Structure of Asymmetric Diblock Copolymers in Thin Films

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Abstract

The structure of asymmetric poly(styrene-b-2-vinylpyridine) (PS-PVP) diblock copolymers allowed to order in a thin film is observed by a combination of secondary ion mass spectrometry and scanning force microscopy. The surface/interface-induced ordering persists over a surprisingly long range (more than 1 μm). The 2-D structure in the layer parallel to a surface is mainly a distorted hexagonal structure similar to that of the (110) plane of a body-centered cubic structure. In contrast to the long-range order in the direction perpendicular to the surfaces, the in-plane structure shows only short-range order. The surface has a very strong effect on the structure only in the direction perpendicular to it. The development of the layered structure induced by the surfaces was also investigated as a function of temperature. When the molecular weight of PS-PVP is low but the copolymer still forms the spherical structure, the layered structure decays more rapidly with distance from the free surface as the temperature is increased. We find that the layered structure changes from an order that is nearly liquid-like near the surface to a long-range-ordered structure continuously as the temperature is decreased, which suggests that the ultimate range of order of the layered structure is governed by thermodynamics. When the molecular weight of PS-PVP is high, the propagation of the layered structure is kinetically limited due to slow diffusion. Similar to symmetric diblock copolymers with a lamellar structure, asymmetric diblock copolymers with a layered spherical domain structure form islands or holes at the free surface when the film is thin and the film thickness does not correspond to an integral number of layers of spheres. As the film becomes thick, the shape of the edge of the island or hole structure changes from a step function to a tanh function, and finally these islands or holes disappear.

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