Smectic ordering in liquid-crystal-aerosil dispersions. I. X-ray scattering.
Smectic ordering in liquid-crystal-aerosil dispersions. I. X-ray scattering.
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液晶气相二氧化硅分散体中的近晶有序。
DOI：
10.1103/physreve.67.011708
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发表时间：
2002
期刊：
影响因子：
--
通讯作者：
G. Iannacchione
中科院分区：
文献类型：
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作者：
R. L. Leheny;Sei Jin Park;R. Birgeneau;R. Birgeneau;J. L. Gallani;Carl W. Garland;G. Iannacchione
全面的X射线散射研究表征了辛基氰基联苯(8CB)在硅胶中的近晶有序性。对于所有密度和所有测量温度，关联仍然是短范围的，表明凝胶施加的无序破坏了向列相(N)到近晶-A(SMA)的转变。近晶关联函数包含两个不同的贡献。第一个项的形式与描述纯8CB在N-SMA相变附近的临界热涨落的形式相同，并且在高温下表现出与纯液晶相似的温度依赖关系。第二项在高温下可以忽略，但在低温下占主导地位，它的形状由热项的平方给出，描述了由凝胶中的限制引起的随机场引起的静态涨落。在热项和无序项中出现的关联长度是相同的，并且在低温下表现出随凝胶密度的强烈变化。静态涨落幅度的温度依赖性进一步表明，随着猝灭无序度的增加，向列相的磁化率被抑制。通过将液晶-气溶胶系统映射到随机场中的三维XY模型上，可以很好地描述总体结果，无序强度随气泡密度线性变化。
Comprehensive x-ray scattering studies have characterized the smectic ordering of octylcyanobiphenyl (8CB) confined in the hydrogen-bonded silica gels formed by aerosil dispersions. For all densities of aerosil and all measurement temperatures, the correlations remain short range, demonstrating that the disorder imposed by the gels destroys the nematic (N) to smectic-A (SmA) transition. The smectic correlation function contains two distinct contributions. The first has a form identical to that describing the critical thermal fluctuations in pure 8CB near the N-SmA transition, and this term displays a temperature dependence at high temperatures similar to that of the pure liquid crystal. The second term, which is negligible at high temperatures but dominates at low temperatures, has a shape given by the thermal term squared and describes the static fluctuations due to random fields induced by confinement in the gel. The correlation lengths appearing in the thermal and disorder terms are the same and show a strong variation with gel density at low temperatures. The temperature dependence of the amplitude of the static fluctuations further suggests that nematic susceptibility becomes suppressed with increasing quenched disorder. The results overall are well described by a mapping of the liquid-crystal-aerosil system onto a three-dimensional XY model in a random field with disorder strength varying linearly with the aerosil density.