A Study of the Phase Transitions of Mixtures of the Nematic Liquid Crystals 5CB and MBBA Using Light Scattering and Microscopic Techniques

A. Christine Rauch

Mixtures of the nematic liquid crystals 5CB and MBBA at various concentrations were investigated by observing phase transitions using light scattering and video microscopy. Transitions were observed as samples were cooled slowly from the isotropic phase to room temperature. Bulk samples ranging in concentration from 20-60% 5CB were observed to change from an isotropic liquid to a spongy, gelatinous substance at room temperature. Thin cells (36 μm) were made of each mixture, and transmitted light intensity was graphed with respect to temperature. Samples ranging in concentration from 20-60% 5CB were shown to exhibit two phase transitions, with all other samples exhibiting only one transition (isotropic-nematic). The nature of the second transition is still under investigation. A coexistence curve with transition temperature versus sample concentration was plotted. Phase transitions were further investigated by photographing samples at constant time intervals and at 58x magnification through a cross-polarizing microscope as temperature was lowered at -0.25 °C/min. Several textures were observed, including schlieren brushes, salt and pepper textures, granitic textures, dendritic growth, and mosaic textures. Growth of droplets during the isotropic-nematic phase transition was measured and graphed with respect to temperature. Finally, a very thin planar service (10 μm) was filled with the 40% 5CB mixture and observed through the cross polarizing microscope as temperature decreased at -0.025 °C/min in order to understand the effects of thickness on the textures observed. Regions of homeotropic alignment were observed in this cell after the mixture had cooled to room temperature, as confirmed by the rotation of the sample between cross polarizers and the observation of a cross pattern when viewed through a cylindrical Bertrand lens.