Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible n-mers.
Identifieur interne : 000423 ( Main/Curation ); précédent : 000422; suivant : 000424Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible n-mers.
Auteurs : Zeinab Parsouzi [États-Unis] ; Greta Babakhanova ; Mojtaba Rajabi ; Rony Saha ; Prabesh Gyawali ; Taras Turiv ; Hao Wang ; Alan R. Baldwin ; Chris Welch ; Georg H. Mehl ; J T Gleeson ; Antal Jakli ; Oleg D. Lavrentovich ; Samuel SpruntSource :
- Physical chemistry chemical physics : PCCP [ 1463-9084 ] ; 2019.
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 (NTB) 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-NTB 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-NTB transition. The N-NTB pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the NTB phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A* phase.
DOI: 10.1039/c9cp00984a
PubMed: 31168534
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<author><name sortKey="Babakhanova, Greta" sort="Babakhanova, Greta" uniqKey="Babakhanova G" first="Greta" last="Babakhanova">Greta Babakhanova</name>
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<author><name sortKey="Rajabi, Mojtaba" sort="Rajabi, Mojtaba" uniqKey="Rajabi M" first="Mojtaba" last="Rajabi">Mojtaba Rajabi</name>
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<front><div type="abstract" xml:lang="en">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<sub>TB</sub>
) phase transition. The elastic constants for director splay (K<sub>11</sub>
), twist (K<sub>22</sub>
) and bend (K<sub>33</sub>
) exhibit the relations K<sub>11</sub>
> K<sub>22</sub>
> K<sub>33</sub>
and K<sub>11</sub>
/K<sub>22</sub>
> 2 over the bulk of the N phase. Their behavior near the N-N<sub>TB</sub>
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<sub>33</sub>
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<sub>33</sub>
for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K<sub>22</sub>
increases pretransitionally in both even and odd n-mers, but more weakly so in the trimer, while K<sub>11</sub>
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<sub>TB</sub>
transition. The N-N<sub>TB</sub>
pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the N<sub>TB</sub>
phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A* phase.</div>
</front>
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