Stuck on Repeat: Dynamic Liquid Crystal Elastomers as (Re)trainable Adhesives

Polymeric networks whose performance can be trained as a function of mechanical inputs provide additional possibilities, as they offer an opportunity to further modulate a material's properties post-synthesis and processing. Herein, disulfide-containing dynamic liquid crystal elastomer (LCE) adhesives exhibiting (re)trainable multistage adhesive character are reported. Synthetic tailoring of the mesogen lateral substituent size allows for direct tuning of the thermal properties (T_g and T_NI) of the network. These dynamic LCEs exhibit pressure-tunable adhesive behavior as a result of their inherent soft elasticity and stimuli-responsive dynamic bonds. Furthermore, higher-strength hot melt adhesive joints can be formed through the activation of the incorporated dynamic disulfide bonds at high temperatures (>150 °C). General adhesive performance can be tuned through mesogen structure, while multicycle probe-tack tests show that their force of adhesion increases with each cycle, demonstrating the trainability of the pressure-tunable adhesive response with repeated use. Thermal disruption of the LC phase resets the adhesive character, which can be subsequently reattained through mechanical training.