Yoshihisa Miyamoto

Kyoto University, Kyoto, Kyoto-fu, Japan

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Publications (61)105.59 Total impact

  • Waki Sakatsuji, Takashi Konishi, Yoshihisa Miyamoto
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    ABSTRACT: The effects of thermal history on the enthalpy relaxation in polystyrene are studied by differential scanning calorimetry. The temperature dependence of the specific heat in the liquid and the glassy states, that of relaxation time, and the exponent of the Kohlrausch-Williams-Watts function are determined by measurements of the thermal response against sinusoidal temperature variation. A phenomenological model equation previously proposed to interpret the memory effect in the frozen state is applied to the enthalpy relaxation and the evolution of entropy under a given thermal history is calculated. The annealing below the glass transition temperature produces two effects on enthalpy relaxation: the decay of excess entropy with annealing time in the early stage of annealing and the increase in relaxation time due to physical aging in the later stage. The crossover of these effects is reflected in the variation of temperature of the maximum specific heat observed in the heating process after annealing and cooling.
    Physical Review E 07/2013; 88(1-1):012605. · 2.31 Impact Factor
  • Takashi Konishi, Waki Sakatsuji, Koji Fukao, Yoshihisa Miyamoto
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    ABSTRACT: The isothermal crystallization from the molten state of poly(butylene terephthalate) is studied by differential scanning calorimetry and small-angle x-ray scattering. The relations among the crystallization temperature, lamella thickness, and melting temperature give two equilibrium transition temperatures. We propose a model of crystalline lamella formation through a mesomorphic state at the growth front by which these equilibrium transition temperatures are interpreted as the transition temperature between the mesophase and the crystal, and the one between the melt and the crystal of infinitely thick lamellae. These results show that the mesomorphic state plays a key role in the ordering process of polymers.
    Physical review. B, Condensed matter 01/2011; 84(13). · 3.66 Impact Factor
  • Takashi Konishi, Yoshihisa Miyamoto
    Polymer Journal - POLYM J. 01/2010; 42(4):349-353.
  • Takashi Konishi, Yoshihisa Miyamoto
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    ABSTRACT: Crystallization from the glass of poly(butylene terephthalate) (PBT) has been investigated by small-angle X-ray scattering and differential scanning calorimetry. When PBT crystallizes from the glassy state, the nodule crystalline structures in nanometer scale forms and the micrometer order objects, i.e., spherulites, are not observed. The nodule size grows and saturates at a given annealing temperature and increases with increasing annealing temperature. Two temperatures are obtained by the linear extrapolation of the temperature dependence of the reciprocal nodule size to naught. The equilibrium melting temperatures by the Gibbs−Thomson plot, i.e., the melting temperature versus reciprocal nodule size, are also the same temperatures. The lower extrapolated temperature corresponds to the equilibrium melting temperature of the α-form of PBT. The higher extrapolated temperature is discussed on the basis of the Strobl crystallization model and can be regarded as the equilibrium mesophase−crystal transition temperature.
    Macromolecules 01/2010; 43(1):375-383. · 5.93 Impact Factor
  • Akira Koyama, Takashi Yamamoto, Koji Fukao, Yoshihisa Miyamoto
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    ABSTRACT: Polymer crystallization from a uniaxially stretched amorphous state was investigated by molecular dynamics simulation using a realistic molecular model of poly(ethylene). Simulations for crystallization were performed at temperatures between 280 and 350 K. Structure formations to crystalline states having lower symmetries than the hexagonal phase of poly(ethylene) were realized above 300 K. It was found that all of the order parameters considered to analyze the crystallization process showed the same time dependence at a given temperature: there was no precursor prior to crystallization. The temperature dependence of the crystallization rate has a bell-shape, and a maximum of the crystallization rate appears around 325 K.
    Journal of Macromolecular Science Part B 02/2007; 42(3 & 4)(pp. 821–831 (2003)):821-831. · 0.63 Impact Factor
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    Y. Tanaka, K. Fukao, Y. Miyamoto, K. Sekimoto
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    ABSTRACT: The relation between fracture surface morphology and the three-dimensional structure of crack fronts is investigated through direct observation of brittle cracks in gels. A key notion in this investigation is the discontinuity of the crack front, whose advancement creates the fracture surface. We discuss the significance of our findings in the studies of general three-dimensional brittle fractures.
    EPL (Europhysics Letters) 01/2007; 43(6):664. · 2.26 Impact Factor
  • Y. Miyamoto, K. Sekimoto
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    ABSTRACT: The viscoelasticity of cis-1,4-polyisoprene rubber is studied in the glass transition region by dynamic viscoelastic measurement. The effect of strain on the complex moduli and on the representative characteristic time is discussed in terms of the change in elastic modulus in the rubbery state.
    05/2006;
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    Ken Taguchi, Akihiko Toda, Yoshihisa Miyamoto
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    ABSTRACT: The growth rate and morphology of isotactic polystyrene crystals grown in ultrathin films have been examined experimentally in terms of the dependences both on the film thickness and on the crystallization temperature. We have found that the thickness dependence of growth rate, G, shows a crossover change when the film thickness becomes comparable with the lamellar thickness of the polymer crystals, irrespective of the temperatures. The morphology of crystals grown in ultrathin films shows a branching typical of dendrites, the growth of which is supposed to be controlled by a diffusion field. The change in the tip width of the dendrites with crystallization temperature follows the expected dependence of the Mullins–Sekerka stability length, ℓMS ∝ (D/G), determined by the diffusion coefficient, D, and the growth rate. The results confirm that a diffusion field plays an essential role in the evolution of the structure.
    Journal of Macromolecular Science, Part B. 01/2006; 45(6).
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    Ken Taguchi, Akihiko Toda, Yoshihisa Miyamoto
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    ABSTRACT: Crystallization of isotactic polystyrene (it-PS) from dilute solution at high supercooling has been inves-tigated by dynamic light scattering (DLS). We successfully obtained simultaneously, in situ in solutions, the time developments of both random coils of it-PS molecules and the growing crystals. The size of coils remains constant during growth, while the crystals pass through two stages, i.e. induction period at the early stage with very slow growth rates and subsequent linear growth stage. It is confirmed that the temperature dependence of the linear growth rates, determined by DLS, agree well with that de-termined by electron microscopy. The temperature dependences of the growth rate and the inverse of induction time are dependent on the viscosity of solvent, which indicates that all dynamics are dominated by the segmental motion of polymer chains in solution at high supercoolings (low temperatures). Two possibilities are proposed for the induction period.
    Journal of Macromolecular Science, Part B. 01/2006; 45(6).
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    ABSTRACT: In order to investigate the detailed structure of a banded spherulite observed by polarized light microscopy, we develop a new image processing technique that can visualize defects (band defects) in the concentric bands and determine the growing directions of crystals everywhere in a spherulite. This technique is applied to a banded spherulite of poly(vinylidene fluoride) and reveals that the spherulite has many defects (colliding defects), on which crystals collide with neighboring ones. It is found that the band defects are included in the colliding defects. The number of colliding defects increases linearly with the radius to give a constant density. Between the defects, the orientations of crystals are well correlated to form a coherent area. On the basis of these findings, a mechanism of the formation of the coherent band pattern is discussed.
    Polymer. 01/2005; 46(20):8717-8722.
  • Yoshihisa Miyamoto, Hiromi Yamao, Ken Sekimoto
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    ABSTRACT: The memory effect is studied in the glass to rubber transition of polyisoprene rubber with the strain as a controlling parameter. A phenomenological model is proposed taking the history of the temperature and the strain into account, by which the experimental results are interpreted. It is shown both experimentally and theoretically that the glassy state memorizes the time-course of strain upon glassification, not as a single parameter but as the history itself.
    04/2004;
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    ABSTRACT: The structure formation of isotactic polypropylene from the uniaxially oriented glass into the smectic mesophase has been studied by small-angle X-ray scattering. Remarkable anisotropy between the directions parallel and perpendicular to the chain axis is found in the development of structure formation. Below −20 °C is observed a structure with density variation having the wave vector, s, perpendicular to the chain axis with a period of 5 to 6 nm. Between −20 and +25 °C, the density variations with s both parallel and perpendicular to the chain axis develop. Above +25 °C, the density variation with s perpendicular to the chain axis develops but disappears eventually, while that parallel to the chain axis keeps on growing.
    Polymer 01/2004; 45(5):1429-1432. · 3.77 Impact Factor
  • Yoshihisa Miyamoto, Hiromi Yamao, Ken Sekimoto
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    ABSTRACT: The crystallization and the melting of vulcanized isoprene rubber under uniaxial deformation have been studied by the measurements of the stress−strain−temperature relation. The melting temperature increases approximately linearly with nominal stress for the rubbers crystallized at a fixed temperature and stress condition. It has been shown that the main origin of the rise in melting temperature with stress is the work of contraction upon melting rather than the entropy reduction on deformation in the molten state. The crystallization rate monotonically increases with nominal stress above the melting stress at a given temperature, while it shows a maximum against temperature at a given stress. The relation between the temperature and the nominal stress at the onset of crystallization is satisfactorily reproduced by the simplified kinetic model.
    Macromolecules. 07/2003; 36(17).
  • Ken Taguchi, Yoshihisa Miyamoto, Hideki Miyaji, Kunihide Izumi
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    ABSTRACT: The periodical striped contrast is observed in the single crystals of isotactic polystyrene and poly(oxymethylene) by transmission electron microscopy. The contrast is attributed to a periodical tilting of the chain stem around the axis normal to the crystal growth plane; the tilting accompanies the surface undulation, which is observed by atomic force microscopy. The wavelength of the undulation increases with the lamellar thickness. In situ observation of the growing crystal by transmission electron microscopy shows that this undulation is introduced during the growth and not through the cooling process of the crystal. It is proposed that the stress of fold surfaces makes a flat lamellar crystal elastically unstable to cause the buckling deformation during the crystal growth.
    Macromolecules 06/2003; 36(14). · 5.93 Impact Factor
  • Yoshihisa Miyamoto, Hideki Miyaji, Kenjiro Asai
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    ABSTRACT: The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δε is analyzed on the basis of the two-site model. A linear relation between Δε/χξ and cos2θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.
    Journal of Polymer Science Polymer Physics Edition 06/2003; 18(3):597 - 606.
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    ABSTRACT: At high supercoolings, isotactic polystyrene and polybutene-1 have a rounded crystal shape, suggesting kinetic roughening. Still, the growth rates of these polymer crystals show the supercooling dependence derived for nucleation controlled growth. On the other hand, isotactic poly-4-methylpentene-1 1,4 trans-polybutadiene at higher crystallization temperatures and polyethylene at high pressures show a rounded crystal shape: thermal roughening. Again, the growth rate is described by the nucleation theory. On the basis of these observations, we propose a crystallization kinetics taking account of the entropic barrier that was originally proposed by Sadler.
    Journal of Macromolecular Science, Part B. 01/2003; 42:867-874.
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    ABSTRACT: The ordering process of poly(ethylene terephthalate) (PET) from the oriented glassy states during the isothermal annealing process was investigated using real-time X-ray scattering experiments with synchrotron radiation sources. The X-ray scattering intensity of the 001' reflection, which is a measure of the smectic order with respect to chain alignment, increases with time and then begins to decrease after reaching a maximum. The intensity of the four-point pattern in the small-angle X-ray scattering region, which is a measure of the fraction of a crystal-like structure with a tilted lamellar structure, increases monotonically with time. Hence, the fraction of PET chains with smectic-like order has a maximum during the isothermal process, while that of the crystal-like structure increases monotonically. The kinetics of structure formation from the nematic-like structure to the crystal-like structure by way of the smectic structure will be discussed.
    Journal of Macromolecular Science, Part B. 01/2003; 42:717-731.
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    Daisuke Tahara, Koji Fukao, Yoshihisa Miyamoto
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    ABSTRACT: The light scattering from the spherulites of polyethylene terephthalate grown near the glass transition temperature has been investigated. The Hv scattering profiles can be reproduced by the sum of the ideal spherulite scattering with the distribution of spherulite radius and the isotropic scattering from randomly oriented crystallites. The ratio of optical anisotropies in the isotropic scattering to the ideal spherulite scattering is obtained by the method established to eliminate the effects of the number density of spherulites and the coefficient depending on the experimental conditions. It is found that the anisotropy ratio is almost independent of the crystallization time and of temperature above 106°C, while it is larger at a crystallization temperature of 103°C. The spherulitic structure is discussed in terms of the anisotropy ratio.
    Polymer 12/2002; 43(26):7461-7465. · 3.77 Impact Factor
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    Yoshihisa Miyamoto, Koji Fukao, Hiromi Yamao, Ken Sekimoto
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    ABSTRACT: The memory effect upon glassification is studied in the glass-to-rubber transition of vulcanized rubber with the strain as a controlling parameter. A phenomenological model is proposed, taking the history of the temperature and the strain into account, by which the experimental results are interpreted. The data and the model demonstrate that the glassy state memorizes the time course of strain upon glassification, not as a single parameter but as the history itself. The data also show that the effect of irreversible deformation in the glassy state is beyond the scope of the present model.
    Physical Review Letters 07/2002; 88(22):225504. · 7.73 Impact Factor
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    Akira Koyama, Takashi Yamamoto, Koji Fukao, Yoshihisa Miyamoto
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    ABSTRACT: The molecular process of crystallization from an oriented amorphous state was reproduced by molecular dynamics simulation for a realistic polyethylene model. The initial oriented amorphous state was obtained by uniaxially drawing an isotropic glassy state at 100 K. By the temperature jump from 100 K to 330 K, there occurred crystallization into the fiber structure, during the process of which we observed the developments of various order parameters. The real-space image and its Fourier transform revealed that a hexagonally ordered domain was initially formed, and then a highly ordered crystalline state with stacked lamellas developed after further adjustment of the relative heights of the chains along their axes.
    Physical Review E 06/2002; 65(5 Pt 1):050801. · 2.31 Impact Factor