Interaction of Ionomers and Polyelectrolytes with Divalent Transition

Publication Date (Web): March 2, 1999 ... the polymer chain depends on the pH: no attachment at low pH (1.5), and progressive bonding as the pH is inc...
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1934

J. Phys. Chem. B 1999, 103, 1934-1943

Interaction of Ionomers and Polyelectrolytes with Divalent Transition Metal Cations (Cu2+ and VO2+): A Study by Electron Spin Resonance (ESR) Spectroscopy and Viscosimetry Krzysztof Kruczala† and Shulamith Schlick* Department of Chemistry, UniVersity of Detroit Mercy, Detroit, Michigan 48219 ReceiVed: October 6, 1998; In Final Form: January 26, 1999

The interactions in aqueous media between ion-containing polymers (ionomers and polyelectrolytes) and the divalent cations Cu2+ and VO2+ were studied by electron spin resonance (ESR) at X and L bands and by viscometry. The polymeric systems studied were poly(ethylene-co-methacrylic acid) (EMAA) ionomer, perfluorinated ionomer (Nafion), poly(acrylic acid) (PAA), and poly(styrene sulfonic acid) (PSSA). ESR spectra indicate immobilization of both cations in EMAA solutions, due to cation ligation to two carboxylic groups from a multichain micelle. In PAA, the attachment of the cations to the polymer chain depends on the pH: no attachment at low pH (1.5), and progressive bonding as the pH is increased. Two types of Cu2+/PAA complexes were detected and were assigned respectively to ligation of the cation to one and to two carboxylic groups in the PAA chain. Only one type of complex with PAA was detected for VO2+ cations in the pH range up to 8.5. No evidence for complexation was detected for both cations in the Nafion and PSSA systems. Viscosity measurements indicate that the micellar dimensions in EMAA were reduced upon progressive addition of Cu2+ cations. We have also detected a time dependence of the reduced viscosity after dilution of the EMAA solutions, possibly due to a redistribution of micelle sizes. The results were examined in light of the models for ion-containing polymers, which have proposed an “ionomer” regime in nonpolar solvents where the ion pairs aggregate into multiplets, and a “polyelectrolyte” regime in polar solvents such as water, where the ion pairs are dissociated. The present study indicates that the interactions responsible for ionomer or polyelectrolyte regimes depend not only on the solvent but also on the type of ionic groups in the polymer, the type of counterions, and the pH.

Introduction Incorporation of ionic groups into polymeric systems is an important method for modification, and improvement, of polymer properties. When the amount of ionic groups is 1015 mol %, the polymer is an ionomer; because of the small amount of ionic groups, ionomers do not normally dissolve in polar solvents and have been studied mostly in bulk or swollen by solvents.1-3 The dominant effect due to the presence of ions in ionomers is the microphase separation into polar and nonpolar domains; the model that is most consistent with experimental results has assumed that in the polar domains the ion pairs are aggregated into multiplets consisting of several (up to ∼10) ion pairs.4 The presence of ionic domains increases significantly the glass transition temperature, Tg, often by hundreds of degrees, and the increase is specific for the type and charge of the counterions.5 Modification of the Tg for a shell of polymeric material in the proximity of the ionic groups has been predicted in a recent model for ionomers4a,b and confirmed experimentally in studies of dynamical processes using nuclear magnetic resonance (NMR)6 and electron spin resonance (ESR)7 spectroscopies. Ionomers have important applications in packaging materials, as ionselective membranes in electrochemical processes, and in fuel cells.8 In contrast to ionomers, polyelectrolytes typically contain ionizable groups on each repeat unit, are soluble in polar * To whom correspondence should be addressed. E-mail: schlicks@ udmercy.edu. † On leave from the Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland.

solvents, and have been studied and used in various applications mainly in solution.9,10 For these reasons, the field of ioncontaining polymers is deeply polarized and ionomers and polyelectrolytes are studied by different groups and using different methods of study without significant overlap.11 Many studies, however, have shown similarities in the behavior of ionomers and polyelectrolytes. In the presence of nonpolar solvents, the ion pairs and multiplets define the “ionomer regime”; in the presence of polar solvents, the ion pairs are dissociated and define the “electrolyte regime”.12 Moreover, solutions of some ionomers (perfluorinated and protiated) have been obtained recently by dissolution in an autoclave, thus making possible the study of the solution structure and comparison with bulk or solvent-swollen ionomers and with polyelectrolytes. Small-angle X-ray and neutron scattering13 and ESR spin probe methods7,14 have indicated that in the solutions the ionomer chains self-assemble into micellar aggregates with dimensions of the order of 100 Å, depending on the solvent, amount and position of the ionic groups in the ionomer, nature of the counterions, and degree of neutralization. The ionomer regime has also been invoked to explain, and provide a model for, the behavior of polyelectrolytes as a function of pH. In an important series of papers, Klooster et al.15 have reported a collapse transition in poly(acrylic acid) (PAA) in methanol neutralized by monovalent counterions, detected by several methods, including viscosimetry, osmotic pressure, light scattering, UV, and conductance results. These and similar studies of poly(methacrylic acid) (PMAA) in solvents of various polarity (water, methanol, dioxane, and their mixtures)12,16,17 led to the conclusion that the chain collapse

10.1021/jp9839697 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999

Interactions of Ionomers and Polyelectrolytes can be described by formation of ion pairs and their association in the supercollapsed state (“ionomer regime”).14 The ionomer regime is established in nonpolar solvents, while titration of PAA or PMAA in polar media leads to chain expansion. The experimental data and theoretical model suggest the close relationship between ionomer and polyelectrolyte behavior in terms of organization of the ionic groups and of changes that can occur due to neutralization, temperature, and presence of solvents and salts. Most theories of the effect of ionic interactions in ion-containing polymers have assumed the presence of noncoordinating monovalent counterions that are treated as point charges.12,18,19 The specificity of the ionic interactions has been demonstrated repeatedly, however. The dependence of Tg of the block polymers poly(ethyl acrylate-co-acrylic acid) on the type and charge of the counterion has been demonstrated.5 In addition, in the papers by Klooster et al.,15 it was clearly shown that the conformational transition in PAA solutions in methanol was detected for Na+ counterions but not for Li+ counterions. In PMAA neutralized by CH3OLi, the decrease of the reduced viscosity with the degree of neutralization is similar to that in PAA/Na, a sharp decrease at R ≈ 0.1, a behavior attributed to the chain collapse due to interactions between the ion pairs.16a Moreover, as early as 1954 it was pointed out that addition of Ca2+ to PAA leads to a larger coil contraction than that of an equivalent amount of Na+.20 More recent viscosity, light scattering, and small-angle neutron scattering studies of polyelectrolytes in the presence of divalent counterions have indicated that the chain dimensions are reduced, compared to an equivalent amount of monovalent counterion, and that the reduction is sensitive to the type of divalent counterion.21,22 The sensitivity of the ionic interactions to the specific counterions and to the nature of the backbone also suggests that the nature of the acid groups (sulfonic or carboxylic) can be expected to have an important role in the conformational behavior of ioncontaining polymers. We present a study of diValent transition metal cations (Cu2+ and VO2+) in aqueous solutions of ionomers and polyelectrolytes. The results are based on ESR spectra at the X-band of Cu2+ (2% neutralization of the ionic groups by cupric acetate or sulfate) and of VO2+ (2% neutralization by vanadyl sulfate) in poly(ethylene-co-methacrylic acid) (EMAA) ionomers, Nafion perfluorinated ionomers, poly(acrylic acid) (PAA), and poly(styrene sulfonic acid) (PSSA). The major objectives were to obtain details on the ionic interactions and to examine the results in view of the expected behavior of ion-containing polymers in a polar solvent. The polymers are shown in Chart 1. Our previous studies of Cu2+ in ionomers have shown convincingly that ESR spectra can provide a detailed picture of the type and number of ligands and of the strength of the interactions of the counterion with the ligands.14,23 ESR spectra of 51V (I ) 7/2) as VO2+ can provide a picture of the dynamics of the complex.14,24 While most conclusions from the present study are based on electron spin resonance (ESR) spectroscopy at the X band (∼9.5 GHz), important additional details and clarifications were obtained from viscosity measurements, and from ESR spectra measured at the L band (1.317 GHz). Experimental Section Materials. The ionomer poly(ethylene-co-methacrylic acid) (EMAA) had a methacrylic acid content of 7.5 mol % and was dissolved in water in an autoclave at 150 °C; the degree of neutralization, by KOH, was 90%. The solution was prepared

J. Phys. Chem. B, Vol. 103, No. 11, 1999 1935 CHART 1: Ionomers and Polyelectrolytes Studied

in the laboratories of DuPont-Mitsui Polychemicals Co. in Chiba, Japan.7 The ionomer content of the solution, 23% w/w, was determined from the weight loss of the samples after drying to constant weight at 390 K. The solutions examined in this study were slightly turbid. The turbidity was due to insoluble residuals (98%, ACS reagent), CuSO4, and VOSO4 were from Aldrich. Isoto-

1936 J. Phys. Chem. B, Vol. 103, No. 11, 1999 pically enriched 63Cu(CH3COO)2‚H2O and 63CuSO4 were prepared from 63CuO (99.73% 63Cu) purchased from Oak Ridge National Laboratory, Oak Ridge, TN, by reaction with acetic acid (glacial, 99.9%) or sulfuric acid (98%) during ∼24 h. Sample Preparation. The paramagnetic cations were in the form of cupric acetate or sulfate and vanadyl sulfate. The starting EMAA solution, whose pH was ∼10, was mixed with the appropriate amount of the cation salt to obtain a 12% w/w aqueous solutions of EMAA with 2% neutralization of the carboxylic groups by the transition metal cations (TMC) Cu2+ or VO2+. The PAA solutions had a polymer concentration of 12% w/w, and 2% of the carboxylic groups were neutralized by the TMCs. Cu2+ spectra were measured in PAA solutions with pH ) 1.5, 3.5, 4.2, 5.0, 6.1, 7.0, 8.3, 9.0, 10.5, 11.5, and >13.5, and VO2+ spectra were measured in PAA solutions with pH ) 1.5, 1.9, 2.8, 3.5, 4.5, 7.5, and 8.5. The Corning Scientific Instruments pH meter model 5 was calibrated by solutions with pH ) 3.0, 7.0, and 10.0. The desired pH was achieved by adding 8 M KOH solution to the PAA solutions. Samples measured at L band were prepared with 63Cu(CH3COO)2‚H2O and with D2O as solvent to reduce the line width broadening due to unresolved proton splittings. The amount of TMC as 63Cu(CH3COO)2 or VOSO4 in PSSA was equivalent to 2% neutralization of the sulfonic groups, and the polymer concentration was 12% w/w. The Nafion solution was mixed with the desired amount of 63Cu(CH COO) to obtain 2% neutralization of the sulfonic 3 2 groups; the final ionomer concentration was 6% w/w. Measurements. ESR spectra at X band were measured in the temperature range 125-300 K with the Bruker ECS106 spectrometer operating at 9.7 GHz (empty cavity) and 100 kHz modulation, using the ESP 3240 system for acquisition and manipulation. Above 273 K, the polymer samples were measured in the flat ESR cell; quartz ESR sample tubes, 4.2 mm o.d., were used at low temperatures. Typical parameters for data acquisition were: modulation amplitude 4 G, microwave power 2 mW, time constant 20 ms, conversion time 40 ms, number of points 2048, number of scans 25, spectral range 2500-3500 G for Cu2+ and 2600-4400 G for VO2+ spectra. Additional experimental details on ESR spectra at X band have been published.7 ESR measurements at L band (1.317 GHz) were performed at the EPR Center, University of Illinois at UrbanaChampaign. Viscosity measurements were performed at 300 K with Ubbelohde viscometers equipped with a range of capillary sizes so as to obtain a flow time longer than 200 s. The flow times were measured by a digital stopwatch with a resolution of 0.01 s. The accuracy of temperature was (0.2 K. The viscosity of EMAA solutions (12% w/w ionomer) was measured for degrees of neutralization of 0.5, 1.0, 2.0, and 3.0%. The density of the EMAA solution at 300 K was 0.9995 g/mL. PAA solutions (12% w/w polymer, 2% degree of neutralization) were measured for pH values of 1.5, 7.0, and 13.5; the corresponding densities were 1.036-1.039, 1.092, and 1.107-1.120 g/mL; the range of densities is due to different batches of PAA purchased. Simulations. ESR spectra were simulated with the program SIM14A.25 The original program was modified by introducing optimization procedures: grid search with adjustable step, the Nelder-Mead simplex method with no restrictions on the range of the ESR parameters and a maximum of 20 optimization parameters, and Monte Carlo. The program calculates the energy levels of the spin system to second order and the transition probabilities to first order. Simulations were typically performed in two steps; the initial parameters were first deduced by the simplex minimization method and then refined by grid search,

Kruczala and Schlick TABLE 1: ESR Parameters for Cu2+ Centersa,b system Cu/water Cu/water Cu/water Cu/water Cu/water Cu/EMAA Cu/EMAA Cu/PAA Cu/PAA

g|

g⊥

2.413 2.368 2.244 2.416 2.243 2.328 2.323 2.406 2.365 2.326 Cu/PAA 2.357 2.317 Cu/PAA 2.264 Cu/Nafion 2.362 Cu/PSSA 2.365 Cu(OH-)4 2.273

2.082 2.074 2.050 2.082 2.049 2.060 2.057 2.081 2.071 2.058 2.058 2.056 2.052 2.070 2.072 2.055

A|(G) A⊥(G) 123 140 189 119 190 145 155 121 134 162 140 157 186 135 132 183

∼5 ∼6 31 ∼4 30 18 22 ∼7 ∼8 14 13 24 30 ∼7 ∼9

comments copper acetate, pH ) 1.5c copper acetate, pH ) 5.4 copper acetate, pH g 13.5 copper sulfate, pH ) 2.5 copper sulfate, pH g 13.5 at 300 K pH ) 1.5 ∼90%, pH ) 3.5 ∼10%, pH ) 3.5 ∼50%, pH ) 7.0 (site 1) ∼50%, pH ) 7.0 (site 2) pH ) 13.5 ref 27

a

All magnetic parameters are for spectra measured at 125 K, unless indicated otherwise. b Gaussian line shapes were used to calculate the spectra. c This pH was obtained by addition of sulfuric acid.

TABLE 2: ESR Parameters for VO2+ Centersa,b center

g|

g⊥

A|(G)

A⊥(G)

comments

VO/water VO/EMAA VO/EMAA VO/PAA VO/PAA VO/PAA VO/PAA VO/PSSA

1.933 1.938 1.939 1.934 1.937 1.939 1.940 1.933

1.977 1.975 1.976 1.979 1.978 1.977 1.977 1.977

201 188 188 201 194 191 185 201

75 65 64 74 69 66 60 73

pH ) 3.4 at 300 K pH ) 1.5 pH ) 3.5 pH ) 7.5 pH ) 8.5 pH ) 4.2

a All magnetic parameters are for spectra measured at 125 K, unless indicated otherwise. b Gaussian line shapes were used to calculate the spectra.

using grid widths of 0.0005-0.0010 for g values, 2 G for A|, 1 G for A⊥, and 1 G for the line width. One spectrum (1000 pts) was calculated in 3-7 s with a COMPAQ computer equipped with a Pentium processor. Results In this section, we will describe the ESR results for Cu2+ and VO2+ in the ion-containing polymers and the viscosity data. The ESR spectra of the cations at X band in the polymer solutions were compared with the spectra of the corresponding paramagnetic salts dissolved in water as reference. To prevent crystallization of water below 273 K, the salts were dissolved in a water/glycerol mixture (85:15 v/v); the salt concentration was equal to that in the PAA samples. The notation for the various samples is Cu/water or Cu/polymer and VO/water or VO/polymer; the notation for samples prepared with the 63Cu isotope is 63Cu/polymer. The magnetic parameters deduced from the simulated spectra are collected in Tables 1 and 2. The accuracy of the magnetic parameters obtained by simulations was (2 G for A| and A⊥, and (0.001 for g| and g⊥. When A⊥ was not resolved, the margin of error was larger, typically (4 G. ESR Spectra of Cu2+. ESR spectra at X band of cupric acetate in the water/glycerol mixture were measured at 300 and 125 K at pH ) 1.5, 5.4, and g13.5; spectra for pH ) 5.4 and g13.5 are presented in Figure 1. The paramagnetic center present at 300 K and pH ) 1.5 (by addition of sulfuric acid) is isotropic with giso ) 2.192, as deduced from simulations; the parameters deduced from simulations of spectra measured at 125 K suggest the presence of the fully hydrated cupric complex. The paramagnetic center present at 300 K and pH ) 5.4 has an isotropic signal with weakly resolved lines from the hyperfine

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Figure 1. X-band ESR spectra of Cu2+ (as the acetate salt) at 300 and 125 K in water/glycerol 85:15 v/v mixture at pH ) 5.4 (a) and at pH g 13.5 (b). Dotted lines show the spectra calculated with the g and A tensor components given on the figure and in Table 1. Values for giso and Aiso were also deduced from simulations. Vertically expanded portions of the spectra are shown in b, and arrows point to the parallel signals (mI ) -3/2) for the two Cu isotopes, 63Cu and 65Cu.

splittings (hfs) of Cu nuclei (I ) 3/2) and giso ) 2.182. The anisotropic spectrum detected at 125 K was simulated (dotted lines in Figure 1a) with the magnetic parameters given in the figure and in Table 1; the parameters are typical of acetate groups as ligands.26 The ESR spectrum disappears in the pH range 7-13, most likely due to the formation of ESR-silent dimeric species. At pH g 13.5, the spectrum reappears (Figure 1b); the lines at 300 and 125 K at this pH are narrower compared to those detected at pH ) 5.4 and allow the resolution of separate signals from the 65Cu and 63Cu isotopes, as indicated by arrows in the vertically expanded spectrum measured at 125 K and shown in Figure 1b. The parameters used to simulate this spectrum are typical for OH- as ligands.27 ESR spectra of Cu2+ (as 63CuSO4) in the water/glycerol mixtures led to deductions that are consistent with the results obtained for aqueous solutions of cupric acetate; at low pH values (2.5) the magnetic parameters are typical of the hydrated cupric complex,28 and at high pH (14), the parameters indicate Cu2+ surrounded by OH- as ligands. X-band ESR spectra of Cu2+ as the acetate salt in EMAA solutions allowed the detection of separate signals for the parallel components of 65Cu and 63Cu isotopes; the 65Cu splittings were removed by 63Cu enrichment. The ESR spectra of 63Cu/EMAA shown in Figure 2 are dramatically different compared to those for Cu/water.29 Even at 300 K, the spectra are typical of a completely immobilized copper complex. The rotational correlation time of the cupric complex is too long for dynamical averaging to occur due to the association of the cation to the EMAA micelles.30 The immobilization of the counterion was detected even in spectra measured at 350 K. The spectra in the parallel region are typical of the “strain” effect, which is due to a distribution of the g and hyperfine splittings in the parallel orientation that lead to different widths of each signal of the parallel component, depending on the mI value;23 this feature

Figure 2. X-band ESR spectra of 63Cu/EMAA at 300 and 125 K. Dotted lines show the spectra calculated with the g and A tensor components given on the figure and in Table 1. Cu2+ was added as the acetate salt.

was not reproduced in the simulations because the codes did not include a distribution of g| and A| values. Identical spectra were obtained when the salt added was 63CuSO4.

1938 J. Phys. Chem. B, Vol. 103, No. 11, 1999

Figure 3. X-band ESR spectra of 63Cu/PAA at 300 K as a function of pH. The smooth dotted line is the spectrum calculated with the g and A tensor components given on the figure and in Table 1. Values for giso and Aiso were also deduced from simulations. Cu2+ was added as the acetate salt.

X band ESR spectra of Cu2+ as the acetate or sulfate salts in PAA solutions are sensitive to the pH. Selected experimental and simulated spectra for the 63Cu-enriched acetate salt are shown in Figure 3 (spectra at 300 K) and Figure 4 (spectra at 125 K). At 300 K and pH ) 1.5, the isotropic spectrum and the magnetic parameters are identical to those detected in neat water at the same pH. In the pH range 3-8 (degree of dissociation of PAA 15 to 95%), the spectra become anisotropic and the number and type of complexes, and their magnetic parameters, depend on the pH. The spectrum for pH ) 7 measured at 125 K is clearly a superposition of two spectral components; the simulated spectrum shown in Figure 4b was obtained by adding equal contributions from two complexes, site 1 and site 2, with the magnetic parameters given in Table 1. The spectrum measured at pH ) 3.5 also suggests a superposition of two components clearly seen in the shoulder accompanying the low-field signal and the perpendicular feature that differs from that detected at pH ) 1.5. The simulated spectrum shown in Figure 4b for this pH is a sum of contributions from two complexes in the intensity ratio of ∼9: 1; the uncertainty in the magnetic parameters is significant, however, because the resolution into the two spectral components is not as good as at pH ) 7.0. As the pH increased above 7.0 the signal intensity decreased but did not disappear completely. The spectrum at pH g 13.5 is well resolved with isotropic hyperfine splittings at 300 K, and the magnetic parameters are typical of complexation with OH-.27 Taken together, the experimental and simulated spectra of Cu/PAA shown in Figures 3 and 4 reflect the evolution of the magnetic parameters and dynamics of the counterions as the pH increases: complexation to water as ligands at low pH, binding to the polymer (and anisotropic spectra even at 300 K) at higher

Kruczala and Schlick pH, and binding to OH- (accompanied by isotropic spectra at 300 K) at and above pH ) 13.5. The ESR spectrum at L band (1.317 GHz) at 77 K of Cu2+ as the 63Cu acetate salt in PAA at pH ) 7 is shown in Figure 5. The effect of strain is reduced at this frequency,23a,b,d and the narrower lines show clearly the presence of the two sites, in agreement with the conclusions based on X-band spectra. X-band ESR spectra of Cu2+ as the acetate in Nafion solutions (pH ≈ 9) and in PSSA solutions (pH ≈ 5.5) are shown in Figure 6. Only one type of signal is observed, and the magnetic parameters listed on the figure and in Table 1 indicate that the ligands are acetate groups, as detected for Cu/water at pH ) 5.4; no indication of complexation to the sulfonic groups of the polymers was detected. ESR Spectra of VO2+. X-band ESR spectra at 300 and 125 K of VO2+ (as the sulfate) in the water/glycerol mixture at pH ) 3.4 were simulated with parameters typical of the hydrated complex.28 Figure 7 presents the ESR spectra of VO2+ in EMAA; the line shapes at the two temperatures are essentially the same, indicating that the counterions are immobilized by attachment to the polymeric micelles, as also observed for Cu2+ in this system. While the slower dynamics upon complexation is the most dramatic effect, the attachment to the carboxylic groups also leads to a lower value of A|, from 202 G in the hydrated complex to 188 G in the EMAA solutions. In Figure 8, we present X-band ESR spectra at 300 K of VO2+ in PAA for the indicated value of the pH. While at pH ) 1.5, the magnetic parameters are identical to those for VO/water, at pH ) 3.5, the spectrum becomes anisotropic, clearly due to attachment to the carboxylic groups of the polymer chain; from the simulated spectrum corresponding to pH ) 3.5 (dotted spectrum in Figure 8) we obtain the hfs tensor components and Aiso ) 111 G, compared to 115 G in the hydrated complex. The isotropic spectrum detected at pH ) 8.5 shows evidence for the replacement of carboxylic ligands by hydroxyl ligands, and is reflected in a much lower value of the isotropic hfs, Aiso ) 98 G. Ligand exchange is accompanied by a color change, from blue (for carboxylic ligands) to brown (for hydroxyl ligands). ESR spectra of VO/PAA measured at 125 K (not presented) reflect the changes in the rigid limit values for the magnetic parameters as a function of pH. As the pH increases, the values of g| and g⊥ vary slightly, but larger variations are seen in the A| and A⊥ values; A| changes from 202 to 194 G, to 191 G, and to 185 G, and A⊥ changes from 75 to 69, to 66, and to 60 as the pH varies from 1.5 to 3.5, to 7.5, and to 8.5. ESR spectra of VO/PSSA show the presence of the corresponding hydrated complex and OH- ligands at higher pH; as for Cu/PSSA, no bonding to the polymer was detected. Viscosity Measurements. The variation of the viscosity of PAA solutions (concentration 12% w/w) upon addition of Cu2+ as the acetate salt was measured at 300 K as a function of the pH. The amount of counterions corresponded to 2% neutralization of the carboxylic groups in PAA. Within experimental error, the reduced viscosity, ηred, did not change upon Cu2+ addition when the pH was 1.5 or 14. At pH ) 7.0, however, ηred decreased by about 6%; we attribute this result to smaller chain dimensions upon complexation.31 The viscosity of EMAA solutions (12% w/w polymer) was measured at 300 K as a function of the amount of Cu2+ (as the acetate salt) added. We noticed that ηred of the polymer solution and of the solution containing Cu2+ ions decreased with time, rapidly on the time scale of ∼2 days, and more slowly over several additional days, as seen in Figure 9a. To separate the

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J. Phys. Chem. B, Vol. 103, No. 11, 1999 1939

Figure 4. X-band ESR spectra of 63Cu/PAA at 125 K as a function of pH. Vertically expanded portions of the spectra are also shown. Upward arrows point to parallel signals from the two types of complexes at pH ) 7 and in the corresponding simulated spectrum (site 1 solid lines and site 2 dotted lines). Cu2+ was added as the acetate salt.

cation addition is assigned to contraction of the micelles on complex formation. Discussion

Figure 5. L-band (1.317 GHz) ESR spectra of 63Cu/PAA at 77 K at pH ) 7. Upward arrows point to the two spectral components in the parallel direction, corresponding to mI ) -3/2 and +3/2. Cu2+ was added as the acetate salt.

effects of cation addition and of time, we followed the variation of ηred in samples after aging for 2 days; the results are given in Figure 9b. The decrease of the reduced viscosity upon

In this section we will deduce the type of complexation of the paramagnetic counterions in the EMAA and PAA systems, relate our deductions to previous studies of PAA and other polyacrylates neutralized by Cu2+ counterions, examine the ESR spectra of Cu2+ and VO2+ in all the ion-containing polymers studied in view of the proposed models for the ionomer and polyelectrolyte regimes, and analyze the viscosity data and dwell on the additional details provided and questions raised by these measurements. Bonding of the Divalent Counterions to EMAA and PAA. The detection of rigid limit or slow-motional ESR spectra for Cu2+ in EMAA and PAA solutions even at 300 K (Figures 2 and 3) is a clear indication for the complexation of the counterion with the carboxylic groups of the polymers. The evolution of the ESR spectra as a function of pH in the PAA system, Figure 3, mirrors the transition from Cu2+ cations with water ligands at pH ) 1.5 to cations bound to the polymers and anisotropic spectra at pH ) 3.5. Of great significance is the detection of two types of complexes at pH ) 7, seen clearly in the ESR spectra measured at 125 K, Figure 4. Detection was possible because of the high resolution in the 63Cu-enriched samples and was facilitated by the similar relative intensities of the two species. ESR spectra of PAA solutions at pH ) 3.5 also reflect the presence of two spectral components; simulated spectra suggest that the relative intensity of the site with the larger value of the parallel hfs, A|, is about 10% of the total intensity. Site 1, detected at pH ) 7, is assigned to Cu2+ ligated to one chelating carboxylic group;32-34 it is logical to assume that site 2 represents two chelating carboxylic groups, because of the larger

1940 J. Phys. Chem. B, Vol. 103, No. 11, 1999

Kruczala and Schlick

Figure 6. X-band ESR spectra of 63Cu/Nafion (a) and 63Cu/PSSA (b) at 300 and 125 K. Dotted lines show the spectra calculated with the g and A tensor components given on the figure and in Table 1. Cu2+ was added as the acetate salt. Values for giso and Aiso were also deduced from simulations.

Figure 7. X-band ESR spectra of VO/EMAA at 300 and 125 K. Dotted lines show the spectra calculated with the g and A tensor components given on the figure and in Table 2. VO2+ was added as the sulfate.

A| and lower g| values, in accord with the A| vs g| correlation diagrams for copper complexes with oxygen ligands.35 We note that the relative intensity of site 2 increases with the pH, 10% at pH ) 3.5 and 50% at pH ) 7 (Table 1), because more carboxylic groups are ionized and available for complexation.

Figure 8. X-band ESR spectra of VO/PAA at 300 K as a function of pH. The smooth dotted line is a spectrum calculated with the g and A tensor components given on the figure and in Table 2. Values for giso and Aiso were also deduced from simulations.

At high pH values, g13.5, the ESR spectrum in PAA at 300 K becomes isotropic and the magnetic parameters indicate replacement of the carboxylic groups by OH- groups and implicitly the breakdown of the polymer complexes. In Cu/PAA,

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J. Phys. Chem. B, Vol. 103, No. 11, 1999 1941

Figure 9. (a) The reduced viscosity in EMAA (2) and in Cu/EMAA (9) as a function of time after dilution; the cation content in Cu/EMAA corresponds to 2% neutralization of the acid groups. (b) The reduced viscosity of Cu/EMAA as a function of the degree of neutralization by Cu2+ added as the acetate salt: no aging (β); aging during 2 days ([).

the total ESR spectral intensity is about the same at pH ) 1.5 and 7.0, decreases above pH ) 7.0, and at pH ) 13.5 is only ∼10% of the initial intensity obtained by integrating the spectrum at pH ) 1.5; therefore, we must deduce that in Cu/ PAA the majority of, but not all, the cations exist as ESR-silent species between pH ) 7.0 and pH ) 13.5. The ESR-silent complexes are bi- or polynuclear,36 and the intensity of the mononuclear, ESR-visible complex Cu(OH-)4 increases at higher pH. The interpretation of the ESR spectra in PAA is important for deducing the type of complex formed in the EMAA solutions. The micellar solutions are stable at pH ≈ 10, and we observe only one type of complex, with parameters almost identical to those deduced for site 2 detected in PAA as the pH increases; this is an indication that Cu2+ is associated with the polymeric micelles by two carboxylic groups, and is reasonable in view of the high pH of the micellar EMAA solutions and the expected high local concentration of COO- groups in the corona area of the micelles. This ligation scheme also explains the full immobilization of the counterions and the rigid limit ESR spectrum detected at 300 K. In Figure 10, we present the two types of Cu2+ complexes in PAA and the association of Cu2+ to the EMAA micelles. In PAA, we have drawn site 2 as a cupric complex associated with one polymer chain, according to the literature data.32 It is quite probable that the gelation we observed at high Cu2+ content in Cu/EMAA29 is a result of cation ligation by carboxylic groups from different micelles. The results for VO/EMAA and VO/PAA indicate immobilization of the complex by attachment of the cation to the polymeric micelles in EMAA, the detection of only one type of complex in PAA, and the replacement of the carboxylic groups from the ionomer by hydroxyl groups at a lower pH (8.5) compared to Cu/PAA. Immobilization of the cation suggests preferential ligation of VO2+ to two carboxylic groups; we do not rule out, however, the presence of cations ligated to one carboxylic group, which cannot be detected because of low relative intensity and/or similar magnetic parameters. Comparison of the results for the two cations in PAA shows therefore

Figure 10. Suggested complexation of Cu2+ in PAA by one and by two carboxylic groups (sites 1 and 2, respectively) in a PAA chain32 and immobilization of Cu2+ by two carboxylic groups in the multichain EMAA micelles.

that binding to the polymer chain and the breakdown of the polymer complex occur at a different pH value, 13.5 for Cu2+ and 8.5 for VO2+, and reflect the specificity of the cation complexation. Several possible reasons can be invoked in order to explain the different pH dependence for the two cations: the different electronic configurations (d9 vs d1) and the effect of the VdO bond on cation complexation and size. For the PSSA system, we detected no ligation of the cations with the sulfonic groups of the polymers, and the corresponding magnetic parameters are identical to those in the fully hydrated complexes. Nafion is expected to behave in a similar way. Comparison with Other Studies. The interaction of Cu2+ with PAA, PMAA, and other polyacrylates has been studied extensively.32-34 The earlier work32 has presented evidence from IR, UV, and static magnetism studies for the ligation of Cu2+ to two carboxylic groups in the same polymer chain in PMAA. Variation of the degree of neutralization led to the conclusion

1942 J. Phys. Chem. B, Vol. 103, No. 11, 1999 that binuclear complexes are formed at a low degree of neutralization and that these complexes dissociate to give mononuclear complexes on increasing the charge of the polymer chain. The most relevant study for comparison with our results was published recently by Franc¸ ois et al.33 who reported on Cu2+ complexation with PAA and PMAA studied by ESR at X band. A deconvolution procedure of ESR spectra measured at ambient temperature was used to “evaluate the fraction of free copper and bound copper ions”.33 For Cu2+/PAA, the main conclusions based on this method are that both binuclear and mononuclear Cu2+ complexes are formed, and that the mononuclear complexes are dominant at a higher degree of neutralization. The data cover the pH range 3-7.5. The ESR spectra measured at 125 K and the high resolution obtained by 63Cu enrichment in this study allowed the description of the Cu2+ complexation in PAA in greater detail. First, we observed at low pH values the counterions bound to water ligands and the progressive replacement of these ligands by carboxylic groups from the polymer; this gradual process was detected not only from an examination of the magnetic parameters but also from the change of the ESR spectra, from isotropic to anisotropic as the counterions become attached to the polymer chains. Second, we detected two types of complexes at pH ) 7 and assigned them respectively to Cu2+ cations ligated to one and to two chelating carboxylic groups of the polymer; chelation by two carboxylic groups occurs at a higher degree of neutralization of the polymer chain. Third, the decrease in the spectral intensity detected at high pH values was followed by the reappearance of the ESR spectrum due to decomplexation (from the polymer) and replacement of the carboxylic ligands by OH- ligands. In this way, we obtained a complete picture of the bonding of the counterions as a function of the pH, to pH ) 14. In contrast to the conclusion in ref 33, we have no indication that the ESR-silent binuclear complexes are dominant at low pH, and our results indicate that up to pH ≈ 7 the ESR intensity is constant with pH. Our data indicate that the binuclear complexes become dominant above pH ) 7. It is possible that different experimental conditions (polymer concentration, cation content) are responsible for the different conclusions reached in this study and in ref 33. Further studies are necessary to clarify this point. Models for the Aggregation of Ionic Groups. The model of Khokhlov et al.12 has provided the rationale for the decrease in the PAA dimensions upon neutralization with Na+ as the counterions in methanol, a solvent with a low dielectric constant. The ion pairs collapse on the chain (“collapsed state”) and associate into multiplets (“supercollapsed state”); this is the ionomer regime. In water, this process is not expected because the ion pairs are dissociated and the polyelectrolyte regime prevails. For the ion-containing polymers with sulfonic groups, Nafion and PSSA, the present study has not detected any evidence for complex formation and the clear conclusion is that the ion pairs are dissociated, as predicted by the theoretical model. It is possible that the ionic interactions are stronger in the case of the divalent cations, compared to monovalent cations, and can lead to contraction of the chain, as detected for PSSA neutralized by Mg2+.22 The ESR data however cannot address this type of bonding, because this spectroscopic technique, as applied here, gives only local information, on the scale of 7, and at pH g 13.5 only ligation to hydroxyl ions was detected. No evidence for complexation was detected for both cations in the Nafion and PSSA systems. Viscosity measurements show the contraction of polymer micelles (in EMAA) and of polymer chains (in PAA) when polymer complexes are formed. The time dependence of the reduced viscosity in the EMAA solutions after dilution was attributed to a redistribution of micelle sizes. The results obtained in this study were examined in light of the models for ion-containing polymers, which have proposed an “ionomer” regime in nonpolar solvents and a “polyelectrolyte” regime in polar solvents such as water. The present study

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