低微度系統下的相變研究關乎基本物理學及工業上的應用。懸浮液晶薄膜提供了一個實驗上的低微度系統。在64COOBC液晶材料上，我利用高解析度比熱及光反射儀來研究Smectic-A — Hexatic-B 相變。發現這個相變並非KT形式的相變。兩層膜的比熱在相變點是發散並且對稱，且臨界指數 a = 0.38 +- 0.02。在厚度大於七層膜以上的懸浮液晶薄膜，發現非完整的逐層相變行為。並且七、八、九層膜上的內層相變，在比熱上有異常的壓制現象。推測應該是來自於表層Hex-B 形成較穩定結構而對內部熱擾動的限制所造成。 在另一個有手徵的液晶材料上 (MDW1397)，我研究薄膜Smectic-A — Smectic-C* —Smectic-CA* 的相變過程。利用偏光型顯微鏡下的觀察，我們提出了一個多重相變模型。此外溫度及外加電場都能使表層分子傾斜結構有所變化。在比熱及光反射量測上，此材料上的Smectic-A — Smectic-C* 相變是一階相變。同時我們在六到二十一層的比熱訊號上發現到一個有趣的比熱訊號外型的轉變現象。 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 films (FSLCFs) provide an ideal experimental system for the research of reduced-dimension. I have investigated the Sm-A — Hex-B phase transitions of 64COOBC FSLCFs using simultaneous heat-capacity and optical-reflectivity measurements. This observed transition is not of KT-type phase transition but weakly first order that disagree with the KTHNY theory. The heat-capacity anomaly associated with this transition can be well described by a power law with critical exponent a=0.38+-0.02 . For the thicker films (N≧7) undergo an incomplete wetting transition. Unusual highly suppression of the heat capacity at the third transition has been found in 7-, 8-, and 9-layer 64COOBC films. This novel phenomenon is possibly due to the existence of a strong Hex-B surface anchoring field which suppresses the fluctuation of the interior Sm-A layers. We have also studied the phase transition of one chiral liquid-crystal compound — MDW1397. Base on many observations on its optical textures using the depolarized optical microscope, we propose a suitable model involving multi-step Sm-A — Sm-C* phase transitions. Both thermal and external electric field can induce anticlinic—synclinic transitions. And the heat-capacity and optical-reflectivity measurements both confirm the Sm-A to Sm-C* transition to be the first order. The heat-capacity anomaly exhibits a novel layer-dependent crossover behavior for films from 6 to 21 layers.
