NMR microscopy has been applied to two kinds of materials that help us to live in comfort: flexible foams which are used in upholstery of car seats and superabsorbers which are the basic ingredient of many hygienic articles. In the latter application we strongly profit from the ability of NMR microscopy to handle "wet" systems which are difficult to deal with by many other techniques. 3D NMR imaging of poly-urethane foams and swollen poly(sodium acrylates) will be presented. One requirement which is much desirable but so far has not been pursued sufficiently is the quantification of image data. Reducing the large amount of data to just a few parameters characterising the sample is essential to the correlation of image data with physical properties, as is required in product development. An appropriate matching of spatial resolution and the size of structures under investigation is paramount. Data are collected on a commercial microimaging system which is also equipped with a diffusion probe. Standard spin echo, gradient echo and diffusion pulse sequences are used. Images of a spatial resolution of up to 8 m in all three directions are obtained. Using standard as well as home-developed image processing programs, statistical evaluations of the foam structure are made and the results compared to those from optical microscopy. Good agreement between the two techniques is found as far as the statistics of the cell size of open cell foams is concerned. NMR data might be used for a further understanding of the mechanical properties of the foams which are used in the manufacturing of car seats etc. The uptake of water in poly(sodium acrylate) material has also been studied. From a practical point of view its use as very potent water absorber in hygiene products makes such investigations important to industry. Investigating "wet" systems which often are difficult to handle by other techniques poses no problem to NMR. Not only geometrical shapes but also chemical and physical properties can be spatially visualised by parameter selective techniques. T2 maps and diffusion maps prove to be a valuable tool for the characterisation of these materials in the jelly state.
The biopolymer suberin forms a barrier layer in underground structures, wound surfaces, and various internal organs of higher plants. One typical suberin can be obtained from the periderm tissue that forms at the surface of wounded potatoes. Although the suberin is generally believed to contain cell-wall, phenolic and fatty-acid domains that are organized in a lamellar fashion, there remain many unanswered structural questions about this material. First, the patterns of covalent linkages among phenolics in a given domain are not fully understood. Second, the connections between domains, especially those between the cell wall and the phenolics, have never been demonstrated convincingly. Third, the biosynthetic development of suberin domains which are ultimately relevant to suberin function has just begun to be explored. In the present work, several approaches have been taken to address the above issues by solid-state magic-angle-spinning NMR spectroscopy. For biosynthesis and structure of suberin, selectively 13C-labeled precursors such as acetate and phenylalanine have been monitored after exogenous incorporation into the developing suberin polymer. For connections between domains, a modified Wideline Separation (WISE) NMR method has been used to investigate the proximity of the cell-wall polysaccharides, phenolics, and aliphatics. Finally, a resolution-enhanced spin-diffusion method has been used to demonstrate the molecular-level connections between the cell-wall and suberin domains. The results provide a more comprehensive picture of cell-wall suberization and the molecular mechanisms of crop protection.
Fast (25kHz) magic angle spinning (MAS) allows for routine high resolution 19F solid-state NMR with high signal-to-noise ratio. This method has been used to elucidate the structure of a series of fluorocarbon films deposited by pulsed plasma enhanced chemical vapor deposition (pulsed PECVD). Such fluorocarbon films have potential application as low dielectric constant layers in integrated circuits and as biopassivation coatings. Previous 19F-13C heteronuclear correlation spectroscopy by the wideline-separation (WISE) approach at 5 kHz spin speed was able to distinguish different fluorocarbon species to the alpha-carbon level (e.g. between CF2CF2 and CF2CF3). Such information is not available from other thin film characterization techniques but is extremely valuable for developing structure/property/processing relationships. However, prominent and overlapping sidebands in the fluorine dimension prevented definitive structural interpretation. One-dimensional 19F 25 kHz spin echo and TOSS spectra were acquired to differentiate residual sidebands from isotropic peaks and the latter allowed peak assignments to be carried out in a straightforward manner. Broad resonances that remained are attributed to isotropic chemical shift dispersion beyond the alpha-carbon level. Thus, attempts have been made to further differentiate resonances and increase spectral resolution through two-dimensional techniques such as the 19F-19F NOESY experiment in the solid-state. 19F fast MAS has also been used to investigate film properties. Radical concentration in the films was interpreted through T1 relaxation response and spectral changes with thermal annealing offered insight into the decomposition mechanism of these films.
Cyclophosphazene derivatives are able to form stable inclusion compounds with a variety of organic guest components, for which only little information is available. This holds in particular for the guest molecules which should experience a strong impact by the surrounding rigid host lattice matrix. In this contribution we report on variable temperature 2H NMR studies of such guest-host complexes over a large temperature range from 5 K to 370 K. In particular, we focus on the behaviour of a series of guest species (six-membered ring systems) which are distinguished by their molecular symmetry. The experimental lineshapes, relaxation and 2D exchange experiments are quantitatively analysed by taking into account various motional models which are appropriate for the description of the systems studied here. From this, reliable data about the structure and dynamics of the guest molecules embedded in the cyclophosphazene channels could be achieved. The major part of the investigations deals with the low temperature range below 100 K. Here, the relaxation behaviour is found to be dominated by overall motions of the guest species being on a rather fast time-scale. Both the actual type and time-scale strongly depends on the particular guest component (chemical structure, symmetry). Eventually, at temperatures below 30 K such overall types of motions can be frozen in on the NMR time-scale. In addition, the present NMR study could provide information about the orientation of the guest species within the host channels which - due the high mobility - cannot be obtained from X-ray studies.
Volume selection methods in NMR employ frequency selective pulses to selectively excite a well defined slice, plane or volume element. Because frequency selective pulses are necessarily finite in duration they momentarily expose spins to the effects of dephasing and relaxation. In spectroscopy and imaging it is often desirable to selectively store in the z-direction magnetisation from the desired region and selectively excite and destroy coherences in the region complementary to the local volume of interest, thus minimising the length of time the desired magnetisation is subjected to T2 relaxation. For various region-selective pulse sequences we follow the spin trajectories and examine the definition and width of the region of interest both numerically and experimentally. The length of time the desired magnetisation is exposed to transverse relaxation during a (45o)sel-(-90o)hard-(45o)sel pulse sandwich is shown to compare favourably to the situation for other selective excitation sequences. Hardware limitations such as hard pulse duration and power are also discussed. We have applied this technique to gain information about the order and dynamics at well defined sites within shearing and extensional flow fields. In particular we consider flow-induced deformation of the segmental distribution of a polymer chain. Our results in the four roll mill geometry suggest that purely extensional flow changes the orientation of the bond vectors in a high molecular weight polymer, 20% polyethylene oxide (PEO) in D2O.
The understanding of spin relaxation is crucial to the experimental interpretation and methodology development in magnetic resonance. Our recent theoretical and experimental studies on the nonlinear and nonstationary phase-relaxation process, triggered by reaction field and many-body effect, not only reveal some intriguing physics but may also provide solid-state NMR a new dimension: delayed acquisition time. (1) At short and intermediate delay ( 0.5-2T2 ), the FID exhibits collective beats stemming from the cooperative coherent motion of the spin dipoles. This can be modelled by a modified Bloch equation with strongly redistributed spectral diffusion. (2) The joint action of reaction field and lattice fluctuations manifests itself as a self-decoupled long-time tail ( >2T2 ) in the FID. This behavior is experimentally verified and qualitatively explained within a fully Hamiltonian picture, the spin-boson model. Therefore, with various acquisition delays, the FIDs zoom into detailed structural and dynamical information describing the local environment of the spin spy. The usefulness of this approach is demonstrated by numerical simulations and experimental measurements of typical dipole-coupled 1H systems.
In recent years, gas-phase imaging utilizing optically pumped xenon-129 has received a great deal of attention for medical applications, for example in lung imaging. This report is about the other NMR active isotope (-i.e. xenon-131) which exhibits distinctive features for imaging applications in material sciences that are not obtainable from xenon-129. The relaxation times of xenon-131 in gas and liquid phases depend strongly on the surface of the surrounding materials thus leading to a substantial dispersion of relaxation rates. In aerogels - i.e. materials with low density (around 0.1 g/cm3) and large surface areas- the relaxation time T1 decreases from around 110 ms outside the aerogel cavities to 27 ms inside the cavities. Removal of the surface bound water from the aerogel leads to even faster relaxation (T1 = 7 ms) which may be used to yield a surface specific contrast for imaging. The experiments were performed at very high fields (600 MHz for protons or 49 MHz for Xe-131) in order to overcome the sensitivity problems of xenon-131 since optical pumping is not an option due its fast relaxation. In addition liquid xenon close to the critical point (289 K) was chosen, thus providing a high spin density in addition to gas-like relaxational behavior. Examples are shown which demonstrate the potential of this isotope as a contrast agent.
Solid-State nuclear magnetic resonance has shown itself to be an excellent technique for studying molecular motions in solids in the frequency range 1 - 1000 kHz. We present the results of a motional investigation on a series of liquid crystalline compounds. A variety of solid-state NMR experiments are discussed, motional details being obtained by lineshape simulations. The compounds consist of traditional rigid mesogenic groups and flexible alkyl spacers, with their physical properties depending on the parity, odd or even, of the number of methylene groups in the spacer. Using simple thermodynamical arguments, some of the physical properties can be related to the local molecular motions occurring in the solid state.
Deuteron NMR studies were performed on the first three integer generations of deuterated polyamidoamine chloride dendrimers (branched molecules grown from the core >N-CH2-CH2-N
Polyrotaxanes are linear polymers threaded with macrocycles (e.g., crown ethers). Solid-state NMR techniques are proving to be especially useful for elucidating the complex morphologies and molecular dynamics of these materials. The rotational motions of the physically entrapped macrocycles may be important for such bulk polymer properties as impact strength, solid-state extrudability, and gas transport. These localized motions are being investigated in a number of polyrotaxanes using solid-state NMR spectroscopy.
For cavity sizes large enough to be threaded by polymethylene chains, macrocycles with at least 22 atoms must be used; 30-crown-10 (30c10) is commonly employed in the synthesis of polyrotaxanes. The 13C solid-state NMR spectrum of 30c10 contains five peaks which can be related to the different conformational sequences that exist in this crystalline material. 13C solid-state NMR spectra of unthreaded 60-crown-20 (60c20) collected at room temperature reveal peaks attributed to crystalline and amorphous regions, similar to solid-state NMR spectra of polyethylene glycols. Once threaded on to a polyacrylonitrile (PAN) backbone, 60c20 is predominantly amorphous. 1H widelines and 2D WISE spectra of the PAN-60c20 show the crowns to be highly mobile in the polyrotaxane at room temperature, while the polyacrylonitrile backbone remains rigid despite an environment consisting of 75 mass % mobile crown ether.
We recently reported that a continuous flow of laser-polarized xenon can be used for surface NMR studies even under MAS spinning. Polarization transfer to other nuclei by SPINOE or CP has been studied too. Our closed gas-circulation system uses fast optical pumping (high power diode laser) and establishes a circulating flow of laser-polarized xenon without any limitation in time for signal accumulation. Here we present 2D EXSY spectra of laser-polarized xenon in solids. Using xenon partial pressures of only 1-10 torr in the gas mixture, and thereby reducing the xenon-xenon interaction that otherwise strongly influences the chemical shift and its distribution, we are able to detect highly resolved spectra that take only 15 minutes of acquisition time for each 2D experiment. Furthermore, we use the steady-state laser-polarized xenon reservoir provided by our apparatus to perform microimaging and gas flow-imaging studies on porous materials.
The structure and chain mobility of ultradrawn ultra-high molecular weight polyethylene fibers has been investigated by solid-state NMR. Crystallinity measurement by 13C NMR in this system is difficult due to the extremely long 13C crystalline T1 and low amorphous signal. These problems have been overcome by a simple procedure of combining information from various direct-polarization and CP/T1 13C spectra. It yields a crystallinity of 882%, which is confirmed by 1H NMR lineshape decomposition. A second, highly mobile, amorphous phase, making up (0.80.2)% of the sample, is detected by 1H NMR. It forms domains of >5 nm thickness. Being undetectable in the extruded precursor material and in the fibers after melting, it must have been induced by the drawing process. 13C NMR confirms that no low-molecular-weight additives are present on a level above 0.01%. The fraction of mobile and oriented interfacial material in the fiber is found to be less than 4%. 180 degree chain flips with a rate of ~ 150 Hz at 360 K have been detected in the crystallites by 13C satellite narrowing and 1H T1rho relaxation. This motion is compared with the faster chain flips in unoriented 13C-13C labeled high-density polyethylene investigated by 2D exchange spectra, stimulated-echoes and 1D lineshape changes, which have yielded the activation energy and quantitative dependence of 1H T1rho on the correlation time. The relation of chain mobility and drawability of semicrystalline polymers is discussed.
A reduced four-dimensional (4D) NMR experiment with a novel 13C magnetization transfer sequence is used to estimate the size of domains of slow and fast segmental motion in a supercooled polymer melt close to the glass-transition temperature Tg. After selecting the signals of slow units in a stimulated echo, 13C magnetization is transferred from these slow units to their surroundings by optimized 13C spin diffusion. The mobility of the segments is analyzed by stimulated echoes. The transfer pulse sequence consists of 180- and 90-degree pulses which generate spin diffusion of transverse 13C magnetization under an average Hamiltonian that conserves the total transverse magnetization (except for T2 relaxation). Radio-frequency heating is minimized by a low duty cycle of <5 % on the 13C channel, and ~70-kHz continuous-wave proton decoupling. The experiment is applied to a sample of poly(vinyl acetate) with 40% of the COO sidegroups enriched with 13C. For these groups, T2 = 40 ms. Two-dimensional exchange experiments show that within 20 ms of the transfer sequence, the magnetization re-equilibrates over a volume of (0.9 nm)3. On the same time-scale of 20 ms, only little re-equilibration among domains of different mobilities is observed in the reduced 4D experiment at 15 K above the glass transition. This means that the typical smallest diameter of the domains of slow units is significantly larger than 1 nm.
Blends of Poly(ethylene-co-vinyl acetate) have been investigated by 2D HETCOR solid state NMR. The pure polymers were also studied. The 2D data obtained from EVA/PVAc 60/40 have provide evidence supporting the short interaction between both polymers in the blends, which can b an indicative that the blends present compatibility at molecular level.
We have measured the locally resolved Knight shift tensors and spin-lattice relaxation times in variable temperature 13C MAS NMR spectra of the quasi-2D superconducting organic metal (ET)2Cu(NCS)2. The problem with the interpretation of these data and also published results for the other organic metals is that the Korringa relation is not applicable because it is valid only for the case of the isotropic Knight shift. We performed calculation of the nuclear relaxation in anisotropic metal under conditions of magic angle spinning. The obtained formula differs from the Korringa relation by replacement of the Knight shift square K2 by (Kxx2 + Kyy2 + Kzz2)/3 where Kxx, Kyy and Kzz are the principal components of the Knight shift tensor.
The derived relation successfully correlates experimental Knight shift and relaxation data in 1D and 2D organic metals. For example, in quasi-1D metal (FA)2PF6 [1] the Korringa product was found within the experimental error to be constant for all the nine non-equivalent carbon sites. However, the experimental relaxation time is significantly shorter than predicted by the theory. This indicates presence in organic metals of electron correlations which enhance the fluctuations of the hyperfine interaction. For 2D metals (ET)2Cu(NCS)2 and (ET)2I3 [2] the relaxation rate increase is about 5 times and may be attributed to antiferromagnetic fluctuations.
1. K.F.Thier and M.Mehring, Phys. Rev. B 50, 2142 (1994).
2. T.Klutz, I.Henning, U.Haeberlen, and D.Schweitzer. Appl. Magn. Res. 2, 441 (1991).
Large number of transition metal (Cr, Mo, W, Fe, Rh ) complexes with substituted polycyclic
aromatics (naphthalenes, phenalenes, biphenylenes, fluorenes, indenes, carbazoles, indoles etc.) were
investigated by means of multinuclear (1H, 13C, 2H, 15N, 29Si, 119Sn) high resolution and MAS NMR.
Dynamic behaviour of these systems including metalotropic rearrangements, [1,5]H- and [1,5]Siand Sn shifts, organometallic groups rotations and inversions were investigated in solution, solid state and phisisorbed on silica. Kinetic and thermodynamic data for these processes were evaluated with the help of precise integration and line form analysis. All reactions proved to be intramolecular and activation energies are in a broad range from 10 to 30 kcal/mol.
For some systems adsorbed on silica MAS NMR including 2D spectra (COSY, NOESY,
HETCOR) were registrated.
NMR spectroscopy is used to determine fat content and internal temperature of hamburger, pork sausage, and various cuts of chicken during cooking. The temperature is determined by the frequency difference of the water and fat lines within the NMR spectrum. This method is more accurate than the absolute measurement of either peak alone. Furthermore, because the measurement relies on a frequency difference, an NMR lock is unnecessary. The hamburger and sausage data indicate an accuracy of 1C. The temperature resolution for chicken is 5C. The difference in resolution is due to the lower fat content of chicken. Chicken bone marrow, however, has a high fat content which could be used to measure its temperature. The speed and accuracy of this technique admits the possibility of using NMR spectroscopy on the process line to determine when these cuts of meat are sufficiently cooked and thus safe to consume.
We have employed several solid-state NMR techniques to probe the molecular structure of poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDT/PSS) films, a unique water-soluble conjugated conducting polymer complex that is useful in several thin film electronic devices. Magic angle spinning (MAS) techniques have been used to characterize PEDT/PSS in the context of varying processing parameters such as the pH of the aqueous solution from which films are cast. In particular, we have utilized spectral editing via interrupted dipolar decoupling to assign resonance shift values and simplify the solid-state MAS spectra. The dipolar dephasing technique eliminates contributions to the NMR spectrum from spins with large heteronuclear dipole-dipole couplings. Using this technique, we have isolated the isotropic shift of PEDT from PSS and studied the effect of solution pH on film conductivity. A strong correlation has been observed between the PEDT isotropic shift values measured under CPMAS conditions and the pH of PEDT/PSS solutions on which the film conductivities strongly depend. Combining bulk conductivity measurements with molecular scale NMR measurements allows the relationship among synthesis/processing conditions, molecular structure, and macroscopic material properties to be established and improved, according to the application(s) intended.
Fluoropolymers exhibit a number of excellent properties, such as chemical resistance, high-temperature stability and unique surface properties. In order to use these properties in conjunction with other materials, the compatibility of fluoropolymers with other materials has to be improved. One possible way of achieving this is radiation modification with high-energy electrons.
Fluorine-19 is an attractive probe nucleus for NMR because of the large chemical shift range, the high magnetogyric ratio and a natural abundance of 100%. The latter two result in a high receptivity but as well in strong dipolar couplings, which have to be averaged by multipulse or MAS in order to acquire high-resolution solid-state spectra. The application of high-speed MAS in addition averages the anisotropy of the chemical shift. When very small signals are to be recorded together with strong ones, spinning sideband suppression sequences like TOSS become problematic because of possible artefacts from incomplete sideband suppression. At 300MHz and sample rotoation frequencies in excess of 30kHz, spinning sidebands are separated by more than 100ppm which is sufficient here.
In order to elucidate connectivities and spacial information, different recoupling techniques, which for certain periods of the experiment reintroduce the dipolar coupling, are applied, to obtain spin exchange or double quantum spectra. In the systems which contain protons as well, special the heteronuclear coupling requires special attention in the recoupling experiments.
In addition information about the molecular dynamics can by obtained from two-dimensional spectra correlating the fluorine chemial shift from high-resolution solid-state spectra under MAS in one dimension with wideline spectra which are dominated by the dipolar coupling, which can be recorded under a CPMG sequence in the second dimension. The assignment of the resulting structures is assisted by a two-dimensional fluorine-carbon chemical shift correlation.
Experiments are demonstrated on low-molecular-weight compounds and are then applied to PTFE and coplymers containing PTFE such as FEP and PFA to study the effect of the modification.
Deuterium NMR measurements of single crystals of 2,3-dimethylnaphthalene (DMN), deuteriated in the aromatic sites, are presented. Two types of orientational disorder exist in the structure: polar disorder, in which the direction of the long molecular axis alternates between "up" and "down", and alignment disorder, in which this axis fluctuates about an average direction. The former had been detected by x-ray and neutron diffraction, but is not directly visible in the NMR spectra. The latter showed up as nonisotropic linebroadening in Deuterium NMR spectra of single crystals. It is argued that the two types of disorder are linked and that their coexistence suggests that the polar disorder is statistical rather than correlated.
2D-magnetization transfer experiments showed spin exchange between all peaks of the spectra with sub second rates. Using one 1D magnetization transfer experiment of quadrupolar order, the effects of spin diffusion and molecular jumps could be separated. The observed molecular jumps include self diffusion between different sites. Further 1D magnetization transfer experiments identified the existence of several self diffusion pathways. These self diffusion processes cause both disorders to be dynamic, thus ruling out the possible existence of ordered domains. The exchange rate of one pathway was determined to be 0.3s-1 at 338 K.
Due to small signals and spin lattice relaxation times in excess of 100s the use of 1D magnetization transfer experiments was crucial to answer specific questions. In these 1D-magnetization transfer experiments several sequences of soft adiabatic inversion pulses were used to mark a subset of deuterons in the spectra with quadrupolar and/or Zeeman order.
Molecular-scale mixing in compatible polymer blends has been studied extensively using magnetic resonance methods. However, the vast majority of these investigations have focused on blends in which at least one of the components is a polar or unsaturated polymer. Saturated blends present unique problems for study by magnetic resonance, since relaxation parameters are often very similar for the homopolymers, and the absence of any functional groups (and therefore enthalpic contributions to the free energy of mixing) precludes the use of resolved HETCOR methods for following spin diffusion. We have used variable-temperature methods to selectively follow Rothwell-Waugh broadening in blends of atactic polypropylene and polyethylene copolymers. Results are consistent with T1pH measurements, which define the change in chain dynamics which occur in mixtures of miscible components relative to copolymers that lead to phase separation. 2D carbon exchange experiments, in which spectral spin diffusion leads to polarization transfer, are also used to probe domain sizes. Our results are compared with those from neutron scattering.
Poster Slot Number: 274
Solid-State NMR Investigation of Morphology and Dynamics of Polyrotaxanes
Institute: Georgia Institute of Technology, Atlanta;
Poster Slot Number: 275
Circulating Laser-Polarized Xenon: 2D-EXSY and Gas Flow-Imaging of 129Xe in Porous Materials
Institute: Department of Chemical Engineering, University of California, Berkeley;
Poster Slot Number: 276
Phase Structure, Chain Dynamics, and Drawability of Ultraoriented UHMWPE Fibers Investigated by Solid-State NMR
Institute: Department of Polymer Science & Engineering, UMass, Amherst, MA 01003;
Poster Slot Number: 277
Reduced 4D Exchange NMR with 13C Transverse Spin Diffusion for Studying Dynamic Heterogeneities in Polymers above Tg
Institute: Department of Polymer Science & Engineering, UMass, Amherst, MA 01003;
Poster Slot Number: 278
2D Solid State NMR Study of EVA/PVAc Blends
Institute: Instituto de Macromoleculas, UFRJ, Rio de Janeiro, BRAZIL;
Poster Slot Number: 279
Spin-lattice relaxation in magic angle spinning NMR of anisotropic metals.
Institute: Ecole Polytechnique, Palaiseau, France;
Poster Slot Number: 280
MULTINUCLEAR NMR INVESTIGATION OF DYNAMIC BEHAVIOUR OF TRANSITION METAL COMPLEXES WITH POLYCYCLIC AROMATIC COMPOUNDS
Institute: NMR Lab., Department of Chemistry, Moscow State University, ;
Poster Slot Number: 281
New Method for Determining Internal Temperature of Cooking Meat via NMR Spectroscopy
Institute: University of California, Davis, CA;
Poster Slot Number: 282
Poly(3,4-ethylene dioxythiophene)/Poly(styrene sulfonate): Conductivity, pH, and NMR Shift Correlations
Institute: University of California at Santa Barbara;
Poster Slot Number: 283
High-speed MAS investigations on modified fluoropolymers
Institute: Institut für Polymerforschung Dresden e.V. ;
Poster Slot Number: 284
Dynamic Disorder in 2,3 Dimethylnaphthalene. A Single Crystal Deuterium-NMR Study
Institute: MPI for Medical Research, Heidelberg; now Schlumberger SPC, Sugar-Land TX;
Poster Slot Number: 285
Miscibility Measurements in Saturated polymer Blends Using Solid State NMR
Institute: Exxon Chemical Polymer Science Division;
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