Microphase formation of an asymmetric diblock copolymer (with f≈0 25) confined in a thin film was investigated by using strong segregation theory.The stability of microphases, such as surface-parallel lamella, surface-parallel and surface-perpendicular cylinders, was determined.It was found that for neutral substrates the surface-perpendicular cylinders are always stable as the film thickness is varied.However, for substrates that prefer the minor component, the stability of these microphases is crucially influenced by the composition of the diblock and the surface interactions between the diblock and the two substrates.Therefore, varying the thickness of the thin film, surface-parallel lamella, including both odd and even layers, surface-parallel and surface-perpendicular cylinders could alternatively exist.

Key words: diblock copolymer, thin film, microphase, lamella, cylinder, strong segregation theory