5266
Macromolecules 2009, 42, 5266–5271 DOI: 10.1021/ma900205s
Determination of the Fddd Phase Boundary in Polystyrene-block-polyisoprene Diblock Copolymer Melts Myung Im Kim,† Tsutomu Wakada,† Satoshi Akasaka,† Shotaro Nishitsuji,† Kenji Saijo,† Hirokazu Hasegawa,† Kazuki Ito,‡ and Mikihito Takenaka*,†,‡ †
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan, and ‡Structural Materials Science Laboratory SPring-8 Center, RIKEN Harima Institute Research 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan Received January 30, 2009; Revised Manuscript Received June 1, 2009
ABSTRACT: We determined the phase boundary of the Fddd phase in the phase diagram of polystyreneblock-polyisoprene (SI) diblock copolymer by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). We prepared eight SI diblock copolymers having different volume fractions of polyisoprene, fPI, in the range of 0.627 e fPI e 0.653 by living anionic polymerization. At the lowest fPI, 0.627, SI diblock copolymer showed only the disorder-lamellar (L) transition without any order-order transitions. SI diblock copolymer having the highest fPI, 0.653, exhibited disorder-gyroid (G)-L transitions with decreasing temperature but did not have the Fddd structure. Except for two samples described above, SI diblock copolymers exhibited disorder-G-Fddd-L transitions with decreasing temperature. It was found that the location of the Fddd region agreed with those obtained by using self-consistent-field theory calculation and Ginzburg-Landau theory.
I. Introduction Diblock copolymers which are composed of two kinds of chemically different polymers connected by a covalent bond can self-assemble into various microdomain structures via microphase separation.1,2 The morphologies of the microdomain structures of diblock copolymers depend on several parameters such as diblock copolymer composition (f ), Flory-Huggins interaction parameter between two constituent polymers (χ), and polymerization index of diblock copolymer (N). The phase diagram of diblock copolymers has been well investigated both theoretically and experimentally over several decades.3-6 Matsen and Schick examined the phase diagram of diblock copolymer by using self-consistent-field theory (SCFT) and found four morphologies: lamellae (L), gyroid (G), hexagonally packed cylinders (C), and spheres on the body-centered lattice (S).5 Khandpur et al. studied the phase diagram of polystyreneblock-polyisoprene (SI) diblock copolymer near the order-disorder transition experimentally and observed five morphologies: S, C, L, hexagonally perforated layer (HPL), and G.6 Subsequent work by Hajduk et al. found that the HPL structure is metastable.7 Recently, we reported the Fddd structure exists as an equilibrium structure in polystyrene-block-polyisoprene (SI) diblock copolymer.8,9 The Fddd structure which is a noncubic network morphology with a 3-fold symmetry was found in polyisoprene-block-polystyrene-block-poly(ethylene oxide) (ISO) triblock terpolymer by Bailey et al. for the first time.10 They characterized ISO triblock terpolymers with fPS =fPI and 0 < fPEO e 1/3 and demonstrated the existence of the Fddd phase between two-domain lamellar and three-domain lamellar phases. Subsequently, Epps et al. confirmed the stability of the Fddd structure and explored the phase diagram further within a composition 0.38
