Macromolecular dynamics in biological systems, which play a crucial role for biomolecular function and activity at ambient temperature, depend strongly on moisture content. Yet, a generally accepted quantitative model of hydration-dependent phenomena based on local relaxation and diffusive dynamics of both polymer and its adsorbed water is still missing. In this work, atomistic-scale spatial distributions of motional modes are calculated using molecular dynamics simulations of hydrated xyloglucan (XG). These are shown to reproduce experimental hydration-dependent C-13 NMR longitudinal relaxation times (T-1) at room temperature, and relevant features of their broad distributions, which are indicative of locally heterogeneous polymer reorientational dynamics. At low hydration, the self-diffusion behavior of water shows that water molecules are confined to particular locations in the randomly aggregated XG network while the average polymer segmental mobility remains low. Upon increasing water content, the hydration network becomes mobile and fully accessible for individual water molecules, and the motion of hydrated XG segments becomes faster. Yet, the polymer network retains a heterogeneous gel-like structure even at the highest level of hydration. We show that the observed distribution of relaxations times arises from the spatial heterogeneity of chain mobility that in turn is a result of heterogeneous distribution of water-chain and chain chain interactions. Our findings contribute to the picture of hydration-dependent dynamics in other macromolecules such as proteins, DNA, and synthetic polymers, and hold important implications for the mechanical properties of polysaccharide matrixes in plants and plant-based materials.

The binding of K+ and Ba2+ cations to short poly(ethylene oxide) (PEO) chains with ca. 4-25 monomeric units in methanol was studied by determining the effective charge of the polymer through a combination of electrophoretic NMR and diffusion NMR experiments. These cations were previously found to bind to long PEO chains in a similar strong manner. In addition, H-1 chemical shift and longitudinal spin relaxation rate changes upon binding were quantified. For both systems, binding was stronger for the short chains than that for the longer chains, which is attributed mainly to interactions between bound ions. For K+ ions, the equilibrium binding constant of a cation to a binding site was measured. For both cations, the binding site was estimated to consist of ca. six monomeric units that coordinated with the respective ions. For the systems with barium, a significant fraction of the bound ions are (BaAnion)(+) ion pairs. This leads to a strong anion effect in the effective charge of the oligomers acquired upon barium ion binding. For K+, the coordinating oligomer segment remains rather mobile and individual oligomers exchange rapidly (<<s) between their free and ion-complexing states. In contrast, segmental dynamics slows significantly for the oligomer section that coordinates with the barium species, and for individual oligomers, binding and nonbinding sections do not exchange on the time scale of seconds. Hence, oligomers also exchange slowly (>s) between their free and barium complexing states.

The self-diffusion of neat water, dimethyl sulfoxide (DMSO), octanol, and the molecular components in a water-DMSO solution was measured by H-1 and H-2 NMR diffusion experiments for those fluids imbibed into controlled pore glasses (CPG). Their highly interconnected structure is scaled by pore size and shows invariant pore topology independent of the size. The nominal pore diameter of the explored CPGs varied from 7.5 to 72.9 nm. Hence, the about micrometer mean-square diffusional displacement during the explored diffusion tithes was much larger than the individual pore size, and the experiment yielded the average diffusion coefficient Great care was taken to establish the actual pore: volumes of the CPGs. Transverse relaxation experiments processed by inverse Laplace transformation were performed to verify that the liquids explored filled exactly the available pore volume. Relative to the respective diffusion coefficients obtained in bulk phases, we observe a reduction in the diffusion coefficient that is independent of pore size for the larger pores and becomes stronger toward the smaller pores. Geometric tortuosity governs the behavior at larger pore sizes, while the interaction with pore walls becomes the dominant factor at our smallest pore diameter. Deviation from the trends predicted by the Renkin equation indicates that the interaction with the pore wall is not a just simple steric one but is in part dependent on the specific features of the molecules explored here.

Highly filled EPDM rubber used in cable transit seals in nuclear power plants were exposed to gamma radiation at a high dose rate at 23 degrees C in media with different oxygen partial pressures (1-21.2 kPa). The motivation of this study was threefold: highly filled polymers are replacing halogen-containing polymers and these materials have rendered less attention in the literature; there is a need to find efficient tools to make possible condition monitoring and extrapolation. Several profiling methods were used: IR microscopy, micro-indentation, micro-sample extraction/gravimetry and non-invasive NMR spectroscopy, and three different deterioration processes were identified: polymer oxidation, migration of low molar mass species, and anaerobic changes in the polymer network. IR microscopy, micro-indentation profiling and the portable NMR method confirmed diffusion-limited oxidation in samples irradiated in air. The inner non-oxidized part of the blocks showed a pronounced change in the indenter modulus by migration of primarily glyceryl tristearate migration was accelerated by the presence of oxygen in the surface layer and anaerobic changes in the polymer network. For extrapolation or for condition monitoring, it is best to use the data obtained by indenter modulus profiling and to use the correlation between indenter modulus and strain-at-break to quantify the sample status. Non-invasive NMR profiling provided useful data but was less precise than the indenter modulus data to predict the strain-at-break. 

In order to establish the potential correlation between the macroscopic ice adhesion and the molecular properties of the premolten layer (PML), the adhesion strength between ice and hydrophilic silica has been measured as a function of temperature. In addition, temperature-dependent molecular properties have been determined using techniques that are sensitive to different aspects of the PML, specifically total internal reflection (TIR) Raman, vibrational sum frequency (VSFS) and NMR spectroscopies. The ice shear adhesion strength was observed to increase linearly with decreasing temperature until -25 degrees C, where a plateau marked the adhesive strength having reached the cohesive strength of ice. Interestingly, at temperatures higher than -20 degrees C the ice samples slid on smooth (R-a < 0.4 nm) silica surfaces. This sliding behavior was not observed on rougher silica surfaces (R-a similar to 6 nm). By varying the penetration depth of the evanescent field, TIR Raman was used to establish an upper limit to the thickness of the PML in contact with silica (<3 nm even at -0.3 K below the bulk melting temperature). Additional quantitative determination of the temperature-dependent thickness of the PML was obtained from H-2 NMR measurements in mesoporous silica particles. Finally, the inherently surface specific technique, VSFS, which probed changes in the hydrogen bond environment, indicated at approximately -25 degrees C the onset of PML, followed by a marked structural change occurring just a fraction of a degree below the melting temperature. Jointly, the experimental approaches link, strongly and consistently, ice adhesion to the PML properties. Specifically, it is inferred that the premolten layer facilitates sliding and contributes to the observed friction behavior, provided its thickness is comparable to the surface roughness of the underlying silica substrate.

The organization of water molecules adsorbed onto cellulose and the supramolecular hydrated structure of microfibril aggregates represents, still today, one of the open and complex questions in the physical chemistry of natural polymers. Here, we investigate by H-2 MAS NMR the mobility of water molecules in carefully H-2-exchanged, and thereafter re-dried, microcrystalline cellulose. By subtracting the spectral contribution of deuteroxyls from the spectrum of hydrated cellulose, we demonstrate the existence of two distinct (H2O)-H-2 spectral populations associated with mobile and immobile water environments, between which the water molecules do not exchange at the NMR observation time scale. We conclude that those two water phases are located at differently-accessible adsorption sites, here assigned to the cellulose surfaces between and within the microfibril aggregates, respectively. The superior performance of H-2 MAS NMR encourages further applications of the same method to other complex systems that expose heterogeneous hygroscopic surfaces, like wood cell walls.

Complex formation in methanol between monodisperse polyethylene oxide (PEO) and a large set of cations was studied by measuring the effective charge acquired by PEO upon complexation. Quantitative data were obtained at a low ionic strength of 2 mM (for some salts, also between 0.5 and 6 mM) by a combination of diffusion nuclear magnetic resonance (NMR) and electrophoretic NMR experiments. For strongly complexing cations, the magnitude of the acquired effective charge was on the order of 1 cation per 100 monomer units. For monovalent cations, the relative strength of binding varies as Na⁺ < K⁺ ≈ Rb⁺ ≈ Cs⁺, whereas Li⁺ exhibited no significant binding. All polyvalent cations bind very weakly, except for Ba²⁺ that exhibited strong binding. Anions do not bind, as is shown by the lack of response to the chemical nature of anionic species (perchlorate, iodide, or acetate). Diffusion experiments directly show that the acetate anion with monovalent cations does not associate with PEO. Considering all cations, we find that the observed binding does not follow any Hofmeister order. Instead, binding occurs below a critical surface charge density, which indicates that the degree of complexation is defined by the solvation shell. A large solvation shell prevents the binding of most multivalent ions.

A method based on H-1 high-resolution magic angle spinning NMR has been developed for measuring concentration accurately in heterogeneous materials like that of ligands in chromatography media. Ligand concentration is obtained by relating the peak integrals for a butyl ligand in the spectrum of a water-saturated chromatography medium to the integral of the added internal reference. The method is fast, with capacity of 10min total sample preparation and analysis time per sample; precise, with a reproducibility expressed as 1.7% relative standard deviation; and accurate, as indicated by the excellent agreement of derived concentration with that obtained previously by C-13 single-pulse excitation MAS NMR. The effects of radiofrequency field inhomogeneity, spin rate, temperature increase due to spinning, and distribution and re-distribution of medium and reference solvent both inside the rotor during spinning and between bulk solvent and pore space are discussed in detail.

Cellulose, one of the most abundant renewable resources, is insoluble in most common solvents but dissolves in aqueous alkali under a narrow range of conditions. To elucidate the solubilization mechanism, we performed electrophoretic NMR on cellobiose, a subunit of cellulose, showing that cellobiose acts as an acid with two dissociation steps at pH 12 and 13.5. Chemical shift differences between cellobiose in NaOH and NaCl were estimated using 2D NMR and compared to DFT shift differences upon deprotonation. The dissociation steps are the deprotonation of the hemiacetal OH group and the deprotonation of one of four OH groups on the nonreducing anhydroglucose unit. MD simulations reveal that aggregation is suppressed upon charging cellulose chains in solution. Our findings strongly suggest that cellulose is to a large extent charged in concentrated aqueous alkali, a seemingly crucial factor for solubilization. This insight, overlooked in the current literature, is important for understanding cellulose dissolution and for synthesis of new sustainable materials.