Phase diagram of a model melt of bent--core molecules

ABSTRACT

The Landau theory of phase transitions with single-order parameter (second rank alignment tensor) has been applied to study the formation of modulated nematic phases in a melt of the symmetric V-shaped molecules. The arms of V-shaped molecule are modelled by rod-like segments composed of monomer units. Molecular architecture is determined by an external angle $\alpha$ between arms. The coefficients of the Landau–de Gennes free energy of melt have been found from microscopic model of V shaped particle. The phase diagram of the system contains regions of stability of the isotropic ($\mathrm{I}$), uniform nematic ${\mathrm{N}}_{\mathrm{U}}$, and modulated nematic phases (${\mathrm{N}}_{\mathrm{T}\mathrm{B}}$ and ${\mathrm{N}}_0$). Principal director (the eigenvector of the alignment tensor, which corresponds to the eigenvalue with the largest absolute value) in the ${\mathrm{N}}_{\mathrm{T}\mathrm{B}}$ phase moves along the helix and forms a fixed acute angle with the helix axis. For the ${\mathrm{N}}_0$ modulated phase the principal director is orthogonal to the helix axis while one of the two remaining directors is parallel to helix axis.

Graphical Abstract

KEYWORDS:

• Nematic twist bend phase
• bent–core molecules
• liquid crystal
• Landau theory of phase transitions

Disclosure statement

No potential conflict of interest was reported by the author(s).