Electroclinic effect in a chiral smectic-Aliquid crystal stabilized by an anisotropic polymer network

Abstract

We have studied the effect of an anisotropic polymer network on the coupling of molecular tilt to applied electric field in the chiral S{A} phase. The polymer network is formed from a photoreactive achiral monomer in a thin planar S{C}{*} cell. Experimental data, obtained from electro-optical measurements near to the S{A}-S{c}{*} transition temperature, T{c} , of the induced molecular tilt, switching time, as well as induced polarization as a function of temperature and electric field strength are presented. The results clearly show that, close to T{c} , the electroclinic effect is largely controlled by the polymer network. The experimental results are discussed in the framework of a simple phenomenological model, extended from the Landau model, which includes the bulk free energy arising from the anisotropic interaction between the polymer network and the liquid crystal director, and the elastic free energy resulting from the anchoring (supposed rigid) of the liquid crystal molecules at the polymer boundaries.

Atomic Force Microscopy Observations of the Polymer Network Structure Formed in Ferroelectric Liquid Crystals Cells

“…This figure shows that Keff linearly increases from 0.5 10 −11 to 2.310 −11 N when the polymer concentration increases from 0% to 7%. The values of Keff found here compare well with those obtained for the same PSFLC systems from the electro-optic measurements [18,20]. In conclusion, the increase in the relaxation frequency and the reduction in the dielectric strength of the Goldstone mode for the PSFLC films seem to be due to the increase in the twist elastic energy, resulting from the strong interaction between liquid crystal molecules and the polymer network liquid crystal molecules and the polymer network.…”

“…This implies that even after polymerization, the characteristic property of the host phase in which polymerisation was carried out was effectively retained. The helical aspect of the fiber structure certainly influences and explains the electro-optical: Deformed Helix ferroelectric liquid crystals (DHF) [18] and electroclinic effects [19,20]. Figures 16 (a) and 16 (b) show examples of the dispersion, '(f) and absorption, "(f) , dielectric spectra obtained in the SmA phase at Tc for each polymer concentration.…”

“…15. In order to examine how the polymer network influences the soft-mode dielectric strength of the PSFLC, we used the model previously developed [20] to explain the electroclinic behavior of PSFLC films. Although polymer networks formed in liquid crystal media generally have a complex structure, they have been previously modeled as an assembly of parallel cylinders randomly distributed within the liquid crystal media [7].…”

“…However, a few information has been reported in the literature about the vertical distance between fibers especially when the polymer network was formed in a ordering helical ferroelectric liquid crystals ( SmC*) and paraelectric SmA phases (PSFLC). The PSFLC composites have been investigated by different methods, electro optic technical [16][17][18][19][20][21][22] and dielectric method [23][24][25][26]. It is know that the dielectric properties of the ferroelectric liquid crystals in the ideal unbounded sample are nowadays well understood.…”

“…From this equation, we conclude that the main parameter which governs the soft mode dielectric strength is the vertical distance which separated between two successive groups of polymer fibers Lc. We used the values of  8.8 10 +3 N m -2 K -1 , a =0.12µm [20], and the values of s presented in the Figure 17(a) at T-Tc=0.1° C, with a reasonable value of the twist elastic constant, typically, K2 10 -11 N, the equation (16) was graphically resolved to evaluate Lc. We obtain a mean inter-fiber Lc.…”