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

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.