Azobenzene-Based (Meth)acrylates: Controlled Radical Polymerization, Photoresponsive Solid–Liquid Phase Transition Behavior, and Application to Reworkable Adhesives

Atom transfer radical polymerization was employed to induce the living polymerization of an azobenzene-containing monomer, 10-[4-(4-hexylphenylazo)phenoxy]decyl acrylate, with the resulting polyacrylate and the corresponding polymethacrylate undergoing a reversible solid–liquid phase transition under isothermal conditions caused by the photoinduced change of the azobenzene moiety shape. Irradiation-induced property changes were investigated by nuclear magnetic resonance, ultraviolet–visible (UV–vis) absorption spectroscopy, and dynamic viscoelasticity measurements, with focus on the effects of the main chain chemical structure and molecular weight. Azobenzene moiety photoisomerization and the concomitant phase transition were faster for the polyacrylate than for the polymethacrylate, which indicated the strong influence of the main chain structure. Finally, phototuning of the adhesion strength using azopolymer-bonded glass substrates was studied by single lap shear tests, with the maximum adhesion strength of >3 MPa being comparable to that of commercial hot-melt adhesives. Irradiation with UV light for only 15 s lowered the adhesion strength to <0.2 MPa, allowing easy debonding upon the application of a small force.