Article pubs.acs.org/Macromolecules
Hierarchical Canopy Dynamics of Electrolyte-Doped Nanoscale Ionic Materials Michael L. Jespersen,†,‡ Peter A. Mirau,*,† Ernst D. von Meerwall,§ Hilmar Koerner,† Richard A. Vaia,† Nikhil J. Fernandes,∥,⊥ and Emmanuel P. Giannelis∥ †
Materials and Manufacturing Directorate, Air Force Research Laboratory, 2941 Hobson Way, Wright-Patterson Air Force Base, Ohio 45433, United States ‡ University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469, United States § Department of Physics, University of Akron, Akron, Ohio 44325, United States ∥ Department of Materials Science and Engineering and ⊥School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States S Supporting Information *
ABSTRACT: Nanoscale ionic materials (NIMs) are organic−inorganic hybrids prepared from ionically functionalized nanoparticles (NP) neutralized by oligomeric polymer counterions. NIMs are designed to behave as liquids under ambient conditions in the absence of solvent and have no volatile organic content, making them useful for a number of applications. We have used nuclear magnetic resonance relaxation and pulsed-field gradient NMR to probe local and collective canopy dynamics in NIMs based on 18-nm silica NPs with a covalently bound anionic corona, neutralized by amine-terminated ethylene oxide/propylene oxide block copolymers. The NMR relaxation studies show that the nanosecond-scale canopy dynamics depend on the degree of neutralization, the canopy radius of gyration, and crowding at the ionically modified NP surface. Two canopy populations are observed in the diffusion experiments, demonstrating that one fraction of the canopy is bound to the NP surface on the time scale (milliseconds) of the diffusion experiment and is surrounded by a more mobile layer of canopy that is unable to access the surface due to molecular crowding. The introduction of electrolyte ions (Na+ or Mg2+) screens the canopy−corona electrostatic interactions, resulting in a reduced bulk viscosity and faster canopy exchange. The magnitude of the screening effect depends upon ion concentration and valence, providing a simple route for tuning the macroscopic properties of NIMs.
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INTRODUCTION Nanoscale ionic materials (NIMs) have received a great deal of recent attention as liquid-like materials containing a high volume fraction of functional nanoparticles (NPs). NIMs are organic−inorganic hybrids containing a core nanoparticle functionalized with a covalently attached corona bearing ionic terminal functional groups, which are neutralized by a canopy of oligomeric counterions (Figure 1). Since the initial reports of polyoxometalate-based liquid salts,1 this class of materials has grown to include NIMs based on nanospheres of metal oxides,2−5 metals,6,7 quantum dots,8 nanorods,9,10 carbon nanotubes,11−13 and more recently, rhombohedral nanoparticles,14 fullerols,15 and proteins.16 NIMs are liquids at or near room temperature in the absence of solvent, enabling unprecedented combinations of liquid properties with the sizedependent properties of the core nanoparticles (e.g., solventless plasmonic fluids9). These materials are of interest for a wide range of commercial applications, including use as lubricants,6,17−19 magnetic fluids,20−22 luminescent materials,8,23,24 and battery electrolytes.25,26 However, the relationship between NIMs structure, the local and collective molecular dynamics, © 2013 American Chemical Society
and the macroscopic properties is poorly understood, inhibiting the rapid optimization of NIMs design and widespread application. NIMs with a wide variety of properties can be produced by changing the NP, corona, or canopy. The physical properties of NIMs depend on the canopy molecular weight and degree of neutralization, and span the range from “soft powders” to waxy solids and viscous liquids (Figure 2a). Furthermore, we have observed that added salts can impact the conductivity18 and fluidity of NIMs (Figure 2b). The structure and dynamics of NIMs have been studied theoretically by molecular dynamics simulations27 and experimentally by NMR, dielectric spectroscopy, rheology, and other methods,12,20−22,28−30 but the effects of structural and compositional variations on the molecular dynamics and bulk properties have not been systematically investigated. We initially reported on the NMR characterization of canopy dynamics in silica-based nanoscale ionic materials29 Received: September 26, 2013 Revised: October 22, 2013 Published: December 2, 2013 9669
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nanoparticle surface and are neutralized by amine-terminated copolymers, for which the radius of gyration may be larger than the distance between charges on the nanoparticle. This could lead to crowding that frustrates charge neutralization and impacts the bulk properties. In these studies we report on the NMR characterization of canopy dynamics in NIMs with different canopy loadings, canopy molecular weights, and electrolyte concentrations. These NIMs (Figure 1) are based on 18-nm silica nanoparticles functionalized with a covalently attached corona of 3(trihydroxysilyl)-1-propanesulfonic acid (TPS), and the sulfonate groups on the corona are neutralized by monoamine ethylene oxide/propylene oxide oligomers as the organic canopy. We compare the NMR properties for NIMs prepared with amine-terminated canopies containing similar propylene oxide blocks (9−10 monomer units) but with short and longer (1 vs 31) ethylene oxide chains to probe the effect of surface crowding on the molecular scale dynamics and the bulk properties. NMR relaxation is used to probe the fast (nanosecond) molecular-level dynamics of the canopy, and pulsed-field gradient NMR is utilized to investigate canopy diffusion on the millisecond time scale. The results show the relationships between surface crowding, canopy exchange, ion concentrations, and the bulk properties. These results have important implications for the design and purification of the next generation of NIMs.
Figure 1. Illustration showing NIMs structure. The NIMs of interest are composed of an 18-nm SiO2 nanoparticle core with a covalently attached corona and an ionically tethered, amine-terminated ethylene oxide/propylene oxide diblock copolymer canopy.
and noted that the local canopy dynamics are relatively insensitive to the presence of the functionalized nanoparticles for neutralized NIMs, while the diffusive behavior is more complex than predicted by the hard-sphere model of diffusion. On the basis of these results, we suggested that crowding at the NP surface and exchange among the canopy layers may play a critical role in the liquid-like behavior of NIMs. These observations suggest that a molecular-level understanding of the dynamic behavior of these materials could guide the rational design of the next generation of NIMs. NIMs are a unique class of ionic liquids in which one charged species is confined to a nanoparticle surface while the counterion is free to diffuse. The molecular dynamics and ion-pair lifetimes in ionic liquids have been the subject of numerous studies, including pulsed-field gradient (PFG) NMR31 and molecular dynamics simulations showing that individual ion pairs are transient species in the bulk liquid phase, with lifetimes on the order of nanoseconds.32 This is attributed in part to the high density of neighboring anions surrounding each cation and charge screening by neighbors in the first solvation shell. The situation is considerably different in these NIMs, where the anions are confined to the
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EXPERIMENTAL SECTION
Silica nanoparticles (LUDOX HS-30 colloidal silica, 30% (w/w) suspension in H2O, 18-nm diameter, Aldrich Chemical Co.), 3(trihydroxysilyl)-1-propanesulfonic acid (30−35% w/w solution in H2O, Gelest, Inc.), sodium chloride (Aldrich), magnesium chloride (Aldrich), and Jeffamine M-600 and Jeffamine M-2070 Polyetheramines (Huntsman Corporation) were used as received. Deionized water (18.2 MΩ-cm) was purified using a Barnstead Nanopure system. Silica nanoparticles were functionalized and mixed with the organic canopy according to a previously reported procedure4,33 and sealed with epoxy for the NMR experiments. NMR spectra and carbon relaxation times were measured on a Tecmag Apollo NMR spectrometer at 125 MHz using a 5 mm Doty Scientific, Inc. NMR probe. Pulsed-field gradient diffusion was measured on a home-built spectrometer at 33 MHz using a gradient strength of 652 G/cm. Additional experimental methods are listed in the Supporting Information.
Figure 2. Images showing variation in macroscopic NIMs behavior (a) as a function of canopy: corona ratio, with 100%, 60%, and 20% neutralization of the corona from left to right, and (b) increase in fluidity with the addition of 1 equiv of NaCl per sulfonate group on the bound corona, demonstrated by flow rates down an upright glass slide after 10 min. 9670
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Figure 3. T1 relaxation times for (a) M-2070 and (b) M-600-based SiO2 NIMs as a function of temperature. The data are shown in (a) for M-2070 (○) and SiO2 NIMs with 100% (▲), 60% (▽), and 20% (▼) neutralization of the SIT corona by M-2070 canopy. In addition, data is shown for 100% NIMs with one sodium cation per sulfonate group (red ▲). The data are shown in (b) for M-600 (○) and SiO2 NIMs with 100% (□) and 80% (■) neutralization of the SIT corona by M-600 canopy.
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RESULTS AND DISCUSSION In order to characterize the local and collective dynamics of the NIMs canopy, we compared the spin−lattice (T1) relaxation and self-diffusion of the bulk canopy and the NIMs. The NIMs are based on 18-nm SiO2 NPs functionalized with sulfonic acid groups and paired with cationic amine-terminated ethylene oxide/propylene canopies with molecular weights of 2070 g/ mol (Jeffamine M-2070) or 600 g/mol (Jeffamine M-600) (Figure 1). The largest peak in the carbon spectra of the NIMs and the bulk canopy is located at ∼74 ppm and is assigned to the overlapping methine and methylene carbons of propylene oxide and the methylene carbons in the ethylene oxide monomers (Figure S1). Resonances from the propyl carbons in the NP corona are broadened by their close proximity to the nanoparticle and are not observed. NMR Relaxation Experiments. The local, molecular-level dynamics of the canopy oligomers in SiO2-based NIMs were studied by measuring carbon spin−lattice relaxation times of the methylene carbon peak at 74 ppm for the bulk canopy and for NIMs with different canopy loadings, as a function of temperature. The carbon T1 relaxation (typically on the order of milliseconds to seconds in polymer melts and solutions) is dominated by dipolar interactions of carbon nuclei with directly bonded protons and depends on the nanosecond time scale fluctuations of the C−H vector.34,35 The T1 minimum is observed when the frequency of molecular motion is on the order of the spectrometer frequency (125 Hz), and the shape of the T1 vs temperature curve provides information about the geometry, rate, and amplitude of molecular motions.36,37 Figure 3a compares the T1 vs temperature curve for the bulk M-2070 canopy and the NIMs prepared with 100%, 60%, and 20% neutralization of the sulfonate groups on the SiO2 surface. The bulk M-2070 shows a T1 minimum near 263 K. Somewhat surprising is the observation that the T1 minima for the NIMs with 100% and 60% neutralization are very close to that measured for the bulk canopy, suggesting that the local polymer chain dynamics are not affected by the presence of the 18-nm SiO2 NP. The T1 minimum for the sample with 20% neutralization of the corona increased from 0.26 to 0.28 s and shifted by 40 to 303 K, suggesting more restricted dynamics in the 20% neutralized sample.
A more quantitative understanding of the local polymer chain dynamics is obtained by fitting the T1 data to an appropriate model in order to estimate the correlation times of the C−H vector as a function of temperature. The isotropic reorientation model for the C−H autocorrelation function predicts a T1 minimum (0.1 s) that does not agree with the experimentally observed T1 minimum for M-2070 (0.26 s). We chose to fit T1 vs temperature data using the Lipari−Szabo (LS)38,39 modelfree approach because it seemed appropriate for the ethylene oxide/propylene oxide M-2070 and M-600 oligomers, which are below the entanglement molecular weights for poly(ethylene oxide) and poly(propylene oxide). One of the key features of the LS model is the generalized order parameter (S2) which is a measure of the relative contributions of rapid librational motions relative to molecular reorientation (Supporting Information). The LS model with an order parameter of 0.36 yielded a good fit to the T1 minima for M-2070, while values of 0.35 and 0.32 were required to fit the data for the 100% and 60% neutralized samples. We can conclude from these fits that the methylene carbon spin−lattice relaxation in the canopy is due to a combination of fast (10 ps) librational motions and slower (0.1−2 ns) molecular motions. The similarities in both the order parameters and the T1 minima for bulk M-2070 and 100% and 60% NIMs suggest that the picosecond and nanosecond time-scale motions in the canopy oligomers are very similar despite differences in canopy loadings. Such a result might be expected if most polymer segments were surrounded by other polymer segments, rather than directly interacting with the surface. The fit for the 20% neutralized sample gave an order parameter of 0.32 at 303 K, indicating that higher temperatures are required to realize the same dynamics. The LS fits were used to calculate correlation times versus temperature, and these fits showed longer correlation times for the 20% NIMs when compared to those calculated for the 100% NIMs, 60% NIMs, and bulk M-2070 (see Supporting Information). The restricted dynamics in the 20% NIMs relative to the 100% and 60% neutralized NIMs and the bulk M-2070 suggest local attractive interactions between the oligomer and the nanoparticle surface. The NMR relaxation data fits a conceptual model in which oligomer crowding at the nanoparticle surface prevents a large 9671
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electrostatic interactions with the nanoparticle surface will affect the canopy dynamics in M-600 relative to M-2070. PFG-NMR Diffusion Experiments. NIMs provide a unique opportunity to examine diffusion in ionic liquids where one component of an ion pair is confined to a nanoparticle surface. In order to examine the collective dynamics of the NIMs canopy, we investigated the canopy self-diffusion in bulk M-2070 and M-2070-based NIMs using PFG-NMR.42,43 Figure 4 shows proton PFG-NMR data for
portion of the canopy oligomers from directly interacting with the nanoparticle surface. Thermogravimetric analysis (not shown) indicates a sulfonic acid density of 4.5 per nm2 on the nanoparticle surface.2 This leads to a situation in which the estimated area per charged sulfonate group (0.2−0.5 nm2) is much smaller than the effective cross-sectional area of an M2070 oligomer (4.9 nm2), estimated from the radius of gyration (Rg = 1.25 nm) of M-2070, assuming a freely jointed chain.40 We note that dynamic light scattering experiments (not shown) yielded an estimated Rg of 1.15 nm for M-2070. The larger Rg relative to the surface charge density results in crowding of canopy molecules at the nanoparticle surface. As a consequence, the greatest direct neutralization would occur with the amine terminus interacting with the sulfonic acid group and the bulk of the chain extended away from the surface. In this arrangement, the large majority of the ethylene oxide groups would be solvated by other M-2070 molecules rather than interacting with the NP surface. This model accounts for the observation that the carbon relaxation times are not sensitive to the presence of the SiO2 NP in NIMs with 100% and 60% neutralization. At lower canopy loadings (e.g., 20% neutralization) the surface is more accessible because the crowding is alleviated, and it is possible for the canopy backbone to fold back onto the nanoparticle surface and interact with sulfonate groups through hydrogen bonding. We note that sulfonated polymers such as Nafion have residual water that is extremely difficult to remove,41 and we have observed strongly bound water molecules in 1H NMR spectra of NIMs with low canopy loadings (Figure S2). The residual water in sulfonic acid-containing polymers is very acidic and appears near 7 ppm, rather than 4.8 ppm. The restricted dynamics in the 20% NIMs may result from hydrogen bonding between acid groups and bound water with the oxygen atoms in the canopy. In order to test the possibility of molecular crowding at the nanoparticle surface, we examined the T1 relaxation of NIMs containing canopy oligomers with a lower molecular weight (Jeffamine M-600, MW ∼600 g/mol) and smaller radius of gyration (∼0.5 nm). The T1 relaxation times measured as a function of temperature for bulk M-600 and NIMs with 100% and 80% neutralization of the TPS corona are shown in Figure 3b. The similar curves observed for bulk M-2070 and M-600 show the typical behavior for polymer segments in a polymeric environment, while the curves for M-600 in the presence of NPs (100% and 80% neutralization) show the effect of NPs on the dynamics. LS fits for the T1 data gave an order parameter of 0.31 for bulk M-600, compared to 0.22 and 0.20 for the 100% and 80% neutralized samples. The T1 minimum shifts from 0.30 s at 258 K for the bulk M-600 to 0.40 s at 308 K and 0.43 s at 313 K for the 100% and 80% neutralized samples, respectively. The significant change in the order parameter for the M-600 NIMs versus the bulk M-600 shows that the distribution of molecular motions responsible for T1 relaxation is affected by the presence of the nanoparticle. These data are consistent with a model in which the smaller Rg of M-600 (∼0.5 nm) relative to M-2070 (∼1.25 nm) partially alleviates crowding at the nanoparticle surface and allows a larger fraction of the canopy molecules to interact directly with the sulfonate groups or bound water molecules at the nanoparticle surface. In addition, the majority of the M-600 chain is closer to the nanoparticle surface compared to the M-2070-based NIMs due to the decreased oligomer size. This makes it more likely that
Figure 4. PFG-NMR self-diffusion data for neat samples of bulk M2070 (○) and SiO2 NIMs with 100% (▲), 60% (Δ), and 20% (red ▲) neutralization of the SIT corona by M-2070 canopy.
neat, dry samples of M-2070 and the SiO2 NIMs with 100%, 60%, and 20% neutralization at 323.5 K. The self-diffusion coefficients and fitting parameters are listed in Table 1. Table 1. Canopy Diffusion Coefficients and Fit Parameters for Bulk M-2070 and SiO2 NIMs with 100%, 60%, and 20% Neutralization of the Corona by M-2070 Canopy corona neutralizaton
D (cm2/s)a
β
bulk M-2070 100%
5.4 ± 0.2 × 10−8 fast: 3.5 ± 0.3 × 10−8 slow: 1.6 ± 0.1 × 10−9 fast: 3.4 ± 0.2 × 10−8 slow: 1.5 ± 0.1 × 10−9 2.6 ± 1.0 × 10−9
0.87 ± 0.02 −
60% 20%
− 0.68 ± 0.13
The diffusion curves were fit to either a double exponential or a stretched exponential. The parameter β is the stretched exponential parameter.
a
The expectation from simple physics is that single exponential decay would be observed for bulk M-2070 rather than the stretched exponential listed in Table 1. We previously reported that the curvature (with a stretch exponent of β = 0.83) can be assigned to self-association in the bulk material, similar to that observed in diffusion studies of Pluronic block copolymers.44 In contrast to this behavior, we observe a double exponential decay for the 100% M-2070 NIMs, revealing two distinct canopy populations that are not exchanging on the time scale (milliseconds) of the diffusion experiment. The diffusion coefficient of the slowly diffusing population gives a hydrodynamic radius of 14 nm, which is close to the predicted 12 nm 9672
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radius of a single NP with a bound corona and a single canopy layer (9 nm NP radius + 0.5 nm TPS corona + 2.5 nm M2070), calculated from the Stokes−Einstein relation using the viscosity of M-2070 at 323 K. This is remarkably good agreement, considering that the Stokes−Einstein relationship assumes a random walk in a dilute medium, which is very different from the “crowded” NIMs solution. In addition, the diffusion coefficient of the more rapidly diffusing population is close to the value for bulk M-2070. Taken together, the data supports a model in which the canopy diffusion is slowed in the NIMs relative to the bulk canopy, but not all canopy molecules are slowed equally due to crowding at the surface (Figure 5).
Figure 6. SAXS data for 100% neutralized NIMs in bulk (black) and diluted to 20% (blue) and 2% (red) in D2O. The scattering curves are offset for clarity.
of strongly bound canopy despite lower canopy loading. The low canopy concentration and short spin−spin relaxation times make it difficult to completely follow the decay in the sample with 20% neutralization. Nonetheless, the data collected for 20% neutralized NIMs shows a single diffusion coefficient which agrees closely with that observed for the slow component of the 100% and 60% neutralized NIMs. At 20% neutralization we observe only the canopy population that interacts strongly with the nanoparticle surface. Since the canopy−corona interactions in NIMs are presumed to be dominated by electrostatics, the introduction of electrolyte ions (e.g., Na+) could screen the electrostatic coupling and affect the dynamics and macroscopic properties of NIMs. Ionic contaminants are ubiquitous and often difficult to remove completely. Contamination is particularly important for NIMs because they are concentrated from a dilute solution, so any impurities present in the nominally pure water will be concentrated in the final material. In order to evaluate the effect of ions on NIMs, neat samples of 100% neutralized M-2070 NIMs were spiked with ionic solutions and dried under vacuum prior to PFG-NMR experiments. Figure 7 compares the PFGNMR diffusion data for NIMs spiked with NaCl relative to the as-prepared NIMs, and the results are summarized in Table 2. ICP-MS analysis (not shown) of as-prepared NIMs showed that even the “unspiked” 100% neutralized NIMs sample contains approximately 0.1 sodium ions per sulfonate group prior further addition of NaCl.33,46 Figure 7 shows that the diffusion rate increases as the NaCl concentration increases, and that the diffusion decay transitions from the slow exchange (double exponential decay) to the fast exchange (stretched exponential decay) limit. We note that the 1:1 spiked sample exhibits stretched-exponential diffusion that is nearly identical to that previously reported for a NIMs sample that we subsequently determined to have one sodium ion per sulfonate.28,29 These results show that contaminant ions screen the canopy−corona electrostatic attraction and increase the rate of exchange between the two canopy populations. This hypothesis is supported by the enhanced diffusion observed upon adding divalent cations, where a NIMs sample spiked with 0.5 mol equiv MgCl2 exhibits nearly identical canopy diffusion as the same NIMs spiked with one molar equivalent
Figure 5. Schematic illustration of NIMs with two canopy populations resulting from oligomer crowding at the nanoparticle surface.
One canopy population interacts strongly with the sulfonate moieties on the nanoparticle surface and diffuses along with the SiO2 NP, while the weakly bound outer sphere canopy molecules diffuse at a rate 1.7 times slower than the bulk canopy. The weakly coupled population of canopy likely exchanges between nanoparticles and plays a key role in the liquid properties of NIMs in the absence of solvent. Small angle X-ray scattering data for neat NIMs shows weak particle interference peaks and preservation of the first-order Bessel oscillation, indicating the nanoparticles are well-dispersed (Figure 6). The absence of an upturn in intensity at low scattering vector indicates that there are no larger-scale structures within the neat NIMs that could contribute to the overall diffusion profile, supporting the hypothesis that the fast and slow populations consist of canopy oligomers interacting strongly and weakly with the nanoparticle surface. The effect of crowding at the nanoparticle surface can also be observed by starving the NIMs of canopy. The 60% neutralized sample shows two-component diffusion, and the diffusion rates for the fast and slow populations are close to those observed for the 100% neutralized samples. The amplitude of the rapidly relaxing component is larger than in the 100% neutralized NIMs, perhaps because of the presence of small amounts of bound water (Figure S2) that partially screens the electrostatic corona−canopy interaction,45 resulting in a smaller population 9673
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the bulk and molecular-level properties. NMR relaxation experiments show that the local canopy dynamics are not affected by the presence of the functionalized nanoparticle until the canopy loading is sufficiently low or the canopy Rg is sufficiently small to allow local attractive (electrostatic) interactions between the canopy main chain and the nanoparticle surface. Higher temperatures are required at low canopy loadings to activate the molecular motions that result in spin−lattice relaxation. The ethylene oxide/propylene oxide segments are able to interact with the surface in low molecular weight canopies due to reduced crowding at the surface. PFG− NMR reveals two populations of diffusing canopy oligomers, which are assigned to a bulk-like component and a component that diffuses with the SiO2 NP. Added electrolyte ions screen the electrostatic corona−canopy interaction and affect the molecular level properties, including the bulk viscosity. We are currently exploring the effects of water, NP size, NP composition, and corona variations on the dynamics of NIMs and the relationship to the macroscopic properties.
Figure 7. PFG-NMR magnetization decay curves for bulk M-2070 (○), NIMs100 (▲), and NIMs100 spiked with 0.5 (Δ), and 1.0 (blue ▲) mol equiv of NaCl, as well as 0.5 mol equiv of MgCl2 (red ■).
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S Supporting Information *
Table 2. Canopy Diffusion Coefficients and Fit Parameters for Bulk M-2070 and M2070-NIMs100 Spiked with NaCl or MgCl2 Na+ (or Mg2+): SO3‑ 0.1:1 Na
:SO3−
+
0.11:1 Na+:SO3− 0.5:1 Na+:SO3− 0.5:1 Mg2+:SO3− 1:1 Na+:SO3−
fast: 2.9 ± 0.1 × 10−8 slow: 1.4 ± 0.1 × 10−9 fast: 3.5 ± 0.3 × 10−8 slow: 1.6 ± 0.1 × 10−9 fast: 2.3 ± 0.3 × 10−8 slow: 3.5 ± 0.1 × 10−9 1.4 ± 0.3 × 10−8 1.2 ± 0.1 × 10−8
Discussion of approach used to calculate correlation times based on the LS model and 1H NMR spectra, along with experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
β
D (cm2/s)a
ASSOCIATED CONTENT
−
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−
*(P.A.M.) Telephone: (937) 255-9155. E-mail: peter.mirau@ us.af.mil.
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AUTHOR INFORMATION
Corresponding Author
Notes
0.94 ± 0.02 0.91 ± 0.01
The authors declare no competing financial interest.
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The diffusion curves were fit to either a double exponential or a stretched exponential. The parameter β is the stretched exponential parameter.
a
ACKNOWLEDGMENTS Funding provided by the Air Force Office of Scientific Research is gratefully acknowledged. The diffusion portion of this work was supported by the National Science Foundation under Grant No. DMR 04 55117. This publication is based on work supported by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). A portion of this research was carried out while M.L.J. was a National Research Council Associate at the Air Force Research Laboratory and an employee of UES, Inc. (Dayton). Jeffamines M-2070 and M-600 were generously donated by Huntsman Corporation (Houston, TX). M. Tchoul contributed GPC in support of this study. The authors would like to thank George Fultz and Timothy Reid (University of Dayton Research Institute) for viscosity and ICP-MS data supporting this research. The authors also thankfully acknowledge Dr. Rajiv J. Berry and Phuong T. Ngo (AFRL/RX) for helpful discussions regarding this work. We would like to thank Dr. Alexander Hexemer and Dr. Eric Schaible for guidance, setup, and data collection at beamline 7.3.3 at the Advanced Light Source/ Lawrence Berkley National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
NaCl (Figure 7). These experiments show that both the concentration and valence of added ions strongly affect the translational diffusion of NIMs canopy, offering a potentially simple route to adjusting NIMs rheological properties (Figure 2b). In light of the observation that ions can affect canopy diffusion, we have also examined the possibility that ions can affect the faster molecular dynamics measured by NMR relaxation. Figure 3a compares the T1 vs temperature curves for 100% M-2070 neutralized NIMs in the absence (▲) and presence (red ▲) of a 1 mol equiv of NaCl. The results show that ion concentrations sufficient to affect the diffusion and viscosity do not have a measurable impact on the molecularlevel motions responsible for T1 relaxation at high canopy loadings. This suggests that the sodium cations interact preferentially with the sulfonate groups on the nanoparticle surface, rather than with the oxygen atoms in the ethylene oxide/propylene oxide monomers of the canopy.
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CONCLUSIONS We have used NMR to investigate the canopy dynamics in nanoscale ionic materials as a function of the canopy loading, canopy molecular weight, and the concentration and valence of electrolyte ions in order to understand the relationship between
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dx.doi.org/10.1021/ma402002a | Macromolecules 2013, 46, 9669−9675
