Raman scattering studies of order parameters in liquid crystalline dimers exhibiting the nematic and twist-bend nematic phases
Raman scattering studies of order parameters in liquid crystalline dimers exhibiting the nematic and twist-bend nematic phases
复制标题
DOI：
10.1039/c5tc02174j
复制
发表时间：
2015-09
影响因子：
6.4
通讯作者：
Zhaopeng Zhang;Vitaly P. Panov;M. Nagaraj;R. Mandle;J. Goodby;G. R. Luckhurst;J. C. Jones;H. Gleeson
中科院分区：
文献类型：
--
作者：
Zhaopeng Zhang;Vitaly P. Panov;M. Nagaraj;R. Mandle;J. Goodby;G. R. Luckhurst;J. C. Jones;H. Gleeson
利用偏振拉曼光谱技术对一系列液晶二聚体的常规向列相(N相)和扭曲向列相(NTB相)的取向有序性进行了定量研究。所研究的二聚体有7、8、9和11个亚甲基连接两个氰基联苯介元，并包括4-戊基-4‘-氰基联苯(5CB)和4-辛基-4’-氰基联苯(8CB)的数据以供比较。对材料的拉曼光谱进行了模拟和测量比较。利用考虑奇二聚体分子弯曲的模型，在向列相温度范围内和紧邻NTB-N相变的正下方，用自旋共振谱确定了<P2>和<P4>有序参数，并对其进行了详细的描述。在向列相中，奇数二聚体表现出较低的级数参数，<P2>取值在0.3-0.5之间，<P4>取值在0.25左右。相反，偶数二聚体显示出极高的序参数值，<P2>取值在0.7到0.8之间，<P4>取值在0.4到0.45之间。对于奇数二聚体，<P2>在NTB相中的值与在N相中的值相似，而<P4>约跳跃了5-10%，然后随着温度的升高而减小。通过比较实验数据和理论预测，我们发现所有材料都与分子场理论定性一致。然而，奇二聚体的<P4>值高于理论模型，这归因于在PRS分析中忽略了分子的柔韧性和双轴性。
Polarized Raman Spectroscopy (PRS) is used to quantify the orientational order in the conventional (N) and twist-bend (NTB) nematic phases of a homologous series of liquid crystalline dimers. The dimers investigated have 7, 8, 9 and 11 methylene groups connecting two cyanobiphenyl mesogens and data for 4-pentyl-4′-cyanobiphenyl (5CB) and 4-octyl-4′-cyanobiphenyl (8CB) are included for comparison. Simulated and measured Raman spectra for the materials are compared. PRS is used to determine both 〈P2〉 and 〈P4〉 order parameters across the nematic temperature range and immediately below the NTB–N phase transition using a model that takes into account the molecular bend of the odd dimers, which is described in detail. In the nematic phase, the odd dimers are found to exhibit rather low order parameters with 〈P2〉 taking values between 0.3 and 0.5 and 〈P4〉 about 0.25. In contrast, the even dimer shows extremely high values of the order parameters with 〈P2〉 taking values between 0.7 and 0.8 and 〈P4〉 between 0.4 and 0.45. For the odd dimers, the values of 〈P2〉 in the NTB phase are similar to those of the N phase, whereas 〈P4〉 jumps by approximately 5–10% and then decreases with temperature. On comparing the experimental data with the theoretical predictions, we find reasonable qualitative agreement for all materials with molecular field theory. The odd dimers, however, show higher 〈P4〉 values than obtained from theoretical models, a factor attributed to the neglect of molecular flexibility and biaxiality in the PRS analysis.