signal is also detected; this component is clearly seen in Figure 5C where a higher modulation amplitude was used, and in the vertically expanded portion of Figure 5A. Two spectral components were obtained for lODSE/FA/S in the entire range of concentrations. In membranes the contribution of the motionally narrowed signal for all three probes is negligible ( lODSA > 10DSE.
IV. Discussion This section consists offour parts. First, we will discuss the results obtained for EtOH and N M F as solvents. Second, we will consider self-assembling of the ionomer in FA. Third, we will compare self-assembling in short-
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2192 Langmuir, Vol. 10, No. 7, 1994
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Figure 6. (A) X-band ESR spectra of 10DSE/J?A/S (22%)(w/w))as a function of temperature. Modulation amplitude was 2 G in the range 125-290 K and 1 G in the range 300-360 K. (B)X-band ESR spectra of 10DSE/FA/M as a function of temperature. Modulation amplitude was 2 G in the range 125-290 K and 0.5 G in the range 300-360 K. The arrow indicates the low field isotropic signal due to the free probe in the excess solvent (see text).
chain perfluorinated surfactants and in perfluorinated ionomers. Finally, we will propose a model for the transition between the rodlike micelles in the ionomer solutions in water and FA, and the reverse micelles in membranes swollen by these solvents. Structure of the Ionomer in EtOH and NMF. The spectra shown in Figure 2 do not reveal formation of large polymer aggregates in EtOH solutions. It could be argued that the absence of the slow-motional component at higher temperatures is due to higher solubilities of the spin probes in the neat solvent. We recall however that the 19FNMR studies, where no external probe is used, also indicate formation of a true Nafion solution in e t h a n ~ l . ~FurJ~ thermore, no changes of the surface tension coefficient in going from neat EtOH to 23% (w/w) Nafion solution were detected (within f l dydcm), while in water a gradual decrease was observed from 0 to 9%(w/w)ofthe polymer.20 It seems that EtOH lacks the polarity and cohesive energy that favor molecular aggregation, even though it has high hydrogen bonding capability (Table 1). Our results are thus in agreement with the suggestion of Binana-Limbele and Zana,22that micellization is governed by the cohesive energy, not by the hydrogen bonding. Lower mobilities of the spin probes below 300 K (Figure 2) are detected in Nafion solutions, compared to the neat solvent. Since the effect is probe dependent, it is logical to assume that the mobilities reflect binding of the probe to unimers. Our results suggest that the binding is more efficient for the neutral probe lODSE, than for the charged BDSA probe.
If phase separation into polar and nonpolar domains existed in the membranes swollen by EtOH, the two spectral components (Figure 3) could be rationalized by spin probes located in the polymer phase and in the EtOH clusters, respectively. The line widths, however, indicate that the probes are significantly less mobile in the membranes than in the bulk solvent. We suggest therefore that EtOH, unlike water or FA (see next section), penetrates into and plasticizes the polymer chains; l9F NMRg and V02+ESR13experiments support this conclusion. The slow motional component may reflect probe molecules located further away from the ionic groups, in regions less accessible to the solvent. The 2Az, values for the swollen membranes are independent of the probe; the extreme separation at 300 K, however, is higher for 5DSA than for lODSA (62.8 G as compared to 60.7 GI. This indicates lower mobility of the former probe and could be explained in terms of its lower solvophobicity and deeper penetration into more rigid regions of the membrane, less penetrated by EtOH. The probe solvophobicity in this relatively nonpolar solvent appears to be in the order BDSA > lODSA > lODSE, which is opposite to the case of FA and water. Similar experiments have been done earlier in this laboratory, for BDSA and 16DSA in NafionlNa membranes swollen by methan01.l~ In this study two-component spectra have been observed at lower temperatures, but at 300 K the motionally averaged signal only has been detected. The different result, compared to the EtOH system, may be related to the lower viscosity ofmethanol, and to its higher
Self-Assembling Surfactants in Nonaqueous Solvents polarity, which leads to reduced penetration into the perfluorinated chains. This explanation is in accord with results obtained for V02+ in membranes swollen by the two alcohols: the rotational correlation time re at 300 K is significantly shorter in methanol than in ethan01.l~ The results for NMF solutions are qualitatively similar to those for EtOH solutions, except when comparing BDSA vs 10DSE data. In neat NMF the spectra at all temperatures are essentially the same for the two probes, but lODSE is less mobile than BDSA in the presence of Nafion. It appears that the spin probes in Nafion solutions associate with the polymer. It is possible, in spite of their solvophilic character, that a dynamic equilibrium exists between free probes and probes bound to unimers and even to aggregates. The presence ofthe latter is suggested by the lower mobility of lODSE/NMF/S in comparison to BDSA/NMF/S. However, the absence of the slow motional component at 300K suggests that the aggregates are much smaller than those in water and FA (see below). Self-Assemblingin FA. As in I, we assign the slow motional and motionally narrowed spectral components, respectively, to spin probes bound to polymer aggregates and in the bulk solvent associated with Nafion unimers. In support of this assignment of the motionally narrowed component is the increased solubility of lODSE (by 1order of magnitude) in 1% (w/w) Nafion solutions in FA, compared to neat FA. This behavior is similar to the increased solubility of hydrophobic probes in solutions of surfactants above the critical micelleconcentration (cmcY4 and indicates formation of solvophobic regions from selfassembled ionomer chains even at low concentrations (1%). Within experimental error (f0.3 G), the isotropic splitting Aisofor the three-line signal in Nafion solutions is the same as in neat FA. The line width is, however, affected by the presence of the polymer. For example, the peak-to-peak width of the m = 1(low field) line of 5DSA increases from 1.11G i n the neat solvent to 1.23Gin 22% (w/w)Nafion solution. For this reason the two-component spectra could not be deconvoluted accurately by subtracting the signal observed in the neat solvent, as described in I for aqueous solutions. Only a lower limit of the slow motional signal contribution was estimated, by performing the spectral titration until the reversed-phase signal appeared. The results for BDSA are shown in Figure 5B; data deduced for 5DSA/water/S in I are also shown, for comparison. Identical results were obtained for the deconvolution for 10DSA/FA/S,within experimental error. The contribution of the slow motional signal for these two probes in 1% (w/w) Nafion solutions in FA is not higher than lo%, which is within the limits of the accuracy. The more solvophobic lODSE probe is less solube in neat FA, and the deconvolution procedure gives 99% and 70 f 10% of the slow signal in 22% and 1% (w/w) Nafion solutions, respectively. In all cases the fraction of the slow component in FA solution is significantlylower than in aqueous solutions of comparable Nafion concentration.20 This may be in part due to higher solubility of the spin probes in the former solvent; we note that lODSE is completely unsoluble in neat water. The fact that for BDSA in the 22% (w/w) Nafion/FA solution the contribution of the slow motional signal is lower than that in 0.5%(w/w) Nafiod HzO solution (70% as compared to 83%) cannot, however, be explained by the different solubilities in neat solvents alone. A more reasonable interpretation of these results is that in ionomer solutions in FA there are fewer micellar (34)Shinoda, K. In Colloidal Surfactants;Shinoda, K,Nakagawa, T., Tamamushi, B., Isemura,T., Eds.;Academichss: New York, 1963; Chapter 1.
Lmgmuir, Vol. 10, No. 7, 1994 2193 Table 2. Extreme Separation (G)of ESB Spectra for FA Systems at 300 K (W,) and at 126 K (M,) 5 DSA 10 DSA 10 DSE % (w/w) Nafion 2A:, 2Azz UZz 2Azz WZz 2Azz 1 69.8 71.2 61.9 70.3 4 70.2 12 69.8 22 54.0 70.0 60.8 71.2 62.2 70.7 ~ 6 560.0 ~ 69.9 63.0 71.0 63.3 71.2 av 69.9 f 0.1 71.1=k 0.1 70.7=k 0.4 a Estimated from the number of solvent molecules per so3- group in the membrane.18
aggregates. In addition, in swollen membranes (which contain =35% (w/w)FA18and where all polymeric material is aggregated)the contribution of the motionally narrowed signal is negligible,
