Ordering in blends of diblock copolymers

The morphology of binary blends of poly(styrene)-poly(2-vinylpyridine) (PS-PVP) diblock copolymers of the type (A-B)_α + (A-B)_β was examined in a thin-film geometry using neutron reflection (NR), transmission electron microscopy (TEM), and self-consistent field (SCF) calculations. Blends of symmetric diblock copolymers (f_A,α ≈ f_A,β ≈ 0.5) revealed a localization of the lower molecular weight diblock copolymer and stretching of the higher molecular weight diblock copolymer near the AB interface that is qualitatively consistent with the data of a prior study by Mayes and co-workers. Blends of asymmetric diblock copolymers of the type f_A,α < 0.5 and f_A,β > 0.5 having similar molecular weights (N_α ≈ N_β) and with ∼50% overall composition of a chemically similar block in the blend, (f_A) ≈ 0.5, revealed molecular-level mixing of the two components and the formation of a single-phase morphology. A lamellar morphology was formed even when the individual components exhibit a nonlamellar morphology in the pure melt state. In general, the morphological characteristics are well anticipated by SCF calculations. After accounting for fluctuations at the interface, spatial distributions of the individual components in the blend obtained by SCF calculations were in close agreement with the NR data. For diblock copolymers near the order-disorder transition, a x value higher than the bulk was necessary to fit the experimental data, suggesting an enhancement of ordering in the thin films near the transition due to the presence of the surfaces.