| Summary: | 碩士 === 大同大學 === 化學工程學系(所) === 96 === The purpose of this research work was an attempt to elucidate the structure-property correlation in the chiral smectic liquid crystals. Optically active (S)-1-propyloxy-2-propanol and (S)-1-(2,2,3,3,3-pentafluoropropyloxy)-2-propanol were designed and prepared by the treatment of (S)-propylene oxide with corresponding alcohols under basic condition. Three homologous series of chiral compounds derived from these alcohols were synthesized, and their mesophases and electro-optical properties were investigated. The effects of changing (1) the linking group in the core structure of the molecule changing from carboxylate group to methyleneoxy group, (2) the chiral group varied from non-fluorinated to semi-fluorinated alkoxy chain length and, (3) the linking groups modified from -CH2O- to deuterated –CD2O- core structure of the molecule were compared and discussed.
The mesomorphic phases and their corresponding transition temperatures were primarily characterized by the microscopic textures and DSC thermograms, and the ferroelectric and antiferroelectric phases were further identified by the measurements of electric switching behavior and dielectric constant ε'.
Compounds I(m=8-11) exhibited the SmA*, SmC* and SmCγ* phases, while compound (m=12) with longer alkyl chain exhibited the SmA*-SmC* phases. Compounds II(m=8,10) with even number of alkyl chain length displayed ferroelectric SmC*, antiferroelectric SmCA* and ferroelectric SmX* (SmI* or SmF*) phases, and compounds II(m=9,11,12) with odd number of alkyl chain length displayed exclusively SmC* phase. It is interesting to found that the deuterated compounds III(m=8-12) suppressed the formation of SmCγ* and resulted in the formation of SmA* and SmC* phases as compared to the non-deuterated compounds I(m=8-11). These results demonstrated that the semi-fluorinated compounds II(m=8-12) facilitate the formation of antiferroelectric phase, and suppressed the formation of SmA* phase. When the linking group changes from -CH2O- to deuterated –CD2O- core structure of the molecules, the temperature range of the SmA* phase and the SmC* phase increase. Comparing structurally similar non-fluorinated compounds PPmPPB(m=8-12) to I(m=8-12), it can be found that when the linking group changes from –COO- to -CH2O-, the temperature range of the SmA* phase and the SmC* phase decrease. Comparing structurally similar compounds II(m=8-12) to MPFPECPmBC(m=8-12), it can be found that compounds with -CH2O- linking group, suppresses the formation of SmA* phase and decreases the temperature range of the SmCA* phase, but increases the temperature range of the SmC* phase.
The physical properties of the chiral compounds in ferroelectric SmC*, SmCγ*, SmX* (SmI* or SmF*) and antiferroelectric SmCA* phases were also measured. The maximum magnitudes of the spontaneous polarization for compounds I(m=8-12), II(m=8-12) and III(m=8-12) are approximately in a range of 17.7-30.9nC/cm2, 80-120.4nC/cm2 and 8-20nC/cm2, respectively, demonstrating that the compounds with semi-fluorinated chiral group have larger Ps values. When the linking group of the chiral compounds changes from -CH2O- to deuterated –CD2O- in the core of the molecule, the maximum Ps values slightly decreases. Comparing I(m=8-12) to PPmPPB(m=8-12), and II(m=8-12) to MPFPECPmBC(m=8-12) indicate that the maximum Ps values are greater in the structurally similar compounds with methyleneoxy linking group than carboxylate group in the core of the molecules.
The maximum apparent tilt angles of series I(m=8-12), II(m=8-12) and III(m=8-12) are approximately in a range of 18°-22°, 23°-34.5° and 21°-27.5°. The results showed that the compounds with semi-fluorinated chiral group have larger apparent tilt angles. When the linking group changes from -CH2O- to deuterated –CD2O- in the core structure of the molecule, the optical tilt angles increase. Comparing I(m=8-12) to PPmPPB(m=8-12), and II(m=8-12) to MPFPECPmBC(m=8-12), it can be found that there has no any signification correlation of the maximum apparent tilt angles.
In conclusion, the chiral compounds with -COO- or -CD2O- linking group on the main core structure of the molecule suppress the formation of the SmCγ* phase. And when the chiral compounds with –CH2O- linking group on the main core structure of the molecule having semi-fluorinated alkyl chain in chiral tail facilitate the formation of SmCA* phase but suppress the formation of the SmA* phase. Regardless of the compounds with -COO- or -CH2O- linking group on the main core structure of the molecule, the results show that the compounds with semi-fluorinated alkyl chain in chiral tail largely enhance the maximum Ps value.
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