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

Liquid crystalline elastomers (LCEs) undergo reversible shape changes in response to stimuli, which enables a wide range of smart applications, in soft robotics, adhesive systems or biomedical medical devices. In this study, we introduce new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCEs to enable processing (director alignment, remolding, and welding). Unlike the traditional siloxane based LCEs, which were produced by other reaction schemes with irreversible bonds (e.g. hydrosilylation), here we use a much more robust reactions (thiol-acrylate/thiol-ene ‘double-click’ chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces exchangeable LCEs (xLCEs) with tunable properties, low glass transition (-30C), controllable nematic to isotropic transition (33 to 70C), and a very high vitrification temperature (up to 250C). Accordingly, this class of dynamically crosslinked xLCEs shows unprecedented thermal stability within the working temperature range (-50 to 140C), over many thermal actuation cycles without any creep. Finally, multiple xLCEs sharing the same siloxane exchangeable bonds can be welded into single continuous structures to allow for composite materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.ERC H202