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The dependence of the orientation of lamellar microdomains in thin films of symmetric diblock copolymers of polystyrene-b-polymethyl methacrylate was quantitatively investigated by small angle neutron scattering, transmission electron microscopy (TEM), and reflectance optical microscopy. A mixed orientation of the lamellar microdomains was found when the difference in the interfacial energies between each block and the substrate was not strong enough to force the orientation of the lamellae parallel to the substrate throughout the film. A limiting film thickness was found, below which a parallel alignment of the lamellar microdomains was seen throughout the film. This film thickness was found to depend on the strength of the interfacial interactions of each block with the substrate. TEM images indicate that the surface field suppresses the fluctuations in the microdomain, reduces defects, and consequently …