Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

Abstract

We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay ( 11 K ), twist ( 22 K ), and bend ( 33 K ) elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behavior to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rodlike mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behavior, described by Haller. The elastic constants also follow the standard behavior, with their ratios being practically temperature-independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N-Ntb phase transition. 33 K decreases strongly to 0.4 pN near the N -Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase. Keywords: dimeric nematic; elastic constants; birefringence; negative dielectric anisotropy; twist-bend nematic. N phase. Uniform bend, however, cannot be realized in space without other types of deformations, either splay or twist. Thus two different variations of the nematic with spontaneous bend have been proposed by Meyer [8] and Dozov [9]: a twist-bend nematic and a splay-bend nematic (yet to be discovered experimentally). The relative stability of the two is controlled by the ratio of the splay 11 K to twist 22 K constants. Namely, in the twist-bend phase, 11 22 /2 KK  , while in the splay-bend case, 11 22 /2 KK  [9, 10]. As for the ChOH state, Meyer [11] predicted that it can be induced by an external electric or magnetic field in a cholesteric liquid crystal only when 33 22 KK  . In calamitic nematics formed by rod-like molecules, the latter condition is not satisfied; the expected and universally observed trend follows the inequalities 33 22 11 K K K  [13]. Prior research indicates that the dimeric materials feature elastic properties very different from those of rod-like nematogens. Atkinson et al. [14] demonstrated a strongodd-even effect in the behavior of 33 K . Two dimers, one with an even number of alkyl groups in the spacer chain and another one with an odd number, were characterized by the splay Frederiks transition technique. In this approach, 11 K is determined from the threshold field of reorientation, and 33 K is obtained by extrapolating the voltage dependence of capacitance far above the threshold. 33 K of the odd homolog was about (3-4) pN, much smaller (by a factor 10 ) than the corresponding value for the even homolog; the results were in good agreement with the theoretical consideration of the odd-even effect by...

Physics of liquid crystals of bent-shaped molecules

“…The most spectacular deviation of DTC5C9 from the classic picture of nematic elasticity and from a naïve interpretation of Dozov's model is demonstrated by 33 K , Figure 37(e,f). On cooling, after reaching a maximum, 33 K decreases to a very low but clearly nonzero value of 0.4 pN near UTB T , Figure 37(f) (Cukrov et al 2017). The same feature was reported for a shorter homolog DTC5C7 by Sebastián (Nerea Sebastián et al 2016) and for a mixture of dimers by Adlem et al (Adlem et al 2013).…”

“…Experimental measurements by light scattering (Parsouzi, Pardaev, et al 2016) yield B in the range of 10 3 -10 4 Pa, which is substantially lower than the compressibility modulus in SmA with density modulation, ( C~10 6Pa). The splay constant K is expected(Challa et al 2014) to be close to the value of 11 K in the N U phase near UTB T , i.e., on the order to 10 pN (Cukrov et al 2017) (Babakhanova et al 2017).…”

“…Since the tilt direction is degenerate, it results in umbilics, i.e., nonsingular defects of winding numbers -1 and +1(P. G. de Gennes and Prost 1995). The +1 umbilics show an in-plane bend of n , which is expected, as K 33 << K 11 )(Cukrov et al 2017. The 1993).…”

Dielectric properties of liquid crystalline dimer mixtures exhibiting the nematic and twist-bend nematic phases

“…The nature, length and parity of the flexible spacer decide the angle between the two mesogenic units and hence the conformation of the dimer [20]. Dielectric properties have been previously studied for pure dimeric liquid crystals of both positive and negative dielectric anisotropies that exhibit the twist-bend nematic phase [24][25][26]. Dielectric spectroscopy measures the dielectric properties of a material, such as relaxation processes occurring within molecules, as a function of frequency.…”

Developments in liquid-crystalline dimers and oligomers

“…This has motivated the synthesis of a number of novel compounds, and present experimental results indicate that the twist-bend phase is primarily driven by topology [39], with the thermal stability having a dependency on the intermesogen angle [40,41]. Several aspects of the N TB phase have been reviewed recently [28,32,42,43]. It has been demonstrated for materials with nonamethylene and heptamethylene central spacers that there is a linear relationship between T NTB−N and T N-Iso [39].…”