From Epoxy Prepolymers to Tunable Epoxy–Ionic Liquid Networks: Mechanistic Investigation and Thermo-Mechanical Properties

The mechanism involving the curing process of bisphenol A diglycidyl ether (DGEBA) used as an epoxy prepolymer (EP) by the phosphonium ionic liquid, denoted trihexyl (tetradecyl) phosphonium decanoate (P 66614 [dec]), was investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The main mechanisms of the opening of epoxy groups inducing the polymerization were modeled through the reaction between a model substrate, i.e., phenyl glycidyl ether (PGE), and the ionic liquid (IL) by using “in situ” 13 C and 31 P NMR spectroscopy. DSC analysis revealed the latency characteristics of this system. FTIR and size exclusion chromatography analyses confirmed the chain-growth polymerization by an anionic route with the formation of an ether linkage. The combined FTIR and NMR techniques for the EP/P 66614 [dec] and PGE/P 66614 [dec] systems suggest a dual polymerization mechanism involving the initiation with the carboxylate counter anion and also the Wittig ylide generated during the heating process of the IL. The epoxy network cured with this system presents higher glass-transition temperatures (134–155 °C) by using a small amount of IL. The liquid nature of the IL and its low vapor pressure allow the development of epoxy networks with environmental safety and easy processing, making these systems very promising for employing as coatings, adhesives, and other applications.