Films of a symmetric A-B diblock copolymer and A-coated An nanoparticles cast from a selective solvent for A develop a lamellar surface layer. The surface layers have a low volume fraction of nanoparticles in the A lamellae atop a substrate layer of B cylinders/spheres in an A matrix containing a larger volume fraction of nanoparticles (see Figure). This arrangement thus induces an order-order transition between block-copolymer phases as their concentration is increased.