Reversible Fe3+-induced urushi organogels in gamma butyrolactone

A multinetwork structured urushi organogel was created using low-volatility gamma butyrolactone (GBL) solvent and Fe ³⁺ ions. The urushi organogels were fabricated through chemical crosslinking with the oxidative polymerization of urushiol as well as physical crosslinking due to the coordination bonds with Fe ³⁺ ions in the Fe ³⁺/urushiol/GBL solution. The role of Fe ³⁺ ions is to provide physical crosslinking sites via the coordination bond with the catechol functional moiety of urushiol and also to serve as the catalysts for the polymerization of urushiol. The gelation and characteristics of urushi organogels in GBL with different amounts of Fe ³⁺ ions were investigated. The formation of urushi organogel by chemical crosslinking and physical coordination crosslinking was investigated by FT-IR, Raman spectroscopy and X-ray photoelectron spectroscopy. The presence of a physical coordination bond between the Fe ³⁺ ion and the catechol moiety of urushiol provides reversible adhesive properties, which provide consistent tensile strength after reattaching the urushi organogel film. The stretchability as well as the reversibility in mechanical strength can provide the possibility for various applications of eco-friendly natural urushi. This research provides not only a method to prepare multinetwork natural urushi organogels using Fe ³⁺ ions but also the great possibility of expanding the application area of natural urushi(ol)s.