Ionic Liquid-Cured Epoxy with Nanostructures of Self-Assembled Block Copolymer—Impact of Different Ionic Liquids on the Material Properties

Block copolymers (BCPs) self-assembly have garnered significant interest for several decades due to their ability to produce ordered structures with a variety of morphologies, such as spheres, cylinders, lamellas, vesicles, and other complex or hierarchical assemblies. Integrating the nanostructures of self-assembled BCPs into epoxy resin enables tuning many critical material properties, such as the rheological behavior, mechanical strength, and toughness. However, there are a limited number of affordable block copolymers that can self-assemble in traditional amine-cured epoxy resins. Ionic liquid (IL) can not only initiate the polymerization of epoxy via ring opening polymerization but also induce the self-assembly of a wide range of BCPs, including the commercially available Pluronic poly(ethylene oxide-<i>b</i>-propylene oxide-<i>b</i>-ethylene oxide) BCPs. The influence of ILs on the thermomechanical properties of the cured epoxy and the self-assembly of BCPs have been independently reported. However, there has been limited work on the co-design of the materials systems to direct the self-assembled structures, curing reactions, and final thermomechanical properties. We are interested in extending this research on understanding the critical influence of the ILs as both a curing agent and a structure-directing agent.<br/>In this work, we utilized 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA), 1-ethyl-3-methylimidazolium acetate (EMI-Ac), trihexyltetradecylphosphonium dicyanamide (THTDP-DCA), and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (THTDP-BTP) to observe their impact on the self-assembled nanostructures of the Pluronic BCPs (P-123) and the curing efficiency of bisphenol A diglycidyl ether (BADGE). To thoroughly investigate the effects, we prepared several fixed compositions of BADGE, P123, and nanoclay but with different ILs. Small-angle X-ray scattering (SAXS) analysis was employed to characterize and compare the nanostructures induced by different ILs before curing. We also investigated the effects of the ILs on the curing process through the use of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). We also observed strong structure-property relationships that showed that the BCP nanostructures influenced both the storage modulus and the flow point during rheological testing as well as the mechanical properties during dynamic mechanical analysis and tensile testing. These results improve our understanding of the role of the IL chemistry as a curing agent and a structure directing agent and how those roles influence the rheological and mechanical properties of the materials. These understandings will help us design future systems more efficiently.