Eco-Friendly Photoreversible Adhesives Derived from Coumarin-Functionalized Epoxy Soybean Oil

Theincreasing demand for high-performance thermosetting adhesivematerials is causing the continuous depletion of fossil reserves andhas led to environmental pollution by accumulating recalcitrant plasticwaste as thermosetting polymer networks are hard to recycle or reuse.This is particularly true in the case of their use as adhesives, wherenot only can a crosslinked adhesive not be recovered but multi-materialcomponents and devices can be difficult to readily separate, makingreuse and recycling complex. Bio-based feedstocks and reusable polymericmaterials can start to address these issues and reduce their environmentalimpact. In this work, we developed sustainable green adhesives whichare reversible in architecture (changing from thermoset to non-crosslinkedmaterial) by combining bio-renewable epoxidized soybean oil (ESO)and non-toxic coumarins, which can readily undergo reversible [2 +2] cycloaddition reaction under the appropriate wavelength of UV light.A series of coumarin-modified ESO materials were synthesized and chemicallycharacterized by H-1 NMR, C-13 NMR, Fourier-transforminfrared (FTIR) spectroscopy, and gel permeation chromatography. Theextent and kinetics of the polymerization of all systems were monitoredand calculated using UV-visible spectroscopy. The photoreversibilityof these synthesized polymer networks was also chemically investigatedand quantified with UV-visible spectroscopy and FTIR techniques.The physical properties of the materials were tested as well as theirability to heal defects such as scratches on the surface when stimulatedby the appropriate UV light. Systems with a more flexible coumarinarm showed a superior adhesion strength with a measurable lap shearstrength of 3.1 MPa with a minimum of 0.045 J cm(-2) dose of 365 nm UV irradiation and with an excellent reuse efficiencyof 94%.