Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible -mers

• Published in: Physical chemistry chemical physics : PCCP 2019-06, Vol.21 (24), p.1378-1389
• Main Authors: Parsouzi, Zeinab, Babakhanova, Greta, Rajabi, Mojtaba, Saha, Rony, Gyawali, Prabesh, Turiv, Taras, Wang, Hao, Baldwin, Alan R, Welch, Chris, Mehl, Georg H, Gleeson, J. T, Jakli, Antal, Lavrentovich, Oleg D, Sprunt, Samuel
• Format: Article
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c9cp00984a

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 ( K 11 ), twist ( K 22 ) and bend ( K 33 ) exhibit the relations K 11 > K 22 > K 33 and K 11 / K 22 > 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 K 33 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, K 33 for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K 22 increases pretransitionally in both even and odd n -mers, but more weakly so in the trimer, while K 11 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. 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.