Polymer Chain Dynamics and NMR

Ionogel electrolytes incorporating exfoliated hexagonal boron nitride (hBN) nanoplatelets are promising materials for next-generation energy storage systems. However, detailed understanding of their ion transport properties at the molecular level remains limited. This study employs diffusion and relaxation nuclear magnetic resonance (NMR) techniques, including fast-field cycling (FFC) NMR, to investigate the dynamics of ionic species in hBN-ionogels. By spanning a broad frequency range from 30kHz using FFC NMR to high-field NMR (500-800MHz), we reveal distinct relaxation mechanisms governing ion dynamics in ionogels with and without lithium salts. Our results highlight the role of hBN in modulating molecular rotation and translational motion, significantly affecting 1H and 19F relaxation profiles. The presence of Li+ alters the dynamic behavior in ionogels, enhancing anion mobility at the interface. Notably, 7Li relaxation reveals strong interactions with the hBN surface that cannot be detected by diffusion NMR. These findings underscore the importance of spanning a broad frequency range in NMR studies of ionogels and provide critical insights into optimizing their design as novel electrolytes.

The potential of Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) for non-invasively monitoring the subcellular and intercellular redistribution of water in cellular tissue during drying and freezing processes is assessed and it is concluded that despite exciting advances in NMR micro-imaging and NMR microscopy, nonspatially resolved NMR relaxation and diffusion techniques still provide the best probes of subcellular water compartmentation in tissue. The power of the NMR relaxation technique is illustrated by using the changes in the distribution of NMR water proton transverse relaxation times to monitor the subcellular compartmentation of water and ice during the drying and freezing of parenchyma apple tissue. The NMR drying data are analysed with a numerical model of the cell and show that mild air-drying in a fluidized bed results in loss of water from the vacuolar compartment, but not from the cytoplasm or cell wall regions. The loss of vacuolar water is associated with overall shrinkage of the cell and only a slight increase in air space. During freezing the vacuolar compartment is found to be the first to freeze, with the cytoplasmic and cell wall compartments only freezing at much lower temperatures. Freeze-drying apple tissue gives much lower water contents than fluidized bed drying, but the NMR data confirms that it destroys membrane integrity and causes cell wall collapse.

This exploratory LDRD targeted the use of a new high resolution spectroscopic diffusion capabilities developed at Sandia to resolve transport processes at interfaces in heterogeneous polymer materials. In particular, the combination of high resolution magic angle spinning (HRMAS) nuclear magnetic resonance (NMR) spectroscopy with pulsed field gradient (PFG) diffusion experiments were used to directly explore interface diffusion within heterogeneous polymer composites, including measuring diffusion for individual chemical species in multi-component mixtures. Several different types of heterogeneous polymer systems were studied using these HRMAS NMR diffusion capabilities to probe the resolution limitations, determine the spatial length scales involved, and explore the general applicability to specific heterogeneous systems. The investigations pursued included a) the direct measurement of the diffusion for poly(dimethyl siloxane) polymer (PDMS) on nano-porous materials, b) measurement of penetrant diffusion in additive manufactures (3D printed) processed PDMS composites, and c) the measurement of diffusion in swollen polymers/penetrant mixtures within nano-confined aluminum oxide membranes. The NMR diffusion results obtained were encouraging and allowed for an improved understanding of diffusion and transport processes at the molecular level, while at the same time demonstrating that the spatial heterogeneity that can be resolved using HRMAS NMR PFG diffusion experiment must be larger than ~μmmore » length scales, expect for polymer transport within nanoporous carbons where additional chemical resolution improves the resolvable heterogeneous length scale to hundreds of nm.« less

Heparin and the related glycosaminoglycan, heparan sulfate, are polydisperse linear polysaccharides that mediate numerous biological processes due to their interaction with proteins. Because of the structural complexity and heterogeneity of heparin and heparan sulfate, digestion to produce smaller oligosaccharides is commonly performed prior to separation and analysis. Current techniques used to monitor the extent of heparin depolymerization include UV absorption to follow product formation and size exclusion or strong anion exchange chromatography to monitor the size distribution of the components in the digest solution. In this study, we used 1H nuclear magnetic resonance (NMR) survey spectra and NMR diffusion experiments in conjunction with UV absorption measurements to monitor heparin depolymerization using the enzyme heparinase I. Diffusion NMR does not require the physical separation of the components in the reaction mixture and instead can be used to monitor the reaction solution directly in the NMR tube. Using diffusion NMR, the enzymatic reaction can be stopped at the desired time point, maximizing the abundance of larger oligosaccharides for protein-binding studies or completion of the reaction if the goal of the study is exhaustive digestion for characterization of the disaccharide composition. In this study, porcine intestinal mucosa heparin was depolymerized using the enzyme heparinase I. The unsaturated bond formed by enzymatic cleavage serves as a UV chromophore that can be used to monitor the progress of the depolymerization and for the detection and quantification of oligosaccharides in subsequent separations. The double bond also introduces a unique multiplet with peaks at 5.973, 5.981, 5.990, and 5.998 ppm in the 1H-NMR spectrum downfield of the anomeric region. This multiplet is produced by the proton of the C-4 double bond of the non-reducing end uronic acid at the cleavage site. Changes in this resonance were used to monitor the progression of the enzymatic digestion and compared to the profile obtained from UV absorbance measurements. In addition, in situ NMR diffusion measurements were explored for their ability to profile the different-sized components generated over the course of the digestion. FigureDOSY spectra of intact (blue) and digested (red) heparin illustrating the differences in their diffusion coefficients.

Counterion condensation and effective charge of poly(styrenesulfonate)

We use nuclear magnetic resonance (NMR) logging to help with the petrophysical evaluation of thin sand-shale laminations. NMR helps to 1) detect thin beds, 2) determine fluid type, and if hydrocarbon is present, 3) establish the hydrocarbon type and volume, and finally 4) determine the permeability of the sands (as opposed to that of the sand-shale system). Formation evaluation in thin sand-shale laminations starts with their detection. NMR vertical resolution is mainly controlled by the antenna aperture, that is, in the case of a high-resolution antenna, 6 in. or 15 cm. Within that distance NMR tools will cumulatively measure all layers of shales and all layers of sands regardless of their individual thicknesses. Because NMR relaxation time in shales is much faster than in the productive sands, thin sand-shale laminations appear on NMR logs with the characteristic bimodal relaxation distribution. The thin laminations are often below the resolution of conventional logs that have a typical vertical resolution of 6 to 12 in. or 15 to 30 cm. This makes fluid typing in the centimeter-thick sands problematic from conventional logs. Also, formation pressure or sampling tools could hit-and-miss the thin sands. In contrast, since gas, oil, and water have different properties, fluid typing techniques that exploit all NMR relaxation times (T1 and T2) and diffusion (D) offer new ways to determine the fluid type in thin layer sands. From the bimodal relaxation distribution of the laminated sand-shale system, it is often possible to determine a cutoff to separate the two components. Porosity in the sand component can then be estimated separately and with it the hydrocarbon pore volume. Conventional high-resolution permeability from NMR is limited to one antenna aperture. If the sand layer thickness is less than that distance, the determined permeability of the sand-shale system will underestimate the true permeability of the sands. Using a fluid flow model, we show that the permeability of the sand component can be estimated separately. Experiments were conducted to verify the characteristic NMR bimodal relaxation distribution in thin beds, and to investigate whether the fraction of sand/shale and the sand porosity could be determined from NMR logs. The results confirmed observations on logs, of which we show case examples of thin sand-shale laminations that are water-bearing, oil-bearing, and gas-bearing respectively. In each case the NMR detection was verified against imaging logs, and the fluid type in the sands was determined from multi-dimensional NMR analysis. The derived hydrocarbon volume was then compared with the results estimated from a full triaxial (3D) induction tool. Permeability of the sand layers was also computed and compared to that of nearby thick sands. Core data in one well was used to validate NMR detection, porosity, permeability and net sand thickness.

Revealing Molecular Mechanisms in Hierarchical Nanoporous Carbon via Nuclear Magnetic Resonance

Chain dynamics of polymer melts was investigated by field cycling and rotating-frame nuclear magnetic resonance (NMR) relaxation spectroscopy in a frequency range from 103 to 3×108 Hz. Far above the critical molecular weight, the frequency dependencies of the spin–lattice relaxation times T1 and T1ρ are characterized by a sequence of power laws ∝ν0.5, ∝ν0.25, and ∝ν0.5 occurring in ranges analogous to the Doi/Edwards limits of the anomalous time dependencies of the mean-square displacement of segments ∝t1/2, ∝t1/4, and ∝t1/2. The T1 dispersion data clearly contradict the dominance of Rouse dynamics within the Doi/Edwards tube. The ν3/4 frequency dependence predicted by de Gennes for the regime of Rouse relaxation along the tube was not observed. The spin–lattice relaxation behavior can, however, be derived from the Doi/Edwards mean-square displacement limits assuming a correlation between segment orientation and displacement direction. A corresponding formalism is presented. On the other hand, the spin–lattice relaxation in dilute polymer solutions and in melts of polymers with molecular weights below the critical value can be described perfectly by the Khazanovich NMR relaxation theory for the Rouse model.

Ligands for Nanoparticles

Biophysical Basis of the Diffusion-Weighted Magnetic Resonance Signal in the Rat Brain

The measurement of translational diffusion coefficients by nuclear magnetic resonance (NMR) spectroscopy is essential in a broad range of fields, including organic, inorganic, polymer, and supramolecular chemistry. It is also a powerful method for mixture analysis. Spatially encoded diffusion NMR (SPEN DNMR)" is a time efficient technique to collect diffusion NMR data, which is particularly relevant for the analysis of samples that evolve in time. In many cases, motion other than diffusion is present in NMR samples. This is, for example, the case of flow NMR experiments, such as in online reaction monitoring and in the presence of sample convection. Such motion is deleterious for the accuracy of DNMR experiments in general and for SPEN DNMR in particular. Limited theoretical understanding of flow effects in SPEN DNMR experiments is an obstacle for their broader experimental implementation. Here, we present a detailed theoretical analysis of flow effects in SPEN DNMR and of their compensation, throughout the relevant pulse sequences. This analysis is validated by comparison with numerical simulation performed with the Fokker-Planck formalism. We then consider, through numerical simulation, the specific cases of constant, laminar, and convection flow and the accuracy of SPEN DNMR experiments in these contexts. This analysis will be useful for the design and implementation of fast diffusion NMR experiments and for their applications.

Direct observation of the active sites in methane dehydroaromatization by NMR

Nuclear magnetic resonance (NMR) spectroscopy is widely used for the monitoring of chemical reactions. Flow NMR methods are being increasingly used to monitor reactions carried out in either batch or flow synthesis mode. Kinetic information is commonly obtained by integration of assigned peaks across a series of spectra. However, the complexity of NMR spectra in reaction mixtures can result in peak overlap and assignment issues, which make it difficult to recover the clean complete spectrum of compounds involved in the reaction. Multiway analysis methods can in principle be used to separate information on compounds in a mixture, but they are demanding on the quality and form of the input data. Herein, it is shown how the multiway analysis of time‐resolved diffusion NMR data can yield the clean spectrum of newly formed compounds, for a selection of click reactions carried out in batch and in flow, when monitored by flow NMR. The use of a fast and robust diffusion NMR approach, together with careful processing, yields high‐quality data, even for continuously flowing samples, which was previously inaccessible. Multiway analysis then yields 1D 1H spectra together with concentration changes. The proposed approach is expected to be particularly useful for reaction monitoring applications.

It is important to characterize drug-albumin binding during drug discovery and lead optimization as strong binding may reduce bioavailability and/or increase the drug's in vivo half-life. Despite knowing about the location of human serum albumin (HSA) drug binding sites and the residues important for binding, less is understood about the binding dynamics between exogenous drugs and endogenous fatty acids. In contrast to highly specific antibody-antigen interactions, the conformational flexibility of albumin allows the protein to adopt multiple conformations of approximately equal energy in order to accommodate a variety of ligands. Nuclear magnetic resonance (NMR) diffusion measurements are a simple way to quantitatively describe ligand-protein interactions without prior knowledge of the number of binding sites or the binding stoichiometry. This method can also provide information about ligand orientation at the binding site due to buildup of exchange-transferred NOE (trNOE) on the diffusion time scale of the experiment. The results of NMR diffusion and NOE experiments reveal multiple binding interactions of HSA with dansylglycine, a drug site II probe, and caprylate, a medium-chain fatty acid that also has primary affinity for HSA's drug site II. Interligand NOE (ilNOE) detected in the diffusion analysis of a protein solution containing both ligands provides insight into the conformations adopted by these ligands while bound in common HSA binding pockets. The results demonstrate the ability of NMR diffusion experiments to identify ternary complex formation and show the potential of this method for characterizing other biologically important ternary structures, such as enzyme-cofactor-inhibitor complexes.

Nuclear magnetic resonance diffusion with surface relaxation in porous media