Understanding the distinctive elastic constants in an oxadiazole bent-core nematic liquid crystal

Abstract

The splay and bend elastic constants of the bent-core oxadiazole material [C5-Ph-ODBP-Ph-OC12] have been investigated as a function of temperature across the nematic phase. The bend constant K(33) is found to take values of ~3.0 pN and to be almost temperature independent, whereas, the splay constant K(11) increases monotonically from ~3.5 pN close to the isotropic phase transition to values of ~9 pN deep in the nematic phase. No pretransitional divergence is observed in either K(11) or K(33) at temperatures approaching the underlying phase. This behavior of the elastic constants is distinct from that observed in rodlike liquid crystal systems but appears to share characteristics with the few other bent-core nematic systems studied to date. We discuss the interdependence of the elastic constants, the birefringence, and the order parameter to allow a comparison of the observed behavior with theory. We show that calculations of the elastic constants via molecular-field theory and atomistic modeling are in excellent qualitative as well as good quantitative (within 2 pN) agreement with the measurements across the temperature range, offering a deeper understanding of the elasticity in bent-core nematic materials than has been, hitherto, available.

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

“…The n value measured in our experiments in the DC phase, at zero electric field, is in excellent agreement with the average refractive index deduced close to N-DC phase transition in Ref. 33.…”

“…Several studies indicate that there are two distinctive (possibly uniaxial and biaxial) regimes in the nematic phase [30][31][32]. Also, the nematic phase shows low splay (K 11 ), twist (K 22 ) and bend (K 33 ) elastic constants compared to other members in the homologues series [33,34]. It has low twist and bend elastic constants compared to calamitics [34].…”

Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series

“…We can thus safely state that in BCNs K 33 < K 11 is due to the bent shape of the molecules rather than arising from the presence of clusters. Our results likewise can be explained by the theory and modeling proposed by Kaur et al [43] and Cestari et al [44]. The bent shape favors the bend distortions of the director as opposed to splay.…”

“…The magnitude of the effective flexoelectric coefficient |(e 1 − e 3 )| is obtained using Eq. The effective flexoelectric coefficient for C4, C6, and C7 is greater than reported for the conventional rodlike molecules that have similar magnitudes of the dipole moments [39,45], but its order of magnitude is comparable to the other BCNs [24,40,43]. However, it is much lower than that found by Harden et al [9] for another BCN LC.…”

“…For an oxadiazole bent-core compound, Kaur et al [43] found K 33 < K 11 . They explained their observations by the molecular field theory and atomistic modeling, suggesting that K 11 is almost independent of the molecular conformation, whereas K 33 is sensitive to changes in the molecular curvature, similar to those calculated for odd-dimeric mesogens [35,44].…”