Elastic and viscous properties of the nematic dimer CB7CB

Abstract

We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4'-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, N_{TB}, and the oblique helicoidal cholesteric, Ch_{OH}, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the N-N_{TB} and N-to-isotropic (I) transitions. The splay constant K_{11} is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K_{33} and twist K_{22} constants are measured by DLS. K_{33}, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and N-N_{TB} pretransitional regions. The measured ratio K_{11}/K_{22} is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the N-N_{TB} transition.

Pitch tuning induced by optical torque in heliconical cholesteric liquid crystals

“…The agreement is again quite good. From the parameter = 2 √ 3 0 Δ we can estimate the bend elastic constant of the used mixture: 3 ≅ 0.6 , which is in good agreement with the values found in similar mixtures exhibiting the oblique heliconical phase at temperature close to the transition to the twist-bend nematic phase 8,16 , thus strengthening our confidence in the good quality of the fit. The parameter Δ Δ 1 0 turns out to be higher than the one calculated but still of the right order of magnitude (0.29 • 10 2 vs the calculated 0.19 • 10 2 V 2 /W).…”

Critical behavior of the optical birefringence at the nematic to twist-bend nematic phase transition

“…Such a behavior is in contrast to that found in the rodlike mesogens, for which usually K 33 > K 11 and K 33 increases on cooling, following the square of the orientation order parameter (S 2 ) [15,16]. Interestingly, precise measurements made by using dynamic light scattering (DLS) [14] showed that in a narrow temperature range of the nematic phase close to the N TB phase, K 33 (and also the twist elastic constant K 22 ) drastically increases due to fluctuations, as locally formed helices constrain the macroscopic twist and bend deformations. Also, the nuclear magnetic resonance (NMR) measurements performed in the nematic phase close to the N TB phase transition suggest that molecules tilt from the magnetic field direction, which is consistent with the formation of a local heliconical structure [17,18].…”

“…Dozov suggested that the spontaneous helical deformation of the director in the N TB phase is driven by the bend elastic constant becoming negative [10] due to pure steric reasons, while it was later suggested that the renormalization of the elastic constants can be due do the flexoelectric effect [11,12]. Experimentally, in the nematic phase of some bent-core materials a very small value of the bend elastic constant (K 33 ) was indeed measured [13], much smaller than the splay elastic constant (K 11 ), with the ratio of K 11 /K 33 as high as 21 [14]. In addition, K 33 decreased with decreasing temperature.…”

Physics of liquid crystals of bent-shaped molecules

“…The parallel component and thus , Figure 38b, are measured by applying a very strong field and extrapolating the measured capacitance to an infinite field limit. The functions , , and measured by Babakhanova et al (Babakhanova et al 2017) are shown in Figure 38. Upon cooling, monotonically increases, Figure 38(a), but first briefly increases near the clearing point, and then decreases over an extended temperature range, Figure 38b, compare to the behavior of in DTC5C9, Figure 37d.…”

“…K , twist 22 K , and bend 33 K constants; (e) detailed data for 33 K . Replotted from (Babakhanova et al 2017) by Greta Babakhanova.…”