||There are many factors affect the properties of ferroelectric (SmC*) and antiferroelectric (SmCA*) liquid crystal phase such as chiral terminal group, rigid core, linking group, terminal chain length and optical purity. For these reasons, the purpose of this research work is an attempt to correlate the various core structures and achiral terminal groups to the formation of mesomorphic phases, especially ferroelectric and antiferroelectric phases in chiral liquid crystal materials.
Thus, the chiral precursor, (S)-1-methyl-2-(2,2,3,3,4,4,4-heptafluorobutoxy) ethanol, was designed and synthesized by reacting (S)-propylene oxide with semi-fluorinated alcohol under basic condition. Consequently, four homologous series of chiral liquid crystal materials derived from this chiral precursor were prepared for the study.
The mesomorphic phases and their corresponding phase 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'.
The results show that rod-like compounds, with the exception of compound III-2 that exhibits N* and SmC* phases, possess both ferroelectric SmC* and antiferroelectric SmCA* phases. The compounds with bend-core structure suppress the formation of the antiferroelectric SmCA* phase and exhibit the phase sequence of Iso.-SmA*-SmC*-Cr..
The thermal stability of ferroelectric SmC* phase for the variation of the core structures in the rod-like compounds displays an order of PhPhCOONa > PhCOOPhPh > PhPhCOOPh > PhCOONa, and that for the variation of achiral terminal end group displays an order of hydroxyalkyl > acryloyl > propionyl > alkenyl > alkyl. Whereas, the thermal stability of antiferroelectric SmCA* phase for the variation of the core structures in the rod-like compounds displays an order of PhPhCOONa > PhCOONa > PhPhCOOPh > PhCOOPhPh, and that for the variation of achiral terminal end groups displays an order of alkenyl > alkyl > acryloyl > propionyl (hydroxyalkyl group suppresses the formation of the antiferroelectric SmCA* phase). For the bend-core compounds, the thermal stability of ferroelectric SmC* phase for the variation of achiral terminal end groups displays an order of alkenyl > acryloyl > alkyl > propionyl (hydroxyalkyl group suppresses the formation of the ferroelectric SmC* phase).
The electro-optical measurements show that the bend-core compounds have much lower Ps values (15.39-49.59 nC/cm2) as compared to the structurally similar compounds with the rod-like structure of the molecule(83.21-130.00 nC/cm2). And rod-like compounds composed of a naphthalene ring in the core structure of the molecule have the maximum Ps values (76.30-84.76 nC/cm2) lower than that composed of the phenyl or biphenyl ring in the core structure of the molecule. The measured maximum Ps values for the variation of achiral terminal functional groups in the rod-like compounds display an order of hydroxyalkyl > alkenyl > alkyl ≒ acryloyl ≒ propionyl at any temperature below Curie point. This may presumably due to the variation of the palarizability at the end group of achiral chain in the molecules.
The maximum optical tilt angles θ for four series of compounds are in the range of 28.6∘-40.0∘, 29.0∘-38.5∘, 37.0∘-41.0∘ and 40.0∘-41.8∘ for the corresponding compounds I(m=1-5), II(m=1-3), III(m=1-3) and IV(m=1-3). These results indicate that the measured optical tilt angles of the compounds in the ferroelectric and/or antiferroelectric phases have no significant correlation to the variations of achiral terminal groups in the molecules.
Keywords: Rod-like compound, bend-core compound, ferroelectric liquid crystal, antiferroelectric liquid crystal.