Comparison of material parameters among flexible dimer, trimer, tetramer of nematic mesogens*

We compare the temperature dependencies of the birefringence, elastic constants and orientational viscosities for four homologous nematic liquid crystal oligomers: dimer (DTC5C9), its associated trimer and tetramer. These materials all exhibit the twist-bend nematic (N_TB) phase. The magnitudes of the splay (K₁₁), twist (K₂₂) and bend (K₃₃) elastic constants, as well as the corresponding orientational viscosities are determined by dynamic light scattering measurements. Interestingly K₃₃ in the dimer and tetramer shows the monotonic decrease with temperature below the nematic-isotropic transition followed by a pretransitional increase close to the nematic to N_TB transition. The trimer, in contrast, shows essentially no pretransitional change in K₃₃ . This behavior can be attributed to odd-even effect associated with the number of monomer unites in oligomers. The orientational viscosities associated with splay, twist and bend fluctuations in the N phase of dimer, trimer and tetramer are comparable to those of nematics formed by rod-like molecules. All three show strong temperature dependence, increasing sharply near the N – N_TB transition.

References
[1] G.Babakhanova, et.al ,Physical Review E, 96 (6), 062704 (2017)