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The availability of a wide range of nano-sized modifiers and particles and their increasing ease of being incorporated into polymer matrices has seen an evolution of conventional composites to a new class of polymer nanocomposites.In many applications, such polymer nanocomposites should possess mechanical (and physical) properties superior to their corresponding microcomposites.Three mechanical aspects are reported herein.First, the toughening mechanics and mechanisms of epoxy thermosets with (a) block ionomer of sulfonated SEBS yielding phaseseparated inclusions with length scales from nanometers to tens of microns, (b) 100 nm nanorubber and (c) 20 nm nano-silica particles are conducted to examine if there is a "nano" effect on toughness [1].Second, fatigue crack growth studies are performed on epoxies filled with different amount of silica, rubber and silica/rubber nanoparticles to ascertain their effects on and fatigue mechanisms associated with the fatigue thresholds and crack growth rates (CGRs) [2].Third, the respective influence of nano-fillers and hierarchical carbon fibers on delamination toughness of composite laminates is evaluated.Recent examples using silica and rubber nanoparticles as well as flame synthesized CNTs@carbon fibers [3] in epoxy/carbon fiber composites are discussed.