Competing Effects of Network Architecture and Composition on Polydomain Liquid Crystal Elastomers

TL;DR

This paper studies how the structure and composition of liquid crystal elastomers affect their phase transition temperatures.

Contribution

The study reveals competing effects of cross-linking and mesogenic dilution on phase transition behavior in LCEs.

Findings

Cross-linking enhances mesogen-backbone coupling, increasing the N–I transition temperature.

High cross-linker concentrations cause deviations from linear T_NI changes due to mesogenic dilution.

Maximum latent heat per gram of mesogen is observed during the N–I transition.

Abstract

Main-chain liquid crystal elastomers (LCEs) are synthesized
to
investigate the interplay of the composition and network structure
on LCE nematic-to-isotropic (N–I) transitions. We focus on
networks synthesized from liquid crystalline oligomers reacted with
tri- or tetrafunctional nonmesogenic cross-linker molecules. We find
that coupling between mesogens and the polymer backbone increases
with the degree of cross-linking. However, this enhanced coupling
competes with mesogenic dilution arising from the cross-linker molecules
to determine the N–I transition temperature (T
NI). When cross-linker molecules are dilute, the degree
of cross-linking directly correlates to the change in T
NI from the oligomer to LCE (ΔT
NI) through mesogen–backbone coupling. In this
regime, ΔT
NI ranges from 2.9 to
12.2 °C and 2.9–13.9 °C for tri- and tetrafunctional
cross-linkers, respectively. At high cross-linker concentrations,
deviations from this linear relationship appear. Further, the fractional
mesogen content within an oligomer chain induces molecular weight-dependent
mesogenic dilution effects arising from the flexible spacer molecules.
Analysis of the N–I transition peak reveals a maximum latent
heat per gram of mesogen (ΔH
NI,mes) for this system.

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