Liquid Crystalline Vitrimers with Full or Partial Boronic-Ester Bond Exchange

In this manuscript, a new vitrimer chemistry strategy (boronic transesterification) is introduced into liquid crystal elastomers (LCEs) to allow catalyst-free bond exchange to enable processing (director alignment, remolding, and welding) in the liquid crystalline (nematic) phase. Additionally, the concept of partial vitrimer network is explored, where a percolating fraction of the network remains permanently cross-linked, hence preserving the integrity of the materials and preventing large creep. This combined strategy allows one to avoid the shortcomings of current methods of aligning LCE, especially in complex shapes. Thiol-acrylate Michael addition reaction is used to produce uniform polymer networks with controllable thermomechanical response and local plasticity. Control of the plasticity is achieved by varying the fractions of permanent and exchangeable network, where a material “sweet spot” with an optimum elastic/plastic balance is identified. Such exchangeable LCE (xLCE) allows postpolymerization processing, while also minimizing unwanted creep during actuation. Moreover, conjoining multiple materials (isotropic and liquid-crystalline) in a single covalently bonded composite structure results in a variety of smart morphing systems that adopt shapes with complex curvature. Remolding and welding xLCEs may enable the applications of these materials as mechanical actuators in reversibly folding origami, in vivo artificial muscles, and in soft robotics

Transesterification in Epoxy-Thiol Exchangeable Liquid Crystalline Elastomers

The incorporation of vitrimer bond-exchange chemistry into liquid crystalline elastomer networks produces ‘exchangeable liquid crystal elastomers’ (xLCE). These materials offer a facile method of material re-shaping and alignment post-polymerisation via the application of a mechanical stress above the temperature of activation of bond exchange. We use di-epoxy mesogenic monomers with thiol-terminated spacers and crosslinker to investigate a range of resulting xLCE. The “click” chemistry of thiols results in good control over the network topology, and low glass transition in this family of materi-als. By combining different spacers, we were able to obtain smectic and nematic phases, and adjust the liquid crystal to iso-tropic phase transition between 42 and 140 °C, while the elastic-plastic transition temperature was maintained close to 200 °C. The broad gap between these temperatures ensures that thermally actuating uniformly aligned elastomers are stable and show no residual plastic creep, making epoxy-thiol xLCE promising for a range of engineering applications.ERC H202