Orientational fluctuations near the smectic A to smectic C phase transition in two "de Vries"-type liquid crystals.
Orientational fluctuations near the smectic A to smectic C phase transition in two "de Vries"-type liquid crystals.
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DOI：
10.1002/cphc.201300358
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发表时间：
2013-09
期刊：
影响因子：
--
通讯作者：
D. Nonnenmacher;S. Jagiella;Qingxiang Song;R. Lemieux;F. Giesselmann
中科院分区：
文献类型：
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作者：
D. Nonnenmacher;S. Jagiella;Qingxiang Song;R. Lemieux;F. Giesselmann
在深入分析近晶单区二维X射线衍射谱的基础上，研究了取向涨落对两种非手性有机硅氧烷“de Vries”型液晶的弱一阶近晶A(SMA)到近晶C(SMC)转变的影响。我们发现，这些材料表现出非常大的分子倾斜波动，幅度高达35°--因此比平均倾斜本身更大。这对于理解这些材料中实际没有近晶层收缩的潜在分子机制是至关重要的：在SMA相，向列序参数非常低(分子涨落相应地高)，并且在SMA到SMC转变时预期的层收缩几乎完全被取向有序度的增加所补偿，因为随着温度的降低，涨落减小。除了一般的倾斜涨落外，我们还观察到了本征软模涨落。它们具有λ形状的温度依赖关系，在SMA-SMC转变处达到峰值，最大幅度约为2°。
On the basis of thorough analysis of 2D X-ray diffraction patterns from smectic monodomains, we examine the influence of orientational fluctuations on the weakly first-order smectic A (SmA) to smectic C (SmC) transitions in two nonchiral organosiloxane "de Vries"-type liquid crystals. We find that these materials exhibit very large molecular tilt fluctuations with magnitudes of up to 35°--thus larger than the average tilt itself. This is essential to understand the underlying molecular mechanism behind the practical absence of smectic layer contraction in these materials: in the SmA phase, the nematic order parameter is very low (molecular fluctuations correspondingly high), and the expected layer shrinkage at the SmA to SmC transition is almost fully compensated by the increase in orientational order, as the fluctuations diminish with decreasing temperature. In addition to the general tilt fluctuations, we observe intrinsic soft-mode fluctuations. They have a λ-shaped temperature dependence that peaks at the SmA-SmC transition with a maximum amplitude of about 2°.