Fabrication of a mechanically tough and strong SiO2@EPOSS modified novolac phenolic network by simultaneously improving its crosslinking inhomogeneity and crosslinking density

Abstract Simultaneously improving mechanical strength and toughness of thermosetting polymer nanocomposites remains a great challenge even though massive efforts have been made by controlling the aggregation and interface interaction of nanoparticles. In fact, crosslinking inhomogeneity and crosslinking density of thermosetting polymer nanocomposites can also profoundly affect the competing mechanical properties. Due to their conflicting influence on the toughness and strength of thermosetting polymer nanocomposites, the difficulty lies in how to coordinate the influence effectively. In this work, a mechanically tough and strong novolac phenolic nanocomposites (ESNH) were successfully constructed by introducing a reactive core-shell POSS with nanosilica as core and epoxy-polyhedral oligomeric silsesquioxane as shell (SiO2@EPOSS) (ES). Via controlling the quantity and the size of self-polymerized ES phase domain, the crosslinking inhomogeneity and crosslinking density of ESNH can be diversely tuned. Especially for 3%ESNH-3%, remarkably enhanced tensile properties and fracture toughness were realized. The self-polymerized ES phase domain with relatively small quantity and size was homogeneously grafted in phenolic networks, which could pin microstructural interfaces and junct nodular domains of phenolic networks, synergistically improving the crosslinking inhomogeneity and crosslinking density. Therefore, the moderately self-polymerized ES phase domain in the phenolic networks was remarkably contributed to coordinating the performance between the fracture toughness and tensile property. This study provides a novel and efficient envision toward simultaneously improving the toughness and strength of thermosetting polymer nanocomposites via tuning their curing behavior and the corresponding microstructure.