Free Surface Confinement of Diblock Copolymer Multilayers

Thin films of symmetric diblock copolymers self-assemble into multilayered structures, due to the strong interactions of one of the blocks with the substrate and/or free surface. If the film thickness, t, is not an integral or half-integral multiple of the bulk period L0, depending upon the boundary conditions, then islands or holes form on the free surface with a step height of L0 to relieve the frustration imposed by the film thickness. It is shown here, however, that a well-ordered multilayer can form prior to hole formation, with a period L that deviates significantly from L0. This structure can be stable for weeks. Topographic features eventually appear on the surface, however, allowing the period to attain its equilibrium value. This indicates that free surface confinement is a kinetic, rather than equilibrium, effect. Confinement of compressed periods is much more effective than for expanded periods, indicating that the activation barrier for the nucleation of holes is larger than that for islands. © 1995, American Chemical Society. All rights reserved.