Showing papers by "Jong Hyun Ahn published in 2000"

Structure of Shear-Induced Perforated Layer Phase in Styrene−Isoprene Diblock Copolymer Melts

Abstract: Diblock copolymers self-assemble into a variety of ordered microstructures due to the immiscibility between the two different blocks comprising the polymer molecule.1 Its phase behaviors depend primarily on three factors: (i) the volume fraction of one monomer unit, f, (ii) the degree of polymerization, N, and (iii) the interaction parameter, ø. For symmetric copolymers with f ) 0.5, the melt is ordered into a lamellar structure. As f is increased, a cylindrical phase and a spherical phase are commonly observed. Besides the classical phases, recent experiments have revealed the existence of new phases in a narrow range of f and øN between the lamellar and cylindrical phases.2-14 The additional phases are the gyroid (G) with Ia3hd space group symmetry 6-12 and hexagonally perforated layers (HPL).2,7-11 The structure of the G phase has been well characterized by small-angle neutron scattering (SANS) and transmission electron microscopy (TEM). Many theoretical studies have also found this phase to be stable.16-18 On the other hand, the structural nature of HPL phase has not been fully resolved. On the basis of small-angle scatterings and transmission electron microscopy studies, it is believed that the HPL structure consists of alternating minority and majority component layers in which hexagonally packed channels of majority component extend through the minority component. The stacking sequences of HPL channels can be modeled as both ABAB... and ABCABC... patterns, as illustrated in Figure 1. Förster et al. have proposed that the perforation of HPL is stacked in ABCABC... pattern for styrene-isoprene diblock copolymers with 0.61 < fPS < 0.65 through the interpretation of SANS data.8 However, the higher order scattering patterns have not been observed to prove clearly ABCABC... stackings. In more recent studies, Vigild et al. have observed long-range oriented HPL structures of ethylenepropylene-dimethylsiloxane diblock copolymer mixtures using the smallangle X-ray scattering (SAXS).14 Unlike the findings of Förster et al., Vigild et al. have reported that the HPL structure is composed of a combination of an ABCABC... and ABAB... arrangement, i.e., as a combination of a face-centered cubic and a hexagonally close-packed structure. In theoretical studies, Qi and Wang have suggested that the HPL structure has an ABAB... stacking sequence by means of a theory based on anisotropic composition fluctuations in the weak segregation limit.15 Thus, the stacking sequence of HPL structure has not been satisfactorily understood until now. In an effort to identify the stacking sequence of HPL, we have investigated the shear-induced HPL structure of the styrene-isoprene diblock copolymer through synchrotron SAXS. Styrene-isoprene diblock copolymer (PSI67) was synthesized by living anionic polymerization using high vacuum techniques. The molecular weight of this sample measured with light scattering was 34 000, and the polydispersity index by GPC was 1.02. The weight fraction of polystyrene was 0.70 by 1H NMR. The volume * E-mail: wczin@postech.ac.kr. Tel.: +82-562-279-2136. Fax: +82-562-279-2399. Figure 1. Hexagonal lattice models for (a) ABAB... and (b) ABCABC... stacking sequence. The letters c and a denote the parameters of lattice.