| Summary: | 博士 === 大同大學 === 化學工程學系(所) === 95 === The primary of this research work was an attempt to elucidate the correlation between structure and property in the chiral smectic liquid crystals. Thus, two new chiral moieties, (S)-2-(metylsulfanyl)ethyl 2-hydroxypropanoate prepared by reacting (S)-lactic acid with 2-methylsulfanyl-1-ethanol, and (S)-[1-methyl-2-(2’-methylsulfanylethoxy)]ethanol prepared by reacting (S)-propylene oxide with 2-methylsulfanyl-1-ethanol under basic condition, were designed and synthesized, and three series of chiral liquid crystal compounds were subsequently prepared for the mesomorphic and physical studies. The target compounds were modified independently by (i) the non-chiral peripheral methylene chain length ‘m’ and (ii) the lateral substitution group in the core.
In first series of chiral materials, MSEmBCPP(m=8-12), all materials displayed enantiotropic mesophases of the SmA*, and SmC* phases. The maximum magnitude of spontaneous polarization (Ps) for materials was around 85nC/cm2. The electro-optical response in the series of chiral materials in the SmC* phase displayed typical ferroelectric hysteresis loop or hysteresis-free, V-shaped switching property upon various applied frequencies and temperatures.
In second series of chiral materials, MMSEEmCB(m=8-12), the formation of smectic phases are significantly depended of the achiral alkyl chain length. For example, material at m=7 displayed SmA*, SmCA* and SmX phases, while materials at m=8-12 display an additional SmC* phase but at m=10-12 the SmCA* phase and at m=11-12 the SmX* were suppressed. The maximum Ps value was observed to be 37.76 nC cm-2, and the maximum tilt angle reached 29.5o.
In the third series of materials, the results obtained from MMSEEmBC2F(m=7–10) with lateral 2-fluoro substitution and MMSEEmBC2F(m=7–10) with 3-fluoro show that all materials display SmA* and SmC* phases. The antiferroelectric SmCA* phase only occurred in two materials, MMSEE7BC2F and MMSEE8BC3F. The results also indicate that lateral fluoro-substituent in the phenyl ring of the materials could depress the formation of higher ordered smectic phases, especially the SmCA* and SmX* phases, lowering the transition temperatures, melting points, clearing points. The measured Ps values, as compared fluoro-substituted materials to non-substtuted materials, have the order of MSEmBCPP > MMSEE7BC2F > MMSEE8BC3F, suggesting that fluoro-substituents depress the Ps values in which 3-fluoro-substituent has more effect than 2-fluoro-substituent.
In conclusion, the results indicated that the chiral materials derived from (S)-2-(metylsulfanyl)ethyl 2-hydroxypropanoate are favorable for the formation of the wide temperature range of the ferroelectric phase, while that from (S)-1-methyl-2-(2’-methylsulphanylethoxy)ethanol are favorable for the formation of antiferroelectic phase. Fluoro-substituents could decrease the transition temperatures but suppress the formation of antiferroelectric phase.
|
|---|
