Distinct differences in the nanoscale behaviors of the twist-bend liquid crystal phase of a flexible linear trimer and homologous dimer

TL;DR

This study compares the nanoscale behaviors of twist-bend liquid crystal phases in a flexible dimer and trimer, revealing distinct temperature dependencies and structural arrangements through experimental characterization and geometric modeling.

Contribution

It provides the first detailed comparison of twist-bend phases in dimers and trimers, highlighting their different temperature-dependent pitches and proposing a geometric model for their molecular arrangements.

Findings

Dimer exhibits temperature-dependent twist-bend pitch (100-170 Å).

Trimer has a temperature-independent pitch of 66 Å.

Geometric model explains molecular arrangements in the twist-bend phase.

Abstract

We synthesized the liquid crystal dimer and trimer members of a series of flexible linear oligomers and characterized their microscopic and nanoscopic properties using resonant soft x-ray scattering and a number of other experimental techniques. On the microscopic scale, the twist-bend phases of the dimer and trimer appear essentially identical. However, while the liquid crystal dimer exhibits a temperature-dependent variation of its twist-bend helical pitch varying from 100 - 170 {\AA} on heating, the trimer exhibits an essentially temperature-independent pitch of 66 {\AA}, significantly shorter than those reported for other twist-bend forming materials in the literature. We attribute this to a specific combination of intrinsic conformational bend of the trimer molecules and a sterically favorable intercalation of the trimers over a commensurate fraction (two-thirds) of the molecular length. We develop a geometric model of the twist-bend phase for these materials with the molecules arranging into helical chain structures, and we fully determine their respective geometric parameters.