arXiv subjects

Yuichi Masubuchi

Publications and source records attributed to Yuichi Masubuchi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Nonmonotonic concentration dependence of the self-diffusion coefficient of surfactants in wormlike micellar solutions

We investigate the concentration dependence of surfactant diffusion in wormlike micellar solutions using dissipative particle dynamics simulations. The simulations show that the self-diffusion coefficient of surfactants exhibits a nonmonotonic dependence on the surfactant concentration, as observed in previous experiments. We quantitatively reveal that this nonmonotonic behavior results from the competition between micellar center-of-mass diffusion and surfactant diffusion within micelles by decomposing the mean-square displacement of surfactants into the corresponding contributions. Furthermore, our detailed analyses demonstrate how the competition between the two diffusion mechanisms is governed by the aggregation number distribution, the dynamics of individual surfactants and micelles, and the kinetics of micellar scission and recombination.

cond-mat.soft↗

arXiv2025

Coarse-Grained Molecular Dynamics Simulations for Oxidative Aging of Polymers under Various O2 Concentration

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 (k_2) to reflect the effects of the O2 concentration. 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.

cond-mat.soft↗

arXiv2024

Persistence length and plateau modulus of semiflexible entangled polymers in primitive chain network simulations

The relationship between the plateau modulus (G_0) and the persistence length (L_p) of entangled semiflexible polymers is still uncertain. Some previous theoretical models have predicted that G_0 decreases with increasing L_p, while experiments indicated the opposite. In this study, we extend the primitive chain network (PCN) model to incorporate bending rigidity at the scale of the entanglement length, consistently with the coarse graining of the model. Simulations investigate the effects of rigidity (and of molecular weight) on chain conformation and relaxation modulus. Our results reveal a relationship described as G_0~L_p^(2/3), indicating that G_0 increases with increasing L_p, consistently with experiments. It should be considered, however, that implicit in our simulations is the condition that the entanglement length is kept fixed with changing L_p.

cond-mat.soft↗

arXiv2024

Brownian simulations for fracture of star polymer phantom networks

Based on 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 study only considered energy-minimized networks without the effects of thermal agitation. We conducted Brownian dynamics simulations at various stretch rates to address this gap. The results showed that even with Brownian motion, the strain and stress at the break obtained for different node functionalities and conversion ratios exhibited master curves if plotted against cycle rank. These master curves were dependent on the strain rate, with the curves tending to approach those observed in energy-minimized simulations as the strain rate decreased, even though the fracture process was affected by the competition against Brownian motion, elongation, and bond degradation.

cond-mat.soft↗

arXiv2024

Reverse non-equilibrium molecular dynamics simulations of a melt of Kremer-Grest type model under fast shear

Although the reverse non-equilibrium molecular dynamics (RNEMD) simulation method has been widely employed, the range of applicability is yet to be discussed. In this study, for the first time, we systematically examine the method against an unentangled melt of the Kremer-Grest type chain. The simulation results indicate that as the shear rate increases, the temperature and density become inhomogeneous. However, the average viscosity remains consistent with the results obtained using the SLLOD method under homogeneous temperature and density. We also confirm that the temperature-density inhomogeneity does not significantly affect polymer conformation.

cond-mat.soft↗

arXiv2024

Phantom chain simulations for fracture of star polymer networks with various strand densities

Despite many attempts, the relation between fracture and structure of polymer networks is yet to be clarified. For this problem, a recent study for phantom chain simulations [Macromolecules, 56, 9359 (2023)] 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. In this study, we extended the simulation to the effect of prepolymer concentration on the relationships between cycle rank and fracture characteristics within the concentration range of 1<c/c^*<8, concerning the overlapping concentration c^*. We created networks from sols of star-branched phantom bead-spring chains via end-linking reaction between different chains through Brownian dynamics simulations with varying the number of branching arms f from 1 to 8, and the conversion ratio ϕ_c from 0.6 to 0.95. For the resultant networks, 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 ε_b, stress at break σ_b, and work for fracture W_b. With the branch point density \u{psion}_br, G/\u{psion}_br, ε_b, σ_b/\u{psion}_br, and W_b/\u{psion}_br of the data for various f and ϕ_c draw master curves if plotted against ξ. The master curves depend on c; as c increases, all the mechanical characteristics monotonically increase. If we plot σ_b/\u{psion}_br and W_b/\u{psion}_br against G/\u{psion}_br, the data for various f and ϕ_c lie on master curves but depending on c. Consequently, the fracture characteristics are not solely described by modulus for the examined energy-minimized phantom chain networks.

cond-mat.soft↗

arXiv2024

Primitive chain network simulations for double peaks in shear stress under fast flows of bidisperse entangled polymers

A few experiments have reported that the time development of shear stress under fast start-up shear deformations exhibits double peaks before reaching the steady state for bimodal blends of entangled linear polymers in specific conditions. To analyze the molecular origin of this phenomenon, multi-chain slip-link simulations based on the primitive chain network model were conducted for the data of a bimodal polystyrene solution reported by Osaki et al. [J Pol Sci B Pol Phys, 38, 2043 (2000)] Owing to the reasonable agreement with the data and the simulation results, the stress was decomposed into contributions from long and short-chain components and decoupled into segment number, stretch, and orientation. The analysis revealed that the first and second peaks correspond to the short-chain orientation and the long-chain stretch, respectively. The results also imply that the peak positions are not affected by the mixing of short and long chains, whereas the second peak intensity depends on the mixing conditions in a complicated manner, dominating the emergence of double peaks.

cond-mat.soft↗

arXiv2024

Phantom Chain Simulations for Fracture of End-linking Networks

Despite numerous studies, the relationship between network structure and fracture remains unclear. In this study, the fracture properties of end-linking networks were compared with those of loop-free analogs made from star prepolymers by performing phantom chain simulations. The networks were created from equilibrated sols of stoichiometric mixtures of linear prepolymers and f-functional linkers through end-linking reactions using Brownian dynamics schemes. The examined networks, with various f values (between 3 and 8) and strand-connection rates (ϕ_s), were evaluated in terms of the primary loop fraction and the cycle rank ξ. These structural characteristics were consistent with mean-field theories that assume independent reactions. Energy minimization and uniaxial stretch were applied to the networks until they broke without Brownian motion. The fracture characteristics, including strain (ε_b), stress (σ_b), and work for fracture (W_b), were obtained from stress-strain curves. The end-linking networks exhibited larger ε_b and smaller σ_b and W_b than those for star networks due to primary loops, at the same set of f and ϕ_s. However, ε_b, σ_b/ν_br and W_b/ν_br (with ν_br being the branch point density) lie on the same master curves as those for star networks if they are plotted against ξ. This result implies that the fracture of end-linking networks is essentially the same as that for star analogs, and the effects of primary loops are embedded in ξ.

cond-mat.soft↗

arXiv2024

Dissipative particle dynamics simulation plus slip-springs for entangled polymers with various slip-spring densities

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 fugacity 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.

cond-mat.soft↗

arXiv2023

Phantom chain simulations for the effect of stoichiometry on the fracture of star-polymer networks

By phantom chain simulations, it has been recently discovered that the fracture characteristics of star polymer networks with different node functionality and conversion ratios can be described by the cycle rank of the networks [Masubuchi et al., Macromolecules, doi: 10.1021/acs.macromol.3c01291]. However, due to the employed simplifications and idealizations of the examined model networks, the results cannot be cast into realistic systems straightforwardly. For instance, the equimolar reaction was assumed in a limited volume for the binary mixture of star prepolymers. For this issue, the present study investigated the effects of stoichiometry by phantom chain simulations. Examined polymer networks were created from binary mixtures of star prepolymers with various mixing ratios by the end-linking reaction via Brownian dynamics simulations. The networks were stretched with energy minimization until the break. From the mechanical response, strain and stress at break and work for fracture were obtained. These fracture characteristics slightly decrease with increasing the contrast of volume fractions of the binary prepolymer blends when the node functionality is small and the conversion ratio is large. For the other cases, the stoichiometry does not impact the fracture behavior. The number ratio of broken bonds and the cycle rank exhibit similar stoichiometry dependence. Consequently, stoichiometry of prepolymer blends does not disturb the previously reported relationships between the fracture characteristics and the cycle rank.

cond-mat.soft↗

arXiv2023

Anisotropy of shear relaxation in primitive chain network simulations for entangled polymers confined between flat walls

Anisotropic shear relaxation is an interesting but rarely discussed issue in polymer dynamics under confinement [Abberton et al., Macromolecules, 48, 7631, 2015]. According to the earlier study of bead spring simulations for an unentangled polymer melt confined between two flat plates, the shear relaxation modulus taken perpendicular to the interface is accelerated by decreasing the distance between plates, whereas the parallel component is unchanged. This study observed similar anisotropic shear relaxation for entangled polymer melts under confinement in multi-chain slip-link simulations (primitive chain network simulations). The analysis demonstrated that the accelerated relaxation in the perpendicular component reflects the Rouse-type constraint release dynamics, for which the coarsening is upper-limited by the geometry. This result suggests a novel mechanism for anisotropic shear relaxation different from the modified chain statistics under confinement considered for unentangled systems.

cond-mat.soft↗

arXiv2023

Phantom chain simulations for fracture of polymer networks created from star polymer mixtures of different functionalities

Fujiyabu et al. have experimentally reported that mixing of 3-arm star prepolymers into 4-arm analog improves the toughness of the resultant polymer networks compared to the base network composed of 4-arm star polymers only. For the mechanism of this phenomenon, this study conducted phantom chain simulations for polymer networks composed of mixtures of star branch prepolymers with equal arm length and different arm numbers, (f_1,f_2 )=(3,4), (3,6) and (3,8), for various f_1=3 prepolymer fractions ϕ_3. The networks were created via end-linking reactions between prepolymers traced by a Brownian dynamics scheme, and the network structure was stored at different conversion ratios ϕ_c, ranging from 0.6 and 0.9. The cycle rank of the gelated networks ξ is fully consistent with the mean field theory, demonstrating that the examined network structure is statistically fair. The networks were stretched with energy minimization until the break, and fracture characteristics including strain at break ε_b, stress at break σ_b, work for fracture W_b, and the ratio of broken strands ϕ_bb, were obtained. ε_b, σ_b/ϕ_bb, and W_b/ϕ_bb data plotted against ξ roughly follow the master curves reported for the base networks without mixing, implying that the change of fracture properties by the mixing of f_1=3 mainly corresponds to a decrease of ξ. The mixing slightly suppresses σ_b/ϕ_bb and W_b/ϕ_bb for large f_2 cases compared to the base networks because of a biased breakage at the network strands without extenders, which are prepolymers with only two reacted arms. The analysis for broken strands exhibited a new master curve for the ξ-dependence of the molecular weight of broken strands.

cond-mat.soft↗

arXiv2023

Modeling for heterogeneous oxidative aging of polymers using coarse-grained molecular dynamics

This study presents a coarse-grained molecular dynamics simulation model to investigate the process of oxidative aging in polymers. The chemical aging effect is attributed to the auto-oxidation mechanism, which is initiated by radicals, leading to the chain scission and crosslinking of polymer chains. In this study, we integrate a thermal oxidation kinetic model in the oxygen excess regime into the Kremer-Grest model, thereby enabling a reactive coarse-grained molecular dynamics simulation to capture the process of oxidative degradation. Our simulation reveals that when the timescale of the characteristic reaction step of oxidative degradation is shorter than the longest relaxation time of polymer chains, the scission sites exhibit spatial heterogeneity. This innovative simulation model possesses the potential to enhance our comprehension of polymer aging phenomena, thus making noteworthy contributions to the realm of polymer science and degradation chemistry.

cond-mat.soft↗

arXiv2023

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

The influence of node functionality (f) on the fracture of polymer networks remains unclear. While many studies have focused on multi-functional nodes with f>4, recent research suggests that networks with f=3 exhibit superior fracture properties compared to those with f=4. To clarify this discrepancy, we conducted phantom chain simulations for star-polymer networks varying f between 3 and 8. Our simulations utilized equimolar binary mixtures of star branch prepolymers with a uniform arm length. We employed a Brownian dynamics scheme to equilibrate sols and induce gelation through end-linking reactions. We prevented the formation of odd-order loops owing to the binary reaction and second-order loops algorithmically. We stored network structures at various conversion ratios (ϕ_c) and minimized energy to reduce computation costs induced by structural relaxation. We subjected the networks to stretching until fracture to determine stress and strain at break and work for fracture, ε_b, σ_b, and W_b. These fracture characteristics are highly dependent on ϕ_c for networks with small f but relatively insensitive for those with large f. Thus, the networks with small f exhibit greater fracture properties than those with large f at high ϕ_c, whereas the opposite relationship occurs at low ϕ_c. We analyzed ε_b, σ_b, and W_b concerning the cycle rank ξ and the broken strand fraction ϕ_bb. We found ε_b, σ_b/ϕ_bb, and W_b/ϕ_bb monotonically decrease with increasing ξ, and the data for various f and ϕ_c superpose with each other to draw master curves. These results imply that the mechanical superiority of the networks with small f comes from their smaller ξ that gives higher ε_b, σ_b/ϕ_bb, and W_b/ϕ_bb than the networks with large f.

cond-mat.soft↗

arXiv2023

Increase in rod diffusivity emerges even in Markovian nature

Rod-shaped particles embedded in certain matrices have been reported to exhibit an increase in their center of mass diffusivity upon increasing the matrix density. This increase has been considered to be caused by a kinetic constraint in analogy with tube models. We investigate a mobile rod-like particle in a sea of immobile point obstacles using a kinetic Monte Carlo scheme equipped with a Markovian process, that generates gas-like collision statistics, so that such kinetic constraints do essentially not exist. Even in such a system, provided the particle's aspect ratio exceeds a threshold value of about 24, the unusual increase in the rod diffusivity emerges. This result implies that the kinetic constraint is not a necessary condition for the increase in the diffusivity.

cond-mat.stat-mech↗

arXiv2023

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

In some binary fluids, mass transport is observed under a temperature gradient. This phenomenon is called the Soret effect. In this study, we discuss the influence of inter-particle interaction. We considered equimolar binary Lennard-Jones fluids with a mass contrast, whereas the interaction was common for all the particle pairs with various cut-off lengths. We performed molecular dynamics simulations of such fluids under equilibrium to obtain the cross-transport coefficients L1q between the fluxes of mass and heat. The simulation revealed that this quantity strongly depends on the cut-off length. Further, we decomposed the heat flux into kinetic and potential contributions and calculated the cross-correlations between decomposed fluxes and the mass flux. The result indicates that the potential contribution dominates L1q, implying that the Soret coefficient is altered by the inter-particle interaction.

cond-mat.stat-mech↗

arXiv2023

Extension of Moving Particle Simulation including rotational degrees of freedom for dilute fiber suspension

We develop a novel Moving Particle Simulation (MPS) method to accurately reproduce the motion of fibers floating in sheared liquids. In conventional MPS schemes, if a fiber suspended in a liquid is represented by a one-dimensional array of MPS particles, it is entirely aligned to the flow direction due to the lack of shear stress difference between fiber-liquid interfaces. To address this problem, we employ the micropolar fluid model to introduce rotational degrees of freedom into the MPS particles. The translational motion of liquid and solid particles and the rotation of solid particles are calculated with the explicit MPS algorithm. The fiber is modeled as an array of micropolar fluid particles bonded with stretching and bending potentials. The motion of a single rigid fiber is simulated in a three-dimensional shear flow generated between two moving solid walls. We show that the proposed method is capable of reproducing the fiber motion predicted by Jeffery's theory being different from the conventional MPS simulations.

physics.flu-dyn↗

arXiv2023

Fluctuating Diffusivity Emerges even in Binary Gas Mixtures

Diffusivity in some soft matter and biological systems changes with time, called the fluctuating diffusivity. In this work, we propose a novel origin for fluctuating diffusivity based on stochastic simulations of binary gas mixtures. In this system, the fraction of one component is significantly small, and the mass of the minor component molecule is different from that of the major component. The minor component exhibits fluctuating diffusivity when its mass is sufficiently smaller than that of the major component. We elucidate that this fluctuating diffusivity is caused by the time scale separation between the relaxation of the velocity direction and the speed of the minor component molecule.

cond-mat.stat-mech↗