Magnetic Resonance in Studying Natural and Synthetic Materials

This chapter introduces into NMR methods as the most important techniques for studying materials. The behaviour of spin system is considered in crossing magnetic fields from “classical” point of view. Basic equations for the vector of the bulk magnetisation M in outer magnetic field (Bloch equations) are discussed in rotating coordinate system. Spin-lattice (longitudinal) relaxation and spin-spin (transverse) relaxation are considered. The chapter describes the NMR experimental techniques for studying relaxation times T1, T2 and cross-relaxation. The role of proton exchange between the water and exchangeable protons of macromolecules is considered. Pulsed field gradient (PFG) NMR techniques are described for application in one- and twodimensions. Double-quantum-filtered (DQF) NMR spectroscopy is introduced as a technique to study the materials with anisotropic motion of molecules.

The chapter presents the results of NMR methods in studying natural silk Bombyx mori with low water content (0.07 g H 2O per g dry mass). The silk fibers have been also studied on mechanical perturbations measuring the stress-strain dependences for silk fibers. The results obtained are compared with published data on mechanical studies of polymer materials and model calculations of stress-strain curves. The free induction decays (FIDs) in the silk samples have been analyzed within the model of two components: a slow relaxation component was associated with water protons whereas the fast one was related to macromolecular protons. The results discovered a slow molecular mobility and strong interaction of water molecules to silk macromolecules. The chapter presents also NMR relaxation methods in studying interaction of water with macromolecules in oriented and nonoriented collagen fibers with different cross-linking level. The NMR relaxation times (T1 and T2) have been studied in the collagen fibers oriented along the static magnetic field B0. Several NMR relaxation experiments have been done on randomly oriented fibers and for collagen samples with varying hydration level. Cross-relaxation effect has been studied on silk and collagen samples with low water content. Correlation times as characteristics of molecular motions have been considered to compare quantitatively the mobility of water and biopolymer macromolecules in natural silk fibers and collagen tissues with different degrees of binding molecules.

The chapter presents the results of one-dimensional NMR methods in studying molecular anisotropy and microstructure of collagen fibers from two connective tissues with different cross-linking level (fibers from adult steer and young calf) at humidity level (HL) of 0.6 g water per g dry mass. The apparent diffusion coefficients (Dapp) have been studied in oriented collagen fibers (along the static magnetic field B0) for two experimental cases: (1) the gradient was applied along the static magnetic field B0 and (2) the gradient was switched on perpendicular to the magnetic field B0. The dependences of Dapp on diffusion time discovered a restriction diffusion of water for both gradient directions. A model of equally spaced parallel planes with permeable barriers, has been used to estimate a restricted distance and permeability coefficient. For both types of collagen fibers (adult and young) anisotropic diffusion of water has been discovered. Moreover, self-diffusion of water in fibers of natural silk (Bombyx mori) with HL=0.18 g H2O per g dry mass has been studied by pulsed field gradient NMR stimulated echo at various diffusion times Δ between 10 and 200 ms. The analysis showed that the decrease in Dapp with the increasing Δ due to the restricted diffusion. The results obtained were compared with published data on restricted diffusion in natural macromolecular systems with low water content.

1H-1H residual dipolar interactions in water molecules in oriented (along the static magnetic field B0) and randomly oriented collagen fibers from two connective tissues with different cross-linking level (fibers from adult steer and young calf) at humidity level (HL) of 0.6 g water per g dry mass were investigated by 1H doublequantum- filtered (DQF) NMR spectroscopy. It was observed that intensities of DQF signal in fibers oriented along outer static magnetic field exceeded significantly the DQF intensities in nonoriented fibers. The results were analysed on the basis of tensor description of DQF signal formation at the action of special sequence of radio frequency pulses with suitable phase cycling. The DQF signals dependencies upon creation time forming the (T2,+1, T2,-1) tensors of the second rank were compared for oriented and nonoriented collagen fibers with different cross-links. A role of exchange has been clarified in an additional study of temperature influence on DQF signals. The registered 1H DQF NMR signals were due to the residual dipolar interactions (RDIs) between the protons of water molecules interacting with oriented collagen fibers. Molecular mobility of water in the fibers of Bombyx mori natural silk has been studied using the DQF NMR technique and single-pulse 1Н NMR. The observed 1Н DQF NMR signal in the natural silk fibers with low water content testified about local order and anisotropic motion of water molecules. DQF spectra in both collagens and silk fibers were analysed within the theory of 1Н DQF NMR on RDIs in the systems with anisotropic mobility. When a humidity level of natural silk fibers was increased to 0.18 g H2O per g dry mass, no DQF signals and RDIs were detected.

In the technological treatment of wood materials, it is important to control water-wood interactions. To measure the water self-diffusion coefficients properly in wetting/drying processes of wood, it is necessary to understand many factors affecting the experiments. For fibrous material, the diffusivities of water could be different along the fiber and in the direction of perpendicular to fiber axis. The chapter presents the data of NMR relaxation and 1-dimensional PFG NMR in studying anisotropic diffusion of water in wood.

This chapter describes how to use 2-dimensional pulsed field gradient (PFG) NMR spectroscopy to explore the anisotropic diffusion of water in wood. Simulations based on the theory of 2D diffusion-diffusion correlations with the application of inverse Laplace transform (ILT) have been carried out for orthogonal and collinear pairs of gradients with parameters of 2D experiment on wood. The algorithm uses analytical solution for the echo attenuation in the 2D PFG experiment with collinear/orthogonal applications of two gradient pulses pairs. Numerical expressions calculated for the gradients and the model diffusion coefficients in both orthogonal directions were treated by 2D ILT on various artificial and experimental data sets. The algorithm is presented in a NMR context for wetting wood when the gradient directions are switched. The results showed how 2D (D1, D2) ILT maps reflected different ratios D1/D2 in two orthogonal directions. When two pairs of gradients were applied orthogonally in 2D experiment on wood, the features of diffusion anisotropy appeared as off-diagonal “wings” in 2D diffusion map whereas in the experiment with two collinear pairs of gradients, anisotropic diffusion of water in wood cells has been observed as two diagonal peaks in 2D map.

The solutions of random copolymers composed from butyl methacrylate (BMA) and methacrylic acid (MAA) with variable molar ratio of BMA/MAA (between 100/0 and 60/40) in isopropanol (IPA) and IPA-water have been studied by the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) method. The self-diffusion coefficients of isopropanol in BMA-MAA solutions were defined from the initial parts of echo-attenuation curves whereas the polymer diffusivities (two orders of magnitude lower than those for solvent) were separately measured applying solvent signal suppression (maximum gradient of 10 T/m). The aim was to study an influence of polymer concentration and BMA/MAA molar ratio on the diffusion constants of solvent and polymer. The diffusion coefficients for IPA increased with increasing BMA content in copolymer. The calculations based on Wang model to describe the solvent diffusion showed that relative mass of bound solvent per mass of polymer increased with increasing MAA content in the copolymer. The model of restricted polymer diffusion have been applied to explain the data on diffusivity of polymer. The macromolecules formed the transient aggregates with effective size that decreased with decreasing MAA content. Some polymer aggregates composed network and resulted in obstructive effect for diffusion of the rest nonconnected copolymer chains. At the study of BMA-MAA copolymers in IPA-water solutions, the focus was on the effects of molecular composition and neutralisation level of methacrylic acid.The results obtained showed that the average diffusion coefficient of solvent in D-IPA/D2O solutions of neutralised copolymer was larger than that in un-neutralised copolymer IPA solutions. Polymer diffusion at infinite dilution in IPA-water allowed an estimate for the effective hydrodynamic radius of unperturbed diffusive polymer as 2.6-4.8 nm.

NMR relaxation and NMR diffusion techniques have been applied to investigate the films from the copolymers of methacrylic acid and butyl methacrylate after their swelling in water. 1H NMR spectra and spin-spin relaxation times have been analysed to estimate the contributions of water protons and the protons of polymer matrix. NMR diffusion study in the films swollen in water discovered a dependence of the echo-attenuation signal on the diffusion time. The data obtained showed a restricted diffusion of water trapped in film pores. The results have been analysed taking into account the known physical models of water diffusion in materials. Applying the Tanner’s approach to the data on water self-diffusion in the copolymer films, the pore size and permeability have been estimated. The increase of the water immersion time resulted in an additional water uptake and increasing molecular mobility of water.

Diffusivity and distribution of water in random copolymer films of butyl methacrylate and methacrylic acid swollen in water have been investigated using magnetic resonance imaging (MRI) and NMR relaxation techniques. The relative contributions of the protons of the polymer matrix and bound water have been analyzed using 1H NMR spectra and MRI data of the swollen films. Pulse Field Gradient (PFG) NMR experiments have been carried out in parallel to the MRI studies. The results have been discussed with reference to published MRI data for water in a number of polymer materials. The free induction decay (FID) and the spin-spin relaxation times (T2) data for water saturated polymer films correlated well with the MRI intensities for the same samples. The results show that by varying the hydrophobic/hydrophilic comonomer ratios in the polymers, the water ingress can be changed. MRI data show how to monitor drying process. Moisture profiles and relaxation data were obtained during film drying. The time dependences of the image slices intensity for the drying films were fitted by a single-exponential function with an average time constant as k = −0.02 min−1.

The chapter considers the data of one-dimensional 1H NMR diffusion studies in white Portland cement paste (the water-cement ratio (w/c) = 0.4) with the age of 1 to 14 days and 1 year. Two-dimensional diffusion-diffusion correlation (DDCOSY) experiments have been carried out in cement paste (w/c= 0.4) aged from 1 to 7 days. It was found that the 1H pulsed field gradient (PFG) NMR experiments measure diffusivities of capillary water. A log-normal pore size distribution and relaxation times dependent on pore size have been applied to fit the data. Mature paste has been characterized by mean capillary pore size of 4.2 μm. The data for cement pastes with the age of 1 week showed similarity. On the basis of data obtained it was suggested that gel porosity and hydrates do not form in the capillary porosity. 2D DDCOSY experiments did not discover any diffusion anisotropy in capillary pores.

This chapter considers how 1H DQF NMR spectroscopy characterises hydrated cement pastes. The increase of the DQF NMR signal during cement hydration has been observed. The mobile single-quantum (SQ) signal was decreasing with increasing hydration time. The measured DQF NMR signal in hydrated white cement has been fitted by a sum of two components. The protons of Ca(OH)2 were responsible for the appearance of the first component. The second component was associated with the protons of water in the planar C–S–H gel pores. A model of water molecules movement near the centres of paramagnetic impurities has been considered. The model was consistent with the level of paramagnetic iron content and experimental data. The DQF spectra in grey cement pastes have been fitted by a sum of three components. First two components in grey cement were considered similar to white cement. The iron-rich phases in grey cements were responsible for an appearance of third component. The experiments with progressively heating cements showed how water leaves from cement sample and how the DQF signals of all components changed with removing water. The behaviour of the DQF signal as function of relative sample mass was in sympathy with bound component of the solid echo experiment. It was discovered that the DQF component assigned to the C–S–H water decreases monotonically, while that associated with the solid Ca(OH)2 first increases before decreasing.