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We have studied the ordering behavior of a (diblock copolymer, poly(styrene-d8)-poly(2-vinylpyridine) (dPS-PVP), in a confined geometry by neutron reflectivity, transmission electron microscopy, and atomic force microscopy. The diblock copolymer was confined between a silicon substrate (Si) on one side and a glassy polymer, poly(2-methylvinylcyclohexane) (P2MVCH), on the other side. In such a geometry, incompatibility between the natural domain period of the diblock copolymer (D*) and the film thickness (L) creates frustration that can be varied by controlling the copolymer film thickness. As the degree of frustration is increased (i.e., film thickness is decreased), the domain periods of the lamellae become progressively distorted from D*, and the lamellae orient with dPS/PVP interfaces parallel to the confining surfaces. The dPS block wets the P2MVCH confining wall and the PVP block wets the Si substrate. There is a limit, however, to the extent of distortion of the lamellar domain period; a further increase in frustration results in a sharp transition to a complex layered morphology that has a heterogeneous in-plane structure adjacent to the P2MVCH confining wall. In this morphology, both the dPS and PVP are located near the P2MVCH confining wall and only PVP is located at the Si confining wall. The sharp transition in the morphology is interpreted in the context of competing surface and bulk interactions. By removal of one of the confining walls, the frustration is relieved and a lamellar structure parallel to the surfaces is recovered with a domain period of D*.

The morphology of binary blends of poly(styrene)-poly(2-vinylpyridine) (PS-PVP) diblock copolymers of the type (A-B)R + (A-B) 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 (fA,R ≈ fA, ≈ 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 fA,R < 0.5 and fA, > 0.5 having similar molecular weights (NR ≈ N) and with ∼50% overall composition of a chemically similar block in the blend, 〈fA〉 ≈ 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 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.

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