Liquid crystal orientational order in confined geometries: A NMR perspective.

摘要

Liquid crystals are a very rich physical system where it is possible to study many phenomena both theoretically as well as experimentally. In almost all applications, liquid crystals exist in contact with some kind of substrate. Liquid crystals properties are greatly affected by a nearby surface: confinement alignment, phase transition temperatures, the critical behavior of the thermodynamic quantities and several other of their properties change. Researching confined liquid crystals to study surface effects will be beneficial for basic physics understanding and provide results perhaps extrapolated to the applied world.;An important concept in a microscopic description of a liquid crystal phase is the order parameter, each of the phases is characterized by one or more such parameters. It is therefore of interest to quantify and measure the degree of order of a particular phase 2H-NMR, as a microscopic measurement at the molecular level, has a number of unique features that make it a useful technique to study liquid crystals. NMR can distinguish between spatial and time averages whereas other methods such as birefringence can not. And, most importantly, deuterium NMR is sensitive to the orientational order present in the system. In fact, through NMR lineshape analysis, we can derive the configuration of the nematic director field, and thus determine liquid crystal alignment in random interconnected host.;In this work I will use thermotropic liquid crystals and confine them in Millipore membranes, silica Aerogel porous glass and silica Aerosil spheres. Millipore membranes are made from pure, biologically inert mixtures of cellulose acetate and cellulose nitrate. It is a randomly interconnected host geometry with a high porosity, and available in a variety of void sizes, for my research I will use sizes from 8.0 mum to 0.025 mum. Silica Aerogel is a connected pore network, available in many different densities. Our work will cover densities ranging from 0.068 to 0.265 g/cc. In Aerogel, the pore size changes from 1000 to 100 A. Aerosil are small SiO2 particles of roughly 70 A diameter. Samples will be prepared with density ranging from 0.015 to 0.3 g/cc. These three porous media offer large surface to volume ratio, so they are suitable to study the surface effect. Measurement will be performed as a function of temperature from 0 to 50°C, covering the isotropic, nematic and smectic phases, as a function of the host media size or the density.;The aim of these systematic measurements and analysis is to study how the different type of confinements: voids, pores or a chain of spherical particles, or spheres, introduce order or disorder in the liquid crystal near the surface and affects the over all liquid crystal alignment. We will explore the possibility of confining size-driven configurational transitions and determine the size dependence for the existence of certain phases. With this study we will determine the effect of different morphologies on the phase transitions of liquid crystal, and particularly, how a size-dependent critical behavior affects the orientational order. The research will be complemented by numerical simulations of the obtained NMR spectrum patterns. (Abstract shortened by UMI.)