Study of Lamellar Liquid Crystals by NMR-Diffusion Techniques

Abstract

This thesis reports on the use of Pulsed Gradient Spin Echo (PGSE) NMR techniques for the measurement of solvent diffusion within lamellar liquid crystals. Throughout this work, various experiments are conducted under shear flow: the lamellar phases, located between the tow cylinders of a Couette cell, are subjected to shear rates up to γ 100s-1. The geometry of the rheological apparatus potentially represents a serious hindrance for the implementation of diffusion measurements: fluid rotation may induce an artifactual flow contribution to the measured displacements. However, a technique is introduced that enables for rapid flow compensation, leaving the effects of diffusive displacements untouched. We demonstrate the efficiency of the technique at shear rates corresponding to the formation of spherical vesicles. Throughout this work, solvent diffusion is used as a probe for the study of the surrounding liquid crystals by interpreting experimental data in the context of the diffusion spectrum formalism. As well as compensating for flow effects, our NMR diffusion technique enables one to access frequencies up to 1 kHz. Practical applications are presented that illustrate the efficiency of the technique. We show that frequency-dependent diffusion coefficients of solvent molecules embedded in a lamellar system at rest may be related to dynamic modes of the membranes as well as misordering of the bilayers. Efficient flow compensation enables the observation of shear effects on the solvent diffusion spectrum. As shear is increased, topological transformations of the bilayers to anisotropic, then isotropic multi-lamellar vesicles are detected. By use of the diffusion spectrum formalism, the shear dependence of the vesicle radius is obtained. Solvent diffusion imaging and velocimetry experiments unveil the existence of different flow regimes with different effects on the vesicles.