Self-healing recyclable polymers based on azobenzenes with thermoset like behaviour

Abstract

Self-healing materials are considered intelligent materials capable of recovering their original shapes fromdamaged or distorted forms after exposure to specific stimuli, such as chemicals, heat, moisture, or light. Theutilization of thermoreversible crosslinks in polymer networks has garnered increasing interest as a method tocreate such materials. In these polymers, damage can be repaired simply by heating them above a gel-liquidtransition temperature. Upon cooling, the material regains its previous cross-linked properties. However,finding alternative external stimuli other than heat presents an intriguing challenge. Among the various possibilities, light is the preferred choice due to its potential for remote and spatially-controlled activation. Additionally, the process can be easily paused and resumed by turning the excitation light on and off.Furthermore, the photo-switching of the glass transition temperature (Tg) in azobenzene-containing polymersinduces reversible transitions from a solid to a liquid state. These reversible trans-to-cis transitions are triggeredby UV and visible light irradiation. This phenomenon offers a novel approach to designing healable polymers andallows for precise control over the mechanical properties of polymers with spatio-temporal resolution.The main objective of this study is to investigate the feasibility of using light as a self-healing trigger in arecyclable polymer based on physically cross-linked epoxy/amine formulations. To achieve this goal, the matrixwas modified with 4-phenylazophenol (PAP), which forms homogeneous blends with the polymer throughhydrogen bond interactions. Films were prepared and characterized using various experimental techniques.Polymerization kinetics of samples with and without PAP was determined using FTIR, which was also employedfor characterizing the final materials. Tg of the resulting products was measured using DSC, revealing a decreasein its value for PAP-modified samples. The transition from a solid to a liquid state during UV light irradiation wasobserved using rheometric analysis. The reversible photoisomerization behaviour of epoxy/amine/PAP wasinvestigated using UV–visible spectroscopy in films prepared by spin-coating THF solutions.The ability of the material to self-heal at room temperature using light as a remote source was confirmedthrough TOM (Transmission Optical Microscopy) observations and quantified by profilometric measurements. Inaddition, surface mechanical properties were evaluated in order to determine the effect of the incorporation ofthe PAP, and the ability of the repaired material to recover its properties. Promising findings indicate that thisrecyclable polymer holds significant potential for multiple applications where its self-healing ability will be ofgreat importance.