Role of flexible spacers in achieving photoinduced phase transitions of azobenzene-based liquid-crystalline polymers at room temperature

The photoinduced solid‒liquid phase transition is a fascinating phenomenon that can be utilized for a range of applications, including debondable adhesives, photolithography, and soft actuators; however, developing polymers with this function is not trivial. In this work, we report an azobenzene (Azo)-containing polymer capable of rapid room-temperature photoliquefaction upon UV irradiation and elucidate the design principles for photoliquefying polymers that harness the photothermal effect. We prepare a series of Azo polymers by coupling diacrylate Azo with dithiol-functionalized flexible spacers of different lengths, such as ethylene glycol (EG), hexa(ethylene glycol) (HEG), and poly(ethylene glycol) (PEG). EG-Azo, with the shortest spacer, has a high melting temperature (Tm) of 78 °C due to the strong interactions among the liquid-crystalline Azo molecules. Owing to the high Tm, EG-Azo does not exhibit a photoinduced solid‒liquid phase transition, although it has the greatest photothermal effect among the polymers (temperature rise to 50 °C). The incorporation of the longer spacers effectively decreases the Tm of the Azo polymers. For example, PEG-Azo possesses a reduced Tm of 40 °C, thereby enabling photoliquefaction at room temperature after only 1 min of UV irradiation. PEG-Azo can be reversibly returned to a solid-state within 5 min after the UV light is turned off. This work shows that the length of flexible spacers in azobenzene (Azo)-based polymers is crucial for achieving room-temperature photoliquefaction (i.e., UV light-induced solid‒liquid phase transition). By adjusting the length of dithiol-functionalized flexible spacers, the melting temperature (Tm) of Azo polymers can be effectively modulated. Incorporating longer spacers decreases the Tm to a temperature achievable by the photothermal effect of Azo molecules, thus enabling photoliquefaction of Azo polymers at room temperature.