Molecular weight regulates reversible adhesion of azopolymers with photoswitchable glass transition temperatures

Molecular weight serves as a critical parameter governing the adhesive properties of polymer adhesives. Nevertheless, its influence on the reversible adhesion of azopolymers exhibiting photoinduced reversible solid-to-liquid transitions remains poorly understood. Here, we systematically elucidate how molecular weight modulates the reversible adhesion behavior of azopolymers. Three azopolymers (P1, P2, and P3) were synthesized by ring-opening metathesis polymerization (ROMP), with molecular weights strategically designed below, near, and above the entanglement molecular weight (Me), respectively. UV irradiation induces trans-to-cis isomerization of azobenzene moieties, while visible light triggers cis-to-trans isomerization. The azopolymers exhibit photoswitchable glass transition temperatures (Tgs) and viscous flow temperatures (Tfs). Crucially, molecular weight dictates the mechanism of photocontrolled reversible adhesion. For P1 and P2, the trans azopolymers exhibit stronger adhesion than the cis form does. However, this trend is reversed for P3, where the cis P3 surpass trans P3 in terms of adhesive performance. These findings establish molecular weight engineering as a critical strategy for developing high-performance stimuli-responsive reversible adhesives.