Odd–even effect on viscoelastic properties of twin-dimer nematic liquid crystals

Abstract

By using a combination of dynamic light scattering and capacitance measurements, we investigate the viscoelastic properties of nematic liquid crystalline α,ω-bis(4-cyanobiphenyl-4′-yloxy)alkane (BCBO-m) dimers and the corresponding 6OCB monomer. The results show that the K₁₁ and K₃₃ elastic constants are larger for the even-m dimers than for the odd-m dimers. K₁₁ and K₃₃ are twice as large for the odd-m dimers than for the 6OCB monomer, and the ratios K₃₃/K₁₁ ~ 2 are similar to that of 6OCB over the entire range of temperature investigated. The even-m dimers are characterised by large K₁₁ and K₃₃, which are respectively 3 and 10 times larger than the corresponding values for 6OCB at T/T_NI = 0.980. Furthermore, K₃₃/K₁₁ = 6 upon decreasing T/T_NI to 0.943. In contrast, the viscosities η_splay and η_bend are not characteristic of dimers. The viscosity η_splay increases with increasing molecular mass and η_bend is comparable to the corresponding value for 6OCB.

Keywords:

• dimer
• nematic
• odd–even effect
• elastic constant
• dynamic light scattering
• viscosity
• Fréedericksz transition