Faculty Profiles - UNEYAMA Takashi

Papers - UNEYAMA Takashi

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Coarse-grained molecular dynamics simulations for oxidative aging of polymers under various O₂ concentrations Reviewed International journal Open Access

Ishida T., Haremaki K., Koide Y., Uneyama T., Masubuchi Y.

Polymer Degradation and Stability Vol. 239 2025.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Polymer Degradation and Stability

Modeling of polymer oxidative aging has been actively studied since the 1990s. Insights from these studies suggest that the transport of oxygen and radicals significantly influences aging heterogeneity, alongside chemical reaction kinetics. A recent simulation study [Ishida et al., Macromolecules, 56(21), 8474-8483, 2023] demonstrated that mesoscale heterogeneity arises when the H-abstraction reaction occurs faster than the relaxation times of polymer chains. In this study, the simulations were extended by modeling the rate of oxygen addition to polymer radicals (k2) to reflect the effects of the O₂ concentration. In this work, polypropylene was chosen as a representative example of the target polymer. Three key aspects of oxidative aging behavior were found to be influenced by the O2 addition rate: (i) reaction kinetics, (ii) the degree of heterogeneity, and (iii) amount of crosslinking. Namely, reducing O2 concentration slows the conversion of polymer radicals into H-abstractable peroxyl radicals. This deceleration delays H-abstraction reactions, increases the number of polymer radicals, and promotes crosslinking reactions between two polymer radicals.

DOI： 10.1016/j.polymdegradstab.2025.111404

Open Access

Scopus

Nonmonotonic concentration dependence of the self-diffusion coefficient of surfactants in wormlike micellar solutions. Reviewed International journal

Koide Y, Ishida T, Uneyama T, Masubuchi Y

The Journal of chemical physics Vol. 162 ( 17 ) 2025.5

Radius of Gyration in Shear Gradient Direction Governs Steady Shear Viscosity of Rouse-Type Model Reviewed

Takashi Uneyama

Nihon Reoroji Gakkaishi Vol. 53 ( 1 ) page： 11 - 19 2025.2

Prediction of induced fluxes in reverse nonequilibrium molecular dynamics. Reviewed International journal

Oishi T, Koide Y, Ishida T, Masubuchi Y, Uneyama T

The Journal of chemical physics Vol. 162 ( 5 ) 2025.2

Coarse-Grained Simulation Model for Crystalline Polymer Solids by Using Breakable Bonds Reviewed International journal

Uneyama T

The Journal of Physical Chemistry B Vol. 129 ( 1 ) page： 506 - 523 2024.12

Reverse Nonequilibrium Molecular Dynamics Simulations of a Melt of Kremer-Grest Type Model under Fast Shear. Reviewed International coauthorship International journal

Oishi T, Koide Y, Ishida T, Uneyama T, Masubuchi Y, Müller-Plathe F

Journal of chemical theory and computation Vol. 20 ( 21 ) page： 9666 - 9672 2024.10

Normal Modes of Rouse-Ham Symmetric Star Polymer Model Reviewed

Takashi Uneyama

Nihon Reoroji Gakkaishi Vol. 52 ( 4 ) page： 255 - 263 2024.9

Phantom chain simulations for the fracture of star polymer networks with various strand densities. Reviewed International journal

Masubuchi Y, Ishida T, Koide Y, Uneyama T

Soft matter 2024.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Soft Matter

Despite many attempts, the relationship between the fracture and structure of polymer networks is yet to be clarified. For this problem, a recent study on phantom chain simulations [Y. Masubuchi et al., Macromolecules, 2023, 56, 9359-9367.] has demonstrated that the fracture characteristics obtained for polymer networks with various node functionalities and conversion ratios lie on master curves if they are plotted against cycle rank, which is the number of closed loops in the network per network node. In this study, we extended the simulation to the effect of prepolymer concentration c on the relationships between the cycle rank and fracture characteristics within the concentration range of 1 ≲ c/c* ≲ 10, concerning the overlapping concentration c*. We created networks from sols of star-branched phantom bead-spring chains via an end-linking reaction between different chains through Brownian dynamics simulations upon varying the number of branching arms f from 1 to 8, and the conversion ratio φc from 0.6 to 0.95. For the resultant networks, the cycle rank ξ was consistent with the mean-field theory. The networks were uniaxially stretched with energy minimization until break to obtain modulus G, strain at break ϵb, stress at break σb, and work for fracture Wb. As reported earlier, ϵb data for various f and φc are located on a master curve if plotted against ξ. The other quantities also draw master curves as functions of ξ if normalized by the branch point density υbr. The master curves depend on c; as c increases, all the mechanical characteristics monotonically increase. If we plot σb/υbr and Wb/υbr against G/υbr, the data for various f and φc lie on master curves but depending on c. Consequently, the fracture characteristics are not solely described by the modulus.

DOI： 10.1039/d4sm00726c

Scopus

PubMed

Effect of Mass on Single File Diffusion Reviewed

Takashi Uneyama

Nihon Reoroji Gakkaishi Vol. 52 ( 3 ) page： 171 - 179 2024.6

Dissipative Particle Dynamics Simulation Plus Slip-Springs for Entangled Polymers with Various Slip-Spring Densities Reviewed International coauthorship International journal

Masubuchi Y., Guo H., Wang F., Khomami B., Boudaghi-Khajehnobar M., Doi Y., Ishida T., Uneyama T.

Macromolecules Vol. 57 ( 10 ) page： 4867 - 4877 2024.5

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Macromolecules

Slip-spring models are valuable tools for simulating entangled polymers, bridging the gap between bead-spring models with excluded volume and network models with presumed reptation motion. This study focuses on the DPD-SS (Dissipative Particle Dynamics-Slip-Spring) model, which introduces slip springs into the standard DPD polymer model with soft-core interactions. By systematically adjusting the activity of slip springs, the density of slip springs within the system is varied. Simulation results demonstrate the compatibility of models with different slip-spring densities in terms of diffusion and linear relaxation modulus when the average number of slip springs per chain is the same. The conversion between DPD-SS models concerning length and time is achieved through Rouse scaling, which utilizes the average number of DPD beads between consecutive anchoring points of slip springs. Additionally, the modulus conversion is accomplished through the plateau modulus that takes account of fluctuations around entanglement. Notably, diffusion and relaxation modulus from the DPD-SS model align with those reported for standard Kremer-Grest and DPD models featuring strong repulsive interactions.

DOI： 10.1021/acs.macromol.4c00280

Scopus

過剰量のシランカップリング剤による鉄粒子の表面処理が MR 流体のレオロジー特性に与える影響 Reviewed

山脇悠矢, 石田崇人, 土肥侑也, 畝山多加志, 増渕雄一

日本レオロジー学会誌 Vol. 52 ( 2 ) page： 99 - 105 2024.4

Comparison of Linear Viscoelastic Behaviors of Langevin- and Jump-Rouse Models Reviewed

Takashi Uneyama

Nihon Reoroji Gakkaishi Vol. 52 ( 1 ) page： 27 - 35 2024.2

Coarse-grained molecular dynamics simulation of oxidative aging of polymers -effect of free radical diffusivity - Reviewed International journal

Ishida T., Doi Y., Uneyama T., Masubuchi Y.

Polymer Journal Vol. 56 page： 1069 - 1078 2024

Language：English Publishing type：Research paper (scientific journal) Publisher：Polymer Journal

Recently, we developed an oxidative aging simulation technique that explicitly considers the transport of radicals by combining oxidation kinetics with coarse-grained molecular dynamics [1]. The current study explores the role of the free radical diffusivity of oxidative aging in polymers, with a focus on its effects on the heterogeneity of spatiotemporal structures and reaction kinetics, through coarse-grained simulations. By examining two distinct systems, including a heterosystem with a high reaction rate and a quasi-homosystem with a low reaction rate, we investigated how the diffusivity of free radicals influences the oxidative aging process. We examined three different diffusivity levels of radicals. For the slowest case, the radical diffusivity is slightly faster than that of the single polymer segment, whereas for the fastest case, the radical diffuses infinitely faster than the polymers. The aging simulations revealed that the variations in free radical diffusivity quantitatively change the reaction kinetics and spatiotemporal structures without qualitatively altering the overall aging behavior. Specifically, in a heterosystem, lower radical diffusivity slows the oxidation kinetics by increasing fluctuations in the local conversion ratios. In contrast, in a quasi-homosystem, reduced radical diffusivity slightly amplifies the heterogeneity of structures in the early aging stages; however, in a high-conversion region, the aged state becomes homogeneous regardless of the degree of radical diffusivity.

DOI： 10.1038/s41428-024-00942-5

Scopus

Brownian simulations for fracture of star polymer phantom networks Reviewed International journal

Masubuchi Y., Koide Y., Ishida T., Uneyama T.

Polymer Journal 2024

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Polymer Journal

According to a recent simulation study [Masubuchi et al., Macromolecules, 56, 9359 (2023)], the cycle rank plays a significant role in determining the fracture characteristics of network polymers. However, the previous studies considered only energy-minimized networks without including the effects of thermal agitation. To address this gap, this study conducted Brownian dynamics simulations at various stretch rates. The results showed that even with Brownian motion, the strain and stress at break obtained for different node functionalities and conversion ratios exhibited master curves when plotted versus the cycle rank. These master curves depend on the strain rate, indicating that the strain and stress at break decrease with decreasing strain rate. The curves are qualitatively similar to those observed in energy-minimized simulations, even though the competition among Brownian motion, elongation, and bond scission affects the fracture process.

DOI： 10.1038/s41428-024-00992-9

Scopus

Modeling for Heterogeneous Oxidative Aging of Polymers Using Coarse-Grained Molecular Dynamics Reviewed

Takato Ishida, Yuya Doi, Takashi Uneyama, and Yuichi Masubuchi

Macromolecules 2023.11

Percolation-induced gel-gel phase separation in a dilute polymer network. Reviewed

Shohei Ishikawa, Yasuhide Iwanaga, Takashi Uneyama, Xiang Li, Hironori Hojo, Ikuo Fujinaga, Takuya Katashima, Taku Saito, Yasushi Okada, Ung-il Chung, Naoyuki Sakumichi and Takamasa Sakai

Nature Materials Vol. 22 ( 12 ) page： 1564 - 1570 2023.10

Effects of attractive inter-particle interaction on cross-transport coefficient between mass and heat in binary fluids Reviewed

Tatsuma Oishi, Yuya Doi, Yuichi Masubuchi, and Takashi Uneyama

Molecular Simulation Vol. 50 ( 1 ) page： 20 - 25 2023.10

Brownian yet non-Gaussian diffusion of a light particle in heavy gas: Lorentz-gas-based analysis Reviewed

Nakai F., Uneyama T.

Physical Review E Vol. 108 ( 4 ) page： 044129 2023.10

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Review E

Non-Gaussian diffusion was recently observed in a gas mixture with mass and fraction contrast [F. Nakai, Phys. Rev. E 107, 014605 (2023)2470-004510.1103/PhysRevE.107.014605]. The mean-square displacement of a minor gas particle with a small mass is linear in time, while the displacement distribution deviates from the Gaussian distribution, which is called the Brownian yet non-Gaussian diffusion. In this work, we theoretically analyze this case where the mass contrast is sufficiently large. Major heavy particles can be interpreted as immobile obstacles, and a minor light particle behaves like a Lorentz gas particle within an intermediate timescale. Despite the similarity between the gas mixture and the conventional Lorentz gas system, the Lorentz gas description cannot fully describe the Brownian yet non-Gaussian diffusion. A successful description can be achieved through a canonical ensemble average of the statistical quantities of the Lorentz gas over the initial speed. Furhter, we show that the van Hove correlation function has a nonexponential tail, which is contrary to the exponential tail observed in various systems.

DOI： 10.1103/PhysRevE.108.044129

Scopus

Extension of moving particle simulation by introducing rotational degrees of freedom for dilute fiber suspensions Reviewed

Enomoto, K; Ishida, T; Doi, Y; Uneyama, T; Masubuchi, Y

International Journal for Numerical Methods in Fluids Vol. 96 ( 2 ) page： 125 - 137 2023.9

Phantom-Chain Simulations for the Effect of Node Functionality on the Fracture of Star-Polymer Networks Reviewed International journal

Masubuchi, Y; Doi, Y; Ishida, T; Sakumichi, N; Sakai, T; Mayumi, K; Satoh, K; Uneyama, T

Macromolecules Vol. 56 ( 23 ) page： 9358 - 9367 2023.9

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Papers - UNEYAMA Takashi