College of Liberal Arts and Sciences

Department of Chemistry

Klaus Schmidt-Rohr

Physical Chemistry of Polymers & Nanocomposites, Analysis of Complex Organic Matter, Solid-state NMR Methods

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Research Interests

Our group studies the molecular structure and dynamics of solid polymers, nanocomposites, and natural organic matter by a variety of selective or two-dimensional nuclear magnetic resonance (NMR) experiments, many of which were developed by us (http://www.public.iastate.edu/~nmrksr/). We have also introduced new methods for quantitative analysis of scattering data of nanostructured materials. This dual approach has enabled us to"solve" important aspects of the structure of the Nafion fuel-cell membrane and of the nanocomposite in bone.

1) Nanocomposites

We have established NMR as a tool for characterizing nanocomposites, typically consisting of inorganic nanoparticles dispersed in an organic polymer, not only in terms of composition, but also in terms of the location of various components relative to the organic-inorganic interface. The composition of the surface layers of both the nanoparticles and the matrix, which are particularly important since they are responsible for the interactions between the components, can be determined by selective NMR techniques, often based on heteronuclear dipolar couplings across the interface.

We have introduced several NMR methods for proving the presence of nanoparticles even in cases where standard electron microscopy fails to provide a specific signature. Spin diffusion from 1H in the nanoparticles to matrix 1H is one option to detect particles in the 1- 10 nm range, X{1H} HARDSHIP NMR another. The shape, size, and size distribution of nanoparticles can be determined from HARDSHIP NMR fits combined with quantitative analysis of small- and/or wide-angle scattering data. Nanocomposites and related systems that we have studied with these methods include:

Our techniques and results on a few of these systems are briefly outlined in the following.

HARDSHIP NMR thickness / depth measurements. NMR can probe the thickness of phosphate, silicate, carbonate, and other nanoparticles in organic-inorganic nanocomposites using the strongly distant-dependent dipolar couplings between the abundant protons in the organic phase and X-nuclei (31P, 29Si, 13C, 27Al, 23Na, etc.) in the inorganic phase. This approach requires pulse sequences with heteronuclear dephasing only by the polymer or surface protons that experience strong homonuclear interactions, but not by dispersed OH or water protons in the inorganic phase, which have long transverse relaxation times T2,H. This goal is achieved by HeteronucleAr Recoupling with Dephasing by Strong Homonuclear Interactions of Protons (HARDSHIP). The pulse sequence alternates heteronuclear recoupling for ~0.15 ms with periods of homonuclear dipolar dephasing that are flanked by canceling 90° pulses. The heteronuclear evolution of the long-T2,H protons is refocused within two recoupling periods, so that 1H spin diffusion cannot significantly dephase these coherences. For the short-T2,H protons of a relatively immobile organic matrix, the heteronuclear dephasing rate depends simply on the heteronuclear second moment. A detailed analysis based on interaction representations shows that homonuclear interactions do not affect the dephasing, even though no homonuclear decoupling is applied, because long-range 1H-X dipolar couplings approximately commute with short-range 1H-1H couplings, and heteronuclear recoupling periods are relatively short. Experiments and simulations show that spherical particles of up to 10-nm diameter can be characterized quite easily.

Simulation of small-angle scattering from nanostructures. NMR can provide many unique insights into the composition and thickness of domains in nanocomposites and other nanostructured materials, but it is not suitable for determining the shape of the nanostructures. This important complementary information, together with independent information on size, can be obtained from small-angle X-ray or neutron scattering (SAXS/SANS) of these materials. We have become interested in simulating the scattered intensity quantitatively for two reasons: (i) For several nanomaterials of interest to us, in particular bone and Nafion, high-quality scattering data have been recorded by other groups but their interpretation has been only qualitative. (ii) The scattered intensity is the Fourier transform of the scattering density. We use numerical multidimensional Fourier transformation routinely in multidimensional NMR and know how to convert between the actual continuous scattering density and the discrete data required by a computer.

Therefore, we have developed a simple numerical approach for calculating the q-dependence of the scattered intensity in SAXS/SANS. For a user-defined scattering density on a lattice, the scattered intensity I(q) is calculated by 3D (or 2D) numerical Fourier transformation and spherical summation in q-space, with a simple smoothing algorithm and an exact, simple correction for continuous rather than discrete (lattice-point) scattering density. Applications to relatively densely packed particles in solids (e.g. nanocomposites) are shown, where correlation effects make single-particle (pure form-factor) calculations invalid. The algorithm can be applied to particles of any shapes that can be defined on the chosen cubic lattice and with any size distribution, while those features pose difficulties to a traditional treatment in terms of form and structure factors. Long parallel rods and platelets of various cross-section shapes are particularly convenient to treat, since the calculation is reduced to two dimensions. We have used the method to demonstrate that the scattered intensity from "randomly" parallel-packed long cylinders is not described by simple 1/q and 1/q4 power laws, but at cylinder volume fractions of more than ~25% includes a correlation peak. The simulations highlight that the traditional evaluation of the peak position overestimates the cylinder thickness by a factor of ~1.5. This is important in the analysis of the size of the hydrophilic clusters in Nafion. It is also shown that a mix of various relatively densely packed long boards can produce I(q) ~ 1/q, usually observed for rod-shaped particles, without a correlation peak. This explains the absence of a correlation peak in the small-angle scattering of bone.

The nanocomposite in bone. The load-bearing material in bone is a nanocomposite of ~ 3-nm thick apatite (calcium phosphate) platelets imbedded in a collagen matrix, with a volume ratio typically near 40:50 and the remaining 10% accounted for by water. The nanocrystalline calcium phosphate is often referred to as hydroxyapatite, but hydroxide has been undetectable in vibrational spectra. Selective 1H{31P} nuclear magnetic resonance (NMR) spectra have enabled quantification of the 5- to 8-fold reduced hydroxide concentration. Carbonate, which is not a component of ideal hydroxyapatite, has been conclusively proven to be fully incorporated in the bioapatite nanocrystals of native bone. A carbonate ion substitutes for about one out of every eight phosphates. Our 13C{1H} and 31P{1H} HARDSHIP NMR experiments show that carbonate is neither a surface deposit nor a central nucleation layer, but has a reduced concentration in the crystallite center. Strongly bound water is proven for the first time and is distinguished from monohydrogen phosphate at the nanocrystal surface. Combining quantitative NMR of PO43-, OH-, CO32-, HPO42-, and bound H2O with information on cation concentrations from energy-dispersive analysis, we are able to provide a comprehensive analysis of the average composition of bone apatite.

Structure of nanodiamonds. Nanodiamonds, crystalline balls of a few thousand carbon atoms often with ~ 2 % nitrogen, are a fascinating material that is present in carbonaceous meteorites and has been synthesized by detonation or shock from carbon-containing precursors. Their surface and interior structure is still a matter of debate: Do they have an buckyball or onion-shell surface ? Or are they fully protonated ? Are there dangling bonds at the surface ? Is the interior hollow ? Why is the diameter of ~4 nm quite independent of synthesis conditions ? Is the lattice distorted, and if so, why ? How common are defects ? Where is the nitrogen ? Does slow growth in the interstellar medium result in the same structure as shock synthesis ? What is the origin of the broad downfield "foot" in the 13C spectrum ?

In order to answer many of these questions, we have studied synthetic nanodiamond with a crystalline core of 4.1-nm diameter by NMR spectroscopy, dipolar-coupling, relaxation and chemical shift anisotropy plus paramagnetic shift anisotropy measurements. The surface layer of these ~4.8-nm diameter carbon crystals is mostly protonated or bonded to OH or NH groups, while sp2-hybridized carbons make up < 1 % of the material. 13C{1H} HARDSHIP experiments, based on H-C dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains 60 % of all carbons, mostly resonate more than 6 ppm downfield (within the 40 - 80 ppm range) from bulk diamond. The location and concentration of unpaired electrons has been studied in detail, based on their strongly distance-dependent effects on T1,C relaxation as well as the paramagnetic-shift anisotropy probed with chemical-shift anisotropy recoupling. Quantitative analysis of these effects shows that the unpaired electrons are at a 0.5-nm distance from the surface. There are ~ 10 unpaired electrons per nanodiamond particle. Based on the information from NMR and WAXD, we have developed a detailed model of the structure of the nanodiamond particles.

2) The Structure of Nafion

The proton-exchange membrane (PEM) is a central, and often performance-limiting, component of all-solid H2/O2 fuel cells. NafionŽ, the most widely used PEM, consists of a perfluorinated polymer that combines a hydrophobic Teflon-like backbone with hydrophilic ionic side groups. It stands out among polymer materials for its high, selective permeability to water and small cations.

High-resolution 13C NMR of fluoropolymers. In spite of the technological significance of perfluorinated polymers such as Nafion, or poly(tetrafluoroethylene), PTFE/TeflonŽ, no 13C solid-state nuclear magnetic resonance (NMR) spectra of these temperature- and solvent-resistant materials could be found in the literature prior to 2001. The standard 13C spectra of Nafion and Teflon are broadened due to the large 19F chemical-shift anisotropy, which prevents on-resonance 19 decoupling. We have obtained the first high-resolution 13C NMR spectra of solid perfluorinated polymers by combining 28-kHz magic-angle spinning (MAS) with rotation-synchronized 19F 180°-pulses. The small line width shows that most Nafion backbone segments are helical and conformationally ordered, even though Nafion is a random copolymer. Conformational disorder is concentrated at the branch points. Furthermore, motional narrowing of 13C-19F dipolar splittings proved that most chain segments between branch points rotate by more than 150° around their helix axes. This rigidity of the backbone excludes many models of Nafion that are based on the assumption of random coiling. Nevertheless, the helices do not pack into well-ordered bundles, according to orientational correlation data from 19F CODEX (centerband-only detection of exchange) NMR.

Figure 1

Figure 1. High-resolution CP/MAS 13C NMR of perfluorinated polymers.

  1. For reference, standard 13C NMR spectrum of Nafion (structure see top of figure), with poor resolution.
  2. High-resolution 13C NMR spectrum of Nafion under fast MAS and pulsed 19F decoupling.
  3. High-resolution 13C NMR spectrum of Teflon (conditions as in b).

Quantitative SAXS simulations of the nanostructure of Nafion. Important aspects of the long elusive nanometer-scale structure that underlies many of these outstanding properties of Nafion have now been conclusively determined by a quantitative analysis of small-angle scattering data, using a novel approach based on numerical Fourier transformation (see above). Nafion is riddled with ~2.3-nm diameter water channels lined with hydrophilic side groups. In this nearly one-dimensional confinement, proton conductivity by a relay (Grotthuss) mechanism can proceed even faster than vehicular transport of other ions, as observed experimentally. The channels, which are locally parallel to their neighbors and can be considered as cylindrical inverted micelles, are stabilized by the rigid polymer backbones proven by NMR. Thin long crystallites act as physical crosslinks. This definitive structural model finally provides a valid target for the design of other, cheaper ionic polymers that could replace Nafion.

3) Structure of Natural Organic Matter

Knowledge of the chemical structure of soil organic matter and other polymers in the environment is very limited, but would be required for a true understanding of soil formation, diagenesis, or retention of water and contaminants. Solid-state NMR provides unique opportunities for analyzing these otherwise nearly intractable systems.

We have been pursuing a worldwide unique program of developing various solid-state NMR methods for characterizing the composition of such complex organic solids. By a suite of new NMR techniques, we can identify more than 36 functional groups (see Fig. 2), and determine their concentrations. Information on the nanometer-scale structure is obtained by various spin-diffusion and relaxation experiments. The focus of these investigations is natural organic matter (NOM), as found in soils, swamps, rivers, oceans, or sediments, but the NMR methods are similarly applicable to many other organic solids. On the basis of greatly increased knowledge on the structure of soil organic matter, we are addressing questions of soil fertility, formation of natural organic matter, and the sorption of nonpolar contaminants in soil.

Figure 2

Figure 2. Top: Typical 13C MAS NMR spectrum of a humic acid, and the limited traditional peak assignments, with significant uncertainties due to overlap with other functional groups (dashed lines). Bottom: More than 30 functional groups that we can identify by advanced solid-state NMR techniques. Ovals refer to identification based on the 13C chemical-shift-anisotropy, dashed circles to dipolar dephasing.

Identification of functional groups. We have developed a set of spectral editing techniques for 13C NMR spectra of complex organic matter. With detection efficiencies sufficient for complex natural organic matter, we can selectively observe 13C NMR signals of:

In addition, we can select signals of CH vs. OH and NH protons (by 1H CSA recoupling). Pulse programs for these methods can be found at http://www.public.iastate.edu/~nmrksr/.

In combination with the 13C isotropic chemical shift, these advanced spectral-editing filters often enable unique identification of functional groups, see Fig. 2. Most of these signals can also be quantified, for instance by means of optimized direct-polarization experiments. Furthermore, we can identify esters, COOH groups, phenols, and aromatic ethers by means of various two-dimensional techniques. Various applications of these methods are briefly discussed in the following.

A likely molecular origin of a rice-yield decline. Intensive cropping of irrigated lowland rice for 20-30 years has led to significant declines in grain yield for high-yielding field trials. This yield decline has been attributed to decreased availability of soil nitrogen (N), which is mostly held in the soil organic matter. In collaboration with Dr. Dan Olk of the National Soil Tilth Laboratory in Ames, and formerly of the International Rice Research Institute (IRRI) in the Philippines, we have performed a detailed structural analysis of a humic acid fraction extracted from a continually submerged, triple-cropped rice soil, and a corresponding humic acid fraction extracted from an aerobic single-cropped rice soil for reference. In particular, we applied our recently developed SPIDER method for selecting the signals of carbons bonded to nitrogen. We have found significant amounts of N directly bonded to aromatic rings in the intensively cropped soil. Since N bonded to aromatics is not readily plant available, this observation may help explain the observed yield decline.

Our quantitative 13C NMR combined with spectral editing have shown that the triple-cropped soil humic acid is rich in lignin derivatives (>45% of all carbon). The chemical shift of the N-bonded aromatic carbons and the peak intensities indicate that these signals are due to amide groups bonded to lignin aromatic rings. In contrast, the single-cropped humic acid has a lower lignin content and showed peak intensities more characteristic of easily degradable peptides, and less N bonded to aromatic carbons.

Lignin in wood, and spectral editing of lignin. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was applied to intact and isolated loblolly pine wood samples in order to identify potential structural changes induced by tree age, milling, lignin extraction, or naturally occurring mutations. Special attention was paid to ketone and aldehyde as well as nonpolar alkyl groups, which could be observed at low concentrations (< 2 in 1000 C) using improved spinning-sideband suppression with gated decoupling. Carbonyl structures were present in intact wood, and there are more keto groups than aldehydes. Their concentrations increased from juvenile to mature wood and with milling time, while extraction did not alter the C=O fraction. Significant amounts of aldehyde and dihydroconiferyl alcohol residues were present in CAD (coniferyl aldehyde dehydrogenase)-deficient wood, confirming solution-state NMR spectra of the corresponding lignin. These results demonstrate the utility of solid-state NMR as an assay for changes in the lignin structure of genetically modified plants.

Absence of mobile carbohydrate domains in dry humic acids proven by NMR, and implications for organic-contaminant sorption. Continuing our investigations of the relations between soil-organic-matter structure and contaminant sorption, we have studied the mobility and domain structure of various standard humic substances by 1H and 1H-13C solid-state nuclear magnetic resonance (NMR) experiments. No major (>3%) mobile components are observed in four dry humic acids, a fulvic acid, a natural organic matter sample, or a whole peat sample by 1H and 1H-13C NMR. In particular, neither polar nor aromatic components show any fast mobility. This directly disproves a previous claim, based on 1H NMR data, of highly mobile carbohydrate domains making up >30% of the humic acid, which were then implicated in dual-mode sorption. A small fraction of mobile nonpolar aliphatic segments identified by us before is the only mobile component, apart from absorbed water that we observe in humic acids exposed to ambient air. 1H-13C wideline separation NMR shows that, contrary to previous claims, the dipolar couplings in the aromatic regions are smaller than in the polar aliphatics, most likely due to differences in local 1H densities. Series of 1H-13C heteronuclear correlation experiments with 1H spin diffusion reveal very close proximity of aromatic and polar aliphatic segments in several humic acids, precluding any polar aliphatic domains on a scale of > 0.5-nm radius. In the standard peat humic acid, aromatic segments also do not form domains of significant size, while nonpolar aliphatic domains, which we had previously shown to correlate with sorption capacity, have been confirmed.

The Maillard reaction. The Maillard reaction between reducing sugars and amino acids is important in food science (like baking cookies: reaction of sugar and egg protein) and has also been considered as a potential humification pathway in soils. In the melanoidin polymers formed, the sugar is completely transformed into complex structures resembling humic acids. The exciting aspect about this "artificial" complex organic matter is the possibility of introducing specific 13C and 15N isotopic label, which enable NMR structural studies in unprecedented detail. We have started exploratory work in this area that looks very promising.