Abstract
During the past decade, ferroelectric liquid crystals (FLCs) have been extensively studied because of their fast response time and memory effect toward an applied electric field. These characteristics make them suitable for electro-optical applications in liquid crystal display, fast switching devices, light shutter, memory devices and pyroelectric sensors. In this work, we demonstrate two major topics. One is the photo-aligne ferroelectric liquid crystal guest-host system and the other is the halogen containing ferroelectric liquid crystals. (1) Photo-aligned ferroelectric liquid crystal guest-host system For past years, surface stabilized ferroelectric liquid crystal (SSFLC) technology (cell gap < 2μm) was proven to be the effective method to make a high performance display. The uniform FLC display would be achieved because the thinning cell gap could provide a boundary force to unwind FLC molecules. However, this technology is not available for large scale display applications. Due to the thinning cell gap and high viscosity of FLCs, filling FLC into a large scale display would be a serious problem and production yield would be down. To circumvent the drawbacks of few gray scales and shock effect, the polymer stabilized FLC system has been developed. A polymer or network (about 1~10 wt%) was added into the FLC materials, respectively. The thermal stability, gray scales, and anti-shock properties usually increase with increasing polymer/network concentration. Nevertheless, this system was not able to resolve all the drawbacks of FLC display applications. First, the polymer could hinder the alignment and rotation of the FLC molecules. Moreover, the compatibility between the FLC molecules and polymer is still considered a serious problem. These brought about the defects of high driven voltage, low contrast ratio, and slow response, which were the major hurdles toward application. As part of our endeavor to pursue the realization of FLC commercial applications in display, it is important to resolve the aforementioned issues. Photo-alignment technique has been reported previously. This approach provides a novel way to prepare rubbing-free production of LC alignment films through photocrosslink-induced alignment. To expose the poly(vinylcinnamate) thin film under linear polarized ultraviolet light (LPUV), a uniaxial reorientation of LC layer could be induced. This is a facile method to make an alignment layer without any mechanical process. The photochemistry of cinnamates group involves two kinds of photoreactions including the (2+2) photodimerization and E/Z photoisomeization. The LC oriented direction (perpendicular or parallel) to the incident LPUV is dependent on the molecular interaction between the alignment layer and LC molecules. In this work, we present a novel photoalignment process of FLC devices which include the advantages of fast switching, high Ps value, well alignment, thick gap cell, and anti-shock properties. Instead of using photo-crosslinkable poly(vinylcinnamate) as the alignment layer, a guest-host system was used to investigate the photo-induced alignment properties of FLCs. This system was consisted of an FLC monomers and cinnamate group containing materials. Various cinnamate containing materials were synthesized via etherification, esterification, and/ or hydrosilation reactions. To realize the photo-alignment mechanisms, thin films were obtained by casting the guest-host mixture on the CaF2 plate. The alignment properties of the films induced by LPUV were investigated by a polarized FT-IR. Furthermore, these photo-induced alignment phenomenon and the variations of electro-optical properties were also discussed. Via painstaking molecular design, a series of cinnamate containing materials reveal crystal, N, SA, SC*, and/or high order smectic phases. Guest-host systems were prepared by mixing these polymers with an FLC monomer, respectively. A photo-induced anisotropy was achieved via LPUV. The FLC monomer was arranged perpendicular to the incidence angle of LPUV. Distinct domains or zigzag defects were observed by POM after filling these guest-host mixtures into test cells. These domains or defects would gradually disappear upon irradiation by LPUV. This is because the smectic layers could be aligned, unwinded and stabilized by the ordered networks. After irradiating by LPUV, these electro-optical properties would be improved. This is because the ordered cross-type network provided molecular interaction perpendicular to alignment direction that could further speed up the switching rate and promote the reorientation properties of the dipoles under the alternating electric field. It is shown that these FLC guest-host materials could exhibit excellent alignment behavior and electro-optical properties in the thick cell. The limitation on SSFLC technique could be removed by the photo-alignment technique. Moreover, the yield of the large scale display application will be increased due to the feasibility in fabricating thicker cell gap. (2)Halogen containing ferroelectric liquid crystals For FLC display, FLC materials have mostly been designed to provide a wide temperature range of FLC phase including room temperature, and large spontaneous polarization (Ps) value. These properties are influenced by the design of molecular structures of FLC mesogens. The fundamental molecular structure of an FLC includes mesogenic group, spacer chain unit, and chiral center unit (terminal chain units). The mesogenic group often consists of at least two linked rigid groups with lateral substituents. The length and chemical structure of the spacer unit appear to be an important factor in FLC phase formation and temperature range. The oxyethylene unit as the spacer unit favors reducing the phase transition temperatures. As the number of oxyethylene unit increases, the transition temperature decreases. Moreover, the chiral center is usually the major part of the terminal chain due to the fact that the Ps value is affected by the asymmetric atom and its position, and the length of the terminal chain unit. In addition, incorporating a polar group (-F, -Cl, -Br, -CN, CF3, etc) onto the chiral center close to the core can maximize the Ps value . In this work, a new series of ferroelectric liquid crystals and side chain liquid crystalline polymers were developed. These liquid crystalline materials consist of various halogens containing chiral moieties (F, Cl, and Br), oligooxyethylene spacers, and ester core unit. The influences of different halogen containing chiral tails and spacer units on formation of mesophases and electro-optical properties were also discussed. This new series of ferroelectric liquid crystal monomers and polymers exhibit chiral smectic C phase except the bromine group containing homopolymers. Wide temperature ranges of chiral smectic (SC*) phase were obtained in these monomer (~90℃ ) and polymers (~100℃). Several compounds such as the MDn12F and MDn12B (n=1 and 2) exhibited chiral smecitc F phase. When the number of oxyethylene spacer units increased, the clearing and phase transition temperatures decreased. The bromine group containing homopolymers possesses few mesomorphic phases and no chiral smectic C phase due to bulky substituted group of chiral center which disturbs the orientation of the side chain liquid crystal polymer. The dielectric properties of a series of FLCPs with different halogen chiral centers were investigated in this work. The collective and molecular relaxation of these FLCPs were also observed. The dielectric constant was enhanced remarkedly because of the occurrence of the Goldstone mode in the Sc* phase. The Goldstone mode appears in the Ch phase for homopolymers PSn12B (n= 1, 2, and 3) due to the electro-induced phase transition effect. Bulky terminal group was loosening the dense packing and increasing the mobility of the tilted smectic phase under alternative electron field. Furthermore, decreasing the size of the chiral center resulted in an increase in the relaxation intensity and decrease activity energy.