Solid-State NMR I Methods

Solid-state NMR spectra generally reflect the sum of several independent interactions such as Zeeman, dipolar and quadrupolar coupling as well as magnetic shielding effects. To obtain the desired information, separation of these effects and their decoding is essential. Within the first part, as a basis of the successful separation of the NMR effects, the common and also the different (tensorial) properties of each interaction are discussed in detail. The second part mainly deals with the experimental design of solid-state high-resolution NMR experiments to suppress certain interactions and with wide-line experiments. Finally the principles of NMR imaging and NMR microscopy are described as an alternative method for investigating solids by NMR.

In this chapter, we will first summarize the technical requirements for obtaining high-resolution solid-state ¹³C NMR spectra of bulk polymers. We will then review the bases of the determination and analysis, in terms of local motions, of the spectrum line shape, proton-carbon dipolar interactions, chemical-shift anisotropies, relaxation times and line widths. A large part of the chapter will be devoted to the description of studies that demonstrate the capability of NMR to investigate the local dynamics of bulk polymers at temperatures above and below the glass transition temperature, respectively. At temperatures below the glass transition temperature, the example of poly(cyclohexyl methacrylate) will illustrate the importance of line width measurements, whereas the chemical-shift parameters and ¹³C-¹H dipolar interactions will be the relevant parameters for the study of aromatic copolyesters and epoxy resins. In the case of bulk polymers at temperatures well above the glass transition temperature, we will first examine the different expressions that have been proposed for the orientation autocorrelation function associated with the polymer local dynamics. Tests of these expressions will be carried out through the determination of the ¹³C spin-lattice relaxation times of a number of elastomers. Results thus obtained permit us to specify the main factors that control the local dynamics in bulk polymers at temperatures well above the glass-transition temperature. Then, NMR studies of compatible blends of polyvinylmethyl ether and polystyrene will show the possibility of investigating the relative influence of intramolecular interactions and intermolecular constraints on the local motions of a given blend component.

Xenon NMR spectroscopy has become an important and widely used tool for characterizing complex chemical systems and host phases. Issues central to new and existing uses of xenon NMR in materials research are emphasized, including xenon mass transport considerations, the sensitivity of xenon’s NMR parameters to local and macroscopic material structure and dynamics, and novel optical polarization techniques. After reviewing the foundations of ¹²⁹Xe and ¹³¹Xe NMR spectroscopy methods, a number of applications are presented, particularly those focusing on new developments in the field.

Two types of stimulated NMR echo experiments, ²H-spin alignment and field gradient NMR, are formulated in terms of a “generalized dynamic scattering function”. The analogies to incoherent quasielastic neutron scattering are discussed. The concept is illustrated by selected examples covering molecular reorientations and self diffusion in molecular crystals and supercooled liquids, anomalous diffusion in linear chain polymers and restricted diffusion of molecules in confined geometries.

Methods and applications of NMR imaging of solids are reviewed. The prime concern of most imaging techniques is the achievement of high spatial resolution. Comparatively little work focuses on improvement and exploitation of NMR parameter contrast. Yet this seems to be the unique feature that makes NMR imaging superior to other imaging techniques, and provides a high potential for localization of previously unknown material heterogeneities.