Abstract
In the nematic (N) phase, the molecular symmetry axis orients on average along one direction denoted as the director. The cholesteric (Ch) phase shows similar orientational order locally. However, the average molecular direction in the Ch phase rotates continuously around a direction perpendicular to the director. The cholesteric blue phase (ChBP) shows a double-twist orientational order that differs from the single-twist order of the Ch phase and also shows self-assembled three-dimensional lattice structure of defect lines of the orientational order in the mesoscopic spatial scale. The helical structure of the molecular orientation in ChBP brings the structural colour and photonic band gap into the wavelength range of visible light. Therefore, ChBP has been studied for applications to photonic elements and fast-response displays. We measured the molecular translational dynamics along the molecular long axis in the Ch phase, ChBP and the isotropic (Iso) liquid phase of the mixture system of the nematic liquid crystal 4′-heptyloxy-4-biphenylcarbonitrile and the chiral dopant (S)-4′-(2-methylbutyl)-4-biphenylcarbonitrile directly at the nanometric molecular scale by using quasi-elastic scattering spectroscopy using Mössbauer gamma ray. We successfully determined the timescale of the molecular translational motion in the Ch phase to be 40 ns, which is similar to the timescale of the N phase of 4′-n-octyl-4-cyanobiphenyl. In the ChBP and Iso phase, molecular motions occur on timescales similar to those of the Ch phase, suggesting that the molecular dynamics is insensitive to the presence of orientational order, the helical structure, and higher-order structure. Our results demonstrate that the molecular dynamics in both the Ch phase and ChBP can be measured by quasi-elastic gamma-ray-scattering spectroscopy, in addition to the time scales of molecular motions in the N and smectic phases. The present results greatly expand the possibility of using this spectroscopic technique for molecular-mobility studies of industrial liquid-crystalline materials, because Ch liquid crystals are widely used for display systems in addition to N liquid crystals.
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Acknowledgements
We wish to thank Prof. Shunji Kishimoto (High-Energy Accelerator Research Organization) for developing the APD detectors. We are also grateful for Mr. Fumitaka Sakai (CFlat Co., Ltd.) for his supplement of images of molecular arrangements of Ch phase, ChBPII and Iso phase (used for Fig. 2). The small-angle X-ray diffraction experiment using Nanopix was performed with a support of Dr. Rintaro Inoue (Kyoto University). The experiment was performed with the approval of the Japan Synchrotron Radiation Research Institute (proposal No. 2017A1096). This work was supported by a Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Scientific Research (S) (Grant No. JP24221005) and Grant-in-Aid for Young Scientists (B) (Grant No. JP15K17736) and by JST CREST (Grant No. JPMJCR1424), Japan. This work was also partially supported by project for construction of the basis for the advanced materials science and analytical study by the innovative use of quantum beam and nuclear sciences in Institute for Integrated Radiation and Nuclear Science, Kyoto University.
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This article is part of the Topical Collection on Proceedings of the International Conference on the Applications of the Mössbauer Effect (ICAME2019), 1-6 September 2019, Dalian, China
Edited by Tao Zhang, Junhu Wang and Xiaodong Wang
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Saito, M., Yamamoto, J., Masuda, R. et al. Microscopic molecular translational dynamics in cholesteric and cholesteric blue phases. Hyperfine Interact 241, 5 (2020). https://doi.org/10.1007/s10751-019-1670-z
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DOI: https://doi.org/10.1007/s10751-019-1670-z