Tunable mechanical properties and photoinduced reversible solid-to-liquid transitions in high-molecular-weight azopolymers via alkyl tail engineering

Photoresponsive azobenzene-containing polymers (azopolymers) exhibiting photoinduced reversible solid-to-liquid transitions show promise for healable materials, reversible adhesives, and nanoimprint lithography. However, balancing robust mechanical properties with efficient photoreponsiveness of azopolymers remains challenging. This work demonstrates a molecular design strategy that combines ring-opening metathesis polymerization (ROMP) to achieve high-molecular-weight entangled azopolymers, with tail-group engineering to modulate mechanical properties and photoinduced reversible solid-to-liquid transitions. Three azopolymers (P1, P2, P3) featuring varied alkyl tail lengths were synthesized via ROMP, resulting in narrow molecular weight distributions and high degrees of polymerization. All three azopolymers exhibit reversible cis - trans photoisomerization. Solid trans azopolymers transform into liquid cis azopolymers through UV-induced trans -to- cis isomerization. Conversely, liquid cis azopolymers revert to solid trans azopolymers via visible light-induced cis -to- trans isomerization. Longer alkyl tails reduced the viscosity of cis azopolymers, enhancing photoinduced solid-to-liquid transition efficiency. Mechanical properties critically depended on azobenzene group aggregation states and polyethylene-like glass transitions governed by alkyl tail length. Consequently, tail-length design enables tuning of tensile strength, elongation, and photoinduced healing capabilities in freestanding films of azopolymers. This study establishes design principles for achieving robust mechanical properties and photoinduced reversible solid-to-liquid transitions simultaneously in azopolymers.