Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible<i>n</i>-mers

Abstract

We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer exhibiting a uniaxial nematic (N) to twist-bend nematic (N_TB) phase transition. The elastic constants for director splay (K₁₁), twist (K₂₂) and bend (K₃₃) exhibit the relations K₁₁ > K₂₂ > K₃₃ and K₁₁/K₂₂ > 2 over the bulk of the N phase. Their behavior near the N-N_TB transition shows dependency on the parity of the number (n) of the rigid mesomorphic units in the flexible n-mers. Namely, the bend constant K₃₃ in the dimer and tetramer turns upward and starts increasing close to the transition, following a monotonic decrease through most of the N phases. In contrast, K₃₃ for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K₂₂ increases pretransitionally in both even and odd n-mers, but more weakly so in the trimer, while K₁₁ increases steadily on cooling without evidence of pretransitional behavior in any n-mer. The viscosities associated with pure splay, twist-dominated twist-bend, and pure bend fluctuations in the N phase are comparable in magnitude to those of rod-like monomers. All three viscosities increase with decreasing temperature, but the bend viscosity in particular grows sharply near the N-N_TB transition. The N-N_TB pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the N_TB phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A* phase.