Unlocking Entropic Elasticity of Nematic Elastomers Through Light and Dynamic Adhesion

Nematic liquid crystal elastomers (LCEs) generally show soft elasticity, masking entropic elasticity inherent in crosslinked networks. They macroscopically deform with little stress increase under straining because of the soft shear mode attained by nematic director rotations. Moreover, the nematic interaction in main-chain LCEs can arrest the ergodic response to cycling strain. It manifests hysteresis, slow relaxation, and increased viscosity, which critically affect their mechanical applications such as actuation and adhesion. Here, it is shown that entropic elasticity fully recovers after photo-isomerization of azobenzene units incorporated in a nematic LCE. With the light-induced bent cis-isomers at room temperature, the degree of soft elasticity, viscosity, and adhesion are tangibly lowered. Mechanical responses rationalize that cis isomers unlock the frozen configurational degree of freedom by reducing nematic order in the same manner observed upon temperature increase. The present results not only unveil how the nematic interaction suppresses entropic elasticity leading to the soft state, but also offer a guide to tuning viscoelasticity toward dynamic mechanical applications.