液晶薄膜光熱現象與結構研究

Abstract

我們使用高解析ac微小訊號熱分析儀、光反射儀和改裝的穿透式電子顯微鏡。分別對正交型液晶(液晶材料為14S5)、鐵電型液晶(液晶材料為C7)，這兩種液晶材料進行液晶薄膜光熱及結構相位性質上的研究。 由電子繞射和光反射實驗研究發現，液晶膜發生相變是以一層接著一層(layer-by-layer)表面層相變的方式，從Sm-A相直接變成Cry-B相。在相變過程中，並沒有經歷Hexatic中間相。此外、由液晶分子間的交互作用的wetting行為來看，我們發現分子與分子間的交互作用力是延遲的凡德瓦爾力(retarded van der waals forces)。 而在使用高解析ac微小熱訊號分析儀研究自由懸浮液晶薄膜，由Sm-A－SmC*相變附近比熱的行為，證實了早期對液晶分子傾角量測工作上，內部液晶層發生由一階相變到二階相變的過程是與液晶膜厚度呈函數關係。並且由不對稱的比熱異常(anomaly)數據來看。在極薄膜時顯現了crossover行為，同時我們亦觀察到液晶膜在低溫時，將由Sm-C相相變為傾斜的Cry-G相。這相是有晶格點存在的。此外、我們也推測因為液晶分子是傾斜的，所以液晶薄膜表面的有序性強度是大過正交型的層狀型(smectic)液晶。

Defect-mediated phase transition has been generally considered as one of the most exciting topics in condensed matter physics in recent years. The phase transitions in reduced-dimensional system have attracted great attention because it involves the research of fundamental physics and technical applications. Free-standing liquid crystal thin films provide an ideal experimental system for the research of reduced-dimension.We have performed AC calorimetry and optical reflectivity studies on various liquid- crystal thin films and reported the following discoveries： (1) I have investigated the Sm-A- Cry-B Phase transition of 14S5 in a liquid-crystal material, using electron diffraction and optical reflectivity have provided the direct confirmation of the existence of layer-by-layer surface transitions, without going through an intermediate hexatic phase. The molecular interactions are found to be through retarded van der waals forces. (2) We have also studied heat-capacity measurements near the smectic-A- smectic-C* phase transition in free standing films of one chiral liquid-crystal compound-C7. The heat-capacity behavior confirms the evolution of the transition in the interior layers from first to second order as function of film thickness suggested earlier in tilt angle measurements. The asymmetry in the heat-capacity anomaly exhibits an interesting crossover in thinner films. The surface ordering strength in these films is found to be much larger than that in other orthogonal-smectic films, which we speculate is due to the molecular tilt. In this thesis, by using our state-of-the-art ac calorimeter and high-resolution optical reflectivity setup, I have systematically studied several phase transitions in 14S5 and C7 liquid-crystal materials. It can’t be denied that the ac calorimetric and optical-reflectivity studies provide a new and important field for the study of the free-standing liquid-crystal films. Many interesting compounds including their phase diagrams remain to be investigated. The substrate-free property also with the quantized layer number makes free-standing liquid-crystal films unique and suitable for the study of the reduced-dimensional system. Chiral LC compounds have many new phase and some interesting physical phenomena, and of course, need further studies.