Azobenzene Molecular Trigger Controlling Phase Transitions of PNIPAm in Ionic Liquids and Light-Controlled Adhesiveness

Photomoieties are generally covalently attached to thermoresponsive polymers to prepare photo/thermoresponsive materials, but the bonded thermo- and photomoieties prevent us from obtaining insights into the effect of the individual response. To clarify the synergetic effect, azobenzene molecules, structurally free from poly(N-isopropylacrylamide) (PNIPAm), are utilized as photoresponsive triggers to tune upper critical solution temperature (UCST) behaviors of PNIPAm in ionic liquids (ILs). PNIPAm in an IL becomes soluble upon trans-to-cis isomerization of azobenzene under UV irradiation and aggregates again under visible light illumination. It is revealed that cis-azobenzene promotes the interaction between PNIPAm and anions but suppresses hydrogen bonding between PNIPAm chains, contributing to lower clouding temperatures under UV irradiation. Furthermore, the light-switchable macroscopic response of PNIPAm/azobenzene/IL is first exploited for an application of photoinduced reversible adhesives. This work sheds light on the intrinsic relationship between the phase transition of PNIPAm and isomerization of azobenzene in multicomponent systems.