Author: Tanya Ostapenko

Publisher:

Published: 2011

Total Pages: 202

ISBN-13:

This dissertation addresses three experimental questions. First, the pretransitional behavior of lyotropic chromonic liquid crystals (LCLCs) is investigated in order to gain further insight into the aggregation mechanism and structure. In order to study the pretransitional behavior of LCLCs in the isotropic phase, a high magnetic field is applied perpendicular to the light propagation direction, which induces birefringence in the material; this is called the Cotton-Mouton effect. The aggregates align with the field, which makes it possible to study how the aggregates form in the isotropic phase. The results of this study indicate that multiple optical effects can be induced, which supports the possibility of a complex aggregate structure. The second part of this dissertation explores the possibility of a biaxial nematic phase (Nb). The geometry of the liquid crystal mesogen is important and it is thought that banana-shaped liquid crystals will have an Nb phase. However, contradicting reports on different bent-core materials have not determined whether this phase exists in them. Optical techniques usually rely on a sample cell rubbing treatment to homeotropically align the main director, n, but optical misidentification of Nb could occur if the material is in a tilted uniaxial phase, which appears the same as a homeotropically-aligned biaxial phase. Using a high magnetic field to completely align n and measuring the magnetic field-induced optical phase difference perpendicular to n gives a conclusive way to determine whether a material has non-zero biaxial order. None of the materials studied appear to have an Nb phase. The last part of this dissertation examines director fluctuations in calamitic and bent-core liquid crystals using dynamic imaging analysis. Dynamic image analysis is a relatively new technique where a measurement of nematic phase fluctuations is made in direct space. These measurements are done using a polarizing microscope, heat stage and CCD camera. The advantage of this technique is that it measures small values of q, making it a complementary technique to dynamic light scattering, where large values of q are easily obtained. Basic material properties may be related to the wave vector and decay time of the fluctuations.