| Due to the unique molecular structure of liquid crystals (LCs), the liquid-crystalline (LC) phase having long-range orientations in one or two dimensions is easy to orient along the applied fields, such as force, electric and magnetic fields. LCs, as functional materials, have been widely applied in displays, optical and electro-optical devices, ordered template, biological detection, etc. Fluidity is an important character of LCs, which enables LC molecules to fast response under an external field. However, it also has imposed great problems and limitations to the further development of LC devices. The research on how to get LC materials having both a fast stimulus response and a certain mechanical stability has become a hotspot in this area. Liquid crystal dispersions are a novel functional composite combining the optical performance and responsiveness of LCs and the mechanical properties and processing advantages of the substrate materials. In particular, LC physical gels are a novel class of stimuli-responsive thermoreversible solid materials, however, their moduli are not high enough and hard to meet the self-supporting requirements, leading to low thermal and voltage resistance, shear thinning and thixotropy. At the same time, low driving voltage and fast response are essential for a photoelectric device. Because there are large amounts of LC-matrix interface in liquid crystal dispersions, the interface control will be beneficial to improve the performance of LCs. In the present dissertation, preparation and structure regulation of LC physical gels, the interfacial control between LCs and polymer matrix, and the effect on properties were studied.Firstly, LC physical gels were prepared through the self-assembly of a series of benzylidene-D-sorbitol derivatives (DBS, MDBS and DMDBS) as gelators in a nematic liquid crystal,4-pentyl-4’-cyanobiphenyl (5CB). The effects of molecular structure and contents of gelators on microstructure, phase behavior, rheological and electro-optical properties of LC physical gels were investigated. The results showed that MDBS and DMDBS with methyl groups on phenyl rings have higher gelation ability than DBS in5CB, and their gels at a fixed gelator content have higher sol-gel transition temperatures and higher thermal resistance. DMDBS with the highest molecular rigidity can form the thickest nano-fibrils in gels and the texture of the three-dimensional brillar networks, thus DMDBS gels exhibit good self-supporting ability with high storage modulus. At the same time, DMDBS gels have lowest driving voltage due to the increase in the size of LC domains.Secondly, LC physical gels were prepared through the self-assembly of DBS as a gelator in a mixed nematic liquid crystal, P0616A. Then, LC physical gels with ordered network structure were successfully achieved in TN cell or under a magnetic field. The effects of the content of DBS on microstructure and electro-optical properties of ordered network LC physical gels were investigated. The formation of the ordered network depends on that the isotropic-nematic transition temperature of a LC gel is higher than its sol-gel transition temperature in order to use LCs as anisotropic templates. Due to the surface orientation effect, LC gels form the directional alignments of self-assembled nano-fibers on the substrate of TN cell and twisted network between two substrates, which enhances the response speed of LC gels. Also, the directional alignments of self-assembled nano-fibers along the direction of magnetic field were formed, and the ordered network perpendicular to the substrate decreases the driving voltage of LC gels.Thirdly, by means of the high-speed shearing method, PVA dispersed liquid crystals were prepared with Gemini surfactant, and their microstructure, phase behavior and electro-optical properties were investigated. The value of critical micelle concentration (CMC) of Gemini surfactant is obviously lower than that of the common surfactant with the same alkyl chain, and the ability to combine with the ion increases, which leads to a lower dosage and a less free ion when forming the micelle. It will be helpful to extend the service life of PVA dispersed liquid crystal films. Additionally, the LC phases are uniformly dispersed in PVA matrix, and the size of LC droplets decreases greatly with the addition of Gemini surfactant. At the same time, the configuration of LC director configuration changes to be radial from bipolar, thus increases the contrast and the response speed of films, and retains a low driving voltage.Finally, PVA dispersed LC physical gels films were successfully achieved via high-speed shearing method, and their microstructure, phase behavior and electro-optical properties were investigated. The results showed that LC physical gels disperse as circular droplets in polymer matrix and show significant birefringence phenomenon under cross polarized light. After adding surfactant of CTAB, the size of LC gels droplets decreases further, but the nano-fiber three-dimensional network still keeps in the dispersed phase. In addition, the phase transition temperatures of PVA dispersed LC physical gels films, such as isotropic-nematic transition and sol-gel transition temperatures show the same change trends as in the bulk LC gels, which will conducive to further adjust and control the fiber network structure using the template effect between LCs and fiber network. Since the dispersed phase-LC physical gels are light scattering state with high contrast, PVA dispersed LC physical gels films have higher contrast than PVA dispersed liquid crystals films, and the response speed obviously accelerates, but the driving voltage increases rapidly with increase in the gelator content. |
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