1H Solid State NMR Study of Poly(methacrylic acid) Hydrogen

Jul 25, 2012 - The 1D and 2D DQ-MAS were acquired by applying one cycle of the back-to-back ... For the 2D 1H–1H DQ sideband experiment, the 0 → Â...
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H Solid State NMR Study of Poly(methacrylic acid) HydrogenBonded Complexes

Blythe Fortier-McGill, Violeta Toader, and Linda Reven* Centre for Self-Assembled Chemical Structures (CSACS-CRMAA), Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, QC H3A 0B8 Canada S Supporting Information *

ABSTRACT: The hydrogen bond structure of a series of poly(methacrylic acid) (PMAA) complexes was studied by solid-state NMR. 13C and 2H labeled PMAA samples were complexed with poly(ethylene oxide) (PEO), poly(vinyl methyl ether) (PVME), poly(acrylamide) (PAAM), poly(vinyl caprolactam) (PVCL) and poly(vinylpyrrolidone) (PVPon). The presence and relative strengths of PMAA’s hydrogen bonds with itself versus those with the complementary polymer was assessed by combining 13C CP-MAS NMR, 1H−13C HETCOR, 1D and 2D DQ 1H MAS NMR experiments. Analyses of 1H DQ spinning sideband patterns gave estimates of the proton−proton distances. Only the polyether−PMAA complexes, PEO and PVME, show resolved 13C and 1H resonances. This spectral resolution is proposed to be due to the selective disruption and stabilization of PMAA’s open and cyclic dimers, respectively. Residual PMAA dimers are detected by 1H NMR for the polylactam complexes, PVCL and PVPon, but both types dimers are weakened, reflecting the greater amount of interpolymer linkages. The PAAM−PMAA complex maintains more of the weaker hydrogen bonds. The role of the different hydrogen bond structures in the relative stabilities and dynamic properties within this series of PMAA complexes and multilayers is assessed.

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hydrogen bonds. 1H MAS NMR was used to characterize the hydrogen bonding to explain the different swelling behaviors of copolymer hydrogels based on N-isopropylacrylamide (NIPAAM) and methacrylic acid (MAA).7 In the case of poly(carboxylic acid) complexes, the hydrogen bonded protons that are close in space, associated with dimer type structures, can be directly detected by 1H double quantum (DQ) MAS NMR. This technique was used to identify two distinct PMAA dimers within a collapsed PMAA hydrogel that differed in their relative acidities and bond distances.8 The hydrogen-bonded arrangements were concluded to reduce the tendency of the involved COOH groups to become ionized such that the more stable hydrogen bonds are the least ionizable. In this paper, we use solid-state 1H and 13C MAS NMR to characterize the hydrogen bonding of a series of PMAA complexes whose chain dynamics were previously studied by wide-line 2H NMR.2 Specifically, we examine the influence of the complementary polymer on the formation of PMAA dimers by 1H DQ MAS NMR.9−11 The stability of the hydrogen bonds within PMAA in the presence of a hydrogen bond acceptor polymer reflects the complexation strength. This information is of particular interest for PAAM complexes with PMAA. In our previous study, PMAA was found to be less mobile when complexed to PAAM as compared to PEO.2 PMAA−PAAM

INTRODUCTION Polymer multilayer films and capsules assembled by the layerby-layer method offer nanometer thickness control as well as the flexibility of a wide variety of substrates and polymers. Although most studies have focused on polyelectrolyte multilayers, attention has shifted toward systems stabilized primarily by hydrogen bonding, driven by potential biomedical applications.1 Likewise, there has been a resurgence of interest in a more complete understanding of the polymer complexes to assist in tailoring the properties of the corresponding multilayer films. In a previous NMR study, we correlated the chain dynamics of water swollen PMAA complexes and multilayers to their macroscopic properties. Specifically, a higher PMAA mobility was associated with a lower dissolution pH value (critical pH), a larger bilayer thickness, and a higher permeability. The trend in the chain mobility was rationalized primarily in terms of the relative hydrophobic nature of the hydrogen-bond acceptor polymer.2 The relative stabilities of the poly(carboxylic acid) complexes arise from the combination of hydrophobic forces and hydrogen bonding. Sukhishvili and Granick demonstrated that the critical pH value is controlled by a balance of internal ionization and the fraction of carboxylic groups that form hydrogen bonds, detected indirectly by Fourier transform infrared (FTIR) spectroscopy, via the degree of ionization.3 The hydrogen bond linkages can also be detected from the 13C chemical shifts of the carbonyl groups.4−6 Solid-state 1H magic angle spinning (MAS) NMR is a more direct probe of the © 2012 American Chemical Society

Received: March 16, 2012 Revised: July 17, 2012 Published: July 25, 2012 6015

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of 1 mL/min and at a temperature of 30 °C. The mobile phase used was a 0.1 M pH 9 phosphate buffer. (The retention time traces are provided in the Supporting Information.) The Mw of PMAA-d3 was found to be 22 kDa with a polydispersity of 1.97 and a dn/dc value of 0.164, as reported earlier.2 Sample Preparation. The typical concentration of the initial polymer solution was 6 mg/mL dissolved in Milli-Q water. The pH of the PMAA solution was adjusted with 1 M HCl to obtain a constant pH value of 1. The complex suspensions were formed upon the addition of 10 mL of the PMAA solution to 10 mL of the hydrogen accepting polymer solution, from which a white precipitate was formed immediately. The pH of these complex suspensions was further adjusted to a constant value of 1 using 1 M HCl. The complex suspensions were concentrated by centrifuging at typically 10 krpm for 20 min. The supernatant was decanted, and the complexes were dried under vacuum for 24 h at 50 °C. NMR Spectroscopy. A Bruker AVANCE 600 WB spectrometer operating at the Larmour frequency of 600.14 MHz for 1H and 150.92 MHz for 13C, was used for acquisitions of all the spectra. A double resonance probe that supports 2.5 mm outer diameter rotors at a spinning speed of 30 kHz was used for the acquisition of all 1H spectra, including the DQ-MAS 1H spectra, as well as the high resolution 13C spectra. The sample temperature at 30 kHz was 324 K unless otherwise stated. All stated temperatures have been calibrated by using the 1H chemical shift difference of ethylene glycol’s methylene and OH protons, (T = (4.637 − Δ(ppm))/0.009967 (K)), to first reference at an exact temperature at low spinning speeds. The subsequent change in temperature at elevated spinning speeds was monitored using the 207Pb chemical shift within Pb(NO3)2 where the temperature dependence of the chemical shift has been calibrated by Beckmann and Dybowski.20 The 1H spectra were referenced to either water, pH 7 (4.85 ppm, 1 H) or adamantane (1.63 ppm, 1H), while the 13C spectra were referenced to glycine (176.03 ppm, carbonyl, 13C). For the 1H spectrum a typical 90° pulse length and recycle delay of 2.5 μs and 3 s, respectively, with 36 transients was applied. The cross-polarization pulse sequence typically used a 1H 90° excitation pulse of 2.5 μs, a contact time of 1.5 ms, a recycle delay of 5 s, with a 1H ramp, a 13C B1 field of 55 kHz and a 1H B1 field of 85 kHz at a spinning speed of 15 kHz, with a sample temperature of 302 K. The 2D 1H−13C HETCOR spectra were acquired using the frequency-switched Lee−Goldberg (FS-LG) applied during the evolution time t1, using an 8 step phase cycle, with a typical 1H 90° excitation pulse of 2.5 μs, a contact time of 1.5 ms, a recycle delay of 2 s, with typically 16 transients for each of the 128 increments, and a spectral width of 30 kHz with 1k of data points at a spinning speed of 15 kHz. . The T1ρ measurements where performed with a shorter contact time of 500 μs at a 1H B1 of either 65 or 85 kHz where the 13C B1 equaled 55 kHz, with otherwise the same pulse parameters as the CP experiment. The intensity of the eight variable pulse spectra were fit as a function of τ values varying between

multilayers have been reported as having both a lower12 and higher13 critical pH as compared to PMAA−PEO multilayers, while the reported bilayer thickness is smaller than that of the less mobile and more stable PMAA−PVME complex.13 The PMAA−PAAM system is also well-known to disassemble rather than stabilize at high temperatures.14 All of this reflects that the hydrogen bonds rather than the hydrophobic forces determine the mobility of PMAA within the PAAM complex. Overall, understanding the structure of the hydrogen bonding will help elucidate the interplay of molecular interactions that determine the properties of these polymer complexes and their multilayers. The organization of the paper and the presentation of the NMR data are as follows. The 13C chemical shifts of the carbonyl group of the PMAA complexes are first presented. The 13C carbonyl chemical shifts of the PMAA complexes with the polyethers (PEO, PVME) are distinct for hydrogen bonds of the complex versus those involved in self-association of PMAA, but only one 13C carbonyl shift is observed for the other complexes (PAAM, PVCL, PVPon). Therefore, the 1H chemical shifts and double quantum coherences (DQC) of PMAA alone and complexed were examined to identify the presence of complex versus self-associated PMAA hydrogen bonded linkages. These assignments are supported by 1H−13C hetero correlation (HETCOR) NMR experiments for the PMAA−polyether complexes. Since the 1H DQC intensities reflect both the populations as well as the proton−proton distances that vary according to the polymer structure, the distances for the PMAA dimers were estimated from 1H DQC spinning sideband analyses.15−17 Finally, the effect of the different complementary polymers on the hydrogen bond structure of PMAA is related to the previously studied mobility and critical pH values of this series of polymer complexes.

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EXPERIMENTAL SECTION

Materials. Poly(methacrylic acid) (PMAA-d5; Mw 6.3 kDa, PI = 1.4) and poly(vinyl caprolactam) (PVCL; Mw 1.8 kDa, PI = 1.3) were purchased from Polymer Source Inc. and were used as received. Poly(ethylene oxide) (PEO; Mw 300 kDa), poly(vinyl methyl ether) (PVME; Mw 1.5 kDa, independently determined by the electro-spray and MALDI−TOF mass spectrometry experiments) as a 50 wt % solution, poly(acrylamide) (PAAM; Mw 10 kDa) as a 50 wt % solution, poly(vinylpyrrolidone) (PVPon; Mw 29 kDa), deuterium depleted water 99.9%, oxalyl chloride (98%), iodomethane, iodomethane-d3, triphenylphosphite, paraformaldehyde, 2,2′-azobis(2-methylpropionamidine) dihydrochloride (ABAP) (97%), and anhydrous dimethyl sulfoxide (DMSO) were all purchased from Sigma-Aldrich and were used as received. The 48 wt % HCl, glacial acetic acid, sodium acetate were all purchased from Fisher, and were used as received. The Milli-Q water used had a measured resistance of 18.2 MΩ cm. The 13C labeled bromoacetic acid was purchased from Cambridge Isotopes Laboratories. Labeled PMAA Synthesis. The method used to synthesize the 13 C labeled methacrylic acid (MAA) was derived from a combination of procedures as reported by Werkhoven et al.18 and Ayrey and Wong,19 see Supporting Information for details. The process for the synthesis of the d3 labeled MAA and its subsequent polymerization has been reported previously.2 The route of polymerization used for the 13 C labeled PMAA was similar to that used for the d3 labeled PMAA. GPC Analysis. Two batches of the 13C labeled carbonyl PMAA-13C were used with either a Mw of 14 kDa with a polydispersity of 1.81 and a dn/dc value of 0.175 or a Mw of 46 kDa with a polydispersity of 2.05. These values were determined using a Polymer Laboratories PL-GPC 50 plus. This is a triple detection instrument, with two PL aquagel− OH 7.5 mm ID columns and a guard column. The instrument was calibrated with Viscotek PEO 22 kDa Narrow Standard at a flow rate

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0.1−40 ms using an exponential decay function I(t) = I(0)e−τ/T1ρ( H), where the standard deviation of the fits was less than 0.05 ms. A table of the carbonyl T1ρ values can be found in the Supporting Information. The tacticity of the PMAA-13C was determined using the pulse saturated transfer magic angle spinning (PST-MAS) to acquire a high resolution 13C spectra.21 The 2.5 mm double resonance probe was used where the sample was spun at 15 kHz, (PMAA-13C), or 25 kHz (PMAA-d5, -d3) with a 13C 90° pulse length of 1.5 ms, a 5 s recycle delay and 256, 10k or 5k transients, respectively, with a spectral width of 38 kHz and 1k of data points. The 1D and 2D DQ-MAS were acquired by applying one cycle of the back-to-back (BABA) recoupling sequence22 during 2 rotor cycles, (N = 2, Figure 1) using a 16 step phase cycle, with typically a 2.5 μs 90° pulse length, a 2−3 s recycle delay, 256 transients for the 1D spectra and 64 transients for each of the 64 increments for the 2D spectra, with a spectral width of 30 kHz in both dimensions and 4k of data points in the direct dimension (F2). For the 2D 1H−1H DQ sideband experiment, the 0 → ± 2 → 0 →1 coherence pathway was selected using a 16 step phase cycle, 512 6016

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Figure 1. 2D 1H DQ MAS NMR correlation pulse sequence.9−11 nonrotor synchronized increments to make up the FID in the indirect time t1, a 2.5 μs 90° pulse length, a 2 s recycle delay, 64 transients, with a spectral width of 500 kHz for the indirect dimension (F1) and 60 kHz for the direct dimension (F2), and the number of rotor periods, N = 4. Phase-sensitive detection in the indirect dimension (F1) was obtained using the States-TPPI method. The DQ sideband patterns were obtained by taking a slice along the 1H chemical shift of interest in the direct dimension (F2) resulting in 1D spectra in the indirect dimension (F1). These indirect dimension (F1) slices were analyzed for determination of the effective dipolar coupling constant. NMR Data Analysis. The simulated 1D DQ spinning sideband spectra were obtained using the MATLAB code graciously provided by Todd M. Alam.16 The DQ sidebands were simulated to fit the experimental indirect dimension (F1) slices by inputing a dipolar coupling constant and its standard deviation in Hz. The dipolar coupling constant (Dij) in units of rad s−1 between protons i and j by a distance rij is given by16,17,23 Dij =

μ0 ℏγγ i j 4πr 3ij

(1)

where μ0 = 4π × 10−7 H m−1, is the vacuum permeability and γ is the gyromagnetic ratio. For an isolated spin-1/2 pair with a distribution of effective dipolar coupling constants, the signal intensity of the DQ sidebands (IDQ) is described by15−17,23 IDQ (t1, ρeff ) =

∑ ρeff cos(2ΔωPCt1) l

×

⎤ ⎡ 3 sin⎢ D ij sin(2β ij) cos(γ ij + ωR t1)NτR ⎥ ⎦ ⎣ π 2 eff

⎡ 3 ⎤ × sin⎢ D ij sin(2β ij) cos(γ ij)NτR ⎥ ⎣ π 2 eff ⎦

Figure 2. Carbonyl region of the 13C−CP-MAS spectra of PMAA and its complexes with a contact time of 1.5 ms. The labels D, S, and C indentify the COOHs involved in ordered dimers (D), disordered (S) intra-PMAA H-bonds and those involved in interpolymer H-bonds or complexed (C) H-bonds.

(2)

Dijeff is the effective dipolar coupling between protons i and j, ΔωPC is the frequency increment for States-TPPI, t1 is the DQ evolution time increment, ωR (=2πνR) is the spinning frequency, N is the number of rotor periods in the excitation/reconversion portion of the BABA sequence, τR (=νR−1) is the rotor period, βij and γij are the Euler angles describing the orientation of the principal axis of the dipolar coupling tensor between spins i and j within the reference frame fixed to the rotor, and the symbol ⟨⟩ represents the Euler angle powder average. The code uses the Grant tiling scheme24 to achieve the powder average over β and γ. The ρeff is the probability of a given effective dipolar coupling Dijeff. The distribution of the dipolar couplings about a mean dipolar coupling value, D̅ ij, with a standard deviation, σ, is given by17 ij

ρeff (Deff ) =

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⎡ ij ij 2 ⎤ 1 1 ⎛ D̅ − Deff ⎞ ⎥ ⎢ ⎟ exp − ⎜ ⎢⎣ 2 ⎝ 2π σ ⎠ ⎥⎦

PVME, present three distinct 13C chemical shifts for carbonyl groups involved in ordered hydrogen bonded dimers (within PMAA) (D, dimer), complexed hydrogen bonds4 (C, complex) and disordered hydrogen bonds (within PMAA)25 (S, disordered). The other polymers (PVCL, PVPon, PAAM) also contain carbonyl groups but the use of 13C labeled PMAA minimizes the contributions of these signals. The relative intensities of each of the three different carbonyl signals are considered to be representative of each carbonyl environment, to within ±5%, as the difference between the t1ρ values of the dimer and the complex peaks, for the PEO and PVME samples, were found to be within 0.5 ms (see Supporting Information, Table S1). For the other samples only one t1ρ could be extracted but the line shape was found to be independent of the contact time, within the range of 0.2 and 5 ms (data not shown). Thus, the lack of resolved signals for PMAA complexes with PVCL, PVPon, and PAAM compared to PMAA complexed to ether PEO or PVME indicates that there are fewer ordered hydrogen bonded PMAA dimers. A more quantitative method to verify this assertion is to use solid-state

(3)

RESULTS C Chemical Shifts. The 13C labels in Figure 2 are based on those of Miyoshi et al.4 for the PMAA−PEO complex and those of Asano et al.25 for the noncomplexed PMAA. The PMAA−poly(ether) complexes, PMAA−PEO and PMAA− 13

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H NMR experiments as the 1H chemical shifts are sensitive to hydrogen bonding and the generation of 1H double quantum (DQ) coherences gives proton−proton proximities. 1D 1H DQ MAS NMR. Double quantum signals can be generated if two protons are sufficiently close to be dipolar coupled, as is the case for carboxylic acid dimers, with interproton distances typically between 0.25 and 0.3 nm.29 For a PMAA hydrogel, it has been shown that two different types of dimers are formed, with proton−proton distances of 0.295 and 0.275 nm, that generate two distinct proton DQ signals.8 These two dimers were also found to differ in their relative abundance, stabilities and acidities. The most stable dimer with the closest interproton distance was the least acidic and the least abundant. The difference between these two dimers was proposed to be related to the PMAA’s tacticity, where the less abundant, and less acidic meso sequences were associated with the more stable dimer.8 However, here we consider the possibility that the stronger dimer is the cyclic type dimer, and that the weaker dimer is an open type dimer as shown in Figure 3. It was Nakashima et al.

Figure 3. Possible hydrogen bonded structures within PMAA.26−28

that proposed that poly(carboxylic acids) could also have chains of open type dimers or of lateral hydrogen bonds.26 This type of hydrogen bonded network was proposed for both PAA and PMAA based on the assumption that polymers should be flexible enough to form structures which are found in their small molecule counter parts. Both the lateral hydrogen bonds and the open dimers, which can also form chains of hydrogen bonds, are considered less stable than the cyclic dimers. In addition the stability of the open dimer and lateral hydrogen bond is expected to depend on the total number of consecutive linkages. A comparison of the 1D 1H MAS and DQF MAS spectra of PMAA and its complexes is shown in Figure 4. The methyl deuterated PMAA-d3 was used to directly compare the 1H NMR results to the previous deuterium NMR study of the same complexes.2 The resolved resonances include two signals in the hydrogen bonding region (at 12.9 and 10.6 ppm), labeled as D1 and D2 for the most and least stable PMAA intracomponent hydrogen bonds, proposed here to be the cyclic and open type dimers (Figure 3). In addition the D2 signal could also include the lateral hydrogen bonded COOH protons which are also considered to be weaker than the cyclic type hydrogen bonds. The labels D1 and D2 only refers the hydrogen bonds that PMAA makes with itself including both intra- and inter- molecular interactions. For our purpose, we refer to hydrogen bonds that PMAA makes with itself as intraPMAA interactions while hydrogen bonds that PMAA makes

Figure 4. 1H-MAS (a) and 1H-DQ-MAS (b) spectra of PMAA-d3 and its complexes. The labels D1and D2 indicate the assignment of the most and least stable PMAA hydrogen bonds, respectively. The label F indicates the non-hydrogen bonded carboxylic acid proton, while the presences of water exchanging with the COOH is labeled as F + H2O. The labels A1 and A2 represent primarily the methylene and methyl ether/ethylene oxide aliphatic protons, respectively.

with a different hydrogen accepting polymer as interpolymer or complex hydrogen bonds. The complex or interpolymer hydrogen bonds are not labeled in the 1D spectra (Figure 4). The other labeled signals at lower frequency correspond to the free (F, 9 ppm), non-hydrogen bonded acid protons and water (F + H2O, 6.5 ppm) exchanging with the free acid, such that the intensity is greatly reduced in the DQF experiment, showing that these exchanging protons are very mobile. The intensity of the aliphatic region at 1−4 ppm varies depending on the amount and type of aliphatic protons of the hydrogen 6018

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accepting polymer. The PMAA-d3 only contains the methylene protons, which like the aliphatic protons of the hydrogen accepting polymers, PVPon, PVCL and PAAM resonate in the A1 region (A1, ∼1.5 ppm). The aliphatic protons of PEO and PVME (A2, ∼3.5 ppm) are shifted to higher frequency because of the adjacent ether group and are more intense due to their mobility. The PMAA complexes with the polyethers have two well resolved peaks at 12.9 and 10.6 ppm. Their increased resolution as compared to noncomplexed PMAA, agrees with the 13C spectra that indicate that complexation with PEO induces more ordered PMAA hydrogen bonds.4 Complexes that contain amide functional groups also show a single broad resonance in the hydrogen bonding region centered around 11 ppm. Although these 1D 1H DQF spectra reflect the quantity and strength of the dipolar couplings, exactly which protons are involved remains ambiguous. Identification of the latter can be obtained from the corresponding 2D 1H−1H DQ MAS NMR spectra. Intrapolymer linkages: PMAA, PMAA-d3 and PMAAd5. Before examining the complexes, the double quantum coherences (DQCs) of PMAA's H-bonded species, specifically those above 9 ppm,30 of PMAA are assigned (Figure 5). The frequencies in the DQ dimension are the sum of the 1H SQ resonant frequencies belonging to the protons close enough in space to be dipolar coupled. DQCs between like spins gives rise to autopeaks on the diagonal that are at twice the frequency of the SQCs, whereas protons with different chemical shifts give rise to pairs of peaks on either side of the diagonal. First we note that there is variation in the intensities of the D1 (12.9 ppm, 25.8 ppm) and D2 (10.6 ppm, 21.2 ppm) autopeaks among the three PMAA samples which is likely due to different water content as discussed below. Second, comparison of the 2D 1H DQ MAS NMR spectra of unlabeled PMAA, the deuterium labeled PMAA-d3 and PMAAd5 shows that the aliphatic−OH cross-peak intensities are strongly reduced in PMAA-d3 and absent in PMAA-d5. The elimination of the intra-PMAA aliphatic−OH cross-peak by using perdeuterated PMAA allows for a clearer identification of cross-peaks between the hydrogen bonded protons (>8 ppm) and the aliphatic protons of the complementary polymer (2−4 ppm). However the PMAA-d5 (Polymer Source Inc.) contains diphenyl end-cap groups producing a strong autopeak at 7.2 ppm corresponding to the aromatic protons. In the case of PMAA-13C and the PMAA-d3, a water-soluble initiator 2,2′Azobis(2-methylpropion amidine) dihydrochloride (ABAP) was used, which gives rise to an autopeak at 9 ppm. The assignment of this extra resonance in addition to that at 7.6 ppm is based on the 2D DQ MAS, 1H−13C HETCOR and solution 1H NMR of the initiator (Supporting Information, Figure S6). As the polymer with the strongest D1 dimer intensity is that with the lowest molecular weight (PMAA-d5, 6.3 kDa, rr:mr:mm = 46:56:0 %) rather than the greatest quantity of meso groups (PMAA 13C, 46 kDa, rr:mr:mm = 44:48:8 %) (Supporting Information), we rule out tacticity as the main source of the two types of dimers. PMAA-d3, which has intermediate molecular weight and low meso dyad content (22 kDa, rr:mr:mm = 77:23:0 %) shows an intermediate D1 intensity. Fourier transform infrared (FTIR) studies of PMAA concluded that cyclic and open type dimers along with lateral chains of hydrogen bonds are present (Figure 3). Nakashima noted that these dimers and chains of H-bonds can be formed

Figure 5. 2D 1H DQ MAS spectra of PMAA-13C, 46 kDa, rr:mr:mm = 44:48:8 % (a), PMAA-d3, 22 kDa, rr:mr:mm = 77:23:0 % (b), and PMAA-d5, 6.3 kDa, r:mr:mm =46:54:0 % (c).

between COOH groups located on different parts of the polymer and do not required adjacent repeat units.26 A 1D 1H DQ NMR study of PAA also detected COOH signals at 12.9 and 10.7 ppm which were assigned to dimers and nonhydrogen bonded protons, respectively.31 However, a 2D DQ NMR study of PAA assigned the signal at 10.7 ppm to that in fast exchange between the dimer and the unbound free COOH. This study also showed that there was a DQ autopeak for the 12.9 and 10.7 ppm signals and that there were no cross-peaks between the two.32 Similar auto peaks are observed for PMAA. The presence of these auto peaks, at 13 and 10.6 ppm, themselves indicate dimer type structures. As there are no cross peaks between the signals at 13 and 10.6 ppm, this would agree with assigning the signals at 13 ppm to the more stable and isolated cyclic type dimers. Although the cyclic dimer contains the more favorable hydrogen bond interactions, the probability of forming this type of structure has been found to be 33% on average within organic crystal structures.33,34 It was suggested that this low probability is due to competing functional groups including water and carboxylate groups, such as those in partially deprotonated poly(carboxylic acid)s.33 The low probability of this cyclic dimer motif could also explain its 6019

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Figure 6. 2D 1H DQ MAS spectra of PMAA-d5 and its complexes in the SQ/DQ region of 0−15 ppm/0−20 ppm. The label COOH-aliphatic indicates the interpolymer PMAA H-bonds and the label D3 indentifies a PAAM-PMAA-d5 hetero type dimer

lower abundance within PMAA noted by Diez-Pena et al.8 Likewise it is believed that the higher molecular weight samples used here retained more water, which would prevent the formation of these cyclic dimers. Similar drying conditions were used for all the samples, but the higher molecular weight samples will have slower dynamics and thus take longer to rearrange and disassociate from water. For the more ambiguous resonance at 10.6 ppm, we believe that the fact that there is a DQ auto peak at (10.6 ppm, 21.2 ppm) implies that this signal also includes dimer type configurations, as previously stated. This would agree with the assignment to open type dimers, D2, but would however exclude lateral type hydrogen bonds. For small molecules, chains of hydrogen bonds are often more favorable as closer packing arrangements can be achieved, as opposed to consecutive cyclic dimers.35 Additionally, these open dimers are analogous to the head-to-tail type hydrogen bounds found in primary liquid alcohols, suggesting that this arrangement can be labile. The idea of labile hydrogen bonds

would agree with Akbey’s assignment of these protons as being in fast exchange between a hydrogen bonded and free state.32 However, we believe that the 10.6 ppm signal is a weaker type of hydrogen bond separate from that at 13 ppm, rather than exchange between the environments at 13 and 9 ppm. An averaged exchange peak is generally associated with kHz motion, which is unlikely to produce a DQ signal. Thus, the chains of open type dimers described by Nakashima appear to be an appropriate candidate for the 10.7 ppm peak. Linkages: COOH−Aliphatic Cross-Peaks of PMAA-d5 Complexes. Figure 6 compares the COOH-aliphatic crosspeaks within the PMAA-d5 and its complexes that depend on the proximity and abundance of the interpolymer linkages as well as molecular motion. The PEO−PMAA-d5 complex (Figure 6b) shows minimal COOH-aliphatic cross-peak intensities reflecting the effects of molecular motion on the dipolar coupling. In contrast, the PVME−PMAA-d5 complex (Figure 6d) contains the largest COOH−aliphatic cross-peaks 6020

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Figure 7. The hydrogen bonded region, SQ/DQ, 9−15 ppm/18−30 ppm, of the 2D 1H DQ-MAS of PMAA-d3 and its complexes. The labels D1 and D2 identify the autopeaks of cyclic and open dimers of PMAA respectively, while the D3 cross-peak indicates the PAAM−PMAA hetero dimer.

dried at pH 1 does not contain resonances in the SQ dimension above 8 ppm (Supporting Information, Figure S8e) and the observation by Dai et al. that the resonance of the amide proton when H-bonded to the oxygen of a carboxylic acid resonates at ∼7.4 ppm.36 Thus, the D3 cross-peak is the DQC between PMAA’s COOH, ∼10.5 ppm, and PAAM’s NH2, ∼7.5 ppm, within a hetero type dimer. 1 H−13C HETCOR NMR. The 1H−13C HETCOR spectra of PMAA and its complexes in Figure 8 provide additional insight into the hydrogen bonded network within these complexes. As mentioned earlier, the stability of the inner hydrogen bonds within the proposed chains of open dimers (Figure 3) is expected to vary depending on the total number of consecutive hydrogen bonds. A distribution of lengths of these hydrogen bonded segments could contribute to the width of the noncomplexed PMAA’s disordered carbonyl resonance centered at 182.5 ppm and the corresponding D2 proton resonance centered at 10.6 ppm (Figure 8c). The 2D DQ NMR spectra show that the polyethers disrupt the open dimers at 10.6 ppm in favor of the more stable cyclic dimers at 12.5 ppm (Figure 8). This correlates with their increased 13C

due to the proximity of the PVME methyl group to the hydrogen bonded COOHs. The PAAM−PMAA-d5 complex also contains D3 cross-peaks that arise from complexation (Figure 6c). Self-Association of PMAA within Complexes: PMAA Dimer Signals. Figure 7 displays the autopeaks of the D1 and D2 dimers within PMAA-d3 and its complexes. The PEO and PVME complexes contain the most intense and well-defined D1 PMAA dimer peaks. This, along with the slight shifts of the autopeaks to higher resonant frequencies, indicates that these D1 cyclic type dimers are more abundant and stable within the polyether PMAA complexes. Although not detected from the 13 C chemical shifts of the carbonyl groups, the 2D 1H DQ NMR spectra clearly show that the other complexes contain PMAA dimers. For the PAAM−PMAA-d3 complex (Figure 7c), the intensity of the D2 autopeak relative to its neighboring D1 autopeak is larger compared to noncomplexed PMAA and the other PMAA complexes. This is primarily due to the overlapping of D3 cross-peak, labeled in Figure 6c. This label was based on the fact that the 2D 1H DQ spectra of the noncomplexed PAAM 6021

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Figure 8. Carbonyl region of the 1H−13C HETCOR spectra of PVME-PMAA-13C (a), PEO-PMAA-13C (b), PMAA-13C (c), PVCL-PMAA-13C (d), PVPon-PMAA-13C (e) and PAAM-PMAA-13C. The spectra were acquired with a spinning speed of 15 kHz, using a contact time of 1.5 ms with a B1(1H) = 85 kHz and a B1(13C) = 55 kHz.

For the PVCL and PVPon complexes, the proton correlation to the carbonyl carbon is dominated by the complex protons (11 ppm) and carbonyls (179 ppm) with a minimal contribution from the D2 intra-PMAA hydrogen bonds (Figure 8, parts d and e). The PAAM−PMAA complex (Figure 8f) has a dominant contribution from the disordered correlation (11.5 ppm, 181 ppm) carbonyl associated with the weaker D2 type hydrogen bonds. Additionally, PMAA’s carbonyl is also associated with the PAAM’s NH2 protons, indicating complexation. Proton−Proton Distances from 1H DQ SpinningSideband Patterns. Spinning sideband analyses have been previously used to determine the interproton distances of Hbonded protons within a PMAA hydrogel and a PMAA-coPNIPAAm copolymer.7,8 The two proton−proton distances extracted from the PMAA hydrogel, 0.295 ± 0.015 nm and 0.275 ± 0.005 nm, were assigned to the weaker D2 (δ = 10.5 ppm) and stronger D1 (δ = 12.5 ppm) dimers, respectively.8 A

resolution, where the D1 cyclic dimer, centered at 12.5 ppm, is correlated with the 13C dimer resonance, centered at 187 ppm (Figure 8, parts a and b). The PEO−PMAA13C complex demonstrates a distinct contribution of the free protons centered at 9 ppm, correlated with the COOH peak at 179 ppm. These free protons are difficult to observe in the 1D 1H spectra (Figure 4) due to overlap with the intense aliphatic protons. In the noncomplexed PMAA, the 179 ppm carbonyl signal is also assigned to the free carboxylic acids, which was based on Asano’s assignment. In the PMAA−polyether complexes, the 179 ppm carbonyl is also assigned to the carbonyls engaged in interpolymer hydrogen bonds. The HETCOR spectrum of the PVME complex supports this assignment as the 179 ppm carbonyl peak is associated with the 10.6 ppm proton signal which, in turn, has COOH−aliphatic cross peaks (Figure 6), the signature DQ signal for the interpolymer linkages in the complexes. 6022

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Figure 9. 2D 1H DQ MAS NMR spectrum of PMAA-d3, using N = 4 cycle excitation/reconversion BABA sequence at νr = 30 kHz. To the right are the experimental (green) and simulated (blue) 1D DQ 1H MAS NMR sideband spectra, for the slices taken at 12.5 ppm (a) and 10.5 ppm (b). The asterisk denotes the DQ spinning sidebands of interest as the unlabeled peaks are the result of the overlapping DQC signal between the H-bonded COOH and aliphatic protons.

Table 1. Extracted 1H−1H Distances (nm) for PMAA and Its Complexes Obtained from Simulation of 1H DQ MAS NMR Spinning Sideband Patterns and Some Literature Reported Tg Values δ = 12.5 ppm

δ = 10.5 ppm

COOH−COOH compound PMAA8 PMAA-d3 PMAA-d5 PMAA-d5(aged) PEO−PMAA-d3 PEO−PMAA-d5 PVME−PMAA-d3 PVME−PMAA-d5 PAAM−PMAA-d3 PAAM−PMAA-d5 PVPon-PMAA-d3 PVPon-PMAA-d5 PVCL−PMAA-d3 PVCL−PMAA-d5

r (nm) D1 0.275 0.274 0.264 (0.280 0.266 0.262 0.261 0.262 0.280 0.270 0.283 0.271 0.280 0.267

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.015 0.015 0.015) 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015

COOH−aliphatic r (nm) D2

0.295 0.292 0.290 (0.300 0.311 0.300 0.308 0.295 0.288 0.292 0.308 0.298 0.308 0.292

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.015 0.022 0.022 0.022) 0.022 0.022 0.022 0.022 0.022 0.022 0.022 0.022 0.022 0.022

smaller distance of 0.270 ± 0.005 nm was extracted from the hydrogen bonded region (δ = 12.3 ppm), within the PMAA-coPNIPAAm complex, which was taken to be evidence of double and/or multiple hydrogen bonds.7 Other examples of extracted proton−proton distances for carboxylic acid dimers, determined by the same method, include 0.279 ± 0.09 nm (δ = 12.1 ppm) for a hexabenzocoronene carboxylic acid derivative29 and 0.22 ± 0.01 nm (δ = 12.5 ppm) for malonic acid.37 Similarly, thedouble quantum coherence signal of the NH−NH protons in uracil was associated with a distance of 0.244 nm, calculated from a geometry optimized crystal structure.38 We have applied the same experiment to the PMAA complexes and analyzed the data using a sideband analysis program provided by T. M. Alam,16 which includes a distribution factor to account for molecular motion. We applied a minimal distribution, σ = 1 Hz, since the dimers are expected to be relatively rigid and we wanted to compare our measurements with those presented in

r (nm)

compound

Tg/°C

PMAA

2104

0.306 ± 0.022

PEO

−734

0.295 ± 0.022

PVME

−2939

0.299 ± 0.022

PAAM

19940

0.299 ± 0.022

PVPon

16041

0.297 ± 0.022

PVCL

14542

0.297 ± 0.022

the literature. In general, larger dipolar coupling constants are required to accurately determine an appropriate distribution. Fits using σ =1 kHz on the systems with the largest dipolar coupling constants are included in the Supporting Information (Figure S13). A representative sideband fit is given in Figure 9 and Table 1 summarizes the extracted proton−proton distances. The simulated spinning sideband pattern of PMAA-d3 alone, calculated by assuming an isolated 1H−1H pair, gave dipolar couplings of 5.9 ± 1.0 kHz and 4.8 ± 1.0 kHz corresponding to proton−proton distances of 0.274 and 0.292 nm, which are comparable to the previously reported values.8 The distances extracted for an aged PMAA-d5 sample are longer (0.280 and 0.300 nm) reflecting the weakening of the hydrogen bonds when the sample is not maintained under sufficiently dry conditions. As the 1H−1H distances do not correlate with the order in which the samples were acquired, the observed trends 6023

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are attributed to the complementary polymer, with a consideration for an additional margin of error associated with the sample age (Supporting Information, Figures S11 and S12). The smaller proton−proton distances of the D1 cyclic dimers of PMAA-d3 complexed to PVME (0.261 nm) and PEO (0.266 nm) as compared to those of noncomplexed PMAA-d3 (0.272 nm) imply that stronger D1 cyclic dimers are formed in the presence of a polyether. The D2 open type dimer distance in the PAAM−PMAA-d3 complex (0.288 nm) is slightly smaller than that of noncomplexed PMAA-d3 (0.292 nm), whereas the other PMAA-d3 complexes display larger D2 1H−1H distances likely due to interpolymer hydrogen bonds disrupting the intraPMAA hydrogen bonds. The D1 and D2 proton−proton dimer distances in PMAA-d5 (0.264 and 0.290 nm) are smaller than in PMAA-d3 (0.274 nm, 0.292 nm). However, the D1 dimer distances of PMAA-d5 do not change significantly when complexed to the polyethers (0.263 nm (PVME), 0. 262 nm (PEO)). The high degree of order and stability of PMAA-d5’s dimers relative to those of PMAA-d3 is likely due to lower water content. The overlapping, unmarked, sidebands corresponding to the dipolar interaction between the aliphatic protons and the hydrogen bonded COOH groups, extracted at 10.5 ppm, have also been evaluated. The extracted distances between the aliphatic-COOH protons were as follows: 0.295 nm (PVME) ≤ 0.297 nm (PVCL, PMAA) ≤ 0.299 nm (PAAM, PVPon) < 0.306 nm (PEO). For comparison, the reported aliphaticCOOH H−H distance for malonic acid is 0.260 ± 0.015 nm.37

Table 2. Trends in the Properties of the Water-Saturated PMAA Complexes and Multilayers and DQ NMR of the Dry PMAA Complexes Critical pH Values13 PEO (4.6) < PAAM (5) < PVME (6.2) < PVPon (6.4) < PVCL (6.95) Bilayer Thickness (nm)13 PEO (20) > PVME (15) > PAAM (7) > PVCL (4.8) ≥ PVPon (4) Permeability (μm/s)43 PEO (0.42) < PVME (0.12) < PVPon (0.066) < PVCL (0.008) PMAA Chain Mobilities44 PEO > PAAM > PVME > PVPon ≥ PVCL COOH−Aliphatic DQC Cross-Peak Intensitiesa PEO (0.3) < PAAM (0.48) < PVCL (0.8) < PVPon (0.91) ≪ PVME (2.83) D1 Cyclic Dimer DQC Intensitiesb PEO (0.95) > PVME (0.85) ≫ PAAM (0.46) > PVCL (0.35) ≥ PVPon (0.34) D2 Open Dimer DQC Intensitiesb PEO (0.28) < PVME (0.31) < PVPon (0.34) ≤ PVCL (0.35) < PAAM (0.53) a

The average integrated COOH-aliphatic cross-peaks were normalized to the phenyl end group of PMAA-d5 within the complex to estimate the number of COOH H-bonds per PMAA-d5 chain. bRelative to the aromatic autopeak of PMAA-d5 and normalized to the noncomplexed PMAA-d5

aliphatic cross-peaks were normalized to the phenyl end group of PMAA-d5 within the complex, thereby giving an estimate of the number of COOH H-bonds per PMAA-d5 chain. This comparison of the cross-peak intensities should be viewed in light of the specific factors that influence the DQC intensities for each complex. Molecular mobility, which will both attenuate the DQC intensity and reduce the effective dipolar coupling constant, needs to be taken into consideration only for the polyethers (PEO; Tg = −55 °C, PVME; Tg = −31 °C). In fact, the large difference in mobility between PEO and PMAA in the solid complex is proposed to facilitate breakage of the interpolymer hydrogen bonding.4 Thus, the COOH−aliphatic cross peak intensity is likely attenuated by PEO’s backbone motion. In the case of PVME, the chain mobility is countered by the additional proton−proton interactions, since the methyl ether group is the hydrogen acceptor group. Because of the proximity of the methyl protons, the PMAA−PVME complex shows the most intense aliphatic−COOH cross-peak of all the complexes. Since the NH peak of PAAM is integrated together with the aromatic peak of PMAA-d5, its relative aliphatic− COOH cross-peak intensity is underestimated. However, with the exception of PVME, the trend in the quantity of interpolymer linkages, as measured by the aliphatic-COOH cross-peak intensities, reflects PMAA complex stabilities as indicated by the critical pH values. This suggests that the pH stability depends primarily on the amount of interpolymer linkages as the interpolymer H−H distances measured here did not vary significantly (Table 1, COOH−aliphatic). The extent to which the self-association of PMAA is modified by complexation with a hydrogen bond acceptor polymer reflects the relative strength and quantity of these interpolymer hydrogen bonds. We find that the PMAA intrapolymer Hbonding is least changed by complexation with PAAM and most disrupted by the other polymers. This conclusion is based on the trends in the DQC cross peak intensities of the cyclic (D1) and open (D2) type intra-PMAA dimers given in Table 2. The dimer intensities, relative to the aromatic autopeak of PMAA-d5 and normalized to that of the noncomplexed PMAAd5, indicates that the intra-PMAA association is weakest for

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DISCUSSION The response of H-bonded polymer complexes to environmental stimuli (temperature, pH, solvent, etc.) will depend on the interplay between the self-association of the component polymers (i.e., intrapolymer H-bonds) and the strength and abundance of the interpolymer linkages (complexation). Therefore, the aim of this work is to characterize these interactions by NMR and relate them to the observed properties, including those of multilayer films built with the same pairs of polymers, as we assume that the H-bonding trends observed in the complex would also apply to the corresponding multilayer film. We believe this is a reasonable assumption since we have previously found that the trends for the PMAA chain mobilities within complexes and supported multilayers are the same. In particular, the H-bond structure can account for the properties of the PMAA−PAAM complex, an outlier in the previously observed correlation between the PMAA chain mobility and bilayer thickness.44 In Table 2, the trends in the properties of the water-saturated PMAA-based complexes and multilayer films (critical pH, PMAA chain mobility, permeability, bilayer thickness) are summarized along with the intra-PMAA and interpolymer hydrogen bonding for the dried complexes as extracted from the NMR data (DQC cross-peak intensities). First we note that the relative number of interpolymer linkages, as deduced from the combined 1H and 13C NMR data, is consistent with the trend in the critical pH values. Whereas the formation of H-bonds between PMAA and the polyethers (PEO, PVME) gives rise to distinct carbonyl 13C chemical shifts, complexation of PMAA-d5 with PAAM, PVCL, and PVPon is detected by the appearance of COOH−aliphatic cross-peaks in the 2D 1H−1H DQ-MAS spectra (Figure 6). To compare the peak intensities, the average integrated COOH− 6024

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dimers. The net result is a higher open dimer population and more dense chain packing for PMAA when it is complexed to PAAM as compared to the polyethers, resulting in a thinner bilayer and reduced PMAA chain mobility.

complexation with PVPon and PVCL and least modified by PAAM. In the case of the polyethers, not only the quantity but also the strength of the intra-PMAA H-bonds, as reflected by the H−H distances (Table 1), is altered differently as compared to PAAM, PVPon, and PVCL. The PEO and PVME selectively disrupt the D2 open type dimers while maintaining and stabilizing the D1 cyclic dimers of PMAA. On the other hand, the relative DQC cross-peak intensities in Table 2 show that PVPon, PVCL, and PAAM equally disrupt both types of dimers, with the polylactames disrupting the most and PAAM the least. The H−H distances of both the cyclic and open-type dimers increase slightly for the PVPon and PVCL complexes, showing that both types of hydrogen bonds are weakened (Table 1) in addition to being greatly reduced in quantity. In the 13C NMR spectra of these complexes (Figure 2), the low population of intra-PMAA H-bonds results only in small shoulders on the broad carbonyl resonances at ∼187 ppm. This is in contrast to the distinct 13C carbonyl peaks for dimer (187 ppm), complex (179 ppm) and disordered (183 ppm) Hbonded COOH signals in the polyether complexes. The fact that the D1 cyclic dimers are stabilized (smaller H−H distances) while the D2 open type dimers are disrupted and weakened is consistent with the observations from 13C and 1H NMR that the intra PMAA H-bonding becomes more ordered (reduced linewidths) and stronger (larger chemical shifts) when complexed to PEO4 or PVME. This picture of the relative abundance and strengths of the inter- and intra-polymer H-bonds in the dry PMAA complexes can be related to the variation in the bilayer thickness, chain mobility and permeability of the hydrated complexes and multilayers. The open type dimers, which can form chains linking at least three COOH groups together (Figure 3), should result in more dense chain packing compared to the cyclic dimers. Thus, the selective disruption of the cyclic dimers when PMAA is combined with either the PEO or PVME as compared to those built with PVCL or PVPon, results in thicker and more porous multilayers. The rearrangement of the intra-PMAA Hbonds also results in a large amount of liberated PMAA chain segments in the case of PEO. This is verified by comparing the HETCOR spectrum of PMAA alone to the spectrum of PMAA−PEO (Figure 8, parts c and b), where the signal intensity assigned to free COOH groups is observed upon complexation of PMAA to PEO. The presence of PMAA chain segments that are neither complexed nor self-associated accounts for the high PMAA chain mobility within the PEO−PMAA complexes as compared to all the other complexes. Finally, we consider the case of the PMAA complex with PAAM, a hydrophilic polymer that can also self-associate and therefore could serve both as a H-bond acceptor (CO) and donor (NH). The weak aliphatic−COOH cross-peaks, combined with the most retention of the original PMAA dimer population, indicate that the PMAA−PAAM complex has the fewest interpolymer COOH H-bonds. This is in good agreement with its low critical pH value of 4.5−5.0. However, as noted earlier, the low critical pH value of PAAM does not fit into the trend in the bilayer thickness or relative PMAA mobilities within the complexes. In spite of having fewer interpolymer COOH linkages, PMAA multilayers with PAAM are thinner than those with PEO and PVME and are less mobile than that of PEO. Like the polylactams though to a far less degree, PAAM equally disrupts the D1 cyclic and D2 open dimers, whereas the polyethers selectively disrupt the open

■

CONCLUSION H and 13C NMR data were used to correlate the relative stability and quantity of H-bond linkages to the dissolution and dynamic properties of PMAA complexes with polyethers, polylactames, and PAAM. The relative quantities of interpolymer COOH H-bonds correlate well with the critical pH values. Two types of intra-PMAA H-bonds were detected and assigned to stronger cyclic, D1, versus weaker open-type, D2, dimers. The thick bilayers, enhanced chain mobilities, and high permeabilities produced when PEO or PVME are deposited with PMAA as multilayer films, were related to the retention and stabilization of the intra-PMAA cyclic dimers. For the polylactams, PVPon and PVCL, which form more stable PMAA complexes, there is a strong reduction and weakening of both types of intra-PMAA dimers. The moderate bilayer thickness and chain mobility but low critical pH value of the PAAM− PMAA complex were attributed to a relatively large population of intra-PMAA open-type dimers combined with the fewest interpolymer H-bonds. Overall, this study indicates that the nature of the polymer self-association after complexation may play a subtle but important role in determining the properties of materials based on hydrogen bonded polymer complexes. 1

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ASSOCIATED CONTENT

S Supporting Information *

Details regarding the synthesis of the labeled PMAA, the solution 1H NMR of the decomposed initiator and the gel permeation chromatography retention time traces, the 13C PST experiments of the different PMAA samples used, the t1ρ (1H) values of the carbonyl, the 1D 1H MAS and 1H DQ-MAS spectra of PMAA-d3 and its complexes dried without heating to illustrate the effects of heating as well as that of the PMAA-d5 and its complexes dried with heating, the 2D 1H−1H DQ-MAS and 2D 1H−13C HETCOR of the initiator to justify the assignment of the PMAA spectrum, the 1H−13C HETCOR of the d5, d3 and 13C labeled PMAA to confirm the assignment of the unbound free acid, the 2D 1H−1H DQ-MAS and 1H−13C HETCOR spectra of the complementary polymers, were included for reference, the 9−15 ppm region of 2D 1H−1H DQ-MAS of the PMAA-d5 and its complexes for comparison to that of PMAA-d3 shown in the paper, the 2D DQ nonsynchronized spectra of the aged PMAA-d5 and the PEO− PMAA-d3 complex to demonstrate the effects of aging, and fits, using a σ ∼ 1 kHz, of PMAA-d5 and its complexes with PEO and PVME, for comparison. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Cedric Malveau for assistance with the NMR at Université de Montréal and Dr. Todd M. Alam, from Sandia National Laboratories, Albuquerque, NM, for providing 6025

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the Matlab code to fit the spinning sideband patterns. This work was funded by NSERC and FQRNT.

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