Phase transitions of block copolymer film on homopolymer-grafted substrate

Morphological transitions such as the order-to-order transition (OOT) and the order-todisorder transition (ODT) for an asymmetric polystyrene-block- polyisoprene (PS-b-PI) were investigated in bulk and film. The PS-rich block copolymer (BCP) bulk possessed the lamella morphology (LAM), which transformed to the gyroid (GYR) and then disordering (DIS) with increasing temperature. Between the LAM and GYR, a perforated layered structure (PL) and phase mixture (Fddd+ GYR) of poorly ordered Fddd and GYR were observed. On the other hand, the film coated on a PS-grafted substrate showed a phase transition from random LAM to epitaxially oriented hexagonally modulated layer (HML) morphology. The HML transformed to GYR with further increasing temperature. The Fddd morphology observed in bulk was not observed at any stage of phase transitions in the film. The BCP film presented not only different OOT pathway but also higher OOT and ODT temperatures. In addition, the d-spacings of layers parallel to a substrate were not decreased at all with increasing temperature except when there was a structural transition, suggesting no relaxation of stretched BCP chains that are normal to the film. These results may be correlated dominantly to the interfacial energy between PS block of BCP and PS brushes on a substrate, which suppresses the compositional fluctuation of BCP in the film especially along the film normal direction, leading to anisotropic variation of d-spacings.