The primary research work of this study is an attempt to correlate the structure-property relationship of the formation of blue phases in chiral liquid crystals in terms of variation of the structure of the rigid cord, achiral alkyl chain length (m) and chiral alkyl chain length (n) possessing ether linkage attached to the chiral tail in the chiral liquid crystal materials.
Thus, the optically active alcohols, 2-octanol, 1-butyloxy-2-propanol, 1-pentyloxy-2-propanol, 1-hexyloxy-2-propanol, were designed and synthesized by reacting (S)-propylene oxide with alkyl alcohols under basic condition. In consequence, five novel homologous series of chiral materials, (R)-6-(1-methylheptyloxy)naphth-2-yl 4’-octyloxybenzoate, I(m), (R)-6-(1-butyloxy-2-propyl)naphth-2-yl 4-octyloxybenzoate, II(m), (R)-6-(1-pentyloxy-2-propyl)naphth-2-yl 4-octyloxybenzoate, III(m), (R)-6-(1-hexyloxy-2-propyl)naphth-2-yl 4-octyloxybenzoate, IV(m), (R)-4-(1-butyloxy-2-propyl)biphenyl 4-octyloxybenzoate, V(m), derived from these alcohols were synthesized for the investigation of the effect of the aliphatic alkyl chain length (m), ether linking group, chiral tail alkyl chain length (n) and rigid core structures on the mesomorphic and electro-optical properties.
The mesomorphic phases and their corresponding transition temperatures were primarily characterized by the microscopic textures and DSC thermograms, and the ferroelectric phases were further identified by the measurements of electric switching behavior and dielectric constant ε'.
The results of compounds I(m) composed of the rigid core structure of PhCOONa and chiral group of optical activity 2-octanol, showed that compound I(m=6) exhibits the mesophases sequence of BPIII-BPII-N*, compound I(m=7) exhibits the mesophases sequence of BPII-BPI-N* and compounds I(m=8,9) exhibits the mesophases sequence of BPII-BPI-N*-TGBA*-SmA*. With the exception of I(m=8) that exists the widest temperature range of enantiotropic BPII and BPI phases, the rest of compounds in series of I(m) possess BPII and BPI phases are monotropic. Compounds I(m) display frustrated phases demonstrated that these compounds possess high chirality.
The results of compound II(m=6, n=4), composed of the same rigid core structure as compound I(m) but differed in the chiral group where an additional ether linkage is introduced, shows the mesophases sequence of BPII-BPI-N*, compounds II(m=7, 8, 9, n=4) exhibit the mesophases sequence of BPII-BPI-N*-TGBA* and compound II(m=8, n=4) displayed an additional SmA* phase. All compounds posses enantiotropic BPII and BPI phases and the thermal stability of BPI and TGBA* phases increase from compounds at m=7 to m=8 and m=9. When extending the alkyl length (n) of chiral tail, the thermal stability of TGBA* phase decreases and when n=5 and 6, the BPI is suppressed. The widest temperature range of frustrated (BP and TGBA*) phases for these compounds appeared at n=4.
The results of compounds V(m=7-9, n=4), composed of the same chiral group but differed in the rigid core structure (PhCOOPhPh) as compounds II(m), show the mesophases sequence of BPII-BPI-N*-SmC*, compound V(m=9, n=4) exhibit an additional BPIII phase. The thermal stability of BPII and BPI phases obtained from compounds V(m) are decreased and the TGBA* phase is suppressed as compared to that of compounds II(m).
The physical properties of the chiral materials in ferroelectric SmC* phases were measured. The maximum magnitudes of spontaneous polarization measured for the materials in the SmC* phase are in the range of 21.7 to 28.5 (nC/cm2). These results also show that the maximum Ps values have no significant correlation to the change of achiral alkyl chain length (m).
In conclusion, the chiral materials composed of the rigid core structure of PhCOONa and the chiral group of an additional ether linkage at terminal tail of the molecules, have higher thermal stability of frustrated (BP and TGBA*) phases. All series of compounds display that achiral alkyl chain length (m) of the molecules have no correlation to the thermal stability of the frustrated phases, nevertheless, chiral alkyl chain length (n) of molecules show an increasing in thermal stability of frustrated phases in decreasing n. Among all compounds, compound II(m=7, n=4) has the widest temperature range of enantiotropic blue phases (cal. 12.5℃).
Keywords: Chiral material, Blue phase, frustrated phases, electro-optical property, switching behavior, dielectric constant ε’