Observation of a Reentrant Twist Grain Boundary Phase

Abstract

We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- ${\mathrm{TGB}}_{A}$. Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso-- ${N}^{*}$-- ${\mathrm{TGB}}_{A}$--Sm- $A$--Re-- ${\mathrm{TGB}}_{A}$-- ${\mathrm{TGB}}_{{C}^{*}}$. Here ${N}^{*}$ and Sm- $A$ stand for the chiral nematic and smectic- $A$ phases, ${\mathrm{TGB}}_{A}$ is the twist grain boundary phase with smectic- $A$ blocks, and ${\mathrm{TGB}}_{{C}^{*}}$ that with smectic- ${C}^{*}$ blocks and exhibiting features of both the smectic- ${C}^{*}$ and TGB phases. The reentrance of the ${\mathrm{TGB}}_{A}$ phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data.