arXiv subjects

Yoshihisa Miyamoto

Publications and source records attributed to Yoshihisa Miyamoto.

10 recordsLinked to original sources

arXiv2001

A molecular dynamics simulation of polymer crystallization from oriented amorphous state

Molecular process of crystallization from an oriented amorphous state was reproduced by molecular dynamics simulation for a realistic polyethylene model. Initial oriented amorphous state was obtained by uniaxial drawing an isotropic glassy state at 100 K. By the temperature jump from 100 K to 330 K, there occurred the 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 highly ordered crystalline state with stacked lamellae developed after further adjustment of the relative heights of the chains along their axes.

cond-mat.soft↗

arXiv2001

Crystal Growth of Isotactic Polystyrene in Ultrathin Films : Film Thickness Dependence

The film thickness dependence of crystal growth is investigated for isotactic polystyrene (it-PS) in thin films, the thickness of which is from 20nm down to 4nm. The single crystals of it-PS grown at 180 in the ultrathin films show the morphology typical in the diffusion-controlled growth: dense branching morphology (DBM), fractal seeweed (FS). The characteristic length of the morphology, i.e. the width of the branch, increases with decreasing film thickness. The thickness dependence of the growth rate of crystals shows a crossover around the lamellar thickness of the crystal, 8 nm. The thickness dependences of the growth rate and morphology are discussed in terms of the diffusion of chain molecules in thin films.

cond-mat.soft↗

arXiv2001

Dynamics of alpha- and beta- processes in thin polymer films: poly(vinyl acetate) and poly(methyl methacrylate)

Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) have been investigated by dielectric relaxation spectroscopy in the frequency range from 0.1Hz to 1MHz at temperatures from 263K to 423K. The $α$-process, the key process of glass transition, is observed for thin films of PVAc and PMMA as a dielectric loss peak at a temperature $T_α$ in temperature domain with a fixed frequency. For PMMA the $β$-process is also observed at a temperature $T_β$. For PVAc, $T_α$ decreases gradually with decreasing thickness, and the thickness dependence of $T_α$ is almost independent of the molecular weight ($M_{\rm w}\le2.4\times10^5$). For PMMA, $T_α$ remains almost constant as thickness decreases down to a critical thickness $d_c$, at which point it begins to decrease with decreasing thickness. Contrastingly, $T_β$ decreases gradually as thickness decreases to $d_c$, and below $d_c$ it decreases drastically. For both PVAc and PMMA, the broadening of the distribution of the relaxation times in thinner films is observed and this broadening is more pronounced for the $α$-process than for the $β$-process. It is also observed that the relaxation strength is depressed as the thickness decreases for both the polymers.

cond-mat.soft↗

arXiv2001

Slow dynamics near glass transitions in thin polymer films

The $α$-process (segmental motion) of thin polystyrene films supported on glass substrate has been investigated in a wider frequency range from 10$^{-3}$ Hz to 10$^4$ Hz using dielectric relaxation spectroscopy and thermal expansion spectroscopy. The relaxation rate of the $α$-process increases with decreasing film thickness at a given temperature above the glass transition. This increase in the relaxation rate with decreasing film thickness is much more enhanced near the glass transition temperature. The glass transition temperature determined as the temperature at which the relaxation time of the $α$-process becomes a macroscopic time scale shows a distinct molecular weight dependence. It is also found that the Vogel temperature has the thickness dependence, i.e., the Vogel temperature decreases with decreasing film thickness. The expansion coefficient of the free volume $α_f$ is extracted from the temperature dependence of the relaxation time within the free volume theory. The fragility index $m$ is also evaluated as a function of thickness. Both $α_f$ and $m$ are found to decrease with decreasing film thickness.

cond-mat.soft↗

arXiv2001

Growth Shape of Isotactic Polystyrene Crystals in Thin Films

The crystal growth of isotactic polystyrene (it-PS) is investigated in very thin, 11 nm thick films. The it-PS crystals grown in the thin films show quite different morphology from that in the bulk. With decreasing crystallization temperature, the branching morphology in a diffusion field appears: dendrite and compact seaweed. The branching morphology is formed through a morphological instability caused by the gradient of film thickness around a crystal; the thicker the film thickness, the larger the lateral growth rate of crystals. Regardless of the morphological change, the growth rate as well as the lamellar thickness depends on crystallization temperature as predicted by the surface kinetics.

cond-mat.soft↗

arXiv2000

Dielectric and Dilatometric Studies of Glass Transitions in Thin Polymer Films

Dielectric relaxation and thermal expansion spectroscopy were made for thin polystyrene films in order to measure the temperature $T_α$ corresponding to the peak in the loss component of susceptibility due to the $α$-process and the $α$-relaxation time $τ$ as functions of film thickness $d$. While the glass transition temperature $T_{\rm g}$ decreases with decreasing film thickness, $T_α$ and $τ$ were found to remain almost constant for $d>d_{\rm c}$ and decrease drastically for $d<d_{\rm c}$ for high temperatures. Here, $d_{\rm c}$ is a critical thickness. Near the glass transition temperature, the thickness dependence of $T_α$ and $τ$ is more prominent. The relation between the fragility index and non-exponentiallity is discussed for thin films of polystyrene.

cond-mat.soft↗

arXiv2000

Fracture energy of gels

To clarify effects of crack speed and cross-link density on fracture energy of acrylamide gels, we evaluated the roughness of the fracture surface and measured the fracture energy taking into account the roughness. The fracture energy increases linearly with crack speed $V$ in a fast crack speed region, and the increasing rate of fracture energy with $V$ decreases with increasing cross link density in the gels. In a slow crack speed region the fracture energy depends on crack speed more strongly than in the fast crack speed region. This indicates that a qualitative change exists in fracture process of the gels.

cond-mat.soft↗

arXiv1999

Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene

The glass transition temperature $T_{\rm g}$ and the temperature $T_α$ corresponding to the peak in the dielectric loss due to the $α$-process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in $T_{\rm g}$ was observed with decreasing film thickness, while $T_α$ was found to remain almost constant for $d>d_{\rm c}$ and to decrease drastically with decreasing d for $d<d_{\rm c}$. Here, $d_{\rm c}$ is a critical thickness dependent on molecular weight. The relaxation time of the $α$-process was found to have a d dependence similar to that of $T_α$. The relaxation function for the $α$-process was obtained by using the observed frequency dependence of the peak profile of the dielectric loss. The exponent $β_{\tiny KWW}$, which was obtained from the relaxation functions, decreases as thickness decreases. This suggests that the distribution of relaxation times for the $α$-process broadens with decreasing thickness. The thickness dependence of $T_{\rm g}$ is directly related to the distribution of relaxation times for the $α$-process, not to the relaxation time itself.

cond-mat.soft↗

arXiv1998

Glass Transition Temperature and Dynamics of $α$-Process in Thin Polymer Films

The glass transition temperature $T_g$ and the temperature $T_α$ corresponding to the peak in the dielectric loss due to the $α$-process have been simultaneously determined as functions of film thickness $d$ through dielectric measurements for thin films of polystyrene. A decrease of $T_g$ was observed with decreasing film thickness, while $T_α$ was found to remain almost constant for $d > d_c$ and decrease drastically for $d < d_c$. Here, $d_c$ is a critical thickness dependent on molecular weight. The thickness dependence of $T_g$ is related to the distribution of relaxation times of the $α$-process, not to the relaxation time itself.

cond-mat.soft↗

arXiv1998

Discontinuous Crack Fronts of Three-Dimensional Fractures

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.

cond-mat.soft↗