Calorimetric Study of Smectic Liquid Crystals

在这个项目中，开发了新的量热计，用于定量测量潜热和过渡前热容。新的量热计可以在宽的频率范围内工作（10.2 HZ到10.3 HZ），因为有证据表明，在一些过渡附近会发生缓慢的动力学，并且因为热容可以是频率的复函数。有了这些增强的量热能力，将有可能通过研究二元混合物来解决SmC-Sml（倾斜的smic -倾斜的hexatic）转变的临界行为，其中转变的性质应该从一级到临界再到超临界。本研究的具体目标是：1)一阶跃迁潜热和预跃迁Cp翼的定量测量；2)二阶和多临界点静态热容临界行为的确定；3)表征大块液晶和多孔介质（气凝胶）中液晶的连续非临界演化或顺序；4)研究液晶跃迁附近的慢动力学，这种跃迁会产生复杂的频率相关热容。液晶，表现出多种长程有序，介于结晶固体和简单液体之间。鉴于液晶显示和器件技术的不断发展，了解这些材料的相变行为具有重要意义。一些具有挑战性的问题仍然存在。对于某些液晶相变附近的慢时间响应几乎一无所知，这将用一种新开发的动态量热计进行研究。手性（不对称分子中的右或左旋性）的影响知之甚少，将在键取向有序和新发现的扭晶界相的研究中进行探索。液晶在气凝胶等随机多孔介质中的行为是一个与液晶在限制几何中的应用密切相关的新领域。对表面效应、有限尺寸约束效应和随机场效应将进行广泛的探索性研究。＊＊＊

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. ***