Polymer composites and polymer nanocomposites are designed and manufactured by choosing surface-modified fillers having a solid surface energy matching the solid surface energy of the polymer, wherein the advancing contact angle of the polymer on the surface-modified filler is less than about 5 degrees. The fillers are surface-modified by adsorption of amorphous polymeric surfactants, consisting of either mono-modal, bimodal, or multi-modal size distribution. The surface-modified fillers of this invention display a measurable yield stress and a viscosity of about 30.000 Pa·s or less at room temperature, and comprise one or more materials selected from the group consisting of minerals, plant material, animal material, carbon fiber, graphite, amorphous carbon, carbon nanotubes, and glass fiber. This invention also provides a method for manufacturing a polymer nanocomposite which includes exfoliating an amorphous organoclay in a crystalline or semi-crystalline polymer by melt compounding the constituents in the absence of added solvents, activators, edge-modifiers, compatibilizers, or hydrotropes.