Formation of a Micellar Pattern on Top of the Cylindrical Morphology in (PS)4-b-PLA Copolymer Thin Films

Abstract

Previously, our thin film study of a block copolymer (BCP) consisting of dibranched polystyrene (PS) and linear polylactide (PLA) demonstrated the thickness- dependent distribution of two different parallel cylindrical orientations, “(10)- on” and “(01)-on”. In this paper, we studied the thin film phase behavior of a more branched BCP in order to understand the effect of degree of branching on the thin film morphology. The BCP employed in this study is composed of tetrabranched PS and linear PLA blocks. The grazing incidence small angle X-ray scattering (GISAXS) analysis indicated that the thin films exclusively preferred the “(10)-on” orientation due to the better wettability of the PLA block on the SiO2 substrate. As a unique phenomenon, in contrast to the cylinder structure in the film interior, micellar domains were revealed on the top layer of the films thicker than 79 nm, as identified by atomic force microscopy. The appearance of micellar domains could be explained by the alleviation of the packing frustration of the highly branched BCP assembly at the film surface.