Cybotactic nematic phase of achiral unsymmetrical bent-core liquid crystals – Quelling of polar ordering and the influence of terminal substituent moiety

One-dimensional theoretical analysis of coupling and confinement effects on the cybotactic clusters of bent-core nematic liquid crystals

“…The cases with nonzero S c at the boundaries are treated separately. This simply describes fixed nematic ordering at the boundaries, consistent with some experimentally reported conditions [21].…”

“…More specifically, if we use γ = 5 as in [19], then the interior values of S c for a confined system are larger than the values for the spatially homogeneous system studied in [19], i.e., one can speculate that confinement cooperates with the coupling mechanisms to boost interior cluster formation. Experimentally, this suggests a strong core to core interaction (among the aromatic cores) between the constituent LCs and therefore an increase in the associated correlation length [21]. Additionally, we observe that an enhanced coupling also results in enhanced values for S g above the nematic supercooling temperature, compared to, say, γ = 0, i.e., it promotes conventional nematic ordering at higher temperatures.…”

“…We denote the cluster molecules as ES (excited state) and the surrounding ones as GS (ground state) and their associated order parameters as S c and S g respectively, using the same notation as Madhusudana [19]. Under usual experimental conditions, LC molecules are confined in a geometry of thickness, typically in the range of few microns (μm), with a definite experimentally imposed molecular alignment at the boundaries (substrate interfaces) [21]. The alignment condition is imposed by coating the two substrate surfaces (glass plates) with a polyimide layer and by gently rubbing the polyimide coated surfaces in a parallel or antiparallel fashion.…”

“…Although the model is able to predict clusters, it is limited in the sense that it assumes a fixed number of molecules inside a cluster which is temperature independent and does not include the effects of spatial inhomogeneities in the order parameters, director fields, boundary effects, or the effects of confinement. Experiments, in contrast, suggest that the size and polarity of these clusters strongly depend on the temperature, and also on confinement effects induced by geometry [21]. Therefore, a sizeable amount of theoretical work is still needed to further build on Madhusudana's elegant phenomenological model.…”

“…In particular, we model Dirichlet boundary conditions, which necessarily means that both S c and S g are fixed on the boundaries. Boundary conditions can be experimentally controlled, so that the Dirichlet conditions are adequate for certain experimental frameworks [21]. We have particularly focused on the effects of a parameter γ , which accounts for the coupling between clusters and the surrounding LC molecules, on the cluster order parameter (S c ).…”