GUS Aerogel Modified Phenolic Nanocomposites: Effects of Inhomogeneous Cross-Linking Characteristics and Interfacial Phase Properties on the Mechanical Behavior

Mechanical properties of thermosetting polymer nanocomposites (PNCs) are largely dominated by the polymer-nanoparticle interfacial phase. However, not much is known about the association of interfacial interaction and interfacial phase thickness with mechanical strength and toughness of thermosetting PNCs. In this work, a model thermosetting nanocomposite (GUSNH) was constructed to investigate the effects of interfacial phase properties on the microstructure (cross-linking density and cross-linking inhomogeneity) and the resulting mechanical properties by a multiscale analysis of BDS, DMA, SAXS, and DQ 1H NMR. By adjusting the interfacial interactions and the interfacial phase thickness between GUS and the phenolic network, the cross-linking microstructure of GUSNH can be controlled flexibly. When the outer interfacial phase is dominant, the cross-linking density and cross-linking inhomogeneity of GUSNH increase as the volume fraction of the outer interfacial phase increases, resulting in higher load transfer efficiency and chain segment mobility. Herein, the flexural strength, tensile strength, elongation at break, and KIC are increased by 56.64%, 65.06%, 78.76%, and 101.02%, respectively. As the volume fraction of the outer interfacial phase decreases, the "brick and mortar"structure and the inner interfacial phase begin to dominate, resulting in a significant decrease in cross-linking density and increase in cross-linking inhomogeneity. The flexural strength and tensile strength of GUSHN are adversely affected by the restricted load transfer and facilitated chain segment. SEM and DIC analyses indicate that crack branching, pinning, deflection, and plastic deformation are the main toughening mechanism of GUSNH.