Provided is a composition comprising a rubber and a polymer nano-particle comprising a poly(mono-vinyl aromatic) core and a poly(mono-vinyl aromatic-conjugated diene) surface layer, wherein the core of the polymeric nano-particle has a glass transition temperature (Tg) of between about 150.degree. C. and about 600.degree. C. Also provided is a composition comprising at least two (mono-vinyl aromatic-conjugated diene) copolymer rubbers and a polymeric nano-particle comprising a poly(mono-vinyl aromatic) core and a poly(mono-vinyl aromatic-conjugated diene) surface layer; wherein the core of the polymeric nano-particle has a glass transition temperature (Tg) of between about 150.degree. C. and about 600.degree. C., and the poly(mono-vinyl aromatic-conjugated diene) surface layer of the polymeric nano-particle comprises a mono-vinyl aromatic content that is between about 50 percent and about 150 percent that of the mono-vinyl content of one of the (mono-vinyl aromatic-conjugated diene) copolymer rubbers.