J . Phys. Chem. 1986,90,6353-6358 withdrawing and electron-releasing substituents: There is an increase in photoelectric sensitivity on a replacement of octamethoxy groups by octacyano substituents. It is shown that the observed effects of substituents in zinc phthalocyanine may be caused by a dependence of the primary quantum yield for initial charge pair generation on the electron-shifting tendency of substituted groups.
6353
Further work is in progress to confirm this hypotheses by additional measurements of carrier generation and transport parameters in substituted phthalocyanines.
Acknowledgment. We thank Vokswagen Foundation for financial support. Registry No. la, 76228-28-9; lb, 104507-32-6.
Detection of Isolated and Paired Cu2+ Ions in Nafion Membranes by ESR Spectroscopy M. G. Alonso-Amigo and Shulamith Schlick* Department of Chemistry, University of Detroit, Detroit, Michigan 48221 -9987 (Received: March 24, 1986; In Final Form: May 19, 1986)
Acid Nafion membranes were neutralized completely by immersion in 0.1 M solutions of Cu2+and Zn2+ ions and studied by electron spin resonance (ESR)spectroscopy as a function of the Cu2+/Zn2+ratio and the amount of water present, in the temperature range 77-300 K. At low Cu2+concentrations, isolated Cu2+ions were detected in two sites, differing in their glland Allvalues. Analysis of the ESR parameters indicates that in site I Cu2+is ligated to water oxygens and to SO3groups of the polymer backbone and is in the “rigid” motional limit in the entire temperature range studied. In site I1 Cu2+ is ligated to water oxygens only. The relative populations of Cu2+in the two sites is very sensitive to variations in the water content of the membranes. All samples were therefore measured under conditions of controlled relative humidity. In the hydrated complexes, ligation to “mobile” and “bound” water is detected by comparing ESR spectra of fully hydrated samples with those in which part of the water was removed by heat treatment. Ligation to “mobile” water leads to averaging of the g and hyperfine anisotropy and is evident from the appearance of an isotropic line in hydrated membranes at 295 K. For CuZ+/Zn2+ ratios of 0.1 and larger, Cu2+-Cu2+ are detected and identified by the Ams = 2 transition at a magnetic field of 1600 G. The signals from the pair suggest that the interion distance is 5.5 A, based on the intensity ratio of the Am, = 2 and 1 transitions.
-
Ionomers are polymeric materials consisting of an organic backbone with some attached ionic groups. In many of the systems investigated so far, the polymers contain pendant acid groups which can be neutralized by addition of metal cations.’ Most ionomers contain less than 10 mol % ionic groups. The presence of ions in these systems has a profound effect on polymer properties such as dielectric response, tensile strength, impact resistance, and elasticity. The properties of ionomers are very sensitive to the amount of water remaining in the system and to the presence of other solvents such as methanol. The morphology of ionomers has been studied by various techniques, including differential scanning calorimetry (DSC), X-ray scattering? and electron mi~roscopy.~The local structure around the cations has been probed by extended X-ray absorption fine structure (EXAFS),4 IR,5 and R a m a d and by study of fluorescence and phosphorescence of organic chromophores incorporated in the ionomers.’-* The effect of the ionic concentration on the chain dimensions has been studied by small-angle neutron scattering (SANS)9 and by nuclear magnetic resonance (NMR).’O (1) MacKnight, W. J.; Earnest, T., Jr. Polym. Sci. Macromolec. Rev.
1981,16, 41. ( 2 ) MacKnight, W. J.; Taggart, W. P.; Stein, R. S. J. Polym. Sci., Polym. Symp. 1974,45, 113. (3) Longworth, R.In Ionic Polymers; Holliday, L., Ed.; Halsted-Wiley: New York, 1975; Chapter 2. (4) Pan, H. K.; Meagher, A.; Pineri, M.;Knapp, G. S.; Cooper, S . L.J . Chem. Phys. 1985.82,1529. (5) Mattera, V. D., Jr.; Risen, W. M., Jr. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 67. (6) Neppel, A,; Butler, I. S.; Eisenberg, A. Macromolecules, 1979,12,948. (7) Lee,P. C.; Meisel, D. Photochem. Photobiol. 1985,41, 21. (8) Szcntirmay, M. N.; Prieto, N. E.; Martin, C. R. J. Phys. Chem. 1985, 89, 3017.
0022-3654/86/2090-6353$01 .50/0
The general conclusion suggested by the numerous studies is that the properties of ionomers can be broadly explained by assuming a degree of ionic aggregation, or clustering, in the organic polymer matrix. The structure of the clusters represents an equilibrium between the electrostatic interactions and the elastic forces of the polymer backbone. Several theoretical models have been advanced in order to explain and interpret the experimental results.” Despite the great research efforts, many questions remain unanswered. For instance, the local environment of the ions, the size of the ionic aggregate, the interion distances, and the effect of aggregation on the chain dimensions have not been defined in detail. In addition, the specific ligation of the ion as a function of water, or solvent, content and the ionic motion as a function of temperature remain open questions. We plan to clarify some of these problems by studying ionomers containing paramagnetic cations, using the method of electron spin resonance (ESR). We report here an ESR study of Cu2+diluted by diamagnetic Zn2+ ions in sulfonated Nafion ionomers as a function of cation concentration, temperature, and water content. The water content is varied by studying samples maintained in air under controlled relative humidity conditions. In samples under a relative humidity of 80% and lower, we obtained the first direct spectroscopic evidence for location of the paramagnetic cation in two sites: Cu2+ is ligated to water oxygens and to sulfonic groups of the polymer backbone in site I and to water oxygens only in site 11. For Cu2+ (9) Forsman, W. C.; MacKnight, W. J.; Higgins, J. S. Macromolecules
1984,17, 490. (10) Boyle, N. G.; McBrierty, V. J.; Eisenberg, A. Macromolecules 1983, 1 1
?L
IO, I>.
(1 1,) Ions
in Polymers; Eisenberg, A., Ed.; American Chemical Society: Washington, DC, 1980.
0 1986 American Chemical Society
6354 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
Alonso-Amigo and Schlick DPPH
in site 11, we can study separately ligation by “mobile” and by “bound” water. This selective study is possible because Cu2+ ligated by mobile water tumbles rapidly above 240 K and averages the anisotropy in the g and hyperfine tensors. By contrast, Cu2+ ligated by bound water is immobilized even at 298 K. Cu2+-Cu2+pairs are observed at high Cu2+concentrations and are identified by the appearance of the Ams = 2 spin-forbidden transition. The interion distance is evaluated from the intensity ratio of the forbidden Ams = 2 and allowed Ams = 1 transitions. Some preliminary results have been reported.12
Experimental Section Great research efforts have been invested in the study of perfluorinated ionomers made by Du Pont because of their commercial application as separation membranes in electrochemical cells.I3 Known by their tradename of Nafions, these ionomers -C
FzCFzCF-
I
-EOCF2CF(CF3) %OCF,CFpSOsH Naf ion
have a Teflon backbone with pendant carboxylic or sulfonic groups attached to a perfluoro ether side chain. These polymers are very attractive for a variety of measurements because of their stability in oxidizing and reducing media. The Nafion 117 membrane, with an equivalent weight of 1100 g/mol of S03H and a thickness of 0.13 mm, was cleaned by soaking in 2-propanol, acidified by soaking in a 9 M H2S04 solution, and rinsed with deionized water. The pretreated membrane was fully equilibrated by immersion in a 0.1 M solution containing a mixture of CuS04 and ZnSO,. The PH of the sulfate solutions varied from 3.3 in pure ZnSO, solutions to 5.5 in pure CuSO, solutions. The concentration of Cuz+in the cation mixture varied in the range 1-100 mol %. After equilibrium the membranes were dry blotted with filter paper and kept at 293 K in an environment with controlled relative humidity, using aqueous solutions of sulfuric acid.’, The samples were quickly transferred to 4-mm-0.d. ESR tubes and sealed. We have noticed that the transfer to ESR tubes, which takes no more than a few minutes, did not alter measurably the water content of the membranes. ESR spectra at the X-band were measured with a Varian E-9 spectrometer operating at 9.3 GHz and with a Bruker 200D SRC spectrometer operating at 9.7 GHz, using 100-kHz modulation. Spectra at 77 K were taken in a liquid nitrogen Dewar inserted in the ESR cavity. In the range 100-300 K, ESR spectra were measured by using the Bruker ER 41 11 variable-temperature unit with liquid nitrogen as the coolant in a flow system. The temperature of the sample was measured by a digital chromelconstantan thermocouple with an accuracy of better than f 2 K. The absolute value of the magnetic field was measured by using a Bruker ER 035M N M R gaussmeter. Calibration of g values was based on DPPH (g= 2.0036) and a 53Cr3+-dopedMgO single crystal (g = 1.9800). The scan was calibrated with a s5Mn2+doped MgO single crystal. The value of 86.7 G for the separation of the center two lines of the hyperfine sextet was used.
Results As described in the previous section, all membranes were fully neutralized with a mixture of Cuz+and ZnZ+. These cations have similar ionic radii and substitute each other in many single cry~ta1s.I~ The procedure we used enables control over the paramagnetic ion concentration and allows characterization of isolated Cu2+species at low Cu2+concentrations while maintaining fully equilibrated membranes. (12) Alonso-Amigo, M. G.; Schlick, S. (Polym. Prepr. Am. Chem. SOC., Diu.Polym. Chem.) 1986, 27, 337.
(13) Perfluorinated Ionomer Membranes; Eisenberg, A., Yeager, H.L., Eds.; American Chemical Society: Washington, DC, 1982. (14) Handbook of Chemistry and Physics; CRC: Cleveland, OH, 1968; p E-37. (15) Schlick, S.; Getz, D.; Silver, B. L. Chem. Phys. Lett. 1975, 31, 555.
V
J w I
SITE SITE
11 I
Figure 1. X-band ESR spectra of Cu2+in Nafion membranes at 28% relative humidity in the temperature range 77-294 K for 1 % molar concentration of Cu2+in the Cu2+-Zn2+mixture. The two sites detected are indicated in the “stick”diagram. Upward arrow shows the magnetic field corresponding to g,,, = 2.192.
ESR spectra of Cuz+are easier to interpret, because of narrower lines, at low temperatures. In this study spectra at 77 K are used to define the cation binding; the mobility of cations can be deduced by analysis of the spectra between 77 and 300 K. ESR spectra of Nafions containing 1 mol % Cu2+in the cation mixture at 28% relative humidity as a function of temperature, between 77 and 300 K, are shown in Figure 1. At the lower temperature two sets of parameters in the parallel orientation are detected and are indicated in the “stick” diagram. These two sets are assigned to Cu2+ in two sites which differ in their gll values and in the value All of the hyperfine coupling constant of the unpaired electron with the Cu nucleus ( I = 3/2). The approximate value of A, was determined by dividing the width of the perpendicular signal by 3; g, was measured by reading the field at which the perpendicular signal crossed the base line. In both sites g, = 2.084 and A , = 0.0020 cm-I. For site I, glll = 2.389, A I - 0.0154 cm-’, and gis2= 2.186. For site 11, gIl1I= 2.409, Alli120.0141 em-’, and g . , ” = 2.192. All parameters were deduced from spectra of membranes containing 1% Cu2+ to minimize the effect of dipolar broadening. The signals corresponding to the parallel orientations in sites I and I1 indicate an mI dependence on the line width, due to a distribution of glland Allvalues.I6 This results in different heights of the lines belonging to the parallel hyperfine quartet and in unequal spacings between the 1ines.I’ The most accurate values of gll and AI,in this case are usually deduced by computer simulation. We plan to analyze this effect in the future, based on ESR studies of Cu2+ enriched in the 63Cu isotope. As the temperature increases, the relative intensity of Cu2+in site I1 defined above decreases; at the same time a signal centered at a magnetic field corresponding to the isotropic ;value of site II appears, grows in intensity, and reaches maximum intensity at 294 K. The results presented in Figure 1 suggest that two sites for Cu2+ exist in Nafions, with *rigid limit” ESR parameters (16) Froncisz, W.; Hyde, J. S. J . Chem. Phys. 1980, 73, 3123. (17) Cannistraro, S.; Giugliarelli, G . Chem. Phys. 1985, 98, 115.
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6355
Cu2+Ions in Nafion Membranes A
DPPH
DPPH
-
I
200 G
Figure 2. X-band ESR spectra at 77 K of Cu2+in Nafion membranes at 40% relative humidity for the indicated percent molar concentrations of Cu2+in the Cu2+-Zn2+mixture. DPPH
Figure 4. X-band ESR spectra at 298 K of Cu2+in Nafion membranes for 1% molar concentration of Cu2+in the Cu2+-Zn2+mixture, in the relative humidity range 28-100%. Upward arrow points to the isotropic signal; downward arrow points to the extreme high field in the anisotropic spectra of isolated paramagnetic cations. Ams=
Figure 3. X-band ESR spectra of Cu2+in Nafion membranes for 10% molar concentration of Cuz+in the cation mixture at 298 K. (A) Samples were dried to lo4 Torr at 350 K for 10 h. (B) Typical spectra for samples at relative humidity of 50-801.
measured at 77 K. Above 77 K part of the Cu2+a t site I1 is able to tumble freely and average the ESR parameters, thus resulting in a signal centered at g = 2.192 which is the gh value of site 11. Some Cu2+observed in site I1 is still immobilized at 279 K. Cuz+ in site I remains immobilized throughout the temperature range investigated. ESR spectra of Cuz+ a t 77 K as a function of Cu2+ concentration in the cation mixture at 40% relative humidity are shown in Figure 2. It is very clear that with increasing Cu2+concentration the signal from site I1 becomes dominant. The effect of the water content on the spectra is presented in Figures 3 and 4. In Figure 3 we compare ESR spectra a t 298 K for samples containing 10 mol 5% Cuz+ in the cation mixture
I
2
DPpH
Figure 5. X-band ESR spectra at 77 K of Cu2+in Nafion membranes at 100%relative humidity. (A) 30 mol % Cu2+in the Cu2+-Zn2+mixture. The Am,= 2 transition was obtained at a higher gain (XlOOO) and a higher (XS) microwave power of 7.9 mW, compared with signals observed around 3000 G. (B) 100 mol % Cu2+. The Ams = 2 signal was obtained at a higher (X200) gain, compared with the signal observed around 3000 G.
maintained under controlled relative humidity with samples containing the same Cu2+ concentration but dried in vacuum at 350 K for about 10 h. In the heat-treated samples, Figure 3A, we observe an increase in the height of the signal corresponding to site I. A very important additional effect of the heat treatment is the disappearance of the isotropic line, which is shown in the spectra of Figure 1 at high temperatures, and the appearance of anisotropic signals corresponding to the parallel and perpendicular
6356 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 orientations of the paramagnetic species. The results presented in Figure 3 indicate that in heat-treated membranes Cu2+cations in both sites are immobilized even at 298 K. ESR spectra of neutralized membranes as a function of the relative humidity are shown in Figure 4. Because the results shown are completely reversible, the change in the spectra can be attributed to changes in the water content of the membranes. The decrease in the relative humidity is associated with a decrease in the intensity of the isotropic signal and with the appearance of signals corresponding to the gll and g, orientations. For Cu2+concentrations of 10 mol % and above in the cation mixture, signals from pairs are observed and identified by the appearance of the electron spin-forbidden Am, = 2 transition1* at a magnetic field of 1600 G. Spectra taken at 77 K are shown in Figure 5 for 30% and 100%Cu2+in the cation mixture at 100% relative humidity. In Nafions containing 30 mol % Cu2+,the signal from the half-field transition is weak and indicates several resolved lines, Figure SA. For 100%Cu2+the half-field transition is more intense but resolution is lost, most likely because of dipolar interaction with neighboring paramagnetic cations.
-
Discussion ESR is a very sensitive method for detection of species containing unpaired electron spins. The magnetic parameters measured in this study will be related to the structure of the paramagnetic species, to the number of ligands, and to the bonding parameters and spatial arrangement of the ligands around the central cation.lg This discussion consists of three parts. In (a) we will suggest the local structure of Cu2+based on the dependence of the spectra taken at 77 K on the relative humidity and on the Cu2+ concentration. In (b) the effect of the temperature on the spectra will be analyzed. Finally, in (c), we will attempt to deduce structural information on the Cu2+-Cu2+ pair. The significance of the results obtained in this study in view of the various models for ionic aggregation will also be discussed. ( a ) Local Structure of Cu2+in Nafons. The values of gll,g,, A,,,and A , measured for sites I and I1 are typical of Cu2+ligated to oxygen ligands in a distorted octahedral c o n f i g ~ r a t i o n . ’ The ~~~~ Nafions studied are expected to contain up to 15% by weight water.4%21,22 This water concentration corresponds to about 1 1 water molecules for each sulfonate group in the membrane. It is therefore reasonable to assume that a hydrated Cu complex Cu(H,O), will be formed at high relative humidity. The ESR parameters measured for site I1 are fully compatible with this assumption. Site 11 is thus assigned to the Cu(H20), complex. The values of gll and AI,for the CuOs group are very sensitive to the details of the binding scheme. The effects of the ligand charge on these values are described in detail in the PeisachBlumberg (PB) plots.” If negatively charged ligands replace some of the neutral ligands, these plots predict an increase in the value of All and a decrease in the value of gll. This is precisely the effect we observed, if the gll and All values for site I are compared to the corresponding values for site 11. The ESR parameters of site I strongly suggest, in view of the PB plots, a Cu complex with a total charge between +! and 0. The existence of C u 0 6 complexes with total charges of +1 and 0 in Nafions can be understood if we assume that Cu2+ is ligated to one or two SO3- groups attached to the polymer backbone, respectively. The PB plots show that ligation schemes with the same total charge have a range of ESR parameters. It is, therefore, difficult to decide whether in site I the cation is ligated to one or two SO< ligands. Complexes C U ( H ~ O ) ~ ( S O ~with ) ~ - , n, = 4 or 5 are thus in accord with the ESR parameters observed for site I. The population ratio of Cu2+ in the two sites is very sensitive to the water content of the membrane, as seen in Figure 4. ~
(18) Smith, T.D.; Pilbrow, J. R. Coord. Chem. Reu. 1974, 13, 173. (19) McGarvey, B. R. In Transition Metal Chemistry;Carlin, R. L., Ed.; Marcel Dekker: New York, 1966; Vol. 3. (20) Peisach, J.; Blumberg, W. E. Arch. Biochem. Biophys. 1974,165,691. (21) Vasquez, R.; Avalos, J.; Volino, F.; Pineri, M. J . Appl. Polym. Sci. 1983, 28, 1093. (22) Escombes, M.; Pineri, M. In ref 13, Chapter 2. p 1 1 .
Alonso-Amigo and Schlick Changes in the local geometry of Cu2+as a function of the degree of hydration have also been detected for Cu2+ in zeolites.23 These suggested structures for Cu2+ligation will now be compared with relevant recent results obtained for Nafions by other techniques. Nafion membranes equilibrated with Zn2+have been studied by the EXAFS technique^.^^ Evidence for the presence of Zn( H 2 0 ) 6species in fully hydrated Nafions has been obtained in these measurements. Ligation of Zn2+to SO3-groups has also been detected24but only in membranes dehydrated at 435 K. We have been able to detect both species in hydrated and heat-treated membranes, as shown in Figures 1-3. In addition we have been able to deduce, from the above figures, the conditions which determine the relative abundance of the two species. We have been able to make these deductions because we can study isolated paramagnetic species in membranes fully equilibrated with a mixture of paramagnetic Cu2+and diamagnetic Zn2+ ions. This method is not possible when using the EXAFS technique. Some support for the assignment of site I to C U ( H ~ O ) ~ ( S O ~ ) ~ is provided by an electron spin-echo study of Cu2+in cation-exchange resins containing sulfonic An important conclusion of this study is that of the sulfonated complexes of Cu2+ the most stable is the one containing two sulfonate groups. Results presented in Figure 2 indicate an increase in the relative intensity of site I1 with increasing Cu2+concentration. This result implies preferential solvation of the cation in the hydrophilic phase of the ionomers. This conclusion must be checked further by measuring ESR spectra of partially equilibrated membranes containing Cu2+ only, starting at very low cation concentrations. ESR spectra of Cu2+in various ionomers, including Nafions, have been studied at 77 K for various Cu2+concentrations and different water contents.21 The main conclusion of this study is that the local concentration of the cation is about 4 times larger than its average concentration. Not many details of Cu2+ligation have, however, been deduced. For example, this study2I has detected neither the presence of two sites nor that of the Cu2+-Cu2+ pair. ( 6 ) Temperature Dependence of the ESR Spectra. Spectra at temperatures above 77 K in Figure 1 indicate that the signal from the hydrated cation, site 11, becomes less intense as the temperature increases. At the same time a new, and isotropic, signal appears, centered at a field corresponding approximately to the value of gisofor site 11. The two signals, the “rigid limit” spectrum assigned to site I1 and the isotropic spectrum, must be assigned to the cation C U ( H ~ O )hydrated ~, by two types of water. We suggest that site I1 represents Cu2+ligated by “bound” water; the isotropic signal is due to Cu2+ligated by “mobile” water, which allows tumbling and averaging of the cation anisotropy. Additional support for the suggested binding of the cation of two types of water is provided in the ESR spectra of the heattreated samples, Figure 3. Two sites in which Cu2+is immobilized are detected even at high temperature, 298 K. Clearly absent is the isotropic signal which we assigned to ligation of Cu2+by mobile water, simply because this water is eliminated by the heat treatment. An important conclusion deduced from the results presented in Figures 1 and 3 is that the heat treatment at 350 K eliminates only the mobile water; the bound water remains. These conclusions are in accord with recent measurements of water sorption isotherms in Nafions, which also indicate two types of water.26 Of the total 15% by weight water in the membrane, about 7-8% is bound and cannot be removed by heating at 320 K, based on DSC and N M R measurements. Important evidence supporting the concept of two types of water in Nafions has been obtained in IR measurement^.^' Two peaks (23) Lee, H.; Narayana, M.; Kevan, L. J . Phys. Chem. 1985, 89, 2419. (24) Pan, H. K.; Knapp, G. S.; Cooper, S. L. Colloid Polym. Sci. 1984, 262, 734. (25) Ichikawa, T.;Miki, H.; Yoshida, H.; Kevan, L. J . Phys. Chem. 1985, 89, 1211. (26) Pineri, M.; Volino, F.; Escombes, M. J . Polym. Sci. Phys. Ed. 1985, 23, 2009. (27) Falk, M. In ref 13, Chapter 8, p 166.
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6357
Cu2+ Ions in Nafion Membranes
have been observed in the region of the OH fundamental stretching frequency. One peak, whose intensity and position does not change with increase of the water content of the membranes, is assigned interion distance in A, and v is the measuring microwave frequency to OH groups located in the vicinity of CF, groups; the hydrogens in GHz. Equation 1 has been extensively tested for a variety of in these water molecules do not participate in hydrogen bonding. dimeric species where the intercation distance has been known The second peak which increases in intensity with increasing water from crystallographic data, and excellent agreement has been content is due to OH groups whose hydrogens participate in obtained with this expression, in spite of its s i m p l i ~ i t y . ~ ~ , ~ ~ hydrogen bonding and whose properties are similar to those in From spectra shown in Figure 5, for Nafions fully equilibrated bulk water. with Cuz+, we obtain Zrel = 8 X and r = 5.5 A. From some N M R and mechanical measurements, it was concluded that above 240 K the bound water becomes m ~ b i l e . ~ ~ . ~ ~The value for ZIcl was obtained by assuming that the signal observed in the region g = -2 is entirely due to the pair. It is, We agree with the conclusion that two types of water exist in however, possible that signals from isolated ions as well as from Nafions and that only one type is removed by heat treatment at larger clusters might contribute to the intensity of this signal. If or below 350 K. Our results indicate, however, that the bound we assume that these species contribute 20% of the total line water remains this way even a t 298 K and the transition intensity and correct Zrel accordingly, we obtain r = 5 . 3 A. The bound-to-mobile water is slow on the ESR time scale at this value of 5.5 A for r is thus considered to be the upper limit of temperature. We must conclude that the rate of exchange between the interion distance. We plan to calculate Ire,as a function of the two types of water is in the range 104-107 s-l. Cu2+ concentration in order to obtain the separate contribution Changes in the mobility of Ti3+ in Nafions at 298 K as a of the pair to the isotropic line. These studies, together with an function of water content have also been detected recently30 in analysis of the splitting from the Ams = 2 transition as a function a study involving use of Nafion membranes as catalytic supports of Cu2+concentration, will indicate whether some or most pairs in complex redox processes, in accord with our results. are part of larger aggregates. We have some indications that the (c) Cu2+-Cu2+Pairs. Dimeric species are known to exist in value of 5.5 A for the interaction distances is a good approxia variety of ionomers.' MBssbauer experiments indicate a high mation. The value of r deduced above can be used to obtain the concentration of cationic dimers in some ionomers. For instance, zero-field splitting parameter D, assuming that D is due only to the amount of Fe3+in the form of dimers is 41% in carboxylic dipolar interaction, without contributions from anisotropic exNafions and 55% in sulfonated Nafions neutralized by iron salts.31 change36(eq 2). In eq 2 D is in ergs and r is in cm. For r = Cu2+dimers have been detected by ESR in butadienemethacrylic acid copolymers neutralized by copper salts,32but the nature of the dimeric bond and the intercation distance have not been elucidated. 5.5 A, gil = 2.409, and g, = 2.084, we obtain D = 4.12 X In this study the signals assigned to the pair are based on the ergs. The sharpest signals of the Ams = 1 transitions consist detection of the Ams = 2 transition at a magnetic field of 1600 usually of two lines separated by D , = D / g , @ . The separation G, Figure 5, which is typical of dimeric species. The assignment D , between this pair of lines of the Ams = 1 transition is 213 G, of the Am, = 1 transition of the pair is not easy because it is close to the value of the line width observed at 100% Cu2+conmasked by the ESR spectra of isolated paramagnetic cations and centration in the region of the Am, = 1 transition, Figure 5. The possibly by higher aggregates. Some broadening and loss of other lines of the Am, = 1 transition are separated by DiI = resolution is observed in the Am, = 2 transition when the con2D/g11b,or 369 G. These lines are usually not as prominent as centration of Cu2+ is increased from 30% to loo%, Figure 5, the other pair because they are split by a hyperfine coupling indicating the effect of the neighboring cations. Despite these constant related to the large AI,value of the isolated ions. difficulties some structural information on the Cuz+ pairs in For lower values of r, D is larger and the Ams = 1 transitions Nafions can be deduced. of the dimer are observed well beyond the range of signals detected It has been shown that the intercation distance can be deterfor isolated ions or for ionic aggregates. We very carefully looked mined by measuring the intensity ratio of the Am, = 2 and 1 for such signals in the entire range of magnetic fields 0-4000 G, transitions. In addition it was found that the shape of the Am, and no signals other than those presented in Figure 5 were de= 2 transition is determined only by the relative orientation of tected. We conclude that the interion distance is close to and not the g and nuclear hyperfine tensors of the two ions in the pair.33-34 much lower than 5.5 A. The shape of the Am, = 2 transition observed in this study is In a study of Cu2+in Y-type zeolites, an isotropic line observed very similar to that detected and simulated by c o m p ~ t e rfor ~ ~ , ~ ~near g = -2 has been assigned to the Ams = 1 transition of a Cu dimers in which the angle t between the symmetry axis of the Cu2+pair.37 The authors justified this assignment by assuming Cu2+ ions of the pair and the interspin vector is 41' or 45'. that the symmetry axes of the two ions ar at 110' to each other. Examination of spectra computed for the Am, = 2 transition in It was concluded that the g tensor of the dimer is isotropic in this many dimeric system^'*^^^-^^ indicates that the results are very case, leading to complete averaging of the anisotropy. We disagree sensitive to the value of e, and we can assume that in our system with this conclusion because an isotropic g tensor for the dimer e is in the range 40-45O. cannot be obtained for any orientation of the ions in the pair; the The interion distance in the pair can be calculated from the g, value of the isolated ions is always a principal g value of the ~ relative intensity Z,,between the Ams = 2 and 1 t r a n s i t i o n ~ ~ ~ . ~dimer. In some cases, when the interion distance is large, one (eq 1). In eq 1, A is a constant with a value of 21 f 2, r is the pair of lines in the Am, = 1 region are close and might coalesce, while the other pair is not easily detected. The dimer signals in the Am, = 1 region appear to be isotropic only because of lack (28) Duplessix, R.; Escoubes, M.; Rodmacq, B.; Volino, F.; Roche, E.; Eisenberg, A,; Pineri, M. In Water in Polymers; Rowland, S . P., Ed.; Amof resolution.
-
erican Chemical Society: Washington, DC, 1982; p 487. (29) Komoroski, K. A.; Mauritz, K. A. J . Am. Chem. Soc. 1978, 100, 7497. (30) Fan, F. R. F.; Liu, H. Y . ;Bard, A. J. J . Phys. Chem. 1985,89,4418. (31) Boyle, N. G.; Coey, J. M. D.; Meagher, A.; McBrierty, V. J.; Nakano, Y . ;MacKnight, W. J. Macromolecules 1984, 17, 1331. (32) Pineri, M.; Meyer, C.; Levelut, A. M.; Lambert, M. J . Polym. Sci., Polym. Phys. Ed. 1974, 12, 115. (33) Eaton, S . S.;More, K. M.; Sawant, B. M.; Eaton, G. R. J. Am. Chem. SOC.1983, 105, 6560. (34) Eaton, S . S.; Eaton, G. R.; Chang, C. K. J . Am. Chem. SOC.1985, 107, 3177. (35) Boyd, P. D. W.; Toy, A. D.; Smith, T. D.; Pilbrow, J. R. J . Chem. Soc., Dalton Trans. 1973, 1549.
Conclusions 1. In Nafions containing low concentrations of Cu2+in the Cu2+ and Zn2+ mixture, two sites for the paramagnetic cation were detected. These sites represent, respectively, ligation to water oxygens and to the polymer network through the sulfonic groups. The relative populations of Cuz+ in the two sites are very sensitive (36) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions;Clarendon: Oxford, 1970; p 508. (37) Chao, C. C.; Lunsford, J. H. J . Chem. Phys. 1974, 57, 2890.
J . Phys. Chem. 1986, 90, 6358-6362
6358
to variations in the water content of the membranes. 2. Cu2+ ligated to bound water is immobilizcd, on the time scale of the ESR experiment, even at 298 K; the complex which represents ligation of Cu2+to mobile water tumbles fast on the same time scale and averages the g and hyperfine anisotropies above 250 K. 3. Preferential solvation of Cu2+ in the hydrophilic phase is observed as the concentration of Cu2+ increases. 4. Heat treatment at 435 K for 10 h at Torr removes the mobile water but not the bound water in the Nafions studied. 5 . Cu2+-Cu2+pairs are observed when the Cu2+concentration in the cation mixture is 10% and above. The upper limit of the interion distance in the pair is 5.5 A.
Acknowledgment. This research was supported by the Research Corp. and by an N S F Grant DMR-8501362 for the purchase of
the Bruker ESR spectrometer. We thank Dr. Robin Hood of Wayne State University for the use of the Varian ESR spectrometer, where the initial spectra were measured. We are grateful to Dr. L. L. Burton of Du Pont for sending us the Nafions used in this study.
Note Added in Proof. As this study was being typed for publication, a quantitative method for determination of the absolute water content in Nafion-H membranes by proton NMR was published (Bunce, N . J.; Sondheimer, S. J.; Fyfe, C. A. Macromolecules 1986,19,333). This study is in agreement with our conclusions that Nafion properties have to be characterized for a known water content or for specified conditions of relative humidity. Registry No. CuS04, 7758-98-7; ZnS04, 7733-02-0; Nafion 117, 66796-30-3.
Dielectric Relaxations in the Liquid and Glassy States of Glucose and Its Water Mixtures R. K. Chan, Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
K. Pathmanathan, and G. P. Johari* Department of Materials Science & Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada (Received: March 24, 1986; In Final Form: July 7 , 1986)
The permittivity and loss of anhydrous glucose and its 10, 15, and 30 wt % water mixtures have been measured from 77 to 350 K in both the glassy and liquid states over a frequency range 1-105 Hz. Two relaxation regions were observed: one above and the second below T,. The T > Tgrelaxation follows the Vogel-Fulcher-Tamman equation and the T < T,, the Arrhenius equation with an activation energy of -60 kJ mol-’. These features are similar to those observed for amorphous polymers. Addition of water raises the strength of the sub-T, relaxation and lowers its rate. On dilution with water the relaxation regions approach each other for two reasons: (i) a decreased glass-transition temperature, and (ii) a decreased rate of the sub-T, relaxation. The increase in the strength of the subT, relaxation peak is attributed to an increased orientational correlation of dipole moments in the localized, high volume, high entropy regions of an otherwise rigid glassy matrix, where molecular reorientation continues to occur.
Introduction Vitrification by supercooling of aqueous solutions is currently receiving much attention for potential use in cryobiological apIn particular, a large number of aqueous solutions, mostly of inorganic solutes, have been vitrified, with varying degree of success, by cooling them at different rates3 Aqueous solutions of organic substances, on the other hand, seem to have been overlooked, despite the greater ease with which they vitrify and their undoubtedly greater relevance to cryobiology and other applications. One objective of this study was to determine the changes in the characteristics of glass transition of a hydrogenbonding organic substance of biological importance when water is added to it. Addition of a fluid solute usually lowers the glass transition temperature, Tg,of the substance. This effect is particularly well-known as ”plasticization”, in amorphous polymers: where the dissolution of a small amount of water or another organic solute in an organic polymer substantially lowers the T, of the polymer. The effect is interpreted on the basis of increased free volume and a weakening of the interchain interactions and it is assumed that interactions in such plasticized polymers are (1) Angell, C. A. Annu. Rev. Phys. Chem. 1983, 34, 593, and references therein. (2) Adrian, M.; Duoochet, J.; Lepault, J.; McDowell, A. W. Nature (London) 1984, 308, 42. Also in Trends Biol. Sci., in press. (3) Angell, C . A.; Sare, E. J. J . Chem. Phys. 1970, 52, 1058. (4) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1980.
0022-3654/86/2090-6358$01.50/0
predominantly intramolecular, or conformational? although the intermolecular interaction between the molecular segment and solvent molecules may be appreciable. The added substance also affects the strength of dielectrically observed secondary relaxations, or the number of molecules, or their segments, capable of undergoing thermally excited motions in the glassy state of the polymer. Recent studiesSv6have shown that neither the thermodynamic nor the relaxational characteristics of amorphous polymers differ from other glasses despite the differences in the strength of interactions and/or the nature of disorder among them. Therefore, it seems that “plasticization” of organic molecular glasses with water may affect their relaxation characteristic in a way similar to that in organic polymers. The second objective of this study was to investigate the effect of such addition on the localized molecular motions in the glassy state. We describe a calorimetric and dielectric relaxation study of the liquid and glassy states of anhydrous glucose, and of its mixtures with water and compare the results with those of pure alcohols and with amorphous polymers. Experimental Methods Analytical grade anhydrous D-(+)-glucose obtained from the British Drug House (Canada) Ltd. was studied without further treatment. The aqueous solutions were prepared by weighing. ( 5 ) Johari, G. P.Ann. N . Y.Acad. Sci. 1976, 279, 117. (6) Johari, G. P. J. Chem. Phys. 1985, 82, 283.
0 1986 American Chemical Society
