Calorimetric Study of Smectic Liquid Crystals

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9311853 Garland In this project, new calorimeters are developed for quantitative measurements of the latent heat and pre-transitional heat capacity. The new calorimeter can operate over a wide frequency range (10.2 HZ to 10.3 HZ) because there is evidence that slow dynamics occurs near some of the transitions, and because the heat capacity can be a complex function of frequency. With these enhanced calorimetric capabilities, it will be possible to resolve the critical behavior at SmC-Sml (tilted smectic-tilted hexatic) transitions by studying binary mixtures, where the nature of the transition should change from first order to critical to supercritical. The proposed research has the following specific goals: 1) quantitative measurement of latent heats and pre-transitional Cp wings for first-order transitions; 2) determination of critical behavior for the static heat capacity at second-order and multicritical points; 3) characterization of the continuous non-critical evolution or order in bulk liquid crystals and liquid crystals in a porous medium (aerogel); and 4) investigation of the slow dynamics near liquid crystal transitions which give rise to a complex frequency- dependent heat capacity. %%% Liquid crystal, exhibit a variety of long range ordering that is intermediate between that of crystalline solids and simple liquids. Understanding the phase transition behavior in these materials is of importance in view of the growing development of liquid crystal display and device technologies. Several challenging problems remain. Almost nothing is known about the slow time response near some liquid crystal phase transitions, and this will be studied with a newly developed dynamic calorimeter. The effects of chirality (right- or left-handedness in asymmetric molecules) are poorly understood and will be expolored in studies of bond-- orientational ordering and the newly discovered twisted-grain- boundary phases. The behavior of liquid crystals in random por ous media like aerogels is a new field that is closely related to applications involving liquid crystals in restrictive geometries. A wide range of exploratory studies concerned with surface effects, finite size confinement effects, and random field effects will be undertaken. ***