We have examined the spatial distributions of polymer chains in blend films of weakly segregated polystyrene-block-poly(methyl methacrylate) [P(S-b-MMA)] and deuterated polystyrene (dPS). By fine-tuning the composition (ϕ PS+dPS = 63.8 vol %) of the total PS/dPS component and annealing temperature (230 and 270 °C), P(S-b-MMA)/dPS blend films mainly form perforated layers with a parallel orientation (hereafter PLs // ). The distributions of dPS in PLs // were probed by grazing-incidence small-angle neutron scattering (GISANS) and time-of-flight neutron reflectivity (ToF-NR). GISANS and ToF-NR results offer evidence that dPS chains preferentially locate at the free surface and within the PS layers for blend films that were annealed at 230 °C. Upon annealing at 270 °C, dPS chains distribute within PS layers and perforated PMMA layers. Nevertheless, dPS chains still retain a surface preference for thin films. In contrast, such surface segregation of dPS chains is prohibited for thick films when annealed at 270 °C.

We have demonstrated the effects of different chain lengths on the phase behavior in films of binary blends of four different weight fractions in a weak segregation limit. The binary blends were composed of nearly symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, and homo-polystyrenes (PS) of three different molecular weights (hereafter hPS 2.8 , hPS 6 , and hPS 17 ). Films of two initial thicknesses were prepared by spin coating of the blends. After thermal annealing at 270 °C (1 h), a series of nanodomains, including perforated layers, double gyroid, cylinders, and spheres, were obtained in sequence as the volume fractions of PS increased in blend films. Perforated layers and double gyroid only exist in PS-b-PMMA-rich blend films, with hPS 2.8 /PS-b-PMMA blends overwhelmingly favoring perforated layers, regardless of film thickness. However, with a small amount of hPS 6 chains, PS-b-PMMA-rich blends preferentially form hexagonally perforated layers in thin films but double gyroid in thick films; at the same composition, PS-b-PMMA-rich blends with hPS 17 chains only form hexagonally perforated layers. Cylinders only formed in blend films of equal weight fractions of PS-b-PMMA and PS. The cylinders with hPS 2.8 and hPS 6 chains favor parallel orientation, but cylinders with hPS 17 PS chains favor a mixed orientation. Spheres predominantly formed in PS-rich blend films, where four types of domain ordering (hexagonal packing, body-centered cubic (BCC), face-centered cubic (FCC), and short-range order) were obtained. For blend films prepared in the PS-rich regions, the four types of ordering depend on chain length and film thickness. The PS-b-PMMA blend films with hPS 2.8 chains predominantly favor hexagonal packing of spheres for thin films but BCC packing of spheres for thick films. Under the same PS-rich region, the PS-b-PMMA blend films with hPS 6 chains predominantly favor hexagonal arrays of spheres in thin films but FCC-packed arrays of spheres in thick films. The PS-b-PMMA blend films with hPS 17 chains predominantly favor short-rangeorder spheres or a mixture of spheres and cylinders with no lattice packing. All of the above findings are explained by the correlation between the variations in chain length and alleviating packing frustration.