Optical, dielectric and visco-elastic properties of a few hockey stick-shaped liquid crystals with a lateral methyl group

Abstract

Measurements of birefringence (Δn), dielectric permittivity (ε_‖, ε_⊥), elastic moduli (K₁₁, K₃₃) and rotational viscosity (γ₁) have been carried out on the nematic (N) phase of a few hockey stick-shaped compounds 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-alkyloxycinnamates with a lateral methyl group inserted between the m-alkyloxy chain and the azomethine connecting group. Interestingly, a dual characteristic (i.e. partially calamitic like and partially bent-core like) is revealed in the N phase of these compounds. The SmC_a–N transition is found to be of first order in nature while the SmC_s–N transition is either second order or weakly first order. All the mesogens exhibit a temperature dependent inversion in the static dielectric anisotropy (Δε = ε_‖ − ε_⊥) from positive to negative values on entering the SmC_a phase. Remarkably in the entire nematic range, the bend elastic modulus (K₃₃) is substantially lower than the corresponding splay modulus (K₁₁). The rotational viscosity coefficient (γ₁) as obtained by extracting the relaxation time (τ₀) values from two precise, independent probing methods, viz. the capacitive decay technique and the optical phase-decay-time measurement method, are slightly higher than those of many known calamitic systems. Moreover, the activation energy (E_a) calculated from the viscosity data is found to be considerably higher in the nematic phase than those obtained for conventional calamitics. The observed behaviours are accounted for by considering the intriguing shape-determined inter-molecular interactions and molecular associations appearing in the mesophase.