A composite structure includes a resin, a plurality of polymer nanoparticles in the resin to form a resin mixture, and a plurality of reinforcing fibers embedded within the resin mixture. At least some of the polymer nanoparticles are either fully soluble in the resin during curing or solidifying of the resin mixture to form a cured composite structure, or semi-soluble in the resin during curing or solidifying of the resin mixture to form the cured composite structure.