Chapter 11. NMR Relaxometry in Liquid Crystals: Molecular Organization and Molecular Dynamics Interrelation

Abstract

In this chapter, a review of proton spin–lattice relaxation studies in liquid crystals is presented. Liquid crystals present a large variety of phases between the isotropic phase and the crystalline phase. Different degrees of molecular positional and orientational order can be found in the mesophases of these materials. The molecular dynamics in these systems is diverse and strongly affected by the degree of molecular self-organization, and to some extent it is possible to identify specific types of collective motions in the mesophases not observed in isotropic and crystal phases. The study of proton NMR spin–lattice relaxation over a broad range of Larmor frequencies (from kilohertz to hundreds of megahertz) has proved to be a very effective method for decoupling the contributions of different relaxation mechanisms. The development of specific theoretical models to account for additional relaxation contributions of collective motions made possible, in some cases, a detailed quantitative analysis and comprehensive interpretation of the experimental results. Details of the specific molecular motions and respective relaxation mechanisms found in these systems are presented. “Fingerprint” relaxation dispersion profiles for the most common nematic and smectic A liquid crystal phases are also described.