pubs.acs.org/Macromolecules Published on Web 01/25/2010 r 2010 American Chemical Society 1958 Macromolecules 2010, 43, 1958–1963 DOI: 10.1021/ma9022229 Phase Transitions of Block Copolymer Film on Homopolymer-Grafted Substrate Hyungju Ahn, † Changhak Shin, Byeongdu Lee,* ,‡ and Du Yeol Ryu* ,† † Department of Chemical and Biomolecular Engineering, Yonsei University, 134 Sinchon-dong, Seodaemun-gu, Seoul 120-749, Korea and ‡ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439 Received October 12, 2009; Revised Manuscript Received December 15, 2009 ABSTRACT: Morphological transitions such as the order-to-order transition (OOT) and the order-to- disorder 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 þ 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. Introduction Block copolymer (BCP) has attracted great attention due to the growing interest for advanced and efficient technologies in various applications such as microelectronic engineering and bioengineering, 1-13 while there are still competitive methods in the fields of electron beam lithography, X-ray lithography, and imprinting method. BCP consisting of chemically different two polymers linked covalently allows us to fabricate various nano- scopic patterns and scaffolds, since it can self-assemble into the ordered arrays such as lamellar (LAM), cylindrical (HEX), and spherical (BCC) morphologies, where the morphologies depend on the volume fraction of one component (f), the degree of polymeri- zation (N), and the Flory-Huggins interaction parameter (χ). 14-23 The complex morphologies, like the gyroid (GYR) 20-27 and hexagonally perforated layers (HPL), 27,28 can be observed in the melt state at a narrow range of f and the product of χN. In addition to those, noncubic network phase, designated as Fddd, was recently reported by Takenaka et al., which motivates efforts to find new morphology in the BCP melts. 29-31 A polystyrene-block-polyisoprene copolymer (PS-b-PI) has been used as a model system to figure out the underlying physics of BCP melt 22-24,29-31 and film 32-34 involving the morphological transition so-called the order-to-order transition (OOT) because it possesses a strong temperature dependence of χ, demonstrating the discrete phase transitions with thermal energy. At a specific composition region from 0.6 to 0.7 of PI volume fraction, it especially presents a sequence of OOT with temperature, in which most results have been well agreed with theoretical considerations by the mean-field approach. 17,18,35 For the BCP films, the interfacial interactions at film/substrate and film/air interfaces significantly influence not only the micro- domain orientation with respect to a substrate but also the phase behavior. Previously we reported that the order-to-disorder transition (ODT) temperature (T ODT ) in BCP films is thickness- and substrate-dependent. For the lamella-forming PS-b-PI that were coated on PS-grafted substrates and bare Si wafers, where the former and latter substrates preferentially interact respec- tively with the PS and PI blocks of the BCP, T ODT ’s in the films thinner than 12L 0 (L 0 : lamellar period) were significantly higher than that in bulk regardless of substrate type, and they gradually decreased as the film thickness increased and reached plateaus at the film thickness 12L 0 . While the T ODT at plateau in the film on a PS grafted substrate was still higher than that in bulk, the plateau T ODT in the film on bare Si wafer became almost identical to that in bulk. 36 The T OOT (the OOT temperature) and OOT pathway in films have also been shown different from those in bulk. For an asymmetric PS-b-PI presenting a sequential phase transition of LAM-HPL-GYR-HEX-DIS (disordering) in the bulk melt, even the weak surface interaction between PI block and a bare Si wafer influences the mesophasic stability in the BCP film, leading to the shift of T OOT ’s toward higher temperature and a loss of HEX morphology in the OOT sequence. 34 More recently, the ODT behavior of a polystyrene-b-poly- (methyl methacrylate) (PS-b-PMMA) coated on a PS-grafted substrate was studied and compared to that of PS-b-PI coated on the same type of substrate. The PS-b-PMMA film also presented higher T ODT than that in bulk, and its T ODT gradually decreased as the thickness of film increased. T ODT of PS-b-PMMA film, however, showed a gentler dependence on the film thickness than those of PS-b-PI. This was attributed to the fact that χ of PS-b- PMMA is less temperature dependent than that of PS-b-PI. 37 *To whom correspondence should be addressed: e-mail dyryu@ yonsei.ac.kr (D.Y.R.); blee@anl.gov (B.L.).