Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers

Abstract

Wide band dielectric spectroscopy of bent-shaped achiral liquid-crystal dimers 1-n-bis(4cyanobiphenyl-4-yl) n-alkanes (CBnCB n=7,9,11) have been investigated in a frequency range 0.1 Hz to 100 MHz using planar-aligned cells of sample thicknesses ranging from 2 to 10 (m) over a temperature range that covers both nematic and twist bend nematic phases.Two peaks in the dielectric spectrum in the higher frequency range are assigned to the molecular relaxation processes. The peak at the highest frequency, ~ 40 to 80 MHz, is assigned to an internal precessional rotation of a single unit of the dimer around the director.The mode in the next lower frequency range of 2 to 10 MHz is assigned to the spinning rotation of the dimer around its long axis. This involves fluctuations of the dipole moment of the bent-shaped conformation that is directed along its arrow direction of the bow shape formed by the dimer. The peak in the frequency range 100 kHz to 1 MHz can be assigned to the collective fluctuations of the local director with reference to the helical axis of the NTB structure. The dependence of its frequency on temperature is reminiscent of the soft mode observed at the SmA* to SmC* phase transition. This result clearly corresponds to the electroclinic effectthe response of the director to the applied electric field in an electro-optic experiment. The lowest frequency mode, observed in frequency range of 0.1 Hz -100 Hz, is identified with the Goldstone mode. This mode is concerned with the long range azimuthal angle fluctuations of the local director. This leads to an alternating compression and expansion of the periodic structure of the NTB phase.

Molecular biaxiality determines the helical structure – infrared measurements of the molecular order in the nematic twist-bend phase of difluoro terphenyl dimer

“…Thus IR absorbance components can, at least in principle, bring information about the ordering in the LC phase, via analyzing the order parameters S and D. In the nematic phase the order parameter, S, follows the Mayer -Saupe model well [39]. However, in the N TB phase, it has been demonstrated that the temperature dependence of the S-order parameter distinctly reverses its monotonic trend of increase to decrease at the transition temperature from the nematic (N) to the twist-band (N TB ) [26][27][28][29][30][31][40][41][42]. This is due to the rearrangement of molecules in the N TB phase as a helical structure appears.…”

“…Such an exception, however, can indicate the possibility that the biaxial nematic phase might appear in between the nematic uniaxial (N U ) and the twist-bend (N TB ) phase [5,10]. A small sample biaxiality, which was found in N TB phase by comparing corresponding perpendicular absorbance components (in both alignment), A Y ≠A h , are likely to be induced by anchoring effect [39][40][41]. It seems, observation of true biaxiality is not possible without application of external field that can align secondary director.…”

How Do Intermolecular Interactions Evolve at the Nematic to Twist–Bent Phase Transition?

“…The experimental study of the structure of the N TB phase has mainly been carried out with nonresonant (SAXS, WAXS) [ 13 , 14 , 15 ] and resonant X-ray scattering [ 10 , 11 , 16 , 17 , 18 , 19 ], freeze–fracture transmission electron and atomic force microscopy (FFTEM, AFM) [ 8 , 9 , 20 , 21 , 22 ], polarized Raman [ 23 ], infrared [ 24 , 25 , 26 ], and nuclear magnetic resonance spectroscopy (NMR) [ 27 , 28 , 29 , 30 , 31 ], as well as with optical methods under applied electric fields [ 32 , 33 , 34 , 35 , 36 , 37 , 38 ] and magnetic fields [ 39 ]. The formation of the N TB structure has been modeled using rigid elements [ 15 , 40 , 41 ].…”

“…Finally, the X-ray evidence of the half-molecular length periodicity along the N TB helix led to a model based on the self-assembly of half-molecule-long blocks as the basic element of the N TB phase [ 10 , 18 , 42 ]. The N TB materials that have been most investigated are the cyanobiphenyl (CB)-based liquid crystal dimers (CBCnCB; n = 7, 9, 11) [ 8 , 9 , 10 , 11 , 13 , 16 , 21 , 23 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 36 , 37 , 38 , 39 ], where cyanobiphenyl units prefer an antiparallel arrangement due to dipole–dipole interactions, [ 43 , 44 , 45 , 46 ]. Up-to-date research has confirmed that molecular curvature is important for phase formation, but its stability increases as the bending angle decreases [ 47 , 48 , 49 , 50 , 51 , 52 ].…”

Twist-Bend Nematic Phase: Role of Third-Order Legendre Polynomial Term in Chiral Interaction Potential

“…The soft materials, composed of highly anisotropic molecules, exhibit nearly limitless partially ordered mesophases. Bentshaped liquid crystals (LCs) [1,2], sometimes descriptively designated as LCs of banana-shaped molecules [3], can form one of these exotic phases, the twist-bend nematic (N TB ) phase [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In recent decades, considerable attention has been given to study the twist-bend nematic (N TB ) phase due to their novel mesostructure, chirality, and intriguing behavior such as elastic properties, which are strongly anisotropic and have an unusual temperature dependence .…”