Effect of optical purity on phase sequence in antiferroelectric liquid crystals

Abstract

We use the discrete phenomenological model to study theoretically the phase diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical purity and temperature. Recent experiments have shown that in some systems the number of phases is reduced if the optical purity is extremely high. In some materials the SmC ⋆ A phase is the only stable tilted smectic phase in the pure sample. In the scope of the presented model this high sensitivity of the phase sequence in the AFLCs to optical purity is attributed to the piezoelectric coupling which is reduced if optical purity is reduced. We limit our study to three topologically equal phases -SmC * , SmC * α and SmC * A and show that the reduction of optical purity forces the system from the antiferroelectric to the ferroelectric phase with a possible SmC ⋆ α between them. The effect of the flexoelectric and quadrupolar coupling is considered as well. If the phase diagram includes only two phases, SmC ⋆ and SmC ⋆ A , the flexoelectric coupling is very small. The materials which exhibit the SmC ⋆ α in a certain range of optical purity and temperature, can be expected to have a significant flexoelectric coupling that is comparable with the piezoelectric coupling. And finally, when temperature is lowered the phase sequence SmA → SmC ⋆ α → SmC ⋆ → SmC ⋆ A is possible only in materials in which quadrupolar coupling is very strong.

Polymer stabilization of the smectic C-alpha* liquid crystal phase—Over tenfold thermal stabilization by confining networks of photo-polymerized reactive mesogens

“…4 One could imagine that the domain boundaries imposed by the confining polymer network affect either the flexoelectric or piezoelectric coupling (or both) considering the discrete phenomenological model reported by Cepič and co-workers. 25,26 This would lead to a progressive disappearance of the ferroelectric SmC* phase leaving only the modulated SmC a * phase with a short pitch.…”

Systems with Competing Interlayer Interactions and Modulations in One Direction: Finding their Structures

“…[33][34][35] It allows for all observed structures having only one temperature dependent coefficient a 0 = a(TÀT 0 ) and a single interlayer interactionã 1 influenced by the magnitude of the tilt. Up to now, intermediate phases as well as the SmC* a phase were found in chiral samples only.…”

“…The method used above allowed for the analysis of a general phase diagram of antiferroelectric liquid crystals. [33][34][35] It allows for all observed structures having only one temperature dependent coefficient a 0 = a(TÀT 0 ) and a single interlayer interactionã 1 influenced by the magnitude of the tilt. All the complexity of modulations having periods of several layers arises due to the competition of various interactions that extend to neighboring layers only.…”

Modifying optical Bragg reflections from an antiferroelectric liquid crystal with photopolymer networks

“…The phase of the order parameter ϕ i characterizes the azimuthal orientation of the tilt plane in the ith layer. Minimization of the free energy with respect to the polarization yields a relation between ξ and P [19][20][21].…”

“…(2) is the quadrupolar NN interaction energy, which represents a barrier energy [22] and accounts for the first-order antiferroelectric-ferroelectric transition. A numerical minimization [4,[19][20][21]23] of the free energy over both the phase ϕ i and the modulus θ i of the order parameter ξ i , for each i can account for the presence and stability of the intermediate phases as well as the fundamental phases.…”