Browsing by Author "Bayomi, Rasha Ahmed Hanafy"

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Grain coarsening on the free surface and in the thickness direction of a sphere-forming triblock copolymer film
(NATURE PUBLISHING GROUP, 2018) Bayomi, Rasha Ahmed Hanafy; Honda, Konomi; Wataoka, Isao; Takagi, Hideaki; Shimizu, Nobutaka; Igarashi, Noriyuki; Sasaki, Sono; Sakurai, Shinichi
Controlling a block copolymer "grain", in which the microdomains are regularly ordered in a single lattice, is important for developing high-performance polymeric materials. This is because the grains, which are several micrometers in size, can directly affect the properties of the materials. In this regard, we focused on grain coarsening on the free surface and in the thickness direction of a sphere-forming triblock copolymer film. We evaluated the grain size on the free surface using atomic force microscopy combined with image processing, and in the thickness direction, we used small-angle X-ray scattering edge-view measurements. It was found that the grain growth in the direction parallel to the free surface was very slow in the early stage of thermal annealing. Then, the grain growth shifted to a rapid growth mechanism with a power-law relationship (grain size similar to t(alpha), with alpha = 0.7) after similar to 30 min. Based on the value of the growth exponent a, the grain growth mechanism is considered to fall between the random and deterministic processes. In contrast, for the thickness direction, a much larger value (alpha = 1.72) was obtained. For such a large a value, it is impossible to consider the growth mechanism of the grain within the conventional to framework of the growth of domains and droplets. Therefore, our results may require a new framework to explain the behavior of the grain growth in the spherical microdomain system. Another notable finding is that the thickness of the oriented layer near the free surface or near the surface in contact with the substrate can be as thick as 9.5 mu m, which is substantially larger than the reported values of the propagation distance of surface-induced orientation of microdomains in block copolymers. Based on the results of the current study, it is speculated that grain growth serves as a propagator for the regular ordering of spherical microdomains and the orientation of the lattice.
Structural analyses of sphere- and cylinder-forming triblock copolymer thin films near the free surface by atomic force microscopy, X-ray photoelectron spectroscopy, and grazing-incidence small-angle X-ray scattering
(ELSEVIER SCI LTD, 2018) Bayomi, Rasha Ahmed Hanafy; Aoki, Takashi; Shimojima, Takuma; Takagi, Hideaki; Shimizu, Nobutaka; Igarashi, Noriyuki; Sasaki, Sono; Sakurai, Shinichi
We investigated surface structures of three kinds of block copolymer (BCP) specimens forming hard spherical microdomains embedded in a soft matrix, by using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and grazing-incidence small-angle X-ray scattering (GISAXS). The three BCP specimens consist of different chemical species such as SEBS (polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene) and MAM (polymethylmethacrylate-block-poly(n-butylacrylate)-block-polymethylmethacrylate). Two of them form spherical microdomains at the thermodynamically equilibrium state (i.e., after fully long time thermal annealing), while the other exhibits spherical microdomains as a frozen state of the non-equilibrium structure (i.e., at the as-cast state using a selective solvent for the solution casting method), which was automatically frozen in by the vitrification of the hard component; the state is far from equilibrium). Nevertheless, such three different BCP specimens (chemically different in the molecular structure and thermodynamically different in the state) showed a common feature of the surface structure, as the outermost surface was completely covered with the single component having a lower value of the surface free energy, as revealed by XPS measurements. This finding significantly indicates that the AFM observation, which implies a coexistence of both BCP components on the free surface, misleads the conclusion. Although such surface coverage is reasonable and well-known, it was further found that the block chains are confined in a thin layer of the outermost region of the specimen. It is striking to find that the block chains are forced to be deformed (compressed) in order to meet the thermodynamic requirement of the surface coverage. (C) 2018 Elsevier Ltd. All rights reserved.