Abstract
Abstract The purpose of this research work was an attempt to elucidate the structure-property correlations in the chiral smectic liquid crystals, because the exciting results have been obtained the field of study. A homologous series of chiral materials MDn2Nx (n = 1, 2, 3; x = 1b, 2b, 2p, 3p) was designed for the synthesis and investigation of the structure-property relationship in the chiral liquid crystal system. The target compounds were independently modified by the length of the oligo(oxyethylene) chain (n) and terminal chiral alkyl chain (x) for the study. A chiral swallow-tailed compounds MDn2N3p (n = 2, 3) have the same phase sequence: Iso-Ch-SA-SCA*-Cr, suggesting that the extending the oxyethylene repeat unit from n=2 to n=3 oligo(oxyethylene) chain length has no remarkable effect on the formation of tilted smectic phases, compound with shorter oxyethylene chain length(n =1; MD12N3p) have the phase sequence: Iso-Ch-SA-SB-Cr. The thermal analysis shows that as n increases, the clearing point falls, and the SA-SCA* transition temperature decreases, resulting in a suppress of the SA phase stability, the temperature range of SCA* phase are generally increased as the n increased. In addition, The MD32N3p material SCA* phase range can be achieved about 50℃(including room temperature). The optical responses in the SCA* phases have been measured. The thresholdless phenomena are found and characterized in these phases. In addition, the maximal spontaneous polarization value (131 nC/cm2) and fast response time (140 μs) are found in MD22N3p, and also found MD32N3p material have maximal spontaneous polarization value (127 nC/cm2) and fast response time (270μs). The dielectric properties for the compounds MD22N3p and MD32N3p were also measured and two different relaxation processes were obtained and characterized as soft mode in the SA phase, and Goldstone mode in the SCA* phases. The results of dielectric investigations indicate that oligo(oxyethylene) chain length has no remarkably effect on the collective or non-collective molecular motions. On the other hand, the molecular modeling shows a similar result with the experimental X-ray diffraction of smectic liquid crystals to demonstrate the correct AFLC structures. Investigating the effect of terminal chiral alkyl chain (x) on the mesomorphic properties show that the stability of tilted smectic phases and the magnitudes of spontaneous polarization in the SC* phases are enhanced as the introduction of second anchiral methyl substitutent at chiral tail. In addition, the phase sequence for compounds MDn2Nx (n = 1, 2, 3; x = 1b, 2b, 2p) is depended on the oligo(oxyethylene) chain length n. A Iso-SA-SB-Cr sequence was obtained at shorter chain length (n = 1). A Iso-SA-SC*-Cr* sequence was obtained at longer chain length (n = 2 and 3).