Dielectric properties of twist grain boundary phases:  Influence of the anchoring and the distance between grain boundaries

Abstract

The dielectric properties of the twist grain boundaries TGB(A) and TGB(C) of liquid crystal phases differ from the smectic-A and smectic-C phase ones: a theoretical model confirmed by experimental results shows that the Goldstone mode of the TGB(C) phase and the soft mode of the TGB(A) phase are strongly reduced. This behavior is due to elastic strain, which is connected to two parameters: the anchoring at the grain boundaries and the distance between the grain boundaries. It is shown quantitatively that a relatively flexible anchoring in the TGB(A) phase becomes rigid in the TGB(C) one. The relaxation frequencies of these modes allow analysis of the rotational viscosity variations.

Twist Grain Boundary Liquid‐Crystalline Phases under the Effect of the Magnetic Field: A Complete2H and13C NMR Study

“…In particular, two types of tilted TGB clearly exist: one with a helical superstructure within each block, that is, a typical SmC* phaselike structure (TGBC*), [4] and another type in which the in-block helical structure does not exist, giving rise to tilted molecules or tilted smectic layers with respect to the helical pitch of the TGB phase (TGBC). [9] Although several studies have been done in order to characterize the structures of TGBC and TGBC* phases by means of calorimetry, [8] dielectric, [10] optical [11] and X-ray measurements, [12,13] no NMR studies have been published so far. Moreover, only few papers are known on structural studies of the TGBA phase by NMR.…”

Characteristics of electrical properties of wide temperature range TGB phases in liquid crystal dimers

“…Similarly in TGBC phase, electric field induces phase fluctuation of the tilt angle and hence Goldstone mode of dielectric relaxation is observed like those in SmC * phase [12,15]. However, ex− perimental evidences suggest that TGB phase relaxation processes have lower amplitudes and higher relaxation fre− quencies as compared to those observed in the classical SmA * and SmC * phases [17].…”

“…The high value dispersive parameter correspond− ing to this mode suggests its polydispersive nature. Ismaili et al [17] proposed a theoretical model verified by some ex− perimental work as well, which suggests that Goldstone mode of TGBC and soft mode of TGBA phases are strongly reduced due to the existence of an elastic parameter (H 2 ) in these phases. Existence of H 2 is connected to the elastic dis− tortion of the director and its amplitude depends strongly on Opto the anchoring parameter b a arising due to the anchoring forces at the grain boundaries and the distance between the grain boundaries l b .…”

“…Dielectric studies of the phases are still in nascent stage and the data on the dielectric behaviour of TGB phases available in literature is not very comprehensive [10,[12][13][14][15][16][17][18][19][20][21] because most of the TGB phases exist over a very short tem− perature range due to their frustrated nature. Initial fre− quency dependent (dynamic) dielectric studies of the TGBA phase [12][13][14][15] show that electric field induces amplitude fluctuation of tilt angle like those of SmA * phase, leading to excitation of soft mode of dielectric relaxation.…”

Frustrated smectic liquid crystalline phases in lactic acid derivatives

“…[23] It can be supposed that both soft and Goldstone modes are active in the individual smectic blocks forming the TGB superstructure, but these modes are not homogeneous within the blocks, as they are clamped at the grain boundaries limiting the blocks. [14,15,24] The clamping results in increase of relaxation frequencies and decrease of the dielectric strength compared to those typical for homogeneous SmA and SmC Ã phases. The difference might reach two orders for each quantity.…”