Accelerating effect of metal ionic liquids for epoxy-anhydride copolymerization

One of the main drawbacks of high-performance epoxy-anhydride thermosets is slow cross-link kinetics requiring high temperature and long curing cycle. Herein, the accelerating effect of imidazolium metal-based ionic liquids (MILs) bearing (FeCl 4 ) - , (ZnCl 4 ) 2- , and (CoCl 4 ) 2- anions on epoxy-anhydride copolymerization was investigated. It was observed that MILs accelerated bisphenol diglycidyl ether (DGEBA) - methylhexahydrophthalic anhydride (MHHPA) cross-linking, better than the reference catalysts (1-methylimidazole and 1-butyl-3-methylimidazolium chloride), especially at low temperatures through their ability to activate a rapid anhydride ring opening and formation of carboxyl groups, which initiates polyesterification. A detailed investigation of the polymerization mechanism revealed the formation of alternating epoxy-anhydride copolymers although several MILs-induced initiation mechanisms were detected. Despite the multiple-initiation consisting of imidazole, counter anion, and polyesterification pathways, the cross-linking kinetics was successfully fitted up to vitrification by the Kamal-Sourour model. Finally, MILs-induced cross-linking leads to homogeneous network build-up enabling to produce thermosetting materials with an increased cross-link density, a glass transition temperature above 150 degrees C, and excellent thermal stability.