Synthesis and Properties of New Chiral Swallow-Tailed Liquid Crystal Derived From (S)-Lactic Acid

• Main Authors: Cho-Ying Lin, 林卓瑩
• Other Authors: Shune-Long Wu
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/55157050454277728527

碩士 === 大同大學 === 化學工程研究所 === 90 === ABSTRACT The primary of this research work was an attempt to elucidate the structure-property correlation in the chiral smectic liquid crystal. A homologous series of chiral materials derived from (S)-lactic acid has been successfully synthesized and the structures-property relationship investigated in the chiral liquid crystal system. The target compounds were modified independently by the (i) nonchiral preipheral length chain, (ii) swallow-tailed group and straight alkyl chain at the chiral tails, (iii) lateral halogen substituents in the core and (iv) the linking groups; ester and ether between chiral tail and core of the molecules of the chiral molecules for the study. Chiral swallow-tailed materials, I(m, 2, 2; m=8~12), displayed enantiotropic mesophases of the SmA* and SmC* phases. An increasing methylene chain lengths m, depressed the temperature of the clear point. Temperature range of the SmA* and SmC* phases decreased as the number of methylene units increased. The electro-optical response in this series of chiral materials in the SmC* phase displayed typical ferroelectric hystersis loop or hysteresis-free, U-shaped switching property upon various applied frequencies and temperatures. The variations of peripheral chain length, m, in this series of compounds, have no remarkable effect on dielectric property and switching behavior in the SmC* phase. Chiral swallow-tailed materials, II(10, n, q; n=q=2~4) displayed SmA* and SmC* phases. An increase of swallow-tailed chain lengths n and q, decreased the temperature of the clear point. Temperature range of SmC* phase decreased as the swallow-tailed chain length increased. Chiral compound II(10, 0, 2) having straight chain in the chiral tail displayed SmA*, SmC* and SmX* phases. As comparison of II (10, 2, 2) with II(10, 0, 2), the material with the straight alkyl chain at chiral tail has more stable SmA* and SmC* phases. The Ps values for compounds are in the order of 1-ethylpropyl＞propyl＞1-propylbutyl＞butylpentyl. The hystersis-free, U-shaped switching is more feasible to form in chiral compounds containing swallow-tailed group. In this series of compounds, the variation of chiral tail groups has no remarkable effect on dielectric property and switching behavior in the SmC* phase. Chiral swallow-tailed materials, III(X, Y) displayed SmA* and SmC* phases. The materials with lateral substitutents in the core could depress the formation of mesophase, lowering the transition temperature, melting point and Ps values. The largest size of chloro substituent disrupted to a great deal for the formation of mesophases. The materials with halogens substuted at 2-position on the ring of the molecule were sterically shielded so that possessed higher transition temperature than that materials at 3- position. The variation of lateral halogen substituents in the core have no remarkable effect on dielectric property and switching behavior in the SmC* phase. The typical character hystersis loop or hystersis-free, U-shaped of ferroelectric phase switching observed at compounds III(H, F), III(H, Cl) and III(F, H) possessing SmC* phase Surprisingly, the optical switching response in SmC* phase of compound III(Cl, H) displayed thresholdless, W-shaped switching . Chiral swallow-tailed materials, IV(m, 2, 2; m=8~12) displayed enantiotropic mesophase of Bp, N*, TGBA*, SmA* and SmC* phases. An increasing methylene chain length m, depressed the temperature of the clear point and transition temperatures of Bp-N*, N*-TGBA* and TGBA*-SmA* phases. An increasing methylene chain length m, could enhance the thermal stability of the N* phase, but has no significant change in the SmA* and SmC* phases. A comparison of I(10, 2, 2) and IV(10, 2, 2) clearly shows that the chiral swallow-tailed materials derived from chiral lactate with ether linking group to the core of the molecule is favorable of forming frustrated phase. V-shaped switching was observed at chiral compound IV(10, 2, 2) possessing SmC* phase, while the other compounds displayed normal feroelectric hysteresis loop. Variation of alkyl chain lengths of the compounds have no remarkable effect on dielectric property and switching behavior in SmC* phase. An achiral swallow-tailed material (P) was mixed (by weight percentage) with a ferroelectric liquid crystal [III (H, F); F] for the study. The binary mixtures of F and P resulted in a phase sequence of SmA*-SmC*-Cr2.-Cr1. The electro-optic response at the amount of achiral swallow-tailed material P greater than 20%, that is, when mixture is 80F/20P, the mixture in ferroelectric SmC* displayed V-shaped switching. The results of this work strongly suggested that thresholdless, V-shaped switching in ferroelectric liquid crystal mixtures could be achieved by mixing an achiral swallow-tailed material with an ferroelectric liquid crystal. In conclusion, the results indicated that the chiral swallow-tailed materials derived from (s)-lactic acid are favorable for the formation of the ferroelectric phase. V-shaped switching behavior achieved in the SmC* phase at chiral compound IV(10, 2, 2). It is worthy emphasizeing again that thresholdless, V-shaped switching in ferroelectric liquid crystal mixtures could be achieved by mixing an achiral swallow-tailed material with an ferroelectric liquid crystal. This finding may provide a new way of assessing the nature of V-shaped switching for display application.