Poly(2-alkyl/aryl-2-oxazoline)-Imidazole\nComplexes\nas Thermal Latent Curing Agents for Epoxy Resins

One-component epoxy resins (OCERs) with improved stability\n(shelf\nlife) and controlled curing temperatures were prepared using epoxy\nresins and polyoxazoline-imidazole (POZ-Im) based thermal latent curing\nagents (TLCs). POZ homopolymers with molar masses of 1000, 2000, and\n5000 g/mol were obtained via cationic ring-opening polymerization\n(CROP) of 2-ethyl-2-oxazoline, 2-propyl-2-oxazoline, 2-pentyl-2-oxazoline,\nand 2-phenyl-2-oxazoline. TLCs were prepared by physically entrapping\nimidazole, the curing agent, in the POZ matrix at the homopolymer/Im\n(HP/Im) ratios of 1:1 and 5:1 and characterized by FTIR and TGA. TLCs\nwere then mixed with bisphenol A diglycidyl ether (DGEBA) to obtain\nOCERs with Im concentrations of 1, 3, and 5 wt %. Dynamic DSC tests\nwere performed to determine the effect of the pendant group and molar\nmass of POZ, the POZ/Im ratio, and Im concentration on the curing\nbehavior of the OCERs, whereas isothermal DSC tests were carried out\nto examine their thermal stability and optimal curing temperatures.\nOptical microscopy was performed to study the compatibility of the\nTLCs with DGEBA. This study showed that the dispersion quality of\nTLCs is highly associated with the compatibility of POZs and DGEBA,\nwhich affected the release of Im, thus left limit temperatures of\ncuring. In addition, higher left limit temperatures were obtained\nwhen the POZ/Im ratio increased. Isothermal DSC results conformed\nto the improved stability and better thermal latency of the samples\nwith a POZ/Im ratio of 5:1. Moreover, the higher left limit temperatures\nwere obtained with the lowest molar mass of POZ due to the better\ninteraction between the −OH end group of POZ and Im. The shelf\nlife of PEOZ 1K-Im 5:1 1% OCER was predicted at −20, 0, and\n20 °C, with an estimated 15.3 days at 20 °C using isothermal\nDSC and rheology at 50, 60, and 70 °C. Overall, this research\ncontributes to the development of OCERs by introducing POZ-Im complexes\nas novel TLCs. The findings shed light on the importance of compatibility\nin achieving optimal dispersion and release of Im and the role of\nthe POZ/Im ratio and POZ molar mass in controlling left limit temperatures,\nultimately influencing the curing behavior of OCERs.