Robustly Curing Epoxy Novolacs by Linear and Cyclic Siloxane-Functionalized Eugenol: Impact of Topologies of Cross-Linkers on Properties of Thermosets

Eugenol-based epoxy resins have been extensively studied, but eugenol-derived curing agents have been less addressed. A curing agent is decisive to fundamental curing mechanisms and greatly affects intrinsic properties of finalized epoxy thermosets. Herein, we synthesize linear (2SiEU) and cyclic (4SiEU) siloxane-functionalized eugenol hardeners to cross-link multifunctional epoxy novolacs of bisphenol A (BNE200) and o-cresol (CNE202). How the linear and cyclic siloxane topologies of 2SiEU and 4SiEU affect the thermal, mechanical, fracture mechanical, and thermomechanical properties, water absorption, and wettability of resultant thermosets is highlighted. 4SiEU endows 4SiEU/BNE200 and 4SiEU/CNE202 thermosets with an elevated glass transition temperature from 52.9 and 53.9 °C to 97.3 and 89.4 °C (DSC), an increased residue (750 °C/N2) from 18.9 and 20.3% to 32.6 and 43.5%, an enhanced tensile strength from 48.1 and 46.8 MPa to 61.8 and 63.9 MPa, and an impact strength from 1.56 and 1.57 kJ·m–2 to 2.70 and 2.22 kJ·m–2, whereas the critical stress intensity factor (KIC) decreases from 1.56 and 1.67 MPa·m1/2 to 1.26 and 1.29 MPa·m1/2, respectively, compared to 2SiEU/BNE200 and 2SiEU/CNE202. All of the thermosets display the high tensile elongation at break, flexural strength, and flexural modulus above 7.3%, 72 MPa, and 2.1 GPa, respectively, with low water absorption (<∼1.8%). In general, the 4SiEU-incorporated cyclic siloxane moieties endow the superior overall performances to the obtained thermosets compared to 2SiEU because they function as additional cross-linkers and increase the content of thermally stable Si–O bonds, and they cause an additional secondary relaxation at a subroom temperature range. Nevertheless, 2SiEU delivers better fracture mechanical properties, which are likely more sensitive to the increased plastic deformation of the resin matrix. Altogether, 2SiEU and 4SiEU are capable of effectively cross-linking the standard epoxy novolacs exhibiting multiple properties of interest, expressing a good promise as robust hardeners for biobased epoxy thermosets.