Ion Environments in Mn2+-Doped Polyelectrolyte Complexes: Dilute

Jul 1, 2016 - Ion Environments in Mn2+-Doped Polyelectrolyte Complexes: Dilute ... the paramagnetic manganese(II) and gadolinium(III) ions with anioni...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCB

Ion Environments in Mn2+-Doped Polyelectrolyte Complexes: Dilute Magnetic Saloplastics Nandita Abhyankar, Yara E. Ghoussoub, Qifeng Wang, Naresh S. Dalal, and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States S Supporting Information *

ABSTRACT: Amorphous hydrated complexes of the polyelectrolytes poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) were doped with the spin-5/2 ion Mn2+. X-band electron paramagnetic resonance (EPR) measurements of the Mn2+ spins within these stoichiometric polyelectrolyte complexes (PECs) revealed an octahedral coordination environment, similar to that observed in aqueous solutions of Mn2+. This octahedral symmetry of the [Mn(H2O)6]2+ complexes, observed in fully hydrated PECs, is somewhat distorted because of the wide range of ion pairs possible with the sulfonate group on PSS. As the Mn2+ concentration was increased, the linewidths broadened, indicating the dominance of dipolar broadening over exchange narrowing in determining the linewidths; that is, any exchange narrowing was masked by the large dipolar broadening. The calculated linewidths were used to estimate the strengths of the dipolar interactions, and hence the distances between the Mn2+ spins, on the basis of a simple model of regularly spaced spins. The distances calculated by this method were roughly comparable to the geometric average distances calculated on the basis of the Mn2+ concentrations and densities of the doped PEC samples. From a comparison of their EPR spectra, the ion environments in the doped, fully hydrated PECs were found to be similar to those in hydrated classical ion exchange resins. EPR spectra before and after drying of the PECs indicate the replacement of octahedrally coordinated water by oxide anions from the polyanion chain and the corresponding loss of the symmetric environment of Mn2+ ions.

■

INTRODUCTION Polyelectrolyte complexes (PECs) are amorphous blends of oppositely charged polymers. Because blending at the molecular level is driven by the release of counterions1 (and a defined number of water molecules2), PEC assembly proceeds rapidly when aqueous solutions of polycations and polyanions are mixed. The loss of counterions is consistent with the pairing of polycation Pol+ and polyanion Pol− repeat units, as shown in eq 1 Pol+Cl−aq + Pol−Na +aq → Pol+Pol−s + Na +aq + Cl−aq

Doping is accompanied by significant softening of the PECs (saloplasticity). For example nPol+Pol−s + Mnaq+ + n A −aq → nPol+A −s + Pol −n M ns + K dop =

(1 −

1+n y)n aMA

→

y1 + n 1+n aMA

as y → 0 (3)

where Kdop is a doping equilibrium constant, y is the fraction of Pol+Pol− ion pairs converted to Pol+A− and Pol−nMn+, and aMA is the solution activity of salt ions. Equation 2 represents a site-specific model in which salt ions are paired with polyelectrolyte repeat units. The equilibrium constants obtained for salt doping may also be explained by a partitioning mechanism that does not concern itself with the exact location of ions within the PEC: ions are either inside or outside the PEC phase under an equilibrium distribution. Using mean-field approximations of continuum electrostatics, the locations of counterions and (charged) polyelectrolyte repeat units are specified by pairwise Coulombic interactions among Pol+, Pol−, and salt ions.16 Two models for ion distribution are illustrated in Scheme 1.

(1)

Early calorimetric measurements yielded no enthalpic changes on complexation,1 supporting the ion release entropic mechanism. Initial studies on PECs, many by Michaels and coworkers,1,3,4 were almost contemporary with studies on ion exchange resins (IERs),5 which are made from crosslinked polyelectrolytes of one charge. It was known that PECs swell in the presence of salt,3 absorbing ions, and that the materials become softer and rubbery.6 Unlike those on classical ions exchangers, which are currently used in vast quantities, very few quantitative measurements were made on PECs for decades. Recently, we2,7 as well as others8−15 have explored the relationship between solution salt concentration and the ion content of PECs. Simple equilibrium expressions derived from eq 1 appear to describe the swelling or “doping” of PECs by salts well. © 2016 American Chemical Society

y1 + n

(2)

Received: March 15, 2016 Revised: June 22, 2016 Published: July 1, 2016 6771

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777

Article

The Journal of Physical Chemistry B

46104; Ondeo-Nalco), manganese (99.98%), p-toluene sulfonic acid monohydrate (Aldrich), Amberlite IRN-77 cation exchange resin, and MnCl2·4H2O (Fisher) were used as received. All solutions were prepared using deionized water (18 MΩ, Barnstead, E-pure). Manganese tosylate was synthesized following the reaction Mn + 2HOTs·H2O + 3H2O → [Mn(OH2)4](OTs)2·H2O + H2. Manganese powder and p-toluene sulfonic acid monohydrate were added to water, and the mixture was heated to reflux. After filtration of the solution, the white [Mn(OH2)4](OTs)2·H2O crystals obtained were dried under vacuum and stored under inert conditions. PEC. The PEC was prepared by simultaneously mixing stoichiometric amounts of 0.125 M PSSNa and PDADMAC solutions in 0.25 M NaCl, as described previously.21 The fully hydrated (i.e., wet with water), salt-free PEC was then extruded (using a Model LE-075 extruder from Custom Scientific Instruments) into fibers.22 Before use, the PEC was dried at 120 °C for ca. 24 h until it reached a constant weight. MnCl2 Doping of PECs. Dried PECs (1.5 g) were doped to equilibrium for 10 days in 15 mL MnCl2 solutions, with concentrations ranging from 0.03 to 0.80 M. The samples of PECs (15−30 mg) were then dropped into 15 mL of water, and the concentration of released MnCl2 was determined by conductivity. The salt-free PECs were then dried and weighed. Mn2+ contents were thus obtained at each salt concentration. EPR. EPR spectra were acquired on a Bruker Elexsys E500 EPR spectrometer with an X-band cavity (9.5 GHz) in a field range of 0.005−1.4 T. PEC fibers of various dopant concentrations were cut into pieces ∼2 mm in length. The EPR spectra of all samples, initially stored in an inert N2 environment, were recorded at room temperature, after full saturation with moisture. To ensure full hydration, the samples were left for 20 days in a sealed EPR tube with a wet Kimwipe inserted at the top of the tube. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) was used as an internal standard to calibrate line positions. Solution spectra were obtained using Mn tosylate. EPR spectra were simulated using the EasySpin toolbox in Matlab. The following parameters were varied to obtain the best fit to the experimental lineshape: giso, Aiso (isotropic hyperfine coupling constant), D (axial zero-field splitting parameter), D-strain, and the pseudo-Voigt linewidth. giso, Aiso, and the pseudo-Voigt linewidth were modulated to obtain the best fit to the hyperfine sextet. On the other hand, D and Dstrain were modulated to obtain the best fit to the “wings” of the spectra, as these parameters do not affect the central part of the spectrum. Pseudo-Voigt lineshapes were used because the experimental lineshapes could not be reproduced by either purely Gaussian or Lorentzian lineshapes. Resin Doping. The cation exchange resin was first soaked in 0.1 M NaCl for 24 h and then in 0.1 M MnCl 2 (“concentrated” resin) or 1% 0.1 M MnCl2/99% 0.1 M NaCl (“dilute” resin) for 48 h. Doped resins were allowed to dry under ambient conditions before acquisition of EPR spectra. Superconducting Quantum Interference Device (SQUID). Magnetic measurements were made on a Quantum Design Magnetic Property Measurement System in an applied field of 5000 G and at temperatures between 200 and 300 K.

Scheme 1. Illustration of Doping of PECs and Ions (a) Breaking Specific Ion Pair Interactions between Charged Polymer Units and (b) Partitioned into the PEC with no Specific Location

At first, Michaels subscribed to the site-specific ion pairing description of PECs,4 as did some of his contemporaries working on classical ion exchangers,17 but then, he transitioned to the emerging electrostatics vocabulary describing ion/ polyelectrolyte interactions.3 The question of whether ions are specifically paired or randomly distributed is not just semantics: the ion content controls all physical properties of a hydrated PEC, such as its modulus and permeability. If Pol+Pol− is viewed as an ion pair representing physical crosslinks, the modulus of the PEC may be described using classical theories of rubber elasticity.18 In addition, the relative hydration level of each ion pair predicts the “strength” of interactions.2 Ion hopping between specific sites describes the dependence of ion transport on the doping level.19 In the present work, we attempt to answer the following question: In PEC doping with a salt, where do the ions go? To do so, we exploit the property that the paramagnetic response of a transition metal ion depends acutely on its short-range environment. We use electron paramagnetic resonance (EPR) spectroscopy to demonstrate that ions in a PEC occupy welldefined locations. In a related previous work, Smirnov and Sen have reported an EPR study of the distance distribution of Gd3+ ions doped into silicate glasses.20 They focused on Gd3+ because it is an Sstate ion (S = 7/2) with little orbital contribution to its magnetic behavior, leading to a relatively simple EPR spectrum, especially at high magnetic fields. In their case, the X-band EPR spectra at a low magnetic field of ca. 0.34 T were broad and complex and could not be utilized for distance estimation. On the other hand, the spectra at the Q-band (ca. 1.2 T, 34 GHz) and W-band (3.4 T, 95 GHz) consisted of single lines whose lineshapes and widths provided a sensitive spin probe for distance estimation using detailed and rigorous lineshape analysis. The probe used in our case, Mn2+, is also an S-state (3d) ion, with electron spin S = 5/2. Unlike the case of Gd3+, Q-band measurements did not provide any significant improvement over X-band measurements; thus, only the X-band study is reported here.

■

■

EXPERIMENTAL SECTION Materials. Poly(4-styrenesulfonic acid, sodium salt), poly(styrene sulfonate) (PSS) (molar mass ca. 200 000 g mol−1, VERSA TL130; AkzoNobel), poly(diallyldimethylammonium chloride) (PDADMAC) (molar mass ca. 400 000 g mol−1, SD

RESULTS AND DISCUSSION The starting materials were dense complexes of poly(diallyldimethylammonium) (PDADMA) and PSS, prepared by “saloplastic” extrusion of solution-precipitated PECs, 6772

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777

Article

The Journal of Physical Chemistry B plasticized by salt water into rods of about 1 mm in diameter.23,24 Previous studies have shown these materials to be stoichiometric and, when rinsed with water, free of counterions.23,24 All studies were performed on fully hydrated (by equilibration with a water-saturated atmosphere) materials, as this is the composition in equilibrium with the salt solution and on which most properties are measured. A stoichiometric PEC immersed in a solution of salt is doped according to eq 2. For a specific pair of positive and negative polyelectrolytes, the doping efficiency of a salt is determined by its place in the Hofmeister series: more hydrophobic ions dope the PEC more extensively.3,8,23 In the present case, MnCl2 was employed as a salt to dope PSS/PDADMA. The doping levels were measured as a function of salt concentration, using conductivity to determine the amount of salt released from the doped PEC following immersion in water.23 Figure 1 shows the equilibrium MnCl2 content of PSS/ PDADMA when immersed in aqueous MnCl2. The doping

Figure 2. X-band spectra for fully hydrated PEC fibers doped with increasing concentrations of MnCl2. The numbers are molar ratios of MnCl2 to the PEC repeat unit. The sharp features near 3500 G are from the standard (DPPH, g = 2.0036).

were stable, with no indication of a change in the number or type of spin over the course of several months. For comparison, spectra were also acquired on aqueous Mn2+ and dilute and concentrated Mn2+-doped IERs. The spectrum for a solution of Mn2+ in water25 is shown in Figure 3a. Figure 3b shows the

Figure 1. Loading of MnCl2 as a function of aqueous [MnCl2] for a PSS/PDADMA PEC at room temperature.

level, y, is given as twice the molar ratio of MnCl2 to polyelectrolyte repeat units. In other words, for y = 0.5, there is 0.25 Mn2+ for each R−SO3− (from PSS) because each Mn2+ nominally occupies two sulfonates. The doping level provides no information on the locations of the ions in the PEC. In classical, fixed-site IERs, y = 1.0 indicates that all sites on the resin are occupied by Mn2+, which is the maximum loading of Mn2+. In contrast, for the PEC in Figure 1, there can be more positive charges from Mn2+ than there are R−SO3− groups (MnCl2/PEC > 0.5). As observed previously for monovalent salts, doping is proportional to [salt] at low concentrations and deviates from linearity at about y > 0.4.21 This deviation is thought to result from additional neutral salt, which does not balance polymer charge.21 The entry of excess salt into ion exchangers is well known.5 Using Figure 1, the Kd for MnCl2 is 0.44. A series of samples with defined y was tested using EPR spectroscopy to probe the environment of the Mn2+ ions. The paramagnetic signal from Mn2+ (spin 5/2) is highly sensitive to the local environment. Figure 2 shows X-band EPR spectra for a series of MnCl2doped PEC fibers with increasing concentrations of MnCl2. A small amount of DPPH was added as an internal standard. EPR spectra of these fully hydrated paramagnetic saloplastics showed a Mn2+ signal at g = 2.001 ± 0.0005. The more dilute samples displayed a hyperfine structure (six lines) typical of Mn2+ in an octahedrally coordinated environment. The spectra

Figure 3. X-band EPR spectra of Mn2+ (manganese II tosylate) in water (a); and dilute (b) and concentrated (c) Mn2+ in an IER. The black lines show the experimental spectra, whereas the red lines show the simulated spectra. Spectra were acquired at room temperature under ambient conditions. The sharp line is from DPPH.

spectrum for a crosslinked styrene sulfonate IER26 containing a small amount of Mn2+ (the rest of the SO3− groups are balanced with Na+ ions). Using a coefficient of 6 for the resin selectivity of Mn2+ ions over Na+ ions in dilute Mn2+ solutions,27 the concentration of Mn2+ in the exchanger was estimated to be about 0.06 Mn2+/SO3−. Figure 3c shows the spectrum for the same resin fully loaded with Mn2+ (0.5 Mn2+/ SO3−). Comparing the X-band spectra of dilute Mn2+ in the PEC, in water and in the IER, significant information can be gleaned from analysis of the lineshapes. In particular, the following discussion covers three main aspects characterizing the Mn2+ spins in these samples: the oxidation state, the local environment, and the distribution of Mn2+ ions within the PEC. At low concentrations, the spectra show the typical six lines caused by hyperfine splitting in a dilute spin system, a result of the interaction of the Mn2+ electronic spin, S = 5/2, with the 6773

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777

Article

The Journal of Physical Chemistry B

are further broadened by a large distribution of D-values, resulting in a large D-strain (discussed below). In Table 1, the D-value of 0 indicates that the D-values are symmetrically distributed over a broad range indicated by the D-strain, resulting in an average value of 0. A striking observation from Figure 1 is that there is no apparent exchange narrowing with an increasing concentration of Mn2+. Thus, a hyperfine structure is visible even at [MnCl2]/ [PEC] ratios of up to 0.3. The loss of hyperfine structure is predominantly attributed to dipolar broadening because the separation between the first and last lines of the −1/2 → +1/2 hyperfine sextet remains unchanged with increasing concentration. However, it is important to note that exchangenarrowing effects may be present but are masked by the large dipolar broadening of the lines. Similar lineshapes and concentration dependence are seen in the EPR spectra for IERs (see Figure 3b,c). Dipolar broadening indicates that Mn2+ ions are bound by site-specific interactions with the anionic polymer, which forms a part of the outer coordination sphere. The environments of Mn2+ in the PEC and resin (Table 1) appear to be similar. Interpretation of EPR Spin Hamiltonian Parameters. Table 2 summarizes the spin Hamiltonian parameters used to simulate lineshapes for increasing [MnCl2]/[PEC] ratios in a fully hydrated PEC (see Figure S1 for fits). As mentioned earlier, the value of the isotropic hyperfine coupling constant,28,29 Aiso, is consistent with an octahedral environment provided by six coordinated water molecules ([Mn(H2O)6]2+). In solution, the motions of [Mn(H2O)6]2+ ions average out anisotropic interactions, including zero-field splitting. In doped PECs, the Mn2+ ions are bound to the PEC matrix, indicated by line broadening caused by dipolar interactions. The broad wings on either side of the hyperfine sextet indicate zero-field splitting, where the outer (±5/2 → ±3/2 and ±3/2 → ±1/2) transitions are broadened because of a distribution of D-values, or D-strain. The values of D are consistent with those obtained for hexaaqua Mn(II) complexes. However, the large D-strain suggests a range of environments, which may be attributed to variations in the outer coordination sphere of the [Mn(H2O)6]2+ ion. It has been shown that the geometry of the coordination octahedron has a significant effect on the value of D.30 The geometry of the dopant [Mn(H2O)6]2+ octahedron may be distorted in several ways, depending on its position in the PEC matrix: a couple of possibilities are illustrated in Scheme 2. We conclude that the D-strain originates from the range of outer coordination environments provided by the local structure of the PEC. This is consistent with discrete [Mn(H2O)6]2+ octahedra, with no through-bond exchange coupling pathways available between them. The increase in line broadening with increasing spin concentration is usually due to dipolar interactions between the paramagnetic ions.31 Dipolar broadening is a through-space effect whose magnitude depends on r−3, where r is the separation between the magnetic dipoles, the Mn2+ ions in this case. The dipolar broadening and D-strain indicate that the [Mn(H2O)6]2+ octahedra are bound to the PEC matrix, likely by ion paring with the sulfonate anionic groups. Therefore, the spatial distribution of Mn2+ ions within the PEC can be used to deduce the distribution of sulfonate groups within the PEC. The formula for calculating dipolar broadening or the dipolar field for an ensemble of identical, isotropically distributed spins in the limit of slow exchange, based on Van

Mn nuclear spin, I = 5/2. The spectra were interpreted using the standard spin Hamiltonian formalism shown below Ĥ = (gβH ·S) + (A S ·I ) + (S ·D·S)

(4) −24

−1

where β is the Bohr magneton (9.27 × 10 J T ), H is the magnetic field, g is the isotropic Landé factor, A is the isotropic hyperfine interaction parameter, and D is the axial zero-field splitting parameter. The cavity tuning indicates a lossy, highly dielectric environment. The values of isotropic g and A obtained from the simulations (see Table 1) reflect a symmetric, octahedral Table 1. Spin Hamiltonian Parameters Derived from Fitting the EPR Spectra of Mn2+ in a PEC, IER, and Aqueous Solution PEC g A (MHz) D (MHz) D-strain (MHz)

2.001 270 0 400

± ± ± ±

0.0005 10 200 100

resin

Mn2+ solution

± ± ± ±

2.0005 ± 0.0005 260 ± 10 0 0

2.001 260 0 600

0.0005 30 300 100

coordination environment for the Mn2+ spin, typical of a hexaaquo complex. This indicates that the Mn2+ ions in the samples corresponding to Figure 2 are in their fully hydrated, octahedrally coordinated state. It is important to note that drying of the PEC may result in loss of water from the octahedral coordination sphere of Mn2+ and, further, its replacement by an anion from the polyanion chain in the PEC. Thus, Mn2+ ions, which are directly complexed to the polyanion in the PEC, are likely to be undetectable, as their spectra are broadened because of longer correlation times and heterogeneity of the system. This is seen by the dramatic decrease in the intensity of the spectra upon drying (see Figure 4). However, the samples reported in Figure 2 were ensured to be fully hydrated by storing in a sealed, humid environment for 20 days or more before measurement (see the Experimental section for further details). The EPR lineshape showed no further change after this period, indicating complete saturation with water. The broad unresolved wings on either side of the hyperfine sextet are due to zero-field splitting, and these wings

Figure 4. EPR spectra for fully hydrated (blue line) and fully dried (green line) samples of MnCl2-doped PECs with [MnCl2]/[PEC] = 0.05. Spectral intensities have been standardized to an internal DPPH standard, to account for the effect of water on cavity Q-factor. 6774

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777

Article

The Journal of Physical Chemistry B

Table 2. Pseudo-Voigt Linewidths (HWHMa, After Correction for Intrinsic Line Broadening) and Mn2+ Separation Distances Calculated on the Basis of the Dipolar Interactions and Concentrations of Fully Hydrated PECs Doped with MnCl2b [MnCl2]/[PEC]

residual HWHM (G)

0.05 0.10 0.20 0.30 0.50

8.5 28.5 34 36.5 51

a

estimated separation from dipolar broadening using S = 5/2 (nm) 2.44 1.63 1.54 1.50 1.34

± ± ± ± ±

estimated separation from dipolar broadening using S = 1/2 (nm)

0.12 0.08 0.08 0.08 0.07

1.62 1.08 1.02 1.00 0.89

± ± ± ± ±

0.08 0.05 0.05 0.05 0.04

estimated separation from concentrations (nm) 2.03 1.62 1.30 1.15 1.0

± ± ± ± ±

0.2 0.2 0.1 0.1 0.1

HWHM, half width at half-maximum. bFor all samples, g = 2.001 ± 0.0005, D = 0 ± 200 MHz, D-strain = 400 ± 100 MHz, Aiso = 270 ± 100 MHz.

Scheme 2. Possible Environments for Mn2+ Within Hydrated PDADMA/PSS PEC, Consistent with EPR Lineshapesa

a

Water occupies the octahedral sites but the field is strained by ion pairing with sulfonates.

polyanion. Realistically, it is likely that the separation distances between Mn2+ ions are distributed over a range. Therefore, the picture of isotropically distributed ions with a single separation distance is a simplified picture. Further experimental evidence through complementary techniques such as SQUID magnetometry is required to draw a more accurate picture of the spatial distribution of Mn2+ ions in PECs. On the basis of broadening of the outer transitions (resulting in broad wings), the possibility that Mn2+ effectively acts as a spin-1/2 system owing to the short lifetimes of the higher-spin states must be considered. Thus, distances were also calculated using an effective value of S = 1/2 in eq 5, as shown in Table 2. It can be seen that the distances calculated using S = 5/2 show a better agreement overall with the distances obtained from the concentrations and conductivity data. The central role of water in maintaining the six-coordinate environment for Mn2+ is further illustrated by the EPR spectra of dried Mn2+-doped PECs. It was observed that spectral intensity diminished considerably on drying, as seen in Figure 4. These changes were not due to changes in Mn oxidation, as SQUID measurements showed that the spin concentration remained constant whether or not the samples were hydrated (Figure S3). In addition to the loss of octahedral coordination due to the loss of water, it is likely that the spins relax too fast to be observed by EPR. More rigid samples are expected to have a shorter spin−lattice as well as spin−spin relaxation time,

Vleck’s formula for the second moment of frequency deviation,32 was provided by Anderson and Weiss (eq 4).33 ⎡ 5.1g 2β 2S(S + 1) ⎤1/6 ⎥ d=⎢ HP2 ⎦ ⎣

(5)

Here, d is the mean separation distance between the spins, Hp is the dipolar field given by the HWHM. Pseudo-Voigt linewidths (HWHM) determined from the simulations were used for this purpose, after correcting for intrinsic linewidth by extrapolation of the plot of linewidth versus concentration to a concentration value of zero. Table 2 shows a comparison of mean Mn2+ separation distances calculated (1) on the basis of simulated linewidths of dipolar broadened lines, using eq 5, and (2) from known Mn2+ dopant concentrations (Figure 1), assuming evenly spaced Mn2+ ions on a cubic lattice and a PEC density of 1.1 g cm−2. The error margins for (1) are calculated using propagation of errors, assuming a maximum of 50% error in the simulated linewidth. This results in a maximum of ∼15% error in the separation distances. As seen from Table 2, the interionic separation distances calculated on the basis of simulated dipolar broadening are in broad agreement with those expected for a homogeneous distribution of Mn2+ ions in the PEC. Thus, there is no evidence of spatial clustering of sulfonate groups, such as the clustering reported for ionomers34,35 and Nafion.36 However, it must be noted that the locations of the Mn2+ ions are determined by the local structure of the amorphous 6775

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777

Article

The Journal of Physical Chemistry B

(6) Yano, O.; Wada, Y. Effect of Sorbed Water on Dielectric and Mechanical Properties of Polyion Complex. J. Appl. Polym. Sci. 1980, 25, 1723−1735. (7) Farhat, T. R.; Schlenoff, J. B. Ion Transport and Equilibria in Polyelectrolyte Multilayers. Langmuir 2001, 17, 1184−1192. (8) Salomäki, M.; Tervasmäki, P.; Areva, S.; Kankare, J. The Hofmeister Anion Effect and the Growth of Polyelectrolyte Multilayers. Langmuir 2004, 20, 3679−3683. (9) Xu, L.; Pristinski, D.; Zhuk, A.; Stoddart, C.; Ankner, J. F.; Sukhishvili, S. A. Linear versus Exponential Growth of Weak Polyelectrolyte Multilayers: Correlation with Polyelectrolyte Complexes. Macromolecules 2012, 45, 3892−3901. (10) Zan, X. J.; Hoagland, D. A.; Wang, T.; Su, Z. H. Ion Dispositions in Polyelectrolyte Multilayer Films. Macromolecules 2012, 45, 8805−8812. (11) Antila, H. S.; Sammalkorpi, M. Polyelectrolyte Decomplexation via Addition of Salt: Charge Correlation Driven Zipper. J. Phys. Chem. B 2014, 118, 3226−3234. (12) Zhang, B.; Hoagland, D. A.; Su, Z. Ionic Liquids as Plasticizers for Polyelectrolyte Complexes. J. Phys. Chem. B 2015, 119, 3603− 3607. (13) Wang, X.-S.; Ji, Y.-L.; Zheng, P.-Y.; An, Q.-F.; Zhao, Q.; Lee, K.R.; Qian, J.-W.; Gao, C.-J. Engineering Novel Polyelectrolyte Complex Membranes with Improved Mechanical Properties and Separation Performance. J. Mater. Chem. A 2015, 3, 7296−7303. (14) Zhang, Y.; Yildirim, E.; Antila, H. S.; Valenzuela, L. D.; Sammalkorpi, M.; Lutkenhaus, J. L. The Influence of Ionic Strength and Mixing Ratio on the Colloidal Stability of PDAC/PSS Polyelectrolyte Complexes. Soft Matter 2015, 11, 7392−7401. (15) White, N.; Misovich, M.; Yaroshchuk, A.; Bruening, M. L. Coating of Nafion Membranes with Polyelectrolyte Multilayers to Achieve High Monovalent/Divalent Cation Electrodialysis Selectivities. ACS Appl. Mater. Interfaces 2015, 7, 6620−6628. (16) Ou, Z. Y.; Muthukumar, M. Entropy and Enthalpy of Polyelectrolyte Complexation: Langevin Dynamics Simulations. J. Chem. Phys. 2006, 124, 154902. (17) Strauss, U. P.; Leung, Y. P. Volume Changes as a Criterion for Site Binding of Counterions by Polyelectrolytes. J. Am. Chem. Soc. 1965, 87, 1476−1480. (18) Jaber, J. A.; Schlenoff, J. B. Mechanical Properties of Reversibly Cross-linked Ultrathin Polyelectrolyte Complexes. J. Am. Chem. Soc. 2006, 128, 2940−2947. (19) Farhat, T. R.; Schlenoff, J. B. Doping-Controlled Ion Diffusion in Polyelectrolyte Multilayers: Mass Transport in Reluctant Exchangers. J. Am. Chem. Soc. 2003, 125, 4627−4636. (20) Smirnov, A. I.; Sen, S. High Field Electron Paramagnetic Resonance of Gd3+-doped Glasses: Line Shapes and Average Ion Distances in Silicates. J. Chem. Phys. 2001, 115, 7650−7656. (21) Wang, Q. F.; Schlenoff, J. B. The Polyelectrolyte Complex/ Coacervate Continuum. Macromolecules 2014, 47, 3108−3116. (22) Wang, Q. F.; Schlenoff, J. B. Tough Strained Fibers of a Polyelectrolyte Complex: Pretensioned Polymers. RSC Adv. 2014, 4, 46675−46679. (23) Ghostine, R. A.; Shamoun, R. F.; Schlenoff, J. B. Doping and Diffusion in an Extruded Saloplastic Polyelectrolyte Complex. Macromolecules 2013, 46, 4089−4094. (24) Shamoun, R. F.; Reisch, A.; Schlenoff, J. B. Extruded Saloplastic Polyelectrolyte Complexes. Adv. Funct. Mater. 2012, 22, 1923−1931. (25) Mcgarvey, B. R. Line Widths in the Paramagnetic Resonance of Transition Ions in Solution. J. Phys. Chem. 1957, 61, 1232−1237. (26) Wiley, R. H.; Jin, J. I.; Reich, E. EPR Spectra of Manganese(II) on Sulfonated Polystyrene Resins Cross-Linked with Pure Divinylbenzene Isomers. J. Macromol. Sci., Part A 1970, 4, 341−348. (27) Bajpai, R. K.; Gupta, A. K.; Rao, M. G. Binary and Ternary IonExchange EquilibriaSodium-Cesium-Manganese-Dowex 50w-X8 and Cesium-Manganese-Strontium-Dowex 50w-X8 Systems. J. Phys. Chem. 1973, 77, 1288−1293.

in agreement with the sharper peaks observed for less rigid samples (as in a solution).

■

CONCLUSIONS This study was undertaken to probe the local environment of a metal ion in an amorphous PEC. Mn2+ was chosen as a dopant because it is a paramagnetic ion with its five valence electrons in a high-spin, S = 5/2, configuration. In addition, 55Mn has a high nuclear spin, I = 5/2, providing a sharp spectroscopic fingerprint of six hyperfine peaks in its EPR spectra. Using EPR measurements on precisely doped samples, we show that EPR spectroscopy can provide detailed information on the oxidation state, local geometry, and positioning (intermolecular distances) of the doped Mn ions. The data support the view that Mn ions dope as Mn2+ and locate in well-defined positions, not as clustered polyions or randomly fluctuating ion dopants. Water molecules sorbed into the hydrophilic complex provide an octahedral coordination for the ion environment, similar to that in classical IERs.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b02697. • Fitting of EPR spectra and SQUID magnetometry of dry and hydrated Mn2+-doped PECs (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +1 (850)-644-3001. Fax: +1 (850)-644-8281. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by grants DMR-1207188 and DMR1506824 from the National Science Foundation. Notes

The authors declare no competing financial interest.

■

ABBREVIATIONS PEC, polyelectrolyte complex; PSS, poly(styrene sulfonate); PDADMA, poly(diallyldimethylammonium)

■

REFERENCES

(1) Michaels, A. S. Polyelectrolyte Complexes. Ind. Eng. Chem. 1965, 57, 32−40. (2) Schlenoff, J. B.; Rmaile, A. H.; Bucur, C. B. Hydration Contributions to Association in Polyelectrolyte Multilayers and Complexes: Visualizing Hydrophobicity. J. Am. Chem. Soc. 2008, 130, 13589−13597. (3) Bixler, H. J.; Michaels, A. S. Polyelectrolyte Complexes. In Encyclopedia of Polymer Science and Technology; Mark, H.F., Gaylord, N.G., Bikales, N.M., Eds.; Interscience: New York, 1969; Vol. 10, pp 765−780. (4) Michaels, A. S.; Miekka, R. G. Polycation-Polyanion Complexes: Preparation and Properties of Poly(vinylbenzyltrimethylammonium Syrenesulfonate). J. Phys. Chem. 1961, 65, 1765−1773. (5) Helfferich, F. G. Ion Exchange; McGraw-Hill: New York, 1962; p 624. 6776

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777

Article

The Journal of Physical Chemistry B (28) Kripal, R.; Shukla, A. K. EPR Studies of Mn2+-doped Diammonium Hexaaqua Magnesium(II) Sulfate. Z. Naturforsch., A 2006, 61, 683−687. (29) Misra, S. K.; Diehl, S.; Tipikin, D.; Freed, J. H. A Multifrequency EPR Study of Fe2+ and Mn2+ Ions in a ZnSiF6·6H2O Single Crystal at Liquid-Helium Temperatures. J. Magn. Reson. 2010, 205, 14−22. (30) Duboc, C.; Phoeung, T.; Zein, S.; Pecaut, J.; Collomb, M. N.; Neese, F. Origin of the Zero-Field Splitting in Mononuclear Octahedral Dihalide Mn-II Complexes: An Investigation by Multifrequency High-field Electron Paramagnetic Resonance and Density Functional Theory. Inorg. Chem. 2007, 46, 4905−4916. (31) Weil, J. A.; Bolton, J. R. Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, 2nd ed.; WileyInterscience: Hoboken, NJ, 2007; p xxiii, 664 pp. (32) van Vleck, J. H. The Dipolar Broadening of Magnetic Resonance Lines in Crystals. Phys. Rev. 1948, 74, 1168−1183. (33) Anderson, P. W.; Weiss, P. R. Exchange Narrowing in Paramagnetic Resonance. Rev. Mod. Phys. 1953, 25, 269−276. (34) Eisenberg, A.; Navratil, M. Ion Clustering and Viscoelastic Relaxation in Styrene-Based Ionomers. IV. X-Ray and Dynamic Mechanical Studies. Macromolecules 1974, 7, 90−94. (35) Castagna, A. M.; Wang, W.; Winey, K. I.; Runt, J. Structure and Dynamics of Zinc-Neutralized Sulfonated Polystyrene Ionomers. Macromolecules 2011, 44, 2791−2798. (36) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104, 4535−4586.

6777

DOI: 10.1021/acs.jpcb.6b02697 J. Phys. Chem. B 2016, 120, 6771−6777