Langmuir 1994,10, 1101-1109
1101
Self -Assembling of Perfluorinated Polymeric Surfactants in Water. 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 Institute of Applied Radiation Chemistry, Technical University of Lodz, 93-590 Lodz, Poland Received September 28, 1993. In Final Form: January 7,1994’ Structural and dynamical information on the self-assembling of perfluorinated ionomers (Ndion) in aqueous solutions was obtained by doping the solutions with nitroxide spin probes and measuring the electron spin resonance spectra as a function of polymer concentration and temperature. The probes are based on stearicacid, either as the lithiumsalt or as the methyl ester. Two spectralsites,assigned respectively to probes associated with large aggregates of low mobility and with faster and smaller objects, have been detected. The smallobjects are tentatively assigned to single chain micelles (unimers). Additionalsupport for this assignment is obtained by surface tension measurements. The spectral parameters of the probes in both sites suggest a polar environment, indicating that the probes ‘drag” part of their solvation shell into the hydrophobic environment. The aggregatesize increases when the ionomer concentrationincreases. The relative intensity of the component associated with the unimers increases with temperature, pointing to atemperature-dependent equilibrium between large aggregates and unimers. Analysis of the temperature variation of the signalassociated with the large aggregatesindicates differentdegrees of probe immobilization, the more hydrophobic probes reflecting more rigid and ordered regions of the aggregates. Only probes associated with large aggregates were detected in the membranes. The results obtainedhave been compared with small-angle scattering and l@FNMR data. This study indicates the advantages and limitations of the nitroxidespin probe method, for the case of protiated probes in solutionsof a perfluorinated polymer surfactant.
I. Introduction Self-assembling of perfluorinated surfactants in water has been extensively studied in recent years, because of the many practical applications of these materials.’ Perfluorinated surfactants are more efficient in controlling the surface properties of aqueous solutions, because of a more pronounced hydrophobicity and significantly lower critical micelle concentration (cmc), compared to the protiated analogs. Self-assembling of polymeric surfactants, however, is more difficult to study. Polymeric surfactants have a very low cmc (=mg/L) or exhibit a gradual decrease of the surface tension with increasing surfactant concentration.2 These results can be rationalizedas follows: although the total polymer concentration is low, the local concentration of hydrophobic structural units for a polymer with a typical molecular mass in the range 1 X 106 to 1X 10s is quite high. For this reason, intramolecular self-assembling is possible, and micelles can be formed from one polymer chain (unimer) or even from parts of a chain. The perfluorinated polymer known as Nafion (1) made by DuPont, consists of a perfluorinated backbone and pendant chains terminated by sulfonic acid, as shown be lo^.^^^ The commercially available polymer has an
* Author to whom correspondence should be addressed. + On leave from the Institute of Applied Radiation Chemistry, Technical University of Lodz,Lodz,Poland. Abstract published in Advance ACS Abstracts, February 15, 1994. (1) Mvers. D. Surfactant Science and Technology;VCH Publishers: Weinhe&, isas. . (2)Anton., P.;-KBberle,. P.:. Laschewsb, - . A. Makromol. Chem. 1993, @
194, i.
(3)Perfluorinated Ionomer Membranes;Eieenberg,A., Yeager, H. D.,
Eds.; American Chemical Society: Washington, DC, 1982.
equivalent weight of 1100 (in grams per mole of sulfonic groups), and the repeating unit contains one CF group and 14CFZgroups. The size of the repeating unit is similar to that of a short chain surfactant. For a molecular weight of -2 X 106, a polymer chain contains =180 pendant sulfonic groups.5 Ndion is known as an ionomer because ! of ionic groups, and it contains less than about 15 mol % .~*~ has been studied mostly as a m e m b r a ~ ~ e Recently, however, a procedure for the solubilization of Ndion membranes has enabled the study of the structure in solution and comparison with perfluorinated surfactants with shorter chaim6P7 Short chain surfactants, and perfluoropolyethers (PFPE), which are terminated by carboxylic groups and have a molecular mass in the range 600-1000, have been recently considered as model compounds for the more complex perfluorinated polymers.a10 -CF&FzCF-
1
OCF2CFOCF2CF2SO3H
I c F3 1
The morphology of Ndion as dry membranes, or swollen by various solvents, has been studied in our group, using (4)Structure and Properties oflonomers; Pineri, M., Eieenberg, A., Eds.; NATO AS1 Series, Reidel: Dordrecht, 1987. (5)Privata communication, from DuPont de Nemours Co. (6)Grot, W. G.; Chadds, F. U S . Patant 0066369,1982. (7)Martin, C.R.; Rhoades, T. A.; Fergueon, J. A. Anal. Chem. 1982, 54 - -, -1RR9. - -- . (8)Ristori, 5.; Martini, G. Langmuir 1992,8, 1937. (9)Ristori, S.;Ottaviani,M. F.; Lenti, D.; Martini, G. Langmuir 1991, 7,1958. (10)Gebel, G.;Ristori, 5.; Loppinet, B.; Martini, G. J. Phys. Chem. 1993, 97,8664.
0743-7463/94/2410-llO1$04.50/00 1994 American Chemical Society
1102 Langmuir, Vol. 10, No. 4, 1994 multifrequency electron spin resonance (MESR),11-14 electron nuclear double resonance (ENDOR),lSand 19F NMR.l6J7 We have suggested that the phase separation into ionicand nonpolar regionsmentioned in the literature3 occurs in membranes swollen by water, and not by less polar solvents such as methanol, ethanol, dimethylformamide (DMF),tetrahydrofuran, or acetonitrile.11J2 ESR studies of paramagnetic cations in membranes swollen by water have also been published by other g r o u p ~ . ~ s J ~ A model for the structure of the ionomers in solutions has been proposed, based on small angle X-ray and neutron Structural scattering, SAXS and SANS, parameters have been deduced in these studies from an analysis of the ionic peak position as a function of polymer concentration in various solvents: water, methanol, ethanol, formamide (FA), N-methylformamide (NMF), and DMF. Best agreement with experimental results for all solutions has been obtained for the model of parallel rods of radius 18-30 A, arranged in a planar hexagonal array. It has been suggested that the rods consist of a central core of perfluorinated backbone, while the ionic charges are on the outer surface, in contact with the solvent.21 The similar behavior of all solvents in the scattering experiments is in contrast with results obtained for the solutions using spectroscopic methods. l9F NMR spectra suggest the formation of true solutions in ethanol and NMF, even at high polymer concentrations, 20% (w/w).16 In water and formamide, however, the line shapes are more complicated and suggest a slower rate of reorientation for one component or superposition of two spectral components.l7 In addition, recent ESR studies of the paramagnetic cationic probe V02+indicate a highly mobile site in aqueous solutions of the ionomer and significant motional constraints in ethanol and methanol solutions.23 We have initiated a study of Ndion solutions in, and membranes swollen by, some of the solvents used for the scattering studies: water, ethanol, FA, and NMF. Structural and dynamical information was obtained by doping the solutions with nitroxide spin probes and measuring the ESR spectra of the probes as a function of polymer concentration and temperature. The spin probes selected (2) are based on stearic acid, either as the lithium salts of 5-doxylstearic acid (5DSA) and 10-doxylstearic acid (10DSA) or as the 10-doxylstearic methyl ester (10DSE). The main objective of this study was to gain additional structural details on the assembling of the polymer chains in solution and to combine the results obtained by ESR, l9F NMR, and scattering methods into a comprehensive picture. The doxy1 spin probes have been evaluated in a (11)Alonso-Amigo,M. G.; Schlick, S.Macromolecules 1989,22,2628. (12)Schlick, S.;Alonso-Amigo,M. G. Macromolecules 1989,22,2634. (13)Bednarek, J.; Schlick, S.J.Am. Chem. SOC. 1990,112,5019;1991, 113,3303. (14)Schlick, S.;Alonso-Amigo,M. G.; Bednarek, J. Colloid Surf. A: Physicochem. Eng. Aspects 1993,72,1. (15)Schlick, 5.;Myers, B. E.; Sjoqvist, L.; Lund, A. 2. Naturforsch. 1992,47a,702. (16)Schlick, S.;Gebel, G.; Pineri, M.; Volino,F.Macromolecules 1991, 24,3517. (17)Avalos, J.; Gebel, G.; Pineri, M.; Schlick, S.;Volino, F. Polym. Prepr. (Am. Chem. SOC. Diu. Polym. Chem.) 1993,34,448. (18)Barklie, R. C.; Girard, 0.;Braddel, 0. J.Phys. Chem. 1988,92, 1371. (19)Martini, G.; Ottaviani, M. F.; Pedocchi, Ristori, S.Macromolecules 1989,22,1743. (20)Aldebert, P.; Dreyfus, B.; Pineri, M. Macromolecules 1986,19, 2651. (21)Aldebert, P.; Dreyfus, B.; Gebel, G.; Nakamura, N.; Pineri, M.; Volino, F. J. Phys. France 1988,49,2101. (22)Gebel, D. Ph.D. Thesis, Universit4Joseph Fourier, Grenoble, 1989. (23)Li, H.; Schlick, S.Polym. Prepr. (Am. Chem. SOC. Diu. Polym. Chem.) 1993,34,446.
Szajdzinska-Pietek et al. large number of protiated micelles and vesicles and have been considered very informative for probing the surfactantlwater interface and for describing the micellar structure and dynamica.24-29 Very few studies of perfluorinated surfactant systems using the spin probe method have been published?~~ and none for perfluorinated polymeric surfactants. For this reason, an additional objective of our study was to find if the spin probes reflect the local structure and dynamics in perfluorinated ionomer solutions. This paper presents results obtained for aqueous solutions of Ndion. Solutions in other solvents, as well as spectral simulations, will be described in subsequent publications.30 Some results obtained in astudyof Ndionl Na membranes doped with 5DSA and 16DSA have been reported.31 C H ~ ( C H ~ ) ~ L , ,(CH2),-2COOX
x 0
Y-
n = 5,lO X=Li(DSA) X = CH3 (WE)
2
11. Experimental Section Materials. The Ndion 117 membranes with an equivalent weightof 1100g of polymer/mol of SOaHand a thickness of 0.178 mm were obtained from DuPont. The membranes as the lithium salt were solubilized in a 1:l (v/v) water/ethanol mixture at 523 K in an autoclave.%% A powder soluble in ethanol, but not in water, was then obtained by evaporation of the solvent at -350 K for 1h. The soluble Ndion powder in the Li+form was a gift from G. Gebel (Grenoble, France). The spin probes, 5DSA and lODSAfromAldrichand lODSE from MolecularProbes, Eugene, OR,were wed as received. Waterwas deionized,doublydistilled, and carefully bubbled with nitrogen. Other chemicals were reagent grade. Sample Preparation. All samples for ESR measurements were prepared in the glovebox, in an oxygen-free atmosphere. Solutions of Ndion/Li in ethanol (23% (w/w)) were prepared fromthe solublepowder; water solutionswereprepared by dialysis of the ethanol solutions against water for 24 h, and the external solvent was exchanged for pure water 8 times. The polymer content in the resulting water solution was 9% (w/w)(onebatch) or 7% (w/w) (second batch), as determined gravimetrically by evaporation of a small sample at 385 K to constant weight. After dialysis, a proper amount of concentrated LiOH in water stock solution was added to ensure neutralization of the polymer. This step is necessary becausethe maximum degree of neutralization of the membranes by Li+is -90 % .s2 Less concentrated aqueous solutions were prepared by dilution with water and thorough stirring, for 2 h at least. The membranes were pretreated as described earlier's and exchangedwith Li+by soaking in 0.1 M LiOH water solution for 24 hat ambient temperature. The neutralized membranes were washed 3 times with water and dried in vacuo (lW Torr) at 373 K. (24)Szajdziika-Pietek, E.;Maldonado, R.; Kevan, L.; Jones, R. R. M. J. Colloid Interface Sci. 1986,110,514.
(25)Haering, G.; Luisi, P. L.; Hauser, H. J. Phys. Chem. 1988,92, 3574. (26)Wikander, G.; J o h o n , L. B.-A. Langmuir 1989,6,728. (27)Denton, J. M.; Duecker, D. C.; Sprague, E. D. J. Phys. Chem. 1993,97,756. (28)Bales, B. L.;Stenland, C. J. Phys. Chem. 1993,97,3418. (29)Bratt, P. J.; Kevan, L. J. Phys. Chem. 1992,96,6849;l993,97, 7371. (30)Szajdzinska-Pietek, E.;Pilar, J.; Plonka, A.; Schlick, S. To be submitted for publication. (31)Lee, K. H.; Schlick,S. Polym. Prepr. (Am. Chem. Soc.Div.Polym. Chem.) 1989,30,302. (32)For divalent counterions such as Cu*+ the maximum degree of neutralization that can be achieved is even lower, 78%. See ref 11 for details.
Self-Assembling of Perfluorinated Ionomers
Langmuir, Vol. 10, No. 4, 1994 1103
The spin probes were dissolved in chloroform to a known concentration and divided into a number of vials. After evaporation of the solvent under a stream of nitrogen, the spin probe f i i was stirred as needed with neat water or the ionomer solution or themembrane suspended in water. In the case of the doxylstearic acid probee, an appropriateamountof LiOH in water stock solution was added to neutralize the spin probes. The stirring time needed for dissolving the spin label ranged from a few h o w to several days and was highest for lODSE, compared with the other probes, under similar conditions. The doped membranes were washed twice with water, dry blotted with filter paper, and insertedinto 4nuq0.d. Pyrextubes. The solutions were measured in capillary tubes made out of disposable pipettes (Pasteur type, soda lime glass). All samples were flame-sealed for ESR measurements. The spin probe concentrationin the solutionswas 50.6mM, which corresponds to the ratio [spin probel/[SOrI in the range 10-"10-9. In the membranes this ratio was =lV,as estimated from the integrated ESR intensity and the weight of the membrane. ESR Measurements. ESR spectra were measured with a Bruker X-band spectrometer, Model ECS 106, equipped with the ESP 3220 data system for acquisition and manipulation,and with the ER4111 VT variable temperature unit. Spectra were measured with 100-kHzmagnetic field modulation, at a microwave power of 2 mW. The modulation amplitude waa in the range 0.62.0 G, with the higher values used for broad lines and/ or weaker signals. Typically 10scans per sample were collected. Surface Tension Measurements. Measurements were performed at 294 K by the Wilhelmy plate method, using the Cahn Dynamic Contact Analyzer Model No. 312. 111. Results ESR spectra of BDSA, lODSA, and lODSE spin probes in water solutions of Ndion/Li and in Ndion/Li membranes swollen by water have been measured in the temperature range 125-360 K. The notation used is probe/S for solutions and probe/M for membranes. Concentration Dependence of ESR Spectra at 300 K. ESR spectra of 5DSA/S at 300 K as a function of polymer concentration are given in Figure 1. At the highest polymer concentration, 9% (w/w), the signal is typical of a slow tumbling probe. The extreme separation (between upward arrows) is 57.2 G, compared with the value of 72.8 G in the rigid limit measured at 125 K, vide infra. With decreasing polymer concentration, a triplet indicative of fast motional reorientation of the probe appears. The relative intensity of this component increases as the polymer concentration decreases. This motionally narrowed triplet is similar to that obtained for 5DSA in neat water at 300 K (see Figure 2),with Ai, = 15.80 0.02 G. The spectral titration methods3 was used in order to deconvolutethe spectra for polymer concentrations lower than 9% (w/w) into two separate components, by subtracting the motionally narrowed component (Figure 2B) from the composite signal (Figure 2A)and integration of the two deconvoluted spectra. The procedure is shown in Figure 2,for 5DSA/S at a polymer concentration of 1% (w/w). The isotropic component subtracted from the composite signal was that of the probe in neat water at 300 K. The slow motional component obtained by this procedure for all concentrations (0.5,1.0,5.0,and 7.0% (w/w)) varies slightly with polymer concentration: the extreme separation 2A',, is larger in more concentrated solutions, as seen in Table 1. The relative intensity of the slow motional component in % vs ionomer concentration is plotted in the inset of Figure 2. Even for 1 % solution the dominant contribution is from the anisotropic signal
3320
3340
3360
3380
3400
3420
IGI Figure 1. X-band ESR spectraat 300K for 6DSNSas a function of ionomer concentration. Spectra are normalized for the same integral intensity and then expanded vertically by the factor given on the right. The interval between the upward mows in the most concentratedsolutionindicatesthe extremeseparation of the anisotropic signal. Downward arrows point to l8C satellites in natural abundance. The position of the C++ g standard (s = 1.9796) is ale0 indicated. Modulation amplitude: 0.6 G.
*
(89%). (33) Jost, P.;Griffth, 0. H.In Spin Labeling. Theory and Applications; Berliner, L.J., Ed.;Academic Press: New York, 1978; p 268.
3320
3340
3360
3380
3400
3420
I11
Figure 2. Deconvolution of the ESR spectrum of 6DSA/S (concentration 1%(w/w)) at 300 K into two componente: (A) compositespectrum;(B) motionallynarrowedcontribution, SDSA in neat water; (C) The difference between (A) and (B). The relative intensity of the anisotropic component (in %) as a function of concentration is plotted in the inset. Modulation amplitude: 0.5 G. ESR spectra of lODSA/S for two ionomer concentrations (0.5% and 7% (w/w)) are presented in Figure 3, and
indicate anisotropic and motionally narrowed componente with an intensity ratio that depends on the polymer concentration. The relative intensity of the anisotropic component is similar for 10DSNS and BDSAIS at the high polymer concentrations. At the lowest concentration,
Szajdzinska-Pietek et al.
1104 Langmuir, Vol. 10, No. 4, 1994 Table 1. Extreme Separation (in G)of ESR Spectra at 300 K (2A”) and at 128 K (2A,) 5DSA
% WIW
lODSA
Nafion Li 2At2. 2.4, 0.5 54.8 13.9 1.0 5.0 7.0 9.0
-W average a
55.0 13.9 55.5 12.9 55.6 51.2 12.8 63.6 13.2 13.3
* 0.5
I lODSE/S
lODSE 2A, 63.9 13.6
M‘,,
2A,
2A’, -62.6
I
13.5
63.5 14.4
64.5 13.8
66.3 14.3 14.1
64.1 13.9 66.5 14.0 13.8
* 0.4
300 K A
I\
I
I
’
* 0.2
Calculated from the water content of the swollen membranes,
given in ref 11.
2h
C 1
1
1
3320
I
I
33M
I
I
3340
I
I
3360
V
I
I
3380
I
I
3400
I I
I
343
1
3340
1
1
3360
I 1
1
1
3380
1
3400
1
l
3420
[GI Figure 4. X-band ESR spectra at 300 K for lODSE/S at two ionomer concentrations, 0.5 and 7% (w/w). Spectra are plotted with the same integral intensity. Experimental A, and Avalues used for the calculation of the order parameter are indicated for the bottom spectrum. Modulation amplitude: 2 G. The inset is the ESR spectrum of lODSE in water at 300 K.
161
Figure 3. X-band ESR spectra at 300 K for 10DSA/S at two ionomer concentrations, 0.6 and 1% (w/w). Spectra are normalized for the same integral intensity and then expanded vertically by the factor given on the right. Modulation amplitude: 0.5 G.
0.5% (w/w), the relative intensity of the anisotropic component in 10DSA/S is lower, only 0.63. We note however that the deconvolution process is more accurate in concentrated solutions, where the slow motional component is dominant. In less concentrated solutions the amplitude of this signal is low and the deconvolution process is consequently less accurate. We estimate that the maximum error in the relative intensity of the slow motional component determined by the spectral titration method is &2% at ionomer concentrations 21% (w/w) and f15% in the more dilute solutions. While the slow motionalspectral component is observed only in the presence of the ionomer, the motionally narrowed components in 5DSA/S and 10DSA/S are practically identical to those detected in corresponding neat water solutions. In order to understand the origin of the two spectral components, we decided to use a spin probe that is similar to 5DSA and lODSA but whose solubility in water is negligible. The lODSE nitroxide spin probe, 2 fulfills this condition; after prolonged stirring of this spin probe in neat water, the ESR spectrum at 300 K (inset, Figure 4) is extremely weak and consists of a broad singlet with a peak-to-peak derivative line width of 23 G,which is assigned to dispersed colloidal particles of the lODSE oil in water. In the presence of the polymer, however, only slow motional spectra are detected even at the lowest polymer concentration, 0.5% (w/w), as seen in Figure 4. The ESR intensities of the lODSE probe in the Ndion solutions increase with polymer concentration; the increase is by a factor of -2.5 in going from 0.5% to 7% and does not change preceptibly when the concentration increases to 9%. This result indicates that the probe is solubilized by the ionomer.
3320
3340
3350
3380
3400
3420 Ill
Figure 5. X-band ESR spectra at 300 K for BDSNM, lODSA/ M, and 10DSE/M. Spectra are plotted with the same integral intensity. Modulation amplitude: 0.5 G.
In Figure 5 we present ESR spectra of the three spin probes in the membranes swollen to equilibrium by water. Only the slow motional component is detected, and the line shapes are different for the three probes, and also different compared with the solutions. Spectra for 10DSNM and 10DSE/M are almost identical, with an extreme separation of 66.5 G; the resolved features at -3370 are due to the highly anisotropicrotational diffusion at 300 K.M The higher mobility for 5DSNM is reflected in a lower extreme separation (63.5 G) and in the significantly larger line widths. Temperature Variation of the ESR Spectra. The evolution of ESR spectra with temperatures in the range 125-360 K (after rapid quenching to 77 K) is shown in Figure 6for lODSNS containing 7% (w/w) ionomer. Minor changes are detected up to 200 K, only a progressive
Self- Assembling of Perfluorinated Ionomers
Langmuir, Vol. 10, No. 4, 1994 1105
I
3320
3340
3360
3380
3400
3420
l
l
l
l
3340
3320
l
3360
l
l
3380
l
l
3400
l
3420
[GI
Figure 6. X-band ESR spectra for 10DSA/S as a function of temperature, for an ionomer concentration of 7% (w/w). Modulation amplitude: 2 G in the range 125-290 K, and 1G in the range 300-360 K.
increase in resolution. At and above 220 K the extreme separation gradually decreases; the motionally narrowed component appears around 275 K and grows in intensity with increasingtemperature. Similarresulta were obtained for all 5DSA/S and 10DSA/S systems. For 5DSA/S containing 9% (w/w) polymer the motionally narrowed signal appears above 300 K, but even at 360 K its contribution is only -3% of the total intensity. In the swollen membranes the motionally narrowed signal is not detected even at 360 K. We noticed that the intensity of the motionallynarrowed component at a given temperature was slightly higher after quenching, as seen by comparing Figure 3 (7%)with Figure 6 (300 K). We did not follow the effect quantiatively, but used fresh samples for each experiment. It is possible that, on quenching, a small amount of probe leaves the aggregate because of reduced solubility at the lower temperatures. For the water-insoluble lODSE probe in the swollen membranes, only the slow motional component is detected in the entire temperature range, as seen in Figure 7. The spectra of 10DSE/S at 360 K for two polymer concentrations, 0.5 and 9.0% (w/w), are given in Figure 8. The signals indicated by the downward arrows in the two spectra correspond to the expected position of the low field component in an isotropic triplet. An upward arrow for the 9% solution indicates the anisotropic component; this component is less intense in the more dilute solution.
IGl
Figure 7. X-band ESR spectra for 10DSE/M as a function of temperature. Modulation amplitude: 2 G in the range 125-290 K, and 0.5 G in the range 300-360 K.
1ODSE/S
I
I
3320
I
I
3340
I
1 ' 1
3360
I
3380
I
I
3400
I
1 1
3420
ill Figure 8. X-band ESR spectra at 360 K for lODSE/S at two ionomerconcentrations,0.5 and 9.0%(w/w). Spectra are plotted with the same integral intensity. Modulation amplitude: 1 G.
IV. Discussion In this section we will discuss the location of spin probes in Ndion solutions and swollen membranes, the case of a protiated probe in a perfluorinated environment, the local dynamics reflected in the ESR spectra of the probes, and comparison of the ESR results with those obtained from scattering experimenta,l9FNMR, and surface tension data.
1106 Langmuir, Vol. 10, No. 4, 1994
Location of the Spin Probes. The anisotropic (slow) component is not detected in neat water solutions of the probes, is the only spectral component detected for the water-insoluble spin probe (lODSE), and is more intense as the polymer concentration increases. These results present compelling evidence that this component represents spin probes associated with polymer chains. The incomplete averaging of the ESR spectral parameters suggests that the probes are part of slowly rotating polymeric species formed in aqueous solutions, which we assign to polymer aggregates. The solubility of lODSE in polymer solutions, and not in neat water, is similar to the solubilization (“sequestering”)of a cholestane spin probe by the nonionic pluronic diol ~ u r f a c t a n t sthe ; ~ ~amount of cholestane that is solubilized increases with increasing surfactant concentration, and its spectrum indicates an environment of low fluidity, as detected in ow study. In Nafion membranes only the anisotropic component is detected in the entire temperature range for all probes, indicating aggregation of all the chains. Location of probes and local polarities are often deduced from the value of the extreme separation (in G) in the rigid limit measured at about 100 K (2A,,), and of the isotropic hyperfine splitting Abo.35 Larger values of these parameters are correlated with a more polar site. The Abo values for the probes associated with the polymer aggregates cannot be measured directly, because complete spectral averaging is not observed even at 360 K, as seen in Figures 6-8. The 2A,, values for the three probes, however, have been measured a t 125 K, and are given in Table 1. The accuracy of the determination of 2A,, is only about &0.5 G, because the lines a t 125 K are rather broad. The polarity of this site can be assessed by comparison with the corresponding values measured for the probes in neat water. The rigid limit spectra of the DSA spin probes in neat water is however very hard to measure, because the solutions prepared at ambient temperature separate on freezing. Only in the case of the lithium salt of lODSA has a spectrum been recorded at 110 K, as shown in Figure 9, where the lines are broadened by the high local concentration of the probe. We estimated 2A,, = 73.5 f 1.0 G. This value, together with the isotropic value Abo = 15.85 f 0.02 G measured at 300 K, is in agreement with the linear dependence of A,, vs Ai, obtained for the spin probes in solvents with a large range of polarities.35 This 2A,, value was not used in the calculation of the order parameter (see below). The 2A,, values for the probes in the Nafion system are very similar (Table l),also indicating a polar environment. We propose that although the probes are solubilized by, and are associated with, the polymer aggregates, the nitroxide group has maintained part of its hydration shell. A similar conclusion has been suggested for the 5DSA and 16DSA probes in ammonium perfluorooctanoate.8 We will come back to this suggestion in the next section. The extreme separation of the slow motional component at 300 K increases with increasing polymer concentration (Table 11, indicating a medium of decreasing fluidity. This result suggests that the localviscosityinside the aggregates increases with polymer concentration. The difference between concentrated and dilute solutions is even more pronounced a t higher temperatures, as seen in Figure 8 for 10DSE/S at 360 K. While at 9% Nafion content it is (34) Cheng, H. Y.; Holl, W. W. J. Pharm. Sci. 1990,79,907. The ESR spectral parametersmeasuredin this study for the sequesteredcholestane probe indicate a micellar environment of low polarity, in contrast to the results obtained in our study. (35) Griffith, 0. H.; Jost, P. In Spin Labeling. Theory and Applications; Berliner, L. J., Eds.; Academic Press: New York, 1976; p 501.
Szajdzimka-Pietek et al. I
I
I
I
I
3300
I
I
3310
I
I
3380
1
I
3420
I
/I 3460 [GI
Figure 9. X-band ESR spectrum at 110 K for lODSA (as the lithium salt)dissolved in water. The interval betweenthe arrows corresponds to 73.5 G. Modulation amplitude: 2 G.
still possibleto distinguish the extreme features that define 2A’,,, the spectrum for the 0.5% Nafion solution is almost isotropic, as expected for smaller objects. The assignment of the motionally narrowed component is not as obvious. The similarity of this component with that detected for the probe in neat water suggests a hydrated size in both cases. The absence of the motionally narrowed component in ionomer solutions containing the spin probe that is insoluble in water, lODSE, supports this logic. On the other hand, the solubilization of the water-insoluble probe (10DSE) by the polymer chains and the large fraction of the slow motional component even for the probes that are somewhat soluble in water (5DSA and 10DSA) indicate that the probes seek the environment of the polymer. This is a very important argument, which encourages us to suggest that a t least part of the intensity of the motionally narrowed component for 5DSA/S and 10DSA/S is due to the association of the probes with unimeric micelles, which are more mobile and lead to complete averaging of the spectral parameters. The appearance of the motionally narrowed component above 300 K and the increase in ita intensity a t even higher temperatures in some of the solutions is additional support for this conclusion: progressive breakdown of the larger polymer aggregates a t higher temperatures leads to the formation of smaller aggregates, possibly single ionomer chains (unimers), and to a motionally narrowed ESR component. We have to remember, however, that the isotropic component is a minor component, and even at the lowest polymer concentration we have studied (0.5%), the fraction of the motionally narrowed component is less than ~ 0 . 2 .Structure 3 represents regions of local hydrophobic aggregation (in circles) that lead to intramolecular micelles. Formation of these micelles is facilitated by the relatively long pendant chains and by the large average distance (14 backbone carbons) between these chains. Location of a Protiated Probe in a Perfluorinated Aggregate. The probe location will now be considered within the broader perspective of the interactions and miscibility between protiated and perfluorinated surfac-
Langmuir, Vol. 10, No. 4,1994 1107
Self-Assembling of Perfluorinated Ionomers
3 tants.= The mutual repulsion between hydrocarbon and perfluorinated chains is evident in the poor miscibility of n-alkanes with fluoroalkane~.~~*= A recent study of the phase behavior of semifluorinated n-alkanes consisting of a perfluorinated A-block connected to a protiated B-block in perfluorinated solvents, by viscosity, dynamic light scattering, and SANS, suggeststhe formation of a protiated micellar core,39 Studies of micelles formed from mixtures of fluorocarbon and hydrocarbon surfactants have indicated that ionic interactions can overcome to some extent the repulsion between the two types of chains. The micellar structure and composition have been studied by various methods, the most dominant being 1H and 19F NMR, surface tension, and scattering, as a function of the total surfactant concentration, the fraction of perfluorinated material, and the charge of the surfactant.% The results represent a range of conclusions, from separation into fluorocarbon-rich and hydrocarbon-rich regions, to the formation of mixed micelles,depending on the specific system and the method of study. Some evidence by NMR and surface tension has been presented for the formation of two types of micelles, or mixed micelles, whose composition deviates from ideal behavior, but the results are sensitiveto the chain lengthsm4Separate micelles in mixed surfactants have been proposed also in a study that combined pulse radiolysis and surface tension methods.41 Uniform mixed micelles have been detected by SANS42143 and by fluorescent probes.44 In the limit of a predominant component (protiated or perfluorinated), however, the micelle composition appears closeto that of the surfactant mixture.4 In view of these results, and considering that the amount of the protiated surfactant (the probe) in our system is very low, it is not surprising that the spin probe was found to be associated with the hydrophobic perflorinated polymer aggregate. It is possible that the probe is located at the water/ polymer interface, wrapped around the polymer aggregate. ~~
~~
~~
(36)Mixed Surfactant System; Holland,P. M.,Rubingh, D. N., Eds.; Symposium Series 501, American Chemical Society: Washington, DC, 1992. (37)Twieg,R.; Ruseell,T. P.;Siemens,R.; Rabolt, J. F. Macromolecules 1985,18,1361. (38)Song, K.;Twieg, R.; Rabolt, J. F. Macromolecules 1991,23,3714. (39)Lo Nostro, P.;Chen, S.-H, J. Phys. Chem. 1993,97,6535. (40)Guo, W.; Fung, B. M.; Chstian, S. D.; Guzman, E. K. In Mixed Surfactant System; Holland, P. M., Rubingh, D. N., Eds.; Symposium Series 501;American Chemical Society: Washington, DC, 1992;Chapter 15, p 244. (41)Aoudia, M.; Hubig, S. M.; Wade, W. H.; Schechter, R. S. In Mixed Surfactant System; Holland, P. M., Rubingh, D. N., Us.; Symposium Series 501;American Chemical Society: Washington, DC, 1992;Chapter 16 n266. --7 r
(42)Burkitt, S.J.; Ottewill, R. H.; Hagter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987,265,628. (43)Caponetti,E.; Martino,D. C.;Floriano, M. A.;Triolo,R. Langmuir 1993,9,ii93. (44)Muto, Y.; Esumi, K.; Megure, K.; h a , R. J. Colloid Interface Sei. 1987,120, 162.
This suggestion is in accord with the conclusions deduced from a recent study of various spin probes, which indicated that the doxyl probes are located at the interface between the solvent (water) and the hydrophobic aggregate (the polymer), even in the case of protiated surfactants.46 Dynamics of the Spin Probe Site. The conclusions discussed above suggest that the dynamics of the spin probe is expected to reflect the dynamics inside the polymer aggregates or at the water-polymer interface. We will focus on the dynamics of the major, anisotropic, component. This component is not completely immobilized: the extreme separation 2A’,, at 300 K can be compared with the rigid limit values 2A,,, measured from the spectra at 125 K (Table 1). This comparison indicates a higher mobility, or lower 2A’,,, for BDSA, compared with the other two probes. In addition, the mobility decreaseswith increasingpolymer concentration, a result that was assigned above to an increase in the aggregation number of the polymer chains. All three probes reveal lower mobilities (higher 2A’,,) in the membranes than in solution. A more quantitative assessment of the effect of the polymer concentration and temperature can be obtained by relating the probe anisotropy to microscopic order, as has been done for doxyl probes in oriented liquid crystals, membranes, and micellar systems.w8 The order parameter S can be calculated according to eq 1,where A,
is half the extreme separation 2A’,, and Amin is half the distance between the first minimum and the last maximum in the slow motional spectra, as shown in Figure 4; A,,, A,, and A,, are the hyperfine tensor components for the respective probes, and Aiso = (Axx + A, + Azz)/3. The average 2A,, values for the probe in the polymer solutions and in the swollen membranes, Table 1, appear to be independent of the polymer concentration. These 2A,, values, together with Abo values measured in neat water, enabled us to calculate the order parameters S for 5DSA/S, 5DSA/M, 10DSA/S, and 10DSA/M systems. To calculate S for 10DSE/S and 10DSE/M, we have made the assumption that the Ai, value is the same as the for the lODSA probe. Averaging of the A,, values for different polymer content gives the following order: A,,(5DSA) < A,,(lODSA) A,,(lODSE); these values suggest that the corresponding Ai, values are in the same order. The order parameters calculated for the spin probes in swollen membranes and in the most concentrated polymer solutions as a function of temperature are shown in Figure 10. At a given temperature, the S values increase from solutions to membranes, and the effect is more pronounced for 5DSA. At a given temperature, the S values change in the order: S(5DSA) < S(1ODSA) IS(lODSE), and the differences between the three spin probes is higher in the polymer solutions than in the membranes. The slopes of the S vs T plots are higher for 5DSA than for lODSA and lODSE, and higher in solutions than in the membranes. This analysis indicates that the dynamics of each of the three probes is different. The least ordered and most
-
(45)Shobba, J.; Srinivas, V.; Balasubramanian, D. J. Phys. Chem.
--.
1989. 93. 17. ----I
(46) Seelig, J. In Spin Labeling. Theory and Applications; Berliner, L. J., Ed.; Academic Press: New York, 1976;p 373. (47)Hubbell, W. L.;McConnell, H. M. J. Am. Chem. SOC. 1971,93, 314. (48)Caldararu, H.; Caragheorgheopol, A.; Dimonie, M.; Donescu, D.; Dragutan, I.; Marinescu, N. J. Phys. Chem. 1992,96,7109.
1108 Langmuir, Vol. 10, No. 4, 1994
Szajdzinska-Pietek et al.
SDSA
'*'.
+
0.40
-1
of 2Azz if we assume that the probes carry ("drag") part of their hydration shell into the aggregatems2 Comparisonwith l9F NMR, Scattering,and Other Data. The two spectral components detected in the ESR spectra are associated with large and small polymer aggregates,respectively. This conclusion can be compared with the SANS results, which detected only the presence of the large polymer aggregates, or rods.20-22 The reason for this apparent discrepancy can, however, be explained. The scattering function of an object is proportional to the square of its volume?3 The SANS data are therefore sensitive to the presence of the larger aggregates; in the system presented here this effect is even more pronounced, because most of the chains are aggregated. The ESR spin probe method is however extremely sensitive to the presence of species having narrow signals and therefore high amplitudes, such as the isotropic signal seen in Figures 1-3, even when the relative intensity of this signal is very low. Now we will consider the results of the NMR experiments." The effect of polymer concentration in Ndion/ Li solutions in water has been recently reported. Two types of spectra were detected as the volume fraction of the polymer increases above a volume content of 3% (equivalent to =6% (w/w)): narrow signals typical of polymer chains in solution, and a broad line typical of swollen membranes. These NMR results are qualitatively similar to the ESR results presented above. The time scales of the two experiments are however different: of the order of lO-4-lod s for the NMR and 10-8-10-9 for the ESR experiments. The appearance of two spectra components in NMR and ESR in the same range of polymer concentration suggests that the nitroxide spin probe and the NMR probe (the lgFnuclei) reflect different regions of the aggregate. It is logical to assume that the reorientation rates for the EPR probes are shorter because the probes are located at the interface polymer/solvent,where the surface is rough and allows for penetration by the solvent. Our suggestion that the small objects are unimers is supported by the detection of narrow signals in l9F NMR experiments at low polymer concentrations; these signals consist of narrow signals typical for single chains. Surface tension measurements at 294 K of aqueous solutions of Ndion with concentrations in the range 0.057.0% (w/w) indicate only a gradual decrease of the surface tension coefficient, from 72.5 dyn/cm for pure water to 62.1 dyn/cm for the most concentrated solution. These results suggest a progressive self-assemblingprocess, even at very low polymer concentration. The previous study of doxyl probes in Nafion from this laboratory3l detected a less rigid ESR spectrum for the 16DSA spin probe in Nafion/Na membranes, compared to 5DSA. This result, together with the present data, suggeststhat the alkyl chain of the probe has on the average a U-shape conformation, similar to that found in some ionic surfactant micelles and vesicles.m@ Furthermore, comparison of 5DSA/M in Ndion/Na and Ndion/Li reveals a counterion effect: in the presence of lithium the spectra are less rigid. This effect can be explained by assuming that the heat group region containingLi+is more
-1a 1: & 0.40
0.30240
mobile is the 5DSA probe, which suggests that the probe is associated with the periphery of the aggregate. The broader ESR lines for BDSA, compared with lODSA and lODSE, are consistent with this interpretation, The other two probes are probably embedded deeper into the aggregates. A similar location for the doxyl stearic acid probes has been suggested in numerous studies of lipid bilayersm*a*49 and surfactant micelle^,^^^^ where the order parameter decreases in going from the surface to the aggregate interior. A similar conclusion has been deduced in a recent determination of the order parameter from fluorescencedepolarization d a h s 1 In polymeric micelles, however, the chains are expected to be entangled and to lead to significant spatial restriction of the motion of a probe buried below the heat group region. As a result, the order parameter is expected to increase toward the micelle interior. Our suggestion that the more hydrophobic probes (10DSA and 10DSE) are located deeper in the aggregate, and further removed from the polar head, can be reconciled with the polar environment suggested by the large values (49) Meirovitch, E.; Nayeem, A.; Freed, J. H. J. Phys. Chem. 1984,88, 3454. (50) Jones,R. R. M.; Maldonado, R.; Szajdzinkda-Pietek,E.; Kevan, L. J. Phys. Chem. 1986,90, 1126. (51) Quitevis,E. L.; Marcus, A. H.; Fayer, M. D. J. Phys. Chem. 1993, 97, 5762.
(52) Dennis, K. J.; Luong,T.;Reshwan, M. L.; Minch, M. J. J. Phys. Chem. 1991,97,8328. Reference 31 in thispaper lists additionaleources that support the pulling of water into the micelle interior. (63)Mortansen, K.; Pedersen, J. S.; Macromolecules 1993, 26, 806. This study presents a study of the micellization process in poly(ethy1ene oxide)-poly(propy1ene oxide)-poly(ethy1ene oxide) triblock copolymers in aqueous solutions by SANS. A thermodynamic equilibrium unimera * micelles exists BB a function of temperature in this system, but only the micelles are detected by SANS. Unlike our system, higher temperatures favor the formation of micelles for the copolymers.
Self-Assembling of Perfluorinated Ionomers hydrated and less compact than in the case of Na+ and may be caused by the higher hydration radius of Li+.Such an interpretation has been proposed for simple surfactant micelles studied by ESEMM and pulse radiolysis.M
V. Conclusions Two componentshave been detected in the ESR spectra of doxy1 spin probes in aqueous Nafion solutions in the temperature range 275-360 K and have been assigned to probes associated respectively with large aggregates of low mobility, and with faster and smaller objects. The small objects are tentatively assigned to single chains (unimers). The spectral parameters for both sites suggest a polar environment,a result that can be rationalized by assuming that the probes drag part of their solvation shell into the hydrophobic aggregates. The size of the aggregates increases with increasing ionomer concentration. The relative intensity of the component associated with the unimers increases with temperature, indicating a temperature-dependent equilibrium between large aggregates and unimers. Analysis of the temperature variation of the signal associated with the large aggregates indicates different degree of immobilization of the three probes; the more hydrophobic probes (10DSA and 10DSE) are more rigidly held by the polymer aggregates, suggesting a location more removed from the solvent/polymer interface. In contrast to the case of simple micelles, the interior of the polymeric micelles is more ordered and more restrictive of probe mobility. Only large aggregates were detected in probe-doped membranes. (64) Szajdzineka-Pietek, E.; Gebicki, J. L.; Kroh, J.Pure Appl. Chem. 1999,66,1617.
Langmuir, Vol. 10, No. 4, 1994 1109 The results obtained have been compared with smallangle scattering, 19F "R,surface tension, and our previous ESR data for Nafion/Na membranes. We note that while the scattering data are more sensitive to large aggregates, the ESR spectra from the probes are very sensitive to the presence of small objects, even if the concentration of these species is low. This study suggests that an analysis of the ESR spectra from prutiated probes can provide useful data on the local structure in perfluorinated ionomer solutions. The analysis of spectra as a function of temperature leads to important information on both the location and the dynamics of the probe site. We caution, however, that the values of 2Azzcannot be used as an indicator of the depth of the probe in the micelle, because the probes appear to ,dragw part of their solvation shell into the polymer aggregates.
Acknowledgment. This research was supported by the National Science Foundation Grants DMR-8718947 and DMR-9224972 (Polymers Program) and INT-8922643 (US-Poland Collaborative Research), by NATO Grant CRG 901093 (US-France Collaborative Research Programme), and by the 1991/92 Founders' Fellowship of the American Association of University Women (AAUW) awarded to S. Schlick. We thank G. Gebel for the soluble Nafion powder, W o n t Company for the perfluorinated membranes, B.Quinn of BASF Laboratories for the surface tension measurements, and J. Pilor for many helpful discussions. We are grateful to one reviewer for his/her profound knowledge of the subject, careful reading of the manuscript, clear comments, and constructive criticism.
