Book Description

This dissertation describes stimuli-responsive liquid crystals and elastomers including thermal/electro-active ionic liquid crystal elastomers, UV responsive twist bend nematic liquid crystal dimmers and fast-switching chiral ferroelectric nematic liquid crystals with detailed studies on its nanoscale structures, electrical and optical properties for possible electric, optical and electro-optical applications. In this dissertation, the first preparation, physical properties, and electric bending actuation of a new class of active materials - ionic liquid crystal elastomers (iLCEs) are described. iLCEs can be actuated by low frequency AC or DC voltages of less than 1 V. The bending strains of the not optimized first iLCEs are already comparable to the well-developed ionic electroactive polymers (iEAPs). Additionally, iLCEs exhibit several novel and superior features. For example, pre-programmed actuation can be achived by patterning the substrates with different alignment domains at the level of cross-linking process. Since liquid crystal elastomers are also sensitive to magnetic fields, and can also be light sensitive, in addition to dual (thermal and electric) actuations in hybrid samples, iLCEs have far-reaching potentials toward multi-responsive actuations that may have so far unmatched properties in soft robotics, sensing and biomedical applications. The following two works are the understanding of the structure of the twist-bend nematic (NTB) phase. The first work presents hard and tender resonant X-ray scattering studies of two novel sulfur containing dimer materials for which we simultaneously measure the temperature dependences of the helical pitch and the correlation length of both the helical and positional order. In addition to an unexpected strong variation of the pitch with the length of the spacer connecting the monomer units, we find that at the transition to the NTB phase the positional correlation length drops. In the second work we use tender x-ray scattering to decipher the variation of the pitch and heliconical bond order of a NTB dimer containing azo groups upon polarized light illumination. It shows the first evidence of manipulation of the nanoscale heliconical structure of a twist bend nematic liquid crystal dimer containing an azo linkage by polarized light. The tender X-ray pattern reveals two different heliconical pitch values of aligned and unaligned domains under polarized light. In addition to the bulk alignment, the value of the heliconical pitch can be also tuned in two timescales by UV-violet light and recovered in a temperature dependent time. Then we studied the electrical, optical, and electro-optical properties of a ferroelectric nematic (NF) LC material doped with commercially available chiral dopants. While the NF phase of the undoped LC is only monotropic, the chiral NF phase is enantiotropic, indicating a chirality induced stabilization of the polar nematic order. Compared to undoped NF material, a remarkable improvement of the electro-optical switching time is demonstrated. The color of the chiral mixtures that exhibit a selective reflection of visible light in the chiral NF phase, can be reversibly tuned by 0.02-0.1V/[mu]m in-plane electric fields, which are much smaller than typically required in full-color cholesteric LC displays. The fast switchable reflection color at low fields has potential applications for LC displays without backlight, smart windows, shutters and e-papers. In the end, there are summaries and outlooks of all the results and the future applications.