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TWIST BEND AND DOUBLE TWIST IN LIQUID CRYSTALS

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2016, PHD, Kent State University, College of Arts and Sciences / Chemical Physics.
Chirality is ubiquitous in the natural world and plays an important role in soft condensed matter, especially liquid crystals. In this dissertation, we investigated the twist bend and double twist structures in thermotropic liquid crystals. The summary of this dissertation is as follows: 1. We demonstrated experimentally a new type of orientational nematic order in which achiral dimer molecules follow the direction of oblique helicoids. The new phase, first considered theoretically by R.B. Meyer and I. Dozov as a twist-bend nematic (Ntb), is a structural link between the well-known uniaxial nematic (N) and chiral nematic (N*) phases. The oblique helicoidal structure of the Ntb phase is evidenced by asymmetric Bouligand arches in freeze-fracture transmission electron microscopy textures. The period of oblique helicoids is 8-9 nm. The unique nanostructure leads to new macroscopic properties of Ntb, such as first-order dielectric reorientation and expulsion of twist and bend deformations of the optic axis. We measured the temperature dependence of the splay (K1 ), twist (K2 ), and bend (K3 ) elastic constants of the uniaxial nematic (N) phase formed by dimeric liquid crystal mixture with negative dielectric anisotropy. The material under study forms the nematic twist-bend (Ntb) phase when cooled down from the uniaxial nematic (N) phase. In the N phase, the bend constant K3 decreases dramatically as the temperature is lowered towards the N-Ntb phase transition. Both K1 and K2 increase as the temperature is lowered; K1 remains about 2-3 times higher than K2 near the transition to the Ntb phase. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that eventually give rise to the Ntb phase. We explored the effects of photo-isomerization on the N-Ntb phase transition using an azobenzene-based non-symmetric liquid crystal dimer. We observed an isothermal, reversible Ntb-N transition, which is attributed to a trans-cis photo-isomerization of the azobenzene fragment on UV irradiation. The cis isomers stabilize the standard nematic phase and the trans isomers stabilize the Ntb phase. 2. We observed experimentally an electrooptic effect in a cholesteric LC with a distinct oblique-helicoidal director deformation. The oblique helicoid, predicted in late 1960-ies, is demonstrated to exist in dimer materials with an anomalously small bend elastic constant. Theoretical, numerical, and experimental analysis establishes that both the pitch and the cone angle of the oblique helicoid increase as the electric field decreases. The effect can enable many applications that require dynamically controlled transmission and reflection of light. We demonstrated the electrooptic applications using the cholesteric heliconical structure, such as the electrically tunable selective reflection of light from ultraviolet to visible and infrared, full color reflective display, and electrically tunable lasing. Further work can be done to achieve the cholesteric heliconical structure with the magnetic field, light irradiation, and surface confinement, besides the electric field. The cholesteric heliconical structure can also find applications in tunable optical filter, smart windows with transparent/opaque/tunable color states. 3. We reported on a fast electro-optic switching (response time 0.1 ms) of a blue-phase-polymer templated nematic with a broad-temperature range of thermodynamic stability and hysteresis-free performance. The nematic fills a polymer template that imposes a periodic structure with cubic symmetry and submicron period. In the field-free state, the nematic in polymer template is optically isotropic. An applied electric field causes non-zero optical retardance. The approach thus combines beneficial structural and optical features of the blue phase (cubic structure with submicron periodicity) and superior thermodynamic stability and electro-optic switching ability of the nematic filler. Further work can be done using liquid crystal twist bend structure as a template, such as Ntb phase and cholesteric heliconical structure, to achieve the fast switching electrooptic performance.
Oleg D. Lavrentovich (Advisor)
125 p.

Recommended Citations

Citations

  • Xiang, J. (2016). TWIST BEND AND DOUBLE TWIST IN LIQUID CRYSTALS [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1469884660

    APA Style (7th edition)

  • Xiang, Jie. TWIST BEND AND DOUBLE TWIST IN LIQUID CRYSTALS. 2016. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1469884660.

    MLA Style (8th edition)

  • Xiang, Jie. "TWIST BEND AND DOUBLE TWIST IN LIQUID CRYSTALS." Doctoral dissertation, Kent State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=kent1469884660

    Chicago Manual of Style (17th edition)