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

By using a combination of dynamic light scattering and capacitance measurements, we investigate the viscoelastic properties of nematic liquid crystalline ,-bis(4-cyanobiphenyl-4-yloxy)alkane (BCBO-m) dimers and the corresponding 6OCB monomer. The results show that the K-11 and K-33 elastic constants are larger for the even-m dimers than for the odd-m dimers. K-11 and K-33 are twice as large for the odd-m dimers than for the 6OCB monomer, and the ratios K-33/K-11 similar to 2 are similar to that of 6OCB over the entire range of temperature investigated. The even-m dimers are characterised by large K-11 and K-33, which are respectively 3 and 10 times larger than the corresponding values for 6OCB at T/T-NI=0.980. Furthermore, K-33/K-11=6 upon decreasing T/T-NI to 0.943. In contrast, the viscosities (splay) and (bend) are not characteristic of dimers. The viscosity (splay) increases with increasing molecular mass and (bend) is comparable to the corresponding value for 6OCB.