Biobased Thermosets Prepared from Rigid Isosorbide and Flexible Soybean Oil Derivatives

A rigid monomer, isosorbide-methacrylate (IM), was synthesized from isosorbide with methacrylate anhydride (MAA) via a solvent-free, ultrasonic-assisted method and then was used to copolymerize with acrylated epoxidized soybean oil (AESO) to formulate a biobased thermosetting resin (IM-AESO). The synthesis of IM was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) by tracking the changes in the functional groups of the reaction system. The AESO was further modified with MAA to replace the hydroxyl groups of AESO with methacrylate groups, generating a resin (IM-MAESO) with an improved degree of unsaturation. The chemical structure of IM and modification of AESO with MAA were characterized using 1H NMR, 13C NMR, and ATR-FTIR analyses. The miscibility of IM with AESO was predicted according to Hansen’s solubility theory and evaluated experimentally. The formulated IM-AESO and IM-MAESO resins were compared with the pure AESO and IM resins in terms of their rheological behaviors, curing kinetic characteristics, flexural properties, dynamic mechanical properties, and thermal stabilities. The results indicated that both the IM-AESO resin and the IM-MAESO resin have much lower viscosities, activation energies, and curing temperatures as well as higher polymerization rates and curing degrees than pure AESO due to the incorporation of IM as a reactive diluent. The combination of stiff IM and flexible AESO results in biobased networks with superior flexural strength, flexural modulus, flexural strain, storage modulus, glass transition temperature, and thermal stability. Furthermore, the MAA modification gives rise to the cross-linking degree and hence stiffness of the IM-MAESO resin as a result of the increase in the unsaturation degree of the MAESO.