Cruceanu F, Liang D, Leheny R L, Garland C W, Iannacchione G S
Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Jan;79(1 Pt 1):011710. doi: 10.1103/PhysRevE.79.011710. Epub 2009 Jan 29.
High-resolution ac calorimetry has been used to study the nematic to smectic- A (N-SmA) phase transition in the liquid crystal octylcyanobiphenyl (8CB) confined in aligned colloidal aerosil gels. A stable and robust nematic alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field. In some ways (such as transition temperature and integrated enthalpy), the dependence of the specific heat peak associated with the N-SmA transition on the aerosil density for aligned gels is consistent with that observed in unaligned (random) gel samples. However, a power-law analysis reveals that the behavior of the critical exponent alpha is quite different. For random gels, alpha varies gradually with aerosil density, whereas we find that alpha for aligned gels shifts abruptly to an XY -like value for the lowest aerosil density studied and remains essentially constant as the sil density increases. This aerosil density independence of alpha is consistent with the critical behavior of the smectic correlation lengths obtained from an x-ray scattering study of 8CB in aligned aerosil gels. The combined calorimetric and x-ray results indicate that the role of quenched randomness in aligned gels of 8CB+sils differs significantly from that in random gels.
高分辨率交流量热法已被用于研究限制在排列好的胶体气凝胶中的液晶正辛基氰基联苯（8CB）从向列相到近晶A相（N-SmA）的相变。通过在强均匀磁场存在下对样品进行反复热循环，实现了稳定且坚固的向列取向。在某些方面（如转变温度和积分焓），与N-SmA转变相关的比热峰对排列好的气凝胶中气凝胶密度的依赖性与在未排列（随机）凝胶样品中观察到的一致。然而，幂律分析表明，临界指数α的行为有很大不同。对于随机凝胶，α随气凝胶密度逐渐变化，而我们发现，对于排列好的凝胶，α在研究的最低气凝胶密度下突然转变为类似XY的值，并随着二氧化硅密度增加基本保持不变。α的这种与气凝胶密度无关的特性与从8CB在排列好的气凝胶中的x射线散射研究获得的近晶相关长度的临界行为一致。量热法和x射线的综合结果表明，8CB + 二氧化硅排列好的凝胶中猝灭无序的作用与随机凝胶中的显著不同。
