(Invited) Stimuli Responsive Liquid Crystal Elastomers/Gels

Abstract

Liquid crystal elastomers (LCEs) are rubbery polymer networks containing the mesogenic groups in their main and/or side chains, and liquid crystal gels (LCGs) are the LCEs swollen by LC solvents. A unique feature of LCEs and LCGs is the strong correlation of the macroscopic shape (deformation) and the LC alignment. This feature enables us to actuate the LCEs and LCGs by various types of external field which change the orientation order of LCs such as temperature change, electric field, solvent uptake etc. Another important feature of LCEs and LCGs is that one can control the mode of induced deformation by programming the type of the alignment of the “director” (the vector representing the sectoral average molecular orientation of LC) in the stage of cross-linking. In addition, LCEs and LCGs exhibit interesting mechanical properties as a result of the coupling of imposed strain with the director realignment. Thus LCEs and LCGs show a rich variety of intriguing stimulus response behavior due to the combination of LCs and elastomers/gels, and they are a promising material for soft actuators and sensors. The cholesteric LCEs and LCGs (Ch-LCEs and Ch-LCGs) possessing helical director configuration with regular pitch behave as “photonic elastomers”, because the cholesteric LCs exhibit unique optical property, i.e., selective reflection for the incident circularly polarized light: They selectively reflects the incident light with circular polarization whose handedness is the same as that of helical director configuration, and whose wavelength corresponds to the helical pitch (Bragg reflection). We demonstrate that the reflection band of Ch-LCEs can be varied by imposed compressive strain [1] as well as temperature change [2] with maintaining the selectivity. Ch-LCGs exhibit pronounced electro-opto-mechanical effect, i.e, finite deformation together with significant change in reflection spectra.[3] Further, the Ch-LCE film with the helical axis normal to the thickness show periodical surface undulation which corresponds to the helical director configuration in response to temperature variation.[4] The nematic LCEs with twist or hybrid director alignment exhibit twist or bending deformation in response to temperature change.[5,6] When these LCEs are allowed to swell in solvents, the temperature change drives the twist or bending deformation coupled to volume change. The total variation and the T-sensitivity of twist pitch or curvature of these LCGs significantly depend on whether the solvent has nematicity or does not.[7] [1] Varanytsia, A., Nagai, H., Urayama, K., Pallfy-Muhoray, P., Sci. Rep., 5, 17739 (2015). [2] Nagai, H., Urayama, K., Phys. Rev. E, 92, 022501 (2015). [3] Fuchigami, Y., Takigawa, T., Urayama, K., ACS Macro Lett., 3, 813 (2014). [4] Nagai, H., Liang, X., Nishikawa, Y., Nakajima, K., Urayama, K., Macromolecules, 49, 9561 (2016). [5] Sawa, Y., Fangfu, Y., Urayama, K., Takigawa, T., Gimenez-Pinto, V., Selinger, R., Selinger, J., PNAS, 108, 6364 (2011). [6] Sawa, Y., Urayama, K., Takigawa, T., DeSimone, A., Teresi, L., Macromolecules, 43, 4362 (2010). [7] Doi, H., Urayama, K., Soft Matter, 13, 4341 (2017).