Conventional bisphenol A (BPA) epoxy resins suffer from intrinsic brittleness and inferior toughness after being transformed into a highly cross-linked network by a curative, and almost all epoxy curatives are from nonrenewable petrochemicals. Here, a series of siloxane-functionalized eugenol-based novolacs is synthesized and applied as the curative and toughener for a standard BPA epoxy (E54). Eugenol and 1,1,3,3-tetramethyldisiloane are connected through hydrosilylation to yield a bisphenol (2SiEU), followed by acid-catalyzed copolycondensation with paraformaldehyde (F) in acetic acid according to the varied molar ratios of F to 2SiEU (X = 0.5, 0.6, 0.7, or 0.8) to afford the novolacs (2SiEUPF-X). As X increases, the aromatic hydrogens (Ph-H) of 2SiEU substituted by carbons (Ph-C) increases with increased viscosities and molecular weights of 2SiEUPF-X. Both the hydroxyl and the methoxyl on 2SiEU moieties could activate the ortho positions on the aromatic rings to facilitate carbocation attack. 2SiEUPF-X has good thermal stability (T-d5% > 330 degrees C) and fluidity at 100 degrees C (<2 Pa<middle dot>s). 2SiEUPF-X is able to cross-link E54 well, and T-g of the resultant 2SiEUPF-X/E54 thermosets increases from 60.5 to 73.7 degrees C as X increases. The thermosets exhibit superior thermal stability (T-d5% > 430 degrees C, N-2), moderate tensile (47-49 MPa) and flexural (78-90 MPa) strengths, good stiffness at room temperature, and low moisture absorption in boiling water (<2%). Furthermore, the thermosets display significantly enhanced fracture toughness (K-IC), fracture energy (G(IC)), and V-notched Charpy impact strength of 2.9-3.2 MPa<middle dot>m(1/2), 3.4-4.9 kJ<middle dot>m(-2), and 5.6-6.5 kJ<middle dot>m(-2), respectively. Altogether, 2SiEUPF-X is readily accessible and endows the cured BPA epoxy thermosets with superb toughness and thermal stability, playing a role in advancing biobased epoxy chemistry and technology.
