DNP NMR Studies of Crystalline Polymer Domains by

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers Ester Verde-Sesto,† Nicolas Goujon,‡ Haritz Sardon,† Pauline Ruiz,‡ Tan Vu Huynh,‡ Fermin Elizalde,† David Mecerreyes,†,§ Maria Forsyth,‡,§ and Luke A. O’Dell*,‡ †

Downloaded via KAOHSIUNG MEDICAL UNIV on October 4, 2018 at 03:45:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

POLYMAT, University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avda. Tolosa 7, 20018 Donostia-San Sebastian, Spain ‡ Institute for Frontier Materials, Deakin University, Geelong, Victoria 3220, Australia § Ikerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain S Supporting Information *

ABSTRACT: Dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy is increasingly recognized as a powerful and versatile tool for the characterization of polymers and polymer-based materials. DNP requires the presence of unpaired electrons, usually mono- or biradicals, and the method of incorporation of these groups and their distribution within the structure is crucial. Methods for covalently binding the radicals to the polymer and controlling their location (e.g., exclusively within a specific phase or at an interface) can allow the selective enhancement of a particular region or the measurement of domain sizes. We have prepared a series of polyurethanes by copolymerization of a nitroxide radical monomer with poly(ethylene glycol) (PEO) and diisocyanate linkers. The PEO is shown to form crystalline domains with the radical monomers in a separate phase, providing DNP enhancements of around 10 and allowing the domain size and morphology to be probed with the aid of X-ray scattering data. Additionally, electron paramagnetic resonance is used to estimate the inter-radical distances and density functional theory calculations are used to refine the PEO crystal structure.

1. INTRODUCTION Dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy is becoming an increasingly popular characterization technique thanks to numerous technological and methodological advances made over the past few decades.1 It allows the much larger Zeeman polarization of unpaired electrons to be transferred to nuclear spins, providing potential NMR signal enhancements of 1 or 2 orders of magnitude.1,2 As solid-state NMR is well-established as a powerful technique for studying polymers,3 capable of providing detailed information about molecular structures, conformations, domain sizes, and dynamics, polymers have long been involved in the development of DNP,4−9 and more recently a growing number of DNP NMR studies of polymeric materials are being reported.10−28 The method of incorporating the polarization source (unpaired electrons, usually in the form of stable radicals) within the polymer is crucial. While the radicals act as the source of polarization enhancement, they will also cause quenching of the NMR signals from nuclei in close proximity. Moreover, the ability to control the location of the radicals can allow the selective enhancement of a particular phase or region, which can be particularly useful in studies of multicomponent polymers. For example, Afeworki and Schaefer studied the interfacial region in polystyrene−polycarbonate blends formed © XXXX American Chemical Society

by solvent casting with radicals introduced only to the former phase and showed that the polarization was transferred across the interface via direct, long-range polarization transfer from the radicals to the protons in the polycarbonate phase, over a distance on the order of 50 Å.8,9 More recently, Emsley and coworkers have measured domain sizes on the order of tens of nanometers in a two-phase polymer system by doping one phase with a radical and monitoring size-dependent spindiffusion mediated changes in polarization buildup times in the other phase. 24,25 Viel and co-workers have discussed preparation methods for polymer DNP experiments, including several different ways of incorporating exogeneous radical molecules into the material such as film casting, glass forming, and incipient wetness impregnation.18,19,21,22 The above approaches have all involved some form of noncovalent molecular doping of the radical molecules into the materials, potentially allowing the diffusion of these species within the polymer domains or across phase boundaries. Alternatively, the covalent attachment of radicals to the polymer can provide a greater level of control over their location and spatial distribution. Previous publications have Received: August 3, 2018 Revised: September 17, 2018

A

DOI: 10.1021/acs.macromol.8b01665 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Synthesis route and molecular structure of the nitroxide-radical containing polyurethane polymers using hexamethylene diisocyanate linkers. When the number of bis-MPA-TEMPO equivalents x is equal to 1, a PU homopolymer based on MPA-TEMPO-HDI is obtained, whereas a MPA-TEMPO-HDI-PEG copolymer is obtained when using x < 1 in which case x represents the number of equivalents of MPA-TEMPO in the final polymer.

covalently “tagged” radicals onto polymeric systems such as polypeptides12 and agarose gels10 for DNP applications. Notably, Münnemann and co-workers synthesized thermally responsive radical-bearing hydrogels by converting carboxylic acid monomer groups into TEMPO-based units,11 providing a hyperpolarization medium for solution-state DNP experiments. In this work, we explore a novel approach for the covalent incorporation of radicals into solid polymer materials for DNP NMR applications. Nitroxide monoradicals based on 2,2bis(hydroxymethyl)propionic acid (MPA) and (2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPO) are incorporated directly into a polyurethane system by direct copolymerization of a bis-MPA-TEMPO monomer29 with poly(ethylene oxide) (PEO) and hexamethylene diisocyanate (HDI) linkers. These materials have subsequently been characterized by differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR), DNP NMR, and small-angle X-ray scattering (SAXS). The latter techniques in combination with the spin diffusion model of Emsley and co-workers24,25 provide insights into the PEO crystallinity, domain size, and morphology, and a preliminary demonstration of the potential use of these materials as a host matrix for studies of other materials by DNP is also presented.

2.2. Differential Scanning Calorimetry (DSC). DSC analysis was performed on a NETZSCH DSC 214 Polyma instrument, which was calibrated using cyclohexane. DSC scans were acquired from the PU-X urethanes as a function of TEMPO concentration, up to 15 mol %. Each sample was first quenched from 20 to −100 °C and then heated to 80 °C, which is beyond the melting points in all cases (first heating scan), at a heating rate of 10 °C min−1. This heating cycle was repeated twice more (second and third scans) by cooling the sample from 80 to −100 °C at a cooling rate of 10 °C min−1. For the quenching experiments, an additional isothermal step of 30 min was added at 80 °C, and a cooling rate of 40 °C min−1 was used. 2.3. Electron Paramagnetic Resonance (EPR). EPR measurements were performed on the solid polymer samples at room temperature using an Adani CMS 8400 X-band EPR spectrometer. The field was swept from 309 to 359 mT with a scan time of 50 s, a microwave frequency of 9.4 GHz, 4096 data points per scan, and four scans accumulated from each sample. The spectra were subsequently fitted using the EasySpin software30 to a “pepper” (static solid state) model including the g tensor, 14N hyperfine coupling tensor, and Lorentzian line broadening. 2.4. Dynamic Nuclear Polarization Nuclear Magnetic Resonance (DNP NMR). DNP NMR experiments were performed at 9.4 T using a Bruker Avance III 400 MHz DNP NMR spectrometer with a 263 GHz gyrotron and a 3.2 mm triple-channel MAS DNP probe. Samples were packed into 3.2 mm sapphire MAS rotors with zirconia drive caps, inserted into the precooled probe, and spun at 8 kHz. The sample target temperature was set to 100 K unless otherwise specified, but it should be noted that the microwave irradiation applied during the DNP experiments will cause some sample heating. 1 H and 13C spectra were recorded using single-pulse and crosspolarization (CP) pulse sequences, respectively, with chemical shifts referenced to TMS at 0 ppm. DNP enhancement factors (ε) for both experiments were measured by running acquisitions with the microwaves switched on and off and comparing the resulting signal intensities. The effective 13C CPMAS longitudinal relaxation times T1* were also measured using a saturation recovery CP pulse sequence and a stretched exponential model.31 Other experimental details are given in the figure captions. 2.5. Small-Angle X-ray Scattering (SAXS). SAXS and wideangle X-ray scattering (WAXS) measurements were performed at the Australian Synchrotron on the SAXS/WAXS beamline. An in-vacuum undulator (22 mm period, 3 m length, Kmax = 1.56) with a beam energy of 16 keV and a 7.0 m camera length were used, allowing a detection range for a momentum transfer of 0.003 < q < 0.205 Å−1. The two-dimensional SAXS patterns were recorded with a 1M Pilatus detector with 981 × 1043 pixel resolution. The one-dimensional WAXS patterns were simultaneously recorded with a 200K Pilatus detector with 981 × 195 pixel resolution. A momentum transfer of 1.30 < q < 4.23 Å−1 was detected for the WAXS measurements. WAXS intensity were normalized using the strong peak at q ∼ 1.36 Å−1.

2. EXPERIMENTAL DETAILS 2.1. Sample Preparation. The synthesis route is summarized in Figure 1. The diol-containing pendant nitroxide, bis-MPA-TEMPO, was synthesized following the procedure previously described by Garmendia et al.29 The poly(MPA-TEMPO-HDI-PEO) urethanes were prepared as follows. Specific molar ratios of bis-MPA-TEMPO and hexamethylene diisocyanate (HDI) were mixed in dried dichloromethane for 15 min prior to the addition of the catalyst, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The reaction was monitored by FTIR, and once around 50% conversion was reached, the required amount of PEO diol was added. The reaction was kept at room temperature for 48 h, after which the isocyanate signal completely disappeared in the FTIR; the catalyst was quenched with benzoic acid, and the polymer was precipitated into cold diethyl ether, isolated by filtration, and dried to obtain an orange solid. Several samples were prepared with different molar ratios of the bis-MPATEMPO and PEO monomers and are referred to hereafter as PU-X, where X is the molar percentage of the radical monomer relative to the PEO. The final composition of the samples was confirmed by 1H NMR and FTIR, and this characterization data is consistent with previous reports.29 A sample of PU-15 was also mixed in water with 25 wt % histidine and then dried in a vacuum oven. Further details on sample synthesis methods and FTIR characterization are provided in the Supporting Information. B


Article

Macromolecules

Figure 2. DSC traces (heating rate of 10 °C min−1) of the polyurethanes as a function of TEMPO concentration (0−15 mol %) after cooling from the melt at (a) 10 °C min−1 and (b) 40 °C min−1. 2.6. Density Functional Theory (DFT) Calculations. DFT calculations were performed on a crystal structure of poly(ethylene oxide)32 using the CASTEP software33 running under the Materials Studio environment. 13C chemical shielding tensors were calculated using Perdew, Burke, and Ernzerhoff functionals, on-the-fly pseudopotentials, a plane wave basis set cutoff of 610 eV, and a 2 × 1 × 1 Monkhorst−Pack k-space grid. In some cases, atomic positions were optimized with the unit cell dimensions fixed (see figure/table captions). The calculated isotropic 13C chemical shieldings (σiso) were converted to chemical shifts (δiso) using the formula δiso = A − σiso, where A was set to 167 ppm to line up the best simulated spectrum with the experimental one. The predicted 13C MAS NMR spectra were constructed using 14 Lorentzian peaks of equal width and height.

results suggest that the addition of the TEMPO monomer promotes the crystallization of the PEO segments. See the Supporting Information for a more detailed discussion of these DSC results. The X-band EPR spectra obtained from the samples at room temperature are shown in Figure 3a alongside fitted simulations (Figure 3b). The spectral line shapes are generally

3. RESULTS AND DISCUSSION Figure 2a shows the DSC traces of PU-X copolymers as a function of TEMPO monomer concentration, up to 15 mol %. For these traces, the samples were cooled from the melt state to −100 °C at a rate of 10 °C min−1 before heating to 80 °C at the same rate. An endothermic peak is observed in all samples above 30 °C, corresponding to the melt transition of the crystalline PEO phase. As the TEMPO monomer concentration increases, this melting point is seen to gradually shift to a higher temperature, increasing from 20.8 to 41.6 °C for the PU-0 and PU-15 samples, respectively, while the measured enthalpy of fusion also increases from 69.1 to 125.3 J/gPEO. From the enthalpy of fusion, the weight percentage of crystalline PEO in the PU-X urethanes can be calculated (see the Supporting Information for details), resulting in a crystalline PEO weight percentage of 33.7% and 61.1% for the PU-0 and PU-15 samples. Both of these observations suggest a larger extent of crystallization of the PEO chains with increasing TEMPO monomer content. This is also consistent with the disappearance of the PEO glass transition (visible in Figure 2a at around −50 °C for the PU-0 sample) when the TEMPO units are introduced. DSC heating traces were also acquired after a faster cooling rate of 40 °C min−1 (Figure 2b). A sharp exothermic event is seen at −33 and −45 °C for the PU-0 and PU-5 samples, respectively, corresponding to the crystallization of the PEO segments and therefore suggesting the formation of some amorphous PEO during quenching. However, the incorporation of 5 mol % TEMPO monomer decreased the enthalpy of crystallization from −32.8 to −16.4 J/gPEO, indicating a reduction of the amount of amorphous PEO present in the system, while at higher TEMPO monomer concentrations no crystallization peak was observed. These

Figure 3. (a) Solid-state X-band EPR spectra obtained from the polymers at room temperature and (b) fitted simulations. (c) Plot of line widths used in each simulation (red circles) and estimated average inter-radical distances (blue squares) as a function of the radical monomer content, with linear fit shown for the former. C


Article

Macromolecules characteristic of the nitroxide radical with splittings arising from the hyperfine interaction with the 14N nucleus and anisotropic effects visible due to the solid-state nature of the sample. The principal values for the g (Zeeman) and A (hyperfine) tensors obtained from the fits were g11 = 1.9962(2), g22 = 2.0007(7), g33 = 2.003(1), A11 = 6(6), A22 = 35(3), and A33 = 90(5), the latter three given as A/h in megahertz. While these values remained roughly constant over all the samples studied, the spectra show significant line broadening as the radical monomer content increases. This could be simulated to good accuracy by incorporating isotropic Lorentzian line broadening in the fits (Figure 3b). We note that separate fits were performed with the inclusion of distributions in the g and A tensor components as well as the existence of unresolved hyperfine couplings, but these resulted in only marginal improvements. The variation of these line broadening values with radical content is highly linear (Figure 3c, red data points) and can be explained by variations in the electron dipolar coupling contribution resulting from the decreasing average distance between the radical monomer units. The latter quantity was estimated using the expression ΔH = (3 × 104)/d3 for a statistical distribution of radicals within a lattice, where ΔH is the dipolar contribution to the EPR line width (in gauss) and d is the average distance between radicals (in angstroms).34 These distances are also shown in Figure 3c (blue data points) and vary from around 23 Å in sample PU-5 down to 11 Å in PU-50. We note that despite apparent variations in the morphology of the separated PEO and TEMPO-containing phases across the range of compositions studied (vide infra), the highly linear variation in the EPR line broadening value suggests that the TEMPO units are quite evenly distributed within the latter component. A DNP-enhanced 13C CPMAS spectrum obtained from sample PU-15 at a temperature of around 100 K is shown in Figure 4a. The most intense signals occur at chemical shifts in the range 70−74 ppm (Figure 4c), arising from crystalline PEO.35 Additionally, a shoulder is present on the lowfrequency side of these peaks at around 62 ppm. This shift is indicative of CH2OH PEO chain-end groups but may also be contributed to by a small amount of amorphous PEO phase.36 The relative quantification of these PEO signals is made difficult by the nonquantitative nature of the DNP experiment, wherein the spin-diffusion-mediated signal enhancement will strongly depend on the sizes of these domains and their distance from the nitroxide radicals. However, it is clear that the vast majority of the PEO component exists in the crystalline form at this temperature. Also visible in the 13C spectrum in Figure 4a are peaks arising from the hexamethylene diisocyanate (HDI) linker groups. Specifically, two distinct peaks centered at 29 and 41 ppm arise from the CH2 groups of the HDI37 and can be fitted to three Gaussian peaks of equal intensity located at chemical shifts of 41.3, 30.5, and 27.0 ppm, representing the outermost, middle, and innermost pairs of CH2 groups in the nominally symmetric HDI linkers. The signal intensity of these peaks decreases with increasing radical content (Figure 4b). This cannot be attributed entirely to the different DNP signal enhancement factors as large differences are observed between PU-5 and PU-15 which show very similar 13C CPMAS enhancement values (Table 1). A more likely explanation is that because NMR signals from carbon sites close to the radical units will be quenched by the dipolar interaction with the unpaired electron, the observed CH2 13C CPMAS signals will

Figure 4. (a) DNP-enhanced 13C CPMAS NMR spectrum obtained from the radical polymer PU-15 at 9.4 T, 8 kHz MAS, and 100 K (400 scans acquired with a recycle delay of 4 s, asterisks indicate spinning sidebands). (b) Expansion of the 13C signals from the HDI linker groups in the DNP-enhanced CPMAS spectra of the samples indicated (PU-X). (c) 13C CPMAS peaks arising from the crystalline PEO phase and (d) a spectrum calculated from a DFT-optimized crystal structure with the 14 distinct carbon peaks shown.

Table 1. Apparent Longitudinal Relaxation Times (T1*) Measured Using a Saturation Recovery 13C CPMAS Experiment with and without Microwave (MW) Irradiation Applied and DNP Enhancement Factors (Signal Intensity with DNP Divided by Signal Intensity without) Measured from 1H MAS and 13C CPMAS NMR Experiments with a Recycle Delay of 4 s C CPMAS T1* (s)

13

polymer

MW off

MW on

PU-5 PU-10 PU-15 PU-20 PU-30 PU-40

1.44 6.50 4.16 2.36 0.31 0.17

1.13 3.91 2.48 1.69 0.28 0.16

DNP enhancement factor 1

H MAS 11.0 9.1 9.5 8.8 6.0 4.2

13

C CPMAS 10.4 10.1 10.5 9.6 7.8 3.7

therefore arise only from HDI linker groups located between two PEO monomers. As the radical monomer content increases, such groups become statistically less likely, and for PU-30 these signals are below the noise level. A similar effect is seen for the carboxyl peak at 157 ppm. To reproduce the observed 13C NMR peak shapes for the crystalline PEO phase (Figure 4c), we generated predicted spectra using density functional theory (DFT) calculations33 performed on the reported XRD crystal structure,32 which consists of parallel PEO helices featuring 14 distinct carbon environments. A poor agreement with the experimental line shape was obtained from the reported crystal structure (see the D


Article

Macromolecules

suggesting smaller PEO domain sizes and much closer average proximity between the bulk crystalline PEO phase and the nitroxide radicals. Interestingly, the PU-5 sample shows a smaller T1* value than the PU-10 and PU-15 samples. This was unexpected due to the lower radical content and is potentially due to morphological differences in the PEO and TEMPO-containing phases present (polyurethanes are known to be phaseseparated polymers). To investigate such effects, we performed SAXS analysis to probe the extent of microphase separation present as well as their respective morphologies. Figure 5a

Supporting Information). However, after an optimization of the atomic positions, the predicted 13C spectrum was found to match remarkably well with the experimental one (Figure 4d). The calculated 13C σiso values for each crystallographic carbon site are given in the Supporting Information, alongside experimental isotropic chemical shifts reported by SchmidtRohr and co-workers.35 These authors also used multidimensional 13C NMR experiments to provide an estimation of the O−C−C−O torsion angles ψ present in the helical chain structure.35 In the reported crystal structure these angles vary significantly (Table 3), however, Schmidt-Rohr’s NMR measurements suggested a much smaller variation (ψ = 74 ± 4°). The ψ values measured from the DFT-optimized structure are shown in Table 2 and lie very close to this range, further validating the accuracy of the DFT-optimized structure. Table 2. O−C−C−O Torsion Angles Measured from the PEO Crystal Structure before (ψunopt) and after (ψopt) DFT Optimization of the Atomic Positions O1−C1−C2−O2 O2−C3−C4−O3 O3−C5−C6−O4 O4−C7−C8−O5 O5−C9−C10−O6 O6−C11−C12−O7 O7−C13−C14−O1

ψunopt (deg)

ψopt (deg)

57.1 67.4 73.8 48.4 92.7 60.0 79.5

68.6 68.0 69.9 75.9 75.2 72.2 70.9

Figure 5. Room-temperature SAXS patterns of the polyurethane samples as a function of TEMPO concentrations indicated (PU-X).

shows the room-temperature SAXS patterns of the PU-X copolymers as a function of TEMPO monomer concentration. The SAXS pattern of the PU-0 sample exhibits a broad hump, suggesting the presence of a very weak microphase separation, with a Bragg d-spacing of 27.5 nm.40−42 As the TEMPO units are introduced (PU-5), the intensity of the diffraction peak increases, suggesting an enhancement of the microphase separation in this system. Additionally, a shift of the diffraction peak toward lower q values is observed, indicating a reduction of the Bragg d-spacing to 16.1 nm. This decrease is observed up to 20 mol % TEMPO and is likely due to the difference in molecular weight of the two monomers (457.3 and 1500 g/ mol for the TEMPO and PEG segments, respectively). Despite the absence of a second-order diffraction peak for sample PU5, a lamellar morphology can be assumed for this sample,40 while PU-10 and PU-20 showed second order peaks with a q2/ q1 ratio of ∼1.41 ± 3%, excluding the possibility of a lamellar phase and indicative of either a bicontinuous or discontinuous cubic phase with an Im3̅m space group. These different morphologies could explain why the T1* values measured in these samples do not show an obvious trend (Table 1). At higher TEMPO content, the second-order peak disappears, suggesting the formation of a less complex two-phase structure with a larger Bragg d-spacing in samples PU-40 and PU-50. Additional low intensity and sharp diffraction peaks are also observed in the lower q range (0.07 Å−1 < q < 0.17 Å−1) for TEMPO concentrations ranging from 5 to 30 mol %. Because these diffraction peaks are not observed in the pristine PU-0 and PU-100 polymers and their positions are not affected by TEMPO concentration, they are likely due to local medium range order in either the PEO or TEMPO phase, induced by the phase separation present in these systems. Interestingly, these peaks are barely visible in the 40−50 mol % TEMPO samples, where microphase separation is still observed, albeit

The DNP signal enhancement factors measured from the samples using 1H MAS and 13C CPMAS are presented in Table 1. For the latter, the most intense 13C signal arising from the crystalline PEO phase was used. The enhancement factors for the other 13C peaks were not quantified due to their very low signal intensities in the MW off spectra. DNP enhancements were observed for all of the samples studied but were generally higher for the lower radical concentrations and decreased significantly for PU-30 and PU-40. The highest 13C CPMAS enhancement was obtained from PU-15 (10.5). It should be noted that these values may be affected by the depolarization of the nuclei under MAS in the absence of microwave irradiation, which would reduce the MW off signal intensities.38,39 The incorporation of the radicals as an intrinsic part of the polymer structure makes this effect difficult to quantify. Also shown in Table 1 are the effective longitudinal relaxation times T1*31 measured using the main 72 ppm peak arising from the crystalline PEO phase by 13C CPMAS saturation recovery in the presence and absence of microwave irradiation. We note that these values in fact indirectly probe the 1H spin network due to the cross-polarization step. The MW off values quantify the 1H spin−lattice NMR relaxation in the crystalline PEO phase, while the MW on values represent a measure of the 1H polarization buildup time in the crystalline PEO phase. Both of these quantities will be potentially affected by the size and morphology of the crystalline PEO domains, with a spatial distribution in 1H T1s and DNP enhancements caused by different proximities to the radicals and the spin diffusion process that transfers the polarization enhancement into the bulk PEO phase. For the lower radical concentrations (PU-5 to PU20), the former values are longer than the latter, indicating spin-diffusion-mediated polarization enhancement in the PEO phase. At higher radical concentrations the T1* values become much shorter and approximately equal, E


Article

Macromolecules with a larger spatial periodicity. This larger spatial periodicity goes against the expected trend, previously observed at lower TEMPO concentration, where d-spacing decreases with increasing TEMPO concentration as the molecular weight of the TEMPO monomer is smaller than that of PEO monomer. This indicates that the PEO and/or TEMPO phases are less dense, which likely disrupts the previously observed local medium range order. The diffraction peak positions, assignments, and extracted lattice parameters are provided in the Supporting Information alongside complementary WAXS data. The organization of these polymers into phase-separated PEO and TEMPO-containing domains allows the quantification of the crystalline PEO domain size using the method proposed by Emsley and co-workers.24,25 The DNP enhancement factors and polarization buildup profiles for the crystalline PEO 13C DNP NMR signal in the presence and absence of microwave irradiation are fitted to a model describing the transfer of DNP-enhanced polarization into the PEO phase via 1H−1H spin diffusion (see Figure 6 and the

Table 3. Crystalline PEO and TEMPO-Containing Domain Sizes Determined by Fitting the DNP Buildup Curves from the Crystalline PEO Signal to the Model Proposed by Emsley and Co-workers24,25 a PEO domain size (nm)

TEMPO domain size (nm)

polymer

spherical model

lamellar model

spherical model

lamellar model

PU-5 PU-10 PU-15 PU-20

50(2) 45(2) 38(2) 25(4)

17(1) 19(1) 21(1) 14(3)

1(1) 1(1) 1(1) 1(1)

1(1) 7(1) 10(1) 8(3)

a

The spherical model assumes a sphere of crystalline PEO surrounded by a layer of TEMPO-containing phase, and the size indicated is the radius of the sphere. The lamellar model assumes a layer of crystalline PEO sandwiched between layers of the TEMPO-containing phase where the sizes indicated are the layer thicknesses. See the Supporting Information for further details of these simulations.

Interestingly, for both models the measured size of the PEO domains are larger than the individual PEO monomer length when in its crystalline, helical conformation (∼9.5 nm),32 suggesting that the short HDI linker chains are incorporated within the crystalline PEO domains. A similar effect has recently been proposed to occur in a poly(octamethylene urea)-co-poly(diethylene oxide) system in which the two isomorphic monomer chains cocrystallized over a wide range of compositions.43 It is also consistent with the observation of significantly broader 13C NMR signals from the PEO phase in the polyurethanes synthesized with a much bulkier linker molecule (isophorene diisocyanate), which could not fit within the PEO structure without severely disrupting its crystallinity (see the Supporting Information). Overall, the results extracted using the lamellar model would appear to provide closer agreement with the X-ray scattering results. In particular, the SAXS data suggest that sample PU-5 exhibits a lamellar phase with a lattice parameter of 16.1 nm, which is within experimental error of that obtained from the DNP analysis (18 ± 2 nm). The remaining samples are suggested by the Xray data to exist as either discontinuous or bicontinuous cubic phases, neither of which are accurately described by the spherical or lamellar models; however, the smaller PEO domain sizes extracted using the latter model more closely match the lattice parameters extrapolated from the SAXS data (see the Supporting Information). Finally, the 13C CPMAS spectra obtained from a sample of PU-15 recrystallized from water with 25 wt % histidine are shown in Figure 7, with and without DNP enhancement. The histidine signals appear as six sharp peaks representing the six distinct carbon sites in the molecule. The peaks are relatively sharp, indicating that the histidine exists as crystalline aggregates. In contrast, the PEO peak is much broader than that of the pure PU-15 sample (Figure 7 inset), indicating a much more disordered or strained PEO structure as one might expect given the incorporation of the histidine. Encouragingly, the DNP enhancement factor for the PEO (9.1) was close to that obtained in the pure PU-15 sample (10.5), and a significant enhancement level was also obtained from the histidine (4.0). This demonstrates the potential of delivering polarization enhancement from these radical copolymers into another material, potentially allowing them to be used as DNP matrices or providing a route to study interface regions between different polymer phases or in composite materials.

Figure 6. Fitted DNP enhancements and normalized signals as a function of the polarization delay for the crystalline PEO 13C CPMAS signal of sample PU-15, using (a, b) the spherical model and (c, d) the lamellar model of Emsley and co-workers.24,25

Supporting Information). The PEO domain sizes extracted in this way are presented in Table 3 for the samples with lower radical monomer contents. Two distinct models were used to fit the data. The spherical model assumes spherical PEO domains surrounded by a layer of radical-containing phase, while the lamellar model assumes layers of PEO sandwiched between TEMPO-containing layers. In the former model, the PEO domain sizes in Table 3 represent the radii of the spherical PEO domains, while in the latter they represent the PEO layer thickness. Good fits to the DNP data could be obtained using both models, as illustrated by Figure 6 (other fits provided in the Supporting Information). For the spherical model, the apparent radii of the PEO domains decreases with increasing radical monomer concentration, while the thickness of the surrounding layer of TEMPO-containing phase remains relatively small (1 nm). Conversely, for the lamellar model the PEO domain thickness varies to a much lesser extent, remaining on the order of 20 nm, while the width of the TEMPO-containing layer appears to increase with the TEMPO monomer content. F


Article

Macromolecules

■

optimized PEO crystal structure, DNP buildup curve fits, NMR spectra for different linker groups (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.A.O.). ORCID

Haritz Sardon: 0000-0002-6268-0916 David Mecerreyes: 0000-0002-0788-7156 Luke A. O’Dell: 0000-0002-7760-5417 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Australian Research Council is acknowledged for funding the DNP NMR spectrometer at the Bio21 Institute, University of Melbourne, through LIEF Grant LE160100120. Dr MarcAntoine Sani is thanked for assistance with the DNP measurements, Dr Jacqui Adcock is thanked for assistance with the EPR measurements, and Dr Aaron Rossini is thanked for advice regarding domain size measurement. The authors thank Dr. Ludovic F. Dumée and Dr. Adrian Hawley for access to the SAXS/WAXS beamline at the Australian Synchrotron, Victoria, Australia, through the funding scheme M13647. M.F. acknowledges the Ikerbasque organisation in the Basque Country for support via the Visiting Ikerbasque Professor scheme.

Figure 7. 13C CPMAS NMR spectra obtained from the PU-15 sample after recrystallization with 25 wt % histidine (9.4 T, 100 K, 8 kHz MAS, 16 scans, 45 s recycle delay). P denotes signals from the polymer PU-15, H denotes signals from histidine, and asterisks denote spinning sidebands. Inset: an expansion of the PEO signal from the DNP-enhanced spectrum is shown (solid line) with the signal obtained from the pure PU-15 sample also shown for comparison (dashed line).

■

4. SUMMARY A series of polyurethanes containing PEO and TEMPO nitroxide radical bearing monomers have been investigated in the context of dynamic nuclear polarization NMR. Both thermal analysis and 13C NMR showed that the majority of the PEO phase in these materials exists in the crystalline form, with the TEMPO units existing in a separate phase. Electron paramagnetic resonance measurements allowed the spatial separation of the radical monomers to be probed, while density functional theory calculations were used to refine the PEO crystal structure, resulting in excellent agreement with 13C NMR data. DNP enhancements on the order of 10 were obtained for the PEO phase, allowing the domain sizes to be estimated using a spin-diffusion-based model. In the case of the PU-5 sample, which showed a lamellar morphology, excellent agreement was found between the domain sizes measured by this DNP approach and independent X-ray scattering data. The latter data also indicated composition-dependent morphologies for the phase-separated domains. Lastly, a brief demonstration of the possibility of using such materials as a host matrix for DNP NMR studies was demonstrated using crystalline histidine as the analyte. Further work is now underway on alternative radical-bearing polymer systems with more easily controllable morphologies for future DNP applications.

■

REFERENCES

(1) Maly, T.; Debelouchina, G. T.; Bajaj, V. S.; Hu, K.-N.; Joo, C.G.; Mak-Jurkauskas, M. L.; Sirigiri, J. R.; van der Wel, P. C. A.; Herzfeld, J.; Temkin, R. J.; Griffin, R. G. Dynamic nuclear polarisation at high magnetic fields. J. Chem. Phys. 2008, 128, 052211. (2) Thankamony, A. S. L.; Wittmann, J. J.; Kaushik, M.; Corzilius, B. Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR. Prog. Nucl. Magn. Reson. Spectrosc. 2017, 102−103, 120−195. (3) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: London, 1994. (4) Hwang, C. F.; Hill, D. A. New effect in dynamic polarisation. Phys. Rev. Lett. 1967, 18, 110−112. (5) Hwang, C. F.; Hasher, B. A.; Hill, D. A.; Markley, F. The use of chemically doped polystyrene as a polarized proton target material. Nucl. Instrum. Methods 1967, 51, 254−256. (6) Wind, R. A.; Duijvestijn, M. J.; van der Lugt, C.; Manenschijn, A.; Vriend, J. Applications of dynamic nuclear polarization in 13C NMR in solids. Prog. Nucl. Magn. Reson. Spectrosc. 1985, 17, 33−67. (7) Wind, R. A.; Zumbulyadis, N.; Young, R. H.; Hung, Y.; Li, L.; Nuttall, R. H. D.; Maciel, G. EPR and 1H and 13C dynamic nuclear polarization studies of a molecularly doped polymer: bisphenol A polycarbonate doped with trianisylamine and trianisylaminium perchlorate. Solid State Nucl. Magn. Reson. 1992, 1, 55−65. (8) Afeworki, M.; McKay, R. A.; Schaefer, J. Selective observation of the interface of heterogeneous polycarbonate/polystyrene blends by dynamic nuclear polarisation carbon-13 NMR spectroscopy. Macromolecules 1992, 25, 4084−4091. (9) Afeworki, M.; Schaefer, J. Mechanism of DNP-enhanced polarization transfer across the interface of polycarbonate/polystyrene heterogeneous blends. Macromolecules 1992, 25, 4092−4096. (10) McCarney, E. R.; Han, S. Spin-labelled gel for the production of radical-free dynamic nuclear polarization enhanced molecules for NMR spectroscopy and imaging. J. Magn. Reson. 2008, 190, 307−315. (11) Dollmann, B. C.; Junk, M. J. N.; Drechsler, M.; Spiess, H. W.; Hinderberger, D.; Münnemann, K. Thermoresponsive, spin-labeled

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01665. Sample synthesis details, FTIR data, DSC data, SAXS and WAXS data, DFT calculated 13C NMR data and G


Article

Macromolecules hydrogels as separable DNP polarizing agents. Phys. Chem. Chem. Phys. 2010, 12, 5879. (12) Vitzthum, V.; Borcard, F.; Jannin, S.; Morin, M.; Miéville, P.; Caporini, M. A.; Sienkiewicz, A.; Gerber-Lemaire, S.; Bodenhausen, G. Fractional spin-labeling of polymers for enhancing sensitivity by solvent-free dynamic nuclear polarisation. ChemPhysChem 2011, 12, 2929−2932. (13) Horii, F.; Idehara, T.; Fujii, Y.; Ogawa, I.; Horii, A.; Entzminger, G.; Doty, F. D. Development of DNP-enhanced highresolution solid-state NMR system for the characterization of the surface structure of polymer materials, J. Infrared Milli. J. Infrared, Millimeter, Terahertz Waves 2012, 33, 756−765. (14) Takahashi, H.; Lee, D.; Dubois, L.; Bardet, M.; Hediger, S.; De Paëpe, G. Rapid natural-abundance 2D 13C-13C correlation spectroscopy using dynamic nuclear polarization enhanced solid-state NMR and matrix-free sample preparation. Angew. Chem., Int. Ed. 2012, 51, 11766−11769. (15) Lee, Y.; Heo, G. S.; Zeng, H.; Wooley, K. L.; Hilty, C. Detection of living anionic species in polymerization reactions using hyperpolarized NMR. J. Am. Chem. Soc. 2013, 135, 4636−4639. (16) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Lesage, A.; Copéret, C.; Emsley, L. Dynamic nuclear polarization surface enhanced NMR spectroscopy. Acc. Chem. Res. 2013, 46, 1942−1951. (17) Blanc, F.; Chong, S. Y.; McDonald, T. O.; Adams, D. J.; Pawsey, S.; Caporini, M. A.; Cooper, A. I. Dynamic nuclear polarization NMR spectroscopy allows high-throughput characterization of microporous organic polymers. J. Am. Chem. Soc. 2013, 135, 15290−15293. (18) Ouari, O.; Phan, T.; Ziarelli, F.; Casano, G.; Aussenac, F.; Thureau, P.; Gigmes, D.; Tordo, P.; Viel, S. Improved structural elucidation of synthetic polymers by dynamic nuclear polarization solid-state NMR spectroscopy. ACS Macro Lett. 2013, 2, 715−719. (19) Le, D.; Casano, G.; Phan, T. N. T.; Ziarelli, F.; Ouari, O.; Aussenac, F.; Thureau, P.; Mollica, G.; Gigmes, D.; Tordo, P.; Viel, S. Optimizing sample preparation methods for dynamic nuclear polarization solid-state NMR of synthetic polymers. Macromolecules 2014, 47, 3909−3916. (20) Can, T. V.; Caporini, M. A.; Mentink-Vigier, F.; Corzilius, B.; Walish, J. J.; Rosay, M.; Maas, W. E.; Baldus, M.; Vega, S.; Swager, T. M.; Griffin, R. G. Overhauser effect in insulating solids. J. Chem. Phys. 2014, 141, 064202. (21) Mollica, G.; Le, D.; Ziarelli, F.; Casano, G.; Ouari, O.; Phan, T. N. T.; Aussenac, F.; Thureau, P.; Gigmes, D.; Tordo, P.; Viel, S. Observing apparent nonuniform sensitivity enhancements in dynamic nuclear polarization solid-state NMR spectra of polymers. ACS Macro Lett. 2014, 3, 922−925. (22) Le, D.; Ziarelli, F.; Phan, T. N. T.; Mollica, G.; Thureau, P.; Aussenac, F.; Ouari, O.; Gigmes, D.; Tordo, P.; Viel, S. Up to 100% improvement in dynamic nuclear polarization solid-state NMR sensitivity enhancement of polymers by removing oxygen. Macromol. Rapid Commun. 2015, 36, 1416−1421. (23) Neudert, O.; Reh, M.; Spiess, H. W.; Münnemann, K. X-band DNP hyperpolarization of viscous liquids and polymer melts. Macromol. Rapid Commun. 2015, 36, 885−889. (24) Schlagnitweit, J.; Tang, M.; Baias, M.; Richardson, S.; Schantz, S.; Emsley, L. Nanostructure of materials determined by relayed paramagnetic relaxation enhancement. J. Am. Chem. Soc. 2015, 137, 12482−12485. (25) Pinon, A. C.; Schlagnitweit, J.; Berruyer, P.; Rossini, A. J.; Lelli, M.; Socie, E.; Tang, M.; Pham, T.; Lesage, A.; Schantz, S.; Emsley, L. Measuring nano- to microstructures from relayed dynamic nuclear polarization NMR. J. Phys. Chem. C 2017, 121, 15993−16005. (26) Viger-Gravel, J.; Berruyer, P.; Gajan, D.; Basset, J.-M.; Lesage, A.; Tordo, P.; Ouari, O.; Emsley, L. Frozen acrylamide gels as dynamic nuclear polarization matrices. Angew. Chem., Int. Ed. 2017, 56, 8726−8730. (27) Gizatullin, B.; Neudert, O.; Stapf, S.; Mattea, C. Dynamic nuclear polarization fast field cycling method for the selective study of

molecular dynamics in block copolymers. ChemPhysChem 2017, 18, 2347−2356. (28) Brownbill, N. J.; Sprick, R. S.; Bonillo, B.; Pawsey, S.; Aussenac, F.; Fielding, A. J.; Cooper, A. I.; Blanc, F. Structural elucidation of amorphous photocatalytic polymers from dynamic nuclear polarization enhanced solid-state NMR. Macromolecules 2018, 51, 3088− 3096. (29) Garmendia, S.; Mantione, D.; Alonso-de Castro, S.; Jehanno, C.; Lezama, L.; Hedrick, J. L.; Mecerreyes, D.; Salassa, L.; Sardon, H. Polyurethane based organic macromolecular contrast agents (PUORCAs) for magnetic resonance imaging. Polym. Chem. 2017, 8, 2693−2701. (30) Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson. 2006, 178, 42−55. (31) Rossini, A. J.; Zagdoun, A.; Hegner, F.; Schwarzwälder, M.; Gajan, D.; Copéret, C.; Lesage, A.; Emsley, L. Dynamic nuclear polarization NMR spectroscopy of microcrystalline solids. J. Am. Chem. Soc. 2012, 134, 16899−16908. (32) Takahashi, Y.; Tadokoro, H. Structural studies of polyethers, (-(CH2)m-O-)n. X. Crystal structure of poly(ethylene oxide). Macromolecules 1973, 6, 672−675. (33) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First principles methods using CASTEP. Z. Kristallogr. - Cryst. Mater. 2005, 220, 567−570. (34) Sackmann, E.; Trauble, H. Crystalline-liquid phase transition of lipid model membranes. II. Analysis of electron spin resonance spectra steroid labels incorporated into lipid membranes. J. Am. Chem. Soc. 1972, 94, 4492−4498. (35) Harris, D. J.; Bonagamba, T. J.; Hong, M.; Schmidt-Rohr, K. Two-dimensional solid-state NMR studies of crystalline poly(ethylene oxide): conformations and chemical shifts. Polymer 2005, 46, 11737− 11743. (36) Spěvácě k, J.; Baldrian, J. Solid-state 13C NMR and SAXS characterisation of the amorphous phase in low-molecular weight poly(ethylene oxide)s. Eur. Polym. J. 2008, 44, 4146−4150. (37) Hiltz, J. A. 13C Solution NMR Spectra of Poly(ether)urethanes, Defence Research and Development Canada, 2002. (38) Thurber, K. R.; Tycko, R. Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples magicangle spinning without microwaves. J. Chem. Phys. 2014, 140, 184201. (39) Mentink-Vigier, F.; Paul, S.; Lee, D.; Feintuch, A.; Hediger, S.; Vega, S.; De Paëpe, G. Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization. Phys. Chem. Chem. Phys. 2015, 17, 21824−21836. (40) Musselman, S. G.; Santosusso, T. M.; Barnes, J. D.; Sperling, L. H. Domain structure and interphase dimensions in poly(urethaneurea) elastomers using DSC and SAXS. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2586−2600. (41) Kulkarni, A. R.; Balaji, R.; Srinavasa, R. S. Structural investigation of polyurethane based gel polymer electrolytes using small angle X-ray scattering (SAXS). J. Phys. Soc. Jpn. 2010, 79, 154− 159. (42) Krakovský, L.; Bubeníková, Z.; Urakawa, H.; Kajiwara, K. Inhomogeneous structure of polyurethane networks based on poly(butadiene)diol: 1. The effect of the poly(butadiene)diol content. Polymer 1997, 38, 3637−3643. (43) Dennis, J. M.; Steinberg, L. I.; Pekkanen, A. M.; Maiz, J.; Hegde, M.; Müller, A. J.; Long, T. E. Synthesis and characterization of isocyanate-free polyureas. Green Chem. 2018, 20, 243−249.

H


DNP NMR Studies of Crystalline Polymer Domains by

Recommend Documents