Robles-Hernández Beatriz, Sebastián Nerea, de la Fuente M Rosario, López David O, Diez-Berart Sergio, Salud Josep, Ros M Blanca, Dunmur David A, Luckhurst Geoffrey R, Timimi Bakir A
Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain.
Otto-von-Guericke Universitat Magdeburg, Institute for Experimental Physics, ANP, 39106 Magdeburg, Germany.
Phys Rev E Stat Nonlin Soft Matter Phys. 2015 Dec;92(6):062505. doi: 10.1103/PhysRevE.92.062505. Epub 2015 Dec 2.
The nature of the nematic-nematic phase transition in the liquid crystal dimer 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and (2)H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N(TB)), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N(TB)-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and (2)H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N(TB) phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. (2)H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.
利用量热法、动态介电响应测量和（2）H核磁共振光谱技术，研究了液晶二聚体1″,9″-双（4-氰基联苯-4'-基）壬烷（CB9CB）中向列相-向列相转变的性质。CB9CB的实验结果表明，与较短的同系物CB7CB一样，所研究的材料呈现出正常向列相，在冷却时会转变为扭曲弯曲向列相（N(TB)），这是一种单轴向列相，由平均弯曲的分子形状促进形成，其中指向矢倾斜并作进动，描绘出一个锥形螺旋。调制差示扫描量热法已被用于分析N(TB)-N相转变的性质，发现该转变为弱一级相变，但接近三临界点。此外，宽带介电谱和（2）H磁共振研究揭示了有关最近发现的扭曲弯曲向列相结构特征的信息。对两个向列相中的动态介电响应进行分析，得出了N(TB)相螺旋结构的锥角估计值。对初始平面排列单元中指向矢电场重新排列的电容测量，得出了高温向列相中的展曲和弯曲弹性常数的值。弯曲弹性常数较小，随着接近扭曲弯曲相，其值随温度降低而减小。这种行为在理论上是预期的，并且在形成扭曲弯曲向列相的材料中也已观察到。（2）H核磁共振测量表征了在扭曲弯曲向列相中识别出的手性螺旋扭曲，还允许确定锥角和相对于指向矢的取向序参量的温度依赖性。
