1H MAS NMR Spectroscopy of Polyethylene Acrylic Acid Copolymers

Chapter 7

1H

Downloaded by PURDUE UNIVERSITY on August 5, 2013 | http://pubs.acs.org Publication Date (Web): October 14, 2011 | doi: 10.1021/bk-2011-1077.ch007

MAS NMR Spectroscopy of Polyethylene Acrylic Acid Copolymers and Ionomers

Todd M. Alam,*,1 Janelle E. Jenkins,1 Michelle E. Seitz,2 C. Francisco Buitrago,3 Karen I. Winey,2 Kathleen L. Opper,4 Travis W. Baughman,4 and Kenneth B. Wagener4 1Electronic

and Nanostructured Materials, Sandia National Laboratories, Albuquerque, NM 87185 2Department of Materials Science and Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104 3Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104 4Department of Chemistry, University of Florida, Gainesville, FL 32611 *E-mail: [email protected]

High speed solid-state 1H MAS NMR spectroscopy has been used to characterize a series of poly(ethylene-co-acrylic acid) P(E-AA) materials where the distributions of the pendant carboxylic acid group along the polymer backbone are precisely controlled. Ionomers obtained from partial neutralization of the carboxylic acid within these P(E-AA) materials with Zn2+ or Li+ were also investigated. Using a combination of 1D 1H MAS NMR and 2D 1H MAS NMR double quantum (DQ) and NOESY correlation experiments, details about the local P(E-AA) structure were obtained. The influence of precise versus random spacing of the carboxylic acid and the impact of Li- and Zn-neutralization on the polymer structure is discussed.

Introduction The control and tailoring of polymer microstructure to suit the final application of the material remains an important objective. In addition, the ability to precisely locate ions within the polymer structure and morphology is predicted to provide © 2011 American Chemical Society In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


control over the performance of ionomer membranes. Advances in the acyclic diene metathesis (ADMET) and ring-opening metathesis polymerization (ROMP) chemistry have now allowed production of polymers where the functional groups are precisely and pseudo-randomly placed along the polyethylene chain. Recent examples include the introduction of alkyl chains, halogens, phosphonic acids, and carboxylic acid groups with very controlled chain spacing (1–4). Understanding the local structure and morphology variations that occur with the inclusion of these functional groups is an area of active research. Characterization of these precise polymer materials has included the use of solid-state NMR spectroscopy; with 13C, 19F and 31P magic angle spinning (MAS) NMR (2, 3, 5), and static 2H NMR (4) investigations exploring the impact of random versus precise functional group incorporation on polymer crystallization. In this chapter, we present then high speed 1H MAS NMR characterization of a series of poly(ethylene-co-acrylic acid) copolymers, P(E-AA), and both Zn- and Li-neutralized P(E-AA) ionomers. The term ionomer has found use in literature to describe polymers that have ionic functional groups in humidified environments. For example, Nafion in considered an ionomer in both the acidic and neutralized salt form, because the sulfonic acid readily ionizes with water. In the present discussion we will refer to the non-neutralized P(E-AA) materials as acid copolymers, and the Li- or Zn-neutralized P(E-AA) systems as ionomers. This distinction arises because the P(E-AA) copolymer acts as a weak acid, with the carboxylic acids not being extensively ionized in a humid environment. The 1H MAS NMR spectra of polymers below Tg are commonly broad unresolved lines due to the presence of strong 1H-1H dipolar coupling (6, 7). Utilizing high speed (> 30 kHz) MAS spinning, this dipolar coupling is significantly reduced producing spectra with increased spectral resolution (8). Even though the 1H MAS NMR spectra for the P(E-AA) materials detailed here are relatively simple, details about the local hydrogen bonding environment and spatial connectivity between different functional groups can readily be obtained using variable temperature (VT) and multi-dimensional NMR spectroscopy correlation experiments as described below.

Experimental Section Material Preparation The synthesis and characterization of the linear poly(ethylene-co-acrylic acid), P(E-AA), materials have been previously described (1). Polymers with precisely spaced carboxylic acid groups were prepared using the ADMET chemistry. These polymer samples are designated as p9AA, p15AA and p21AA, and reflect samples where the carboxylic acid groups are precisely (p) located every 9th, 15th and 21st carbon along the backbone, respectively. The relative acid concentrations are 22, 13.3, and 9.5 mol%, respectively. A ROMP synthesis method was used to produce materials of equivalent acid concentrations, but with the carboxylic acid groups pseudo-randomly distributed along the polymer backbone. The designation r15AA, r21AA and r44AA reflect the random (r) 116 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


nature and the average number of carbons between acid groups. The structures of these random and precise materials are shown in Scheme 1.

Scheme 1. Structures of poly(ethylene-co-acrylic) acid copolymers and the corresponding Li- and Zn-neutralized ionomers. The Zn2+ neutralization of the P(E-AA) copolymers has been previously described (9). These polymer samples are designated as p21AA-56%Zn, p21AA-116%Zn, p15AA-82%Zn and p9AA-66%Zn to reflect the initial precise copolymer and the extent of neutralization. The Li+ neutralized material was prepared by dissolving the acid copolymer in a 1:4 mixture of 1,4-dioxane and 1-butanol at 90 oC, adding the appropriate amount of lithium acetate salt, followed by filtration of the resultant precipitant. These Li-neutralized materials are designated as p22AA-43%Li, p15AA-45%Li and r15AA-31%Li. The extent of Zn2+ or Li+ neutralization achieved was determined using inductively coupled plasma elemental analysis performed by Galbraith Laboratories (Knoxville, TN). The generalized structure for the exchanged P(E-AA) copolymers is shown in Scheme 1. The materials utilized for the NMR studies had previously been hot pressed at 150 oC (~ 0.4 mm film thickness) for X-ray scattering analysis. These polymer films were used as received unless otherwise noted. Solid-State 1H NMR Spectroscopy The solid-state 1D 1H magic angle spinning (MAS) NMR spectra were obtained on a Bruker AVANCE-III spectrometer operating at 600.13 MHz using a 2.5 mm broadband MAS probe, using N2 for spinning. A rotor-synchronized Hahn spin echo pulse sequence was employed (Figure 1a), with 2.5 µs π/2 pulse, 16 scan averages, and a 5 s recycle delay. The rotor spinning speed for analysis was 30 kHz, unless specifically noted. Spin regulation was maintained at ± 1Hz through the experiments. It is known that significant frictional heating occurs at high MAS speeds. The actual sample temperature was calibrated using the 207Pb chemical shift change of a secondary Pb(NO3)2 sample (10, 11), with all temperatures reported in this chapter reflecting this correction. The 1H MAS NMR chemical shifts were referenced to the secondary external standard adamantane, δ = +1.63 ppm with respect to TMS δ = 0.0 ppm. 117 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


The 2D 1H MAS NMR double quantum (DQ) to single quantum (SQ) correlation experiments utilized the chemical shift anisotropy (CSA) and off-set compensated Back-to-Back (BaBa) multiple pulse sequence for the excitation and reconversion of the multiple quantum coherences as shown in Figure 1b (8). The 0 → ±2 → 0 → 1 coherence pathway was selected using a 64 step phase cycle, 128-256 non-rotor synchronized t1 increments, a 2.5 µs π/2 pulse length, 8-64 scan averages, and an excitation/reconversion length τexc of 66.66 µs, and a dephasing delay of τ0 = 10 µs. Phase sensitive detection in the F1 dimension was obtained using the States-TPPI method (12). For this excitation time the observation of signal in the DQ NMR spectra reveals the existence of dipolar coupling between a pair of nuclei with 1H-1H distances < 5 Å. The solid-state 2D 1H NOESY MAS NMR correlation experiments utilized a standard non-rotor-synchronized sequence (Figure 1c) with mixing times τmix ranging to 1 ms. Typically 8 to 16 scan averages were employed with 512 to 1024 t1 increments. Spectral deconvolutions were performed using the DMFIT software package (13). The average 1H-1H dipolar coupling constants were obtained from simulation of the 1H MAS NMR spectra using in house software written in MATLAB 2010a (The Mathworks, Inc.).

Figure 1. Solid-state 1H MAS NMR pulse sequences utilized: a) 1D rotor-synchronized Hahn spin echo, b) 2D DQ-SQ MAS NMR correlation experiment using the DQ BaBa excitation/reconversion sequence, and c) 2D 1H MAS NMR NOESY. In these sequences the rotor period is defined as τR. Ab Initio Calculations The small acid-, ester- and Li-containing clusters were optimized in the Gaussian 09 software (14) (Gaussian Inc., Wallingford CT) using the 6-311++G(2d,2p) basis set (15, 16), and density functional theory (DFT) utilizing 118 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Becke’s three parameter exchange functional (17), and the LYP correlation function (B3LYP). The optimizations utilized a surrounding PCM solvent with a dielectric of ε = 2.3 to represent the surrounding continuum of a PE polymer. The integral equation formalism (IEF-PCM) model was implemented for these calculations (18). The chemical shielding tensors, σ, were calculated using the Gaussian 09 program utilizing the gauge-including atomic orbital (GIAO) method at the DFT level (19). All NMR shielding calculations for the small acid clusters used the same exchange and correlation functionals and basis sets as for the structure optimization. The NMR chemical shift is defined with respect to the chemical shielding of the TMS reference species by

where positive δ values represent environments that are deshielded and resonate at a higher frequency.

Results and Discussion 1H

MAS NMR

The solid state 1H MAS NMR spectra for the different random and precise P(E-AA) materials are shown in Figure 2. As predicted for such a simple copolymer, two major resonances were observed corresponding to protons in a methylene (CH2) environment (δ ~ +1.5 ppm) and to protons in carboxylic acid (COOH) environment (δ ~ +12 ppm). The high chemical shifts suggest that the carboxylic acids are strongly hydrogen bonded and were assigned to acid-acid dimers as shown in Scheme 2. The relative intensity of the COOH resonance scales correctly with the overall acrylic acid mol % (see Figure 2). For example, the p9AA (22 mol %) copolymer has the largest relative COOH intensity. The methine CH resonance (δ ~+2.2 ppm), and the CH2 groups α to the COOH functionality (δ ~ 1.5 to 1.7 ppm) were not resolved below Tg under the current MAS conditions. At very high temperatures in the melt (> Tg or Tm) the methine resonance was observable in the 1D 1H MAS NMR spectra. The 1H NMR chemical shifts and full width at half maximum (FWHM) line widths for the observed resonances are given in Table 1. Minor proton environments were also observed in several of these P(E-AA) materials, and include small amounts of adsorbed water ( δ ~ +4.8 ppm), and a carboxylic ester resonance at δ ~ +3.4 ppm (see r15AA in Figure 2). The assignment of this ester resonances is based on the presence of additional minor OCH3 carbon species, δ(13C) ~ +51 ppm, and an additional minor C=O species, δ = +174 ppm, observed in both the solution and the solid state 13C NMR. It should be noted that this lot of the r15AA polymer had some residual adsorbed methanol remaining from polymer precipitation and processing, which was finally removed after extensive vacuum pumping at 80 oC (see upper spectral inset, Figure 2). For the final (vacuum dried) r15AA sample, the carboxylic ester resonance represents a minor proton species and corresponds to ~1 to 3% (assuming a CH3 integration) of the entire 1H concentration. For samples containing observable carboxylic 119 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


ester, a second carboxylic acid resonance (COOHb) was also observed at δ ~ +7 ppm. The smaller 1H chemical shift of this acid species reflects a reduction in the hydrogen bond strength, and is assigned to carboxylic acids that are free or complexed to the carboxylic acid ester, instead of the strong hydrogen bonded acid-acid dimer structure. Additional discussion concerning the local COOHb proton environment is provided in the section on 2D correlation experiments below. The relative integration of the COOHa and COOHb carboxylic acid resonances are given by the a/b ratio in Table 1, and range between 10 and 3 for the r15AA and r21AA samples, respectively.

Figure 2. The solid-state 1H MAS NMR spectra (νr = 30 kHz, 332 K) for the precise and pseudo-random P(E-AA) copolymers. The NMR spectra are dominated by protons in the methylene and carboxylic acid environments.

Scheme 2. Proposed carboxylic acid dimer structure and carboxylic acid – carboxylic acid ester structure based on small cluster optimizations using ab initio DFT B3LYP/6-311++(2d,2p) methods. Simulation details are given in the experimental section. While the methylene can be deconvoluted into a broad and narrow component resonance under high speed (30 kHz) MAS, there are no clear 1H NMR spectral 120 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


signatures that allow quantification of the amorphous and crystalline morphologies known to be present within these pseudo-random and precise P(E-AA) acidic copolymers. The different contribution to the methylene line width are better resolved by analysis of static 1H spectral line shapes and DQ buildups, but are not discussed here. The crystalline and amorphous morphologies are clearly identifiable in the solid-state 13C MAS NMR spectra, vary with the degree of neutralization and preparation method, but will be presented and discussed in a subsequent publication (Jenkins and Alam, unpublished results). The changes in the crystallinity does not appear to significantly impact the observed 1H NMR chemical shifts. There are some generalized observations concerning the methylene line width that can be made. For the precise copolymer (Figure 2, left side) there is an increase in the CH2 line width as the spacing between carboxylic acids decreases. This inhomogeneous line width most likely reflects differences in the relative concentrations of the crystalline and amorphous component, along with the increased disorder in the CH2 environment for the copolymers with higher acid concentrations. This variation in the methylene line width was not as pronounced in the random copolymers (Figure 2, right side), where the relative contributions to narrow and broad line width components do not change significantly with increased spacing between acid groups. This is also consistent with the invariance in the amorphous/crystalline morphology ratio observed for these random materials using 13C MAS NMR (Jenkins and Alam, unpublished results). It has been shown that 1H MAS NMR can provide a wealth of information concerning hydrogen bonding (20), and in general, the chemical shift of the OH species gets larger with increasing hydrogen bond strength (21). The variation of the 1H NMR chemical shifts for the COOH environment with temperature in the P(E-AA) acidic copolymers is shown in Figure 3a. The δ > +12 ppm resonances have been assigned to acid protons in a strong hydrogen bonding environment such as that present in the acid-acid dimer structure (Scheme 2). For the random materials (r15AA and r21AA), the observation of distinct COOHa and COOHb resonances over the temperature range investigated (270 to 340 K) demonstrates that the proton exchange rate between these two different carboxylic acid hydrogen environments is significantly slower than the NMR time scale of the chemical shift difference (1/τexchange < 3500 Hz). For the precise and random P(E-AA) acidic copolymers studied, the high frequency COOHa resonance shows the largest temperature variation (Figure 3a) ranging from δ = +13.3 ppm to +12.2 ppm (Δδ = 1.1 ppm), and is very similar for all materials. The exception to this trend is the p15AA acidic copolymer which has a shifted chemical shift variation ranging from δ = +12.9 to δ = +11.9 ppm. The low frequency (weakly hydrogen bonded) COOHb environment also exhibits temperature variation (data not shown), but with a greatly reduced variation (Δδ = 0.3 ppm). Several of these acidic copolymers have transitions (Tg and Tm) in the temperature range studied by this NMR study (i.e. p9AA, p21AA and r15AA); which do not appear to have an impact of magnitude of the COOH temperature variation. The presence of semi-crystalline components appears to have little impact on the hydrogen bond strength. The consistent 1H NMR chemical shift temperature behavior between these different acidic copolymers demonstrates 121 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

that the local COOH environments are not significantly changed by the spacing between AA groups, or by the random or precise arrangement of these carboxylic acid functional groups. The lower 1H NMR chemical shift of the p15AA acidic copolymer argues that for this material the hydrogen bonding is weaker, again consistent with the lowest Tg observed (see Table 1). This reduced hydrogen bond strength most likely results from the energetic competition between acid dimer formation and the chain packing in the amorphous phase produced for this acid group spacing.


Table 1. The 1H MAS NMR (332 K) chemical shifts and line widths for the random and precise PE-AA copolymers, and the Zn2+ and Li+ ionomers Mol% AA

Tg(K) [Tm]

δ (ppm) [FWHM]a COOHa

COOHb

CH2

Precise p9AA

22

295c

12.3 [778]

--

2.5 [1403, 23.2%]b 1.5 [1070,76.8% ] b

p15AA

13.3

269c

12.3 [577]

--

1.9 [1660, 36.6%] b 1.1 [670,63.4%] b

p21AA

9.5

318c,d

12.4 [650]

--

1.7 [1564, 56.5%] b 1.6 [333, 44.5%] b

13.3

347 c,d

12.3 [455]

7.3 [365] [a/b ~ 10]e

1.65 [1940, 55.3%]

Random r15AA(dried)

b

1.54 [410, 44/7%] b r21AA

9.5

358 c,d

12.5 [750]

7.23 [360] [a/b ~ 3.2]e

1.51 [1790, 43.9%] b

1.48 [460, 56.1%] b --

4.5

371 c,d

12.2 [925]

p9AA-66%Zn

22

--i

12.2 [1260]

1.53 [1110]

p15AA-82%Zn

13.3

-- i

12.8 [970]

1.62 [975]

p21AA-56%Zn

9.5

326f

12.4 [1275]

1.7 [3560, 11.5%] 1.6 [705, 88.5%]

p21AA116%Zn

9.5

327g

--

1.55 [2365, 40%] 1.55 [745, 60%]

r44AA

1.7 [2130, 45%] b 1.5 [385, 55%]

Zn-Neutralized

Li- Neutralized Continued on next page.

122 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Table 1. (Continued). The 1H MAS NMR (332 K) chemical shifts and line widths for the random and precise PE-AA copolymers, and the Zn2+ and Li+ ionomers Mol% AA

Tg(K) [Tm]

δ (ppm) [FWHM]a


COOHa

COOHb

CH2

i

14.5 [1700] 13.2 [1710]

--

1.59 [1415]

p9AA-43%Li

22

--

p15AA-45%Li

13.3

-- i

14.5 [1700]

--

1.61 [995]

r15AA-31%Li

13.3

-- i

14.5 [1725]

--

1.53 [775]

a

δ = chemical shift (± 0.05 ppm), FWHM = full width at half maximum (±5 Hz ). b The relative % contributions of the different line width components. c Reference (1). d Tm, since this copolymer contained significant crystalline component. e a/b = ratio of integrals between the COOHa and COOHb species. f Reference (9). g Obtained from static 1H NMR line width analysis. i Not measured.

A linear correlation between the solid-state 1H NMR chemical shift and the H···O hydrogen bond length has been described for inorganic crystals (22)

While this relationship was developed for crystalline materials, it can still provide some quantitative information about the hydrogen bonding in these P(E-AA) copolymers and ionomers. For the acidic copolymer, the observed 1H NMR chemical shifts of the COOH environment vary between δ = +13.3 and +12.2 ppm (Figure 3a), with Equation (2) predicting a hydrogen bond length between 1.63 to 1.68 Å. The lengthening of the hydrogen bond at higher temperatures (approaching or above Tg) is consistent with increased local motions of the polymer chains that disrupt hydrogen bonding. Interestingly, these 1H NMR chemical shift-predicted bond lengths are very close to the 1.66 Å predicted from ab initio modeling of small carboxylic acid dimer (Scheme 2). Direct Gaussian GIAO modeling of the acidic dimmers predicts a δ ~ +13.7 ppm. Details for these simulations are provided in the experimental section. These NMR results demonstrate that the majority of the carboxylic acid protons in the P(E-AA) acidic copolymers reside in strongly hydrogen bonded-type dimer complexes. Only in the acidic copolymers, where an observable concentration of carboxylic acid ester was observed (see discussion above), are free or weakly complexed carboxylic acids resonances present. The ab initio predicted structure of the acid-ester dimer complex (Scheme 2) predicts an increase in the hydrogen bond length to 1.81 Å, corresponding to a predicted δ = 9.75 ppm (Gaussian GIAO δ = 10.6 ppm). While this decrease in the 1H NMR chemical shift is in the correct direction, it is not as 123 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


large as that observed experimentally (δ ~ +7.3 ppm), arguing that the hydrogen bonding in the COOHb species is disrupted to a larger extent than predicted using this simple model. The predicted 1H NMR chemical shift for a free, uncomplexed COOH species is δ = +6.6 ppm in a continuum dielectric of ε = 2.3 (polyethylene) using Gaussian GIAO methods. This lack of free acid groups in the P(E-AA) copolymers and Zn- and Li-neutralized ionomers (see those results below) is consistent with previous FTIR studies on poly(ethylene-ran-methacrylic acid) ionomers (23). It should also be noted that at a sample temperature of 270K (the lower limit of our fast MAS VT capabilities) the temperature changes in the chemical shift had not yet reached a plateau, suggesting that the dynamics of these carboxylic acid protons that influence the chemical shift are still dominant at this temperature.

Figure 3. The temperature variation of the COOH proton chemical shift for a) P(E-AA) acidic copolymers and b) Zn-neutralized P(E-AA) ionomers. The Tg and Tm values (Table 1) for different P(E-AA) materials in this temperature range are shown for comparison. The solid-state 1H MAS NMR spectra for the partially Zn- and Li-neutralized P(E-AA) copolymers are shown in Figure 4. The chemical shift and FWHM line width for the different exchanged materials are presented in Table 1. Similar to the acidic copolymers, the CH2 protons give rise to the dominant resonance, with the COOH resonance being reduced in intensity with increasing neutralization. For example, the inset (Figure 4) shows an expansion of the COOH resonance for the p21AA copolymer with different levels of Zn2+ neutralization, clearly revealing the loss of the COOH proton with Zn2+ incorporation into the material. For the p21AA-116%Zn material there is no visible COOH 1H NMR signal remaining showing complete removal of the carboxylic acid proton. For the Zn-neutralized materials the COOH chemical shifts are unchanged with respect to the un-neutralized materials, suggesting that the COOH groups retain strong dimer-type hydrogen bonding and that the neutralization with Zn2+ does not drastically impact the COOH environments. In contrast, the 1H NMR chemical 124 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


shift of the COOH protons in the Li-neutralized ionomers are larger (higher frequency) arguing an increase in the hydrogen bond strength following partial Li-neutralization. Using Equation (1), the observed δ = +14.5 and +13.2 ppm predicts a shortened hydrogen bond distance between 1.56 and 1.63 Å.

Figure 4. The 1H MAS NMR spectra (νr = 30 kHz) for the Zn- and Li-neutralized precise and pseudo-random P(E-AA) copolymers at 332K. The inset shows an expansion of the COOH proton region for the p21AA Zn neutralized materials: 0% (blue), 56% (red) and 116% (black) demonstrates the loss of this proton with increasing exchange.

This is quantitatively captured in the ab initio cluster calculations for the Li-containing dimer and tetramer clusters shown in Scheme 3. For the partially neutralized Li-acid dimer the predicted hydrogen bond length was 1.60 Å (predicted δ = +13.8 ppm). The preferred oxygen coordination of Li+ is 4, such that the tetramer cluster containing the partially Li+ neutralized carboxylic acid groups is perhaps a better description of the Li-acid clusters being formed. This tetramer cluster has a range of hydrogen bond lengths (1.71, 1.62 and 1.51 Å), with the average 1.61 Å hydrogen bond distance predicting a δ = +13.6 ppm. It should be noted that these ab initio clusters are representative of the type of changes that are occurring in these ionomers, and are probably simplistic in the description of the overall structure. For example, in the Zn-neutralized ionomers, large ionic aggregates containing numerous Zn-acid environments have been proposed to described the overall polymer structure, (9) and that the NMR spectra would be an average over these different configurations. Clearly the 1H MAS NMR chemical shift remains a sensitive probe to changes in the hydrogen bond strength in these P(E-AA) ionomers during neutralization. 125 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Scheme 3. Proposed carboxylic acid – Li+ neutralized dimer structure and the carboxylic acid – Li+ neutralized tetramer structure based on small optimized cluster models obtained using ab initio DFT B3LYP/6-311++(2d,2p) methods. The temperature variation of the 1H NMR chemical shifts for the COOHa resonance in the Zn-neutralized P(E-AA) ionomers is shown in Figure 3b. The p21AA-116%Zn ionomer has no observable COOH resonance, and is therefore these values are not included. At low temperatures the chemical shifts are similar to those observed in the acidic copolymers, but show a marked deviation at higher temperatures. This may reflect changes that are occurring during the melting transition, which is in contrast to the temperature invariance of the acidic copolymers during transition. This would suggest differences in the polymer structure between the Zn-neutralized ionomers and the original acidic copolymers. The temperature variation of COOH environment in the Li P(E-AA) ionomers was very complex and revealed multiple COOH domains with differing concentration of coordinated Li+. This differential segregation was explored using 1H-7Li REDOR MAS NMR spectroscopy and will be presented elsewhere (Alam, unpublished results). 2D MAS NMR Correlation Experiments Additional details about the local structural environment are obtained using 2D NMR correlation experiments. Figure 5a shows the 2D 1H MAS NMR DQ-SQ correlation spectrum for the p9AA acid copolymer. Similar results were observed for the p21AA copolymer. These 2D DQ experiments provide a sensitive tool to measure spatial proximity of protons within these materials, with the intensity of the DQ peaks being a function of the distance between interacting spins (~ 1/r6). The 2D DQ-SQ NMR experiments reveal the expected interactions for the proposed structures in these P(E-AA) copolymers. There is a strong autocorrelation peak on the diagonal (dashed line) for the CH2 proton environments, consistent with strong dipolar couplings (close spatial interactions) within the polymer backbone. In these experiments, a lack of an autocorrelation resonance would show that protons of the same chemical shift (i.e. the same environment) are not dipolar coupled to each other, or that local dynamics are large enough to average the 1H-1H dipolar coupling. For the p9AA copolymer, the observation of a COOH autocorrelation resonance proves close proton-proton distances between acid groups, and supports the argument for an acid dimer 126 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


structure in these materials (ab initio predicted 1H-1H distance 2.4 Å). Due to exchange effects for the acid proton, (see previous temperature variation of chemical shift discussion) we are unable to directly measure the 1H-1H distance using the DQ NMR experiments. A water autocorrelation resonance was also observed, and is consistent with either water-water clustering in the material, or strongly absorbed water molecules with very limited mobility (24). The off diagonal resonances in the 2D NMR DQ-SQ correlation experiments reveal spatial contact between different types of protons, including the predicted COOH-CH2 cross peak. By increasing the sample temperature to 332 K (above Tg) the COOH-COOH cross peak completely disappears in the 2D NMR DQ-SQ spectrum, consistent with the motional averaging of the 1H-1H dipolar coupling due to increased polymer backbone dynamics. Figure 5b shows the 2D 1H MAS NMR NOESY correlation experiment, which reveals similar spatial interaction for the off-diagonal resonances. No information about the spatial interaction between similar proton environments (autocorrelation type resonances) can be extracted from these NOESY experiments as the diagonal resonances in these 2D experiments represent magnetization that has not exchanged.

Figure 5. The a) 2D 1H MAS NMR DQ-SQ correlation spectra obtained at 273K (νr = 30 kHz) and the b) 2D 1H MAS NOESY (τmix = 10 ms) at 332K (νr = 30 kHz) for the precise p9AA copolymer. Several different cross peaks resulting from through space conductivities were observed, including the carboxylic acid auto-correlation resonance in the 2D DQ spectra. Additional discussion provided in the text. The 2D 1H MAS NMR DQ-SQ correlation experiments (data not shown) for the pseudo-random P(E-AA) acid copolymers are very similar, with the exception of those containing significant concentrations of the carboxylic ester proton species. Additional information concerning the local structure in these ester-containing materials can be realized by inspection of the 2D 1H MAS NMR NOESY spectra shown in Figure 6. The protons in the carboxylic ester environments (δ ~ 3.8 ppm) are more mobile (have a longer T2), such that the 2D NMR NOESY spectrum tends to emphasizes these spectral features at τmix = 127 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

10ms, allowing strong correlations to be observed even for these minor species. Interestingly, Figure 6 shows that the strongly bonded carboxylic acid protons (COOHa) only have spatial contacts to the CH2 protons (similar to that in Figure 5b), and does not appear to be spatially located near either the weakly hydrogen bonded carboxylic acid (COOHb) or the carboxylic ester. Similarly, the protons in the COOHb species only show through space contact with the ester protons, consistent with the previous assignment of a weakly bonded interaction. These results suggest that these different types of carboxylic acids are not within the same cluster.


Dynamic Heterogeneity The 1H MAS NMR spectra reveal that strong 1H-1H dipolar interaction are present within both the P(E-AA) copolymers and ionomers. Below Tg, relatively broad CH2 resonances with spinning sidebands (SSB) were observed due to strong 1H-1H dipolar couplings not completely removed by fast MAS spinning (30 kHz) or local chain dynamics. These SSB occur at integer values of the spinning speed around the dominant CH2 resonance, as shown in Figure 7a. The previous sections have concentrated on analysis of the isotropic resonance region to extract chemical shift information, and have ignored the SSB patterns present within the NMR spectra. Simulations of the SSB patterns in the 1H MAS NMR spectra can provide an effective or average dipolar coupling values (Deff).

Figure 6. The 2D 1H MAS NOESY (τmix = 10 ms) at 332K (νr = 30 kHz) for the r15AA acid copolymer. The strongly hydrogen bonded carboxylic acid (COOHa) environment reveals only through space proton-proton contacts with the backbone CH2 protons, while the more weakly hydrogen bonded carboxylic acid (COOHb) has dominant through space interactions with the methyl ester protons (OCH3). No contact between the COOHa and COOHb protons is observed. Additional discussion provided in the text. 128 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 7. The influence of inhomogeneous T2 relaxation on: a) The observed 1D 1H MAS NMR line shape for the p15AA-45%Li ionomer as a function of the echo time. b) Expansion of the -1 and -2 SSB spectral region showing this heterogeneous loss, c) Change in overall signal intensity for the COOH and CH2 resonances, d) Variation of the FWHM for the isotropic resonance and e) Change in the effective dipolar coupling constant extracted from simulation of the 1D 1H MAS NMR spectra in (a). The polymer dynamics on the other hand produce averaging of the dipolar coupling, which is readily reflected in changes to the SSB patterns in the MAS NMR spectra. While it is tempting to discuss the local structure and environment of these P(E-AA) copolymers as being uniform, it is important to remember that these co-polymers are heterogeneous in both their structure and dynamics. For example, heterogeneous dynamics were readily observed during 1H MAS NMR echo spin-spin T2 relaxation experiments, where changes in the MAS NMR spectra result with increasing echo times. For example, Figure 7a shows the T2-Hahn echo spectra for the pAA15-45%Li ionomer sample as a function of rotor-synchronized echo delays (Nc = number of rotor cycles, νr = spinning speed). Even though the overall signal intensity for the CH2 and COOH resonances reveals a single exponential decay (Figure 7c), close inspection of the 1H MAS NMR spectra (Figure 7a) reveal heterogeneous changes in the spectra. Most importantly are the change in relative intensity of the ±1 and ±2 spinning sideband (SSB) shown in Figure 7b, where the spectra have been normalized to the isotropic peak intensity. The SSB intensities decrease with increasing echo time as a result of differential T2 relaxation. Simulations of these SSB changes correspond to a decrease in the effective dipolar coupling Deff (Figure 7d). These spectral changes occur because the polymer fractions with smaller Deff (fewer SSB) have longer T2 relaxation times, with this signal from these fractions being preferentially retained at longer echo periods. A reduction in the line width of the 129 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

isotropic resonance as a function of echo time (Figure 7e) was also observed. The MAS line width of the isotropic resonance is known to be impacted by dipolar coupling involving higher order spin correlation, which are diminished (narrower line widths) for those polymer environment that are more dynamic, and have a longer T2 relaxation. These heterogeneous dynamics and the averaging inherent in NMR measurements should always be considered when comparing information obtained using other structural refinement methods (dielectric, neutron scattering, etc.), or to structural details extracted from other NMR experiments employing multiple pulse sequences where heterogeneous relaxation may be involved.


Conclusions High speed solid-state 1H MAS NMR has been presented for a series of precise and pseudo-random P(E-AA) acidic copolymers and the related Li- and Zn-neutralized ionomers. Even though the 1H NMR spectra were relatively simple, a wealth of information concerning the local carboxylic acid hydrogen bonding environment in these materials was obtained. These NMR results demonstrate the formation of strong acid-acid dimers, with very little free acid present within the P(E-AA) materials. While the Zn-neutralization does not produce a large change in the acid-acid hydrogen bonding environment, the introduction of Li+ into the P(E-AA) ionomers actually increases the strength of the acid hydrogen bonding.

Acknowledgments Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The NMR portion of this research was funded entirely through Sandia’s LDRD program. KIW acknowledges funding from the National Science Foundation Polymers Program, Grant DMR 0549116.

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1H MAS NMR Spectroscopy of Polyethylene Acrylic Acid Copolymers

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