Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

TL;DR

This paper introduces a new siloxane-based dynamic covalent chemistry in liquid crystalline elastomers, enabling shape programming, remolding, welding, and high thermal stability for smart applications.

Contribution

It develops a robust thiol-acrylate/thiol-ene chemistry for highly uniform, dynamically crosslinked LCEs with tunable properties and enhanced thermal stability, unlike traditional irreversible siloxane LCEs.

Findings

Achieved low glass transition temperature (-30°C) and controllable nematic-isotropic transition (32-75°C).

Demonstrated high thermal stability over many actuation cycles without creep.

Enabled welding of multiple materials with reversible phase transformations.

Abstract

Liquid crystalline elastomers (LCE) undergo reversible shape changes in response to stimuli, which may enable a wide range of smart applications, such as soft robot, adhesive systems or implantable medical devices. Here we introduce new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCE to enable processing (director alignment, remolding, and welding). Unlike, the traditional siloxane-based LCE, which are produced by a other reaction schemes with irreversible bonds (e.g. hydrosilylation), here we use a much more robust reaction (thiol-acrylate/thiol-ene 'double-click' chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces LCEs with tunable properties, low glass transition (-30C), controllable nematic to isotropic transition (32 to 75C), and a very high vitrification temperature (250C). Accordingly, this class of dynamically crosslinked LCE shows unprecedented thermal stability within the working temperature range (-50 to 140C), over many thermal actuation cycles without any creep. Finally, multiple materials sharing the same siloxane exchangeable bonds can be welded into single structures to allow for materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.