| The discovery of columnar phase behaviors in discotic liquid crystals may potentially be used in a wide range of applications, and this has led to an increased interest in this specific group of liquid crystals. This class of liquid crystals could be used in the field of photonics because they possess uniaxial negative birefringence, and in electronic transfer processes because they act as electron carriers. All of these unique features originate from the disk-like chemical structure in this class of liquid crystals. Hexa-n-octoxyl-triphenylene (C8HET), a disk-like liquid crystal, has been chosen as a representative from the series of discotic liquid crystals studied to illustrate the phase behaviors and determined structures of this class. Differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), polarized light microscopy (PLM), and transmission electron microscopy (TEM) results show that in addition to the hexagonal columnar liquid crystalline phase (Φh) that appears at high temperatures, three crystalline phases (K1, K2 and K3) are observed at relatively lower temperatures. The K 1 phase was obtained by rapidly quenching C8HET from its Φ h phase to below 37°C. Slowly cooling C8HET from the Φ h phase results in the formation of a mixture of both K2 and K3 phases. Upon heating, both the K1 and K 2 phases melt and recrystallize to form the K3 phase. Although the K1 and K2 phases have faster crystallization kinetics than the K3 phase, hermodynamic analysis indicates that both of the K1 and K2 phases are metastable with respect to the K3 phase.; Furthermore, by molecular design, cyanobiphenyl mesogens were introduced as end groups to this series of discotic liquid crystal molecules. The phase transition behavior is also studied with the methods mentioned above. Four phases were found in the sample RD12. The isotropic melt transfers to a nematic liquid crystalline phase upon cooling the system to 131°C and this transition is reversible thermodynamically. The nematic phase could be “frozen” by fast cooling or quenching the material. The “frozen” nematic phase could transfer to the crystalline Cr2 phase upon heating, and then, the Cr2 melt and recrystallize to form another crystalline phase, Cr1. The Cr1 phase can also be obtained by slowly cooling the system from its nematic phase. The Cr1 phase melts at 121°C to enter nematic phase.; The material, RD12, responds to the electric field efficiently. With the increasing of the intensity of electric field, the PLM texture of RD12 nematic phase change to dark. The responding time is less than 1 sec and the critical voltage is 150 V. The electric field leads to the decoupling of the quadrupole moment between the cyanobiphenyl groups and makes it possible to obtain the homeotropic packing, which results in the dark texture under PLM. |
