Light Scattering Study of a Twist Grain Boundary Liquid Crystal

Abstract

Dynamic light scattering is used to probe the fluctuation modes of a liquid crystal exhibiting twist grain boundary (TGB) structure. At the chiral nematic to proposed "chiral line liquid" phase transition, anomalous temperature dependence in the fluctuation spectrum and an instability in the helicoidal director structure signify developing TGB order. At lower temperatures, the behavior of the smectic layer-director fluctuations ("soft" mode) indicates that the previously identified commensurate TGB(A) phase may in fact be a TGB(C) phase with an unusually small tilt angle.

1H NMR Relaxometry Study of a Rod-Like Chiral Liquid Crystal in Its Isotropic, Cholesteric, TGBA*, and TGBC* Phases

“…In particular, the “soft mode” observed in the TGBA* phase is always weaker than that in the SmA phase, and the “Goldstone mode”, which affects the TGBC* phase, is strongly reduced with respect to the SmC* phase. , Moreover, in the range of frequencies where the dielectric relaxation is dominated by the collective motional contributions (<100 kHz), discontinuities have been observed at the transitions between the TGB phases in the N*−TGB−SmC* phase sequence. Similar conclusions could be drawn on the basis of dynamic light scattering studies . Information about the local molecular motions, such as the overall reorientational molecular motions (spinning and tumbling) and molecular translational self-diffusion, can be obtained by means of NMR spectroscopy. , For instance, the line-shape analysis of 2 H NMR spectra recorded by means of a 2D exchange NMR sequence in the TGBA* phase of a chiral compound allowed Zhang et al to estimate the self-diffusion constant D ∥ along the helical pitch to be about 10 −12 m 2 /s .…”

“…Similar conclusions could be drawn on the basis of dynamic light scattering studies. 17 Information about the local molecular motions, such as the overall reorientational molecular motions (spinning and tum- bling) and molecular translational self-diffusion, can be obtained by means of NMR spectroscopy. 18,19 For instance, the line-shape analysis of 2 H NMR spectra recorded by means of a 2D exchange NMR sequence 20 in the TGBA* phase of a chiral compound allowed Zhang et al 11 to estimate the self-diffusion constant D | along the helical pitch to be about 10 -12 m 2 /s.…”

Effect of Central Longitudinal Dipole Interactions on Chiral Liquid-Crystal Phases

“…Previous works have reported that it has a short-range smectic layer formation, like those found in the TGB phases [ 2 , 4 ]. Following these previous works, several studies have been performed to investigate the phase behavior of the chiral line liquid, but these are few in number in comparison with other liquid-crystal phases [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. The recent development of photonic technologies has enabled the creation tunable nanostructures of these phases.…”

Dispersion relations of director fluctuations along the direction perpendicular to the helical axis in cholesteric liquid crystals

“…Later, fluctuations of tensor order-parameter modes parallel to the helical axis are also calculated [9]. In contrast, for the mode perpendicular to the helical axis, there are few investigations, which have only been done for the case in which the wave number of the scattering vector q, which is defined as the difference between the wave number vectors of the incident and the scattered light is much less than that of the helical pitch q 0 , which is defined by q 0 = 2π/P , where P is the helical pitch length (q q 0 ) [6,10]. By the way, order-parameter fluctuations in the isotropic phase above the cholesteric phase are also investigated as the pretransitional behavior [11,12].…”