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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

This study addresses the multiscale analysis of acrylic urethane networks (AUN). To establish the kinetic model for predicting AUN oxidation, this study considered the pure thermal oxidation of AUN at 160, 180, and 200 degrees C. Chemical changes were monitored using infrared spectroscopy. These indicated the presence of an imide, presumably generated from the oxidation of CH2 at the alpha-position of nitrogen. On the macromolecular and macroscopic scales, oxidation was shown to induce predominant crosslinking, leading to a drop in toughness (i. e., embrittlement). The novel kinetic model of AUN thermal aging was developed from a mechanistic scheme previously established for polyamide 11, by adding some extra paths of thermolytic alkyl radical formation, oxidative N-H bonds decomposition and coupling of aminyl radicals.

DOI： 10.1016/j.porgcoat.2021.106654

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

In this work, the role of moisture in photo-ageing was discussed for the case of an acrylic-urethane network. Ageing behaviours were characterised by Fourier transform infra-red spectroscopy, positron annihilation lifetime spectroscopy, sol-gel analysis, and differential scanning calorimetry. Two common features between dry and wet photo-ageing, namely, decrease in the mean free volume and increase in the glass transition temperature were observed as photo-oxidation progressed. The characteristics of the final internal structure of the wet photo-aged sample differed from those of the dry photo-aged sample. Although the dry photo-aged samples suggested network densification due to extensive crosslinking formation, wet photo-ageing led to an inhomogeneous structure wherein the average molar mass between crosslinks (at a scale of several nanometres) increased and some domains crosslinked at the sub-nanoscale. Considering the physical (plasticisation) and chemical (hydrolysis) effects induced by water, a possible mechanism for the evolution of the macromolecular architecture was proposed for photo-ageing in dry and wet environments.

DOI： 10.1016/j.polymertesting.2021.107123

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

The molecular architecture changes of crosslinked urethane during photo-oxidation was investigated by Fourier transform infrared spectroscopy (FTIR) in combination with two-dimensional (2D) correlation analysis. The 2D correlation FTIR spectroscopy provided insight into molecular architectural dynamics during photo-aging, where the sequential order of peak changes was determined upon perturbation with photo-aging. Photo-oxidation was monitored from the decrease in the peak intensity at 1537 cm(-1) (attributed to the urethane group) and the generation of the hydrogen bonded C=O groups at 1698 cm(-1) immediately after urethane decomposition. Several notable peaks appeared (1712,1650, and 1750 cm(-1)) in the later stages, which were attributed to the free photo-oxidative product. In summary, photo oxidation involved a cleavage of urethane crosslink and generation of end groups that formed hydrogen bonds with surrounding hydrogen bonding sites. Further degradation of the free photo products was observed likely because of the small number of accessible hydrogen bonding sites due to the increasing heterogeneity of the internal structure. (C) 2020 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.polymdegradstab.2020.109242

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

Structural changes induced by photo-aging have significant influence on material properties. The prediction of the period prior to the onset of significant morphological changes is useful in determining material service lifetime. We assume "degelation" to be the starting point of significant morphological change during photo-aging. Herein, we demonstrated the relationship between such time and the "degelation" phenomenon for an acrylic-urethane network polymer. We also derived a "degelation" theory considering network imperfections, e. g. dangling chains and looping chains. This provides a more accurate representation of reactive curing polymers widely used in industries. Lastly, we estimated the "imperfection degree" by comparing the number of elastically active chains obtained from a swelling experiment with the theoretically derived number. In this system, samples may originally possess approximately 10% network imperfection and it influences on the degelation period in aging process. Our study may potentially contribute to a more accurate prediction of the onset of significant structural change.

DOI： 10.1016/j.polymer.2019.122035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The structural alteration of acrylic-urethane coatings, induced by photo-aging, and its temperature dependence, was investigated using positron annihilation lifetime spectroscopy and solvent swelling experiments. The results revealed a slight structural alteration due to photo-aging at 20 degrees C. On the other hand, a significant structural alteration occurred due to photo-aging at 60 degrees C, which resulted in a decrease in the pore size and total pore volume. To study the mechanism of such morphological changes, infrared spectra were measured before and after aging the samples. The results revealed the breakage of the urethane crosslink points and the generation of various photo-products including peroxide species that can form hydrogen bonds. This indicates the foundation of new physical and chemical crosslinks (different from the initial urethane linkage). Based on the chemical modifications, probable mechanisms of the internal morphological changes are proposed. Moreover, the changes are mainly due to an enhancement in the chain-mobility, due to the destruction of the polymer network and an increase in the inter-chain interactions. The results of this study reveal that significant internal structural alterations can occur after sufficient damage to the initial network structure. The proposed explanation is reasonable in relation to the temperature dependence of photo-induced structural alterations, and with respect to the molecular mobility and reaction ratio of photo-oxidation. (C) 2019 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.polymdegradstab.2019.02.007

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Society of Polymer Science  

Organic coatings for building exterior walls are attracting a lot of attention because of the requirement for long-term usage of buildings and other constructs. Polyurethane (PU) has been commonly applied on building exterior walls. UV irradiation tests were carried out in a custom made apparatus at low relative humidity (＜ 10%) and several surface temperatures. For irradiated samples, IR and Raman spectra were measured to study change of chemical structure. Irradiated samples did not yellow on exposure and we could not observe an increase of double bond groups (C=C, C=O). In terms of temperature dependent degradation reactions, the Photo-Fries reaction seemingly does not depend on temperature, whereas Norrish type reactions are promoted by higher temperature. Moreover, we investigated the relationship between aggregate-state and change of elastic modulus, in early stages, it shows a better correlation between them than in later stages.

DOI： 10.1295/koron.2017-0082

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Publishing type：Research paper (scientific journal)  

DOI： 10.1002/fld.5258

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Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1002/fld.5235

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arXiv

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Other Link： http://arxiv.org/pdf/2301.03818v1

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.conbuildmat.2023.133411

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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 ({\phi}_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,
{\epsilon}_b, {\sigma}_b, and W_b. These fracture characteristics are highly
dependent on {\phi}_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 {\phi}_c, whereas the opposite
relationship occurs at low {\phi}_c. We analyzed {\epsilon}_b, {\sigma}_b, and
W_b concerning the cycle rank {\xi} and the broken strand fraction {\phi}_bb.
We found {\epsilon}_b, {\sigma}_b/{\phi}_bb, and W_b/{\phi}_bb monotonically
decrease with increasing {\xi}, and the data for various f and {\phi}_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
{\xi} that gives higher {\epsilon}_b, {\sigma}_b/{\phi}_bb, and W_b/{\phi}_bb
than the networks with large f.

DOI： 10.1021/acs.macromol.3c01291

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Other Link： http://arxiv.org/pdf/2307.09832v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Society of Rheology, Japan  

<p>In the multi-chain slip-spring, the dynamics of Rouse chains connected by virtual springs called slip-springs that hop along the chain are considered. Although this approach has been proven to reproduce entangled polymer dynamics efficiently, the settings of the artificially introduced slip-springs should be carefully examined. In this study, we discussed the effect of the slip-spring degeneracy by limiting the number of anchored slip-springs on each Rouse bead to zero and one. According to the altered slip-spring statistics, we derived the compensation potential to eliminate the artifact of slip-springs on the chain statistics. With this potential, we performed numerical simulations. The results demonstrated that although we reasonably retain the Gaussian chain statistics, the dynamics deviate from the established entangled polymer dynamics because of the significant slow-down in the chain motion induced by the correlation between different slip-springs. The observed deceleration can be explained by the fact that the movement of slip-springs can be cast into an asymmetric exclusion process.</p>

DOI： 10.1678/rheology.51.211

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Two-dimensional sheet-shaped poly(methyl methacrylate) (2d-PMMA) with crosslinking only in the two-dimensional direction was synthesized via planar polymerization of MMA monomer in montmorillonite (MMT) nanolayers by using ?-ray irradiation, and the samples obtained were characterized. Size-exclusion chromatography with a multi-angle light scattering (SEC-MALS) measurements of the obtained sample showed bimodal peaks: a main peak with a longer elution time and an apparent molecular weight M-w,M-app and radius of gyration R-g in tetrahydrofuran (THF) similar to those for linear PMMA samples, and a subpeak with a shorter elution time and M-w,M-app and R-g evidently lower than those for the linear PMMA samples. This result indicated that polymers with moderate branching and crosslinking were contained in the subpeak. Atomic force microscopy (AFM) observation of the sample showed the presence of sheet-shaped molecules with a height of ca. 0.6 nm, corresponding to the thickness of one MMA monomer, and a width of several to several tens of nanometers, showing a broad distribution in molecular width. The subpeak portion was isolated via SEC fractionation, and its AFM observation showed thin circular disk-shaped molecules with relatively uniform size (i.e., a width of several tens of nanometers). However, in these molecules, two types of molecular heights (i.e., 0.6 and 1.2 nm) were identified. This result indicated that MMA monomers were filled as two layers within one MMT interlayer, and the polymerization reaction proceeded in that state. The similarity in the molecular size of 2d-PMMA observed via AFM and SEC-MALS was confirmed. These results demonstrated that the desired sheet-shaped polymers were obtained through the above synthesis and fractionation process.

DOI： 10.1038/s41428-023-00795-4

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Other Link： https://www.nature.com/articles/s41428-023-00795-4

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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.

DOI： 10.1103/PhysRevE.107.044604

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arXiv

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Other Link： http://arxiv.org/pdf/2301.02394v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Effects of branch functionality on mechanical properties of polymer networks are yet to be fully elucidated, although multi-functional approaches have been mainly attempted. For instance, Fujiyabu et al. [Sci. Adv. 2022, 8, abk0010] recently reported that polymer networks made from tri-branch prepolymers exhibit superior mechanical properties to tetra-branch analogues. Although they attribute the difference to stretch-induced crystallization observed in tri-branch poly (ethylene glycol) networks, the mechanism still needs to be clarified. In this study, we performed coarse-grained molecular simulations to extract the effect of branch functionality. The prepolymers were replaced by bead-spring phantom chains, and gelation was simulated by a Brownian dynamics scheme. We subjected the resultant networks to energy minimization and uniaxial stretch by introducing breakage for elongated segments. In the stress-strain relation thus obtained, stress and strain at the break were larger for tri-branch networks than that for tetra-branch analogues, consistent with the experiment. The superiority of tri-branch networks is observed in a wide range of the conversion ratio in gelation, molecular weights of prepolymers, and polymer concentrations. The result implies that the mechanical superiority of tri-branch networks to tetra-branch ones is due to a fundamental structural difference generated during gelation.

DOI： 10.1021/acs.macromol.3c00047

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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.

DOI： 10.1103/PhysRevE.107.014605

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Other Link： http://arxiv.org/pdf/2207.05424v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SOC RHEOLOGY, JAPAN  

We derive the stress tensor of a rigid dumbbell by using the virtual work method. In the virtual work method, we virtually apply a small deformation to the system, and relate the change of the energy to the work done by the stress tensor. A rigid dumbbell consists of two particles connected by a rigid bond of which length is constant (the rigid constraint). The energy of the rigid dumbbell consists only on the kinetic energy. Also, only the deformations which do not violate the rigid constraint are allowed. Thus we need the dynamic equations which are consistent with the rigid constraint to apply the virtual deformation. We rewrite the dynamic equations for the underdamped SLLOD-type dynamic equations into the forms which are consistent with the rigid constraint. Then we apply the virtual deformation to a rigid dumbbell based on the obtained dynamic equations. We derive the stress tensor for the rigid dumbbell model from the change of the kinetic energy. Finally, we take the overdamped limit and derive the stress tensor and the dynamic equation for the overdamped rigid dumbbell. We show that the Green-Kubo type linear response formula can be reproduced by combining the stress tensor and the dynamic equation at the overdamped limit.

DOI： 10.1678/rheology.50.313

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Other Link： http://arxiv.org/pdf/2205.09374v1

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Many natural exposure sites have been developed to ensure the reliability of materials intended for outdoor use. However, the effects of local climate on aging have not been completely understood. This study aimed to elucidate the regional characteristics of natural aging. Non-stabilized and stabilized polycarbonates were monitored in terms of their appearance (yellowing and loss of gloss) during natural weathering at five exposure sites (Tokyo, Kagoshima, Okinawa, Florida, and Arizona) in conjunction with climate fluctuation for up to 24 months. Three approaches were employed to characterize the natural aging behaviors: (i) modeling the rate function of degradation, (ii) evaluating the contribution ratio of individual degradational factors, and (iii) estimating the “synchronicity” by cross-correlation analysis with the climate dataset. The aging rates were the highest in Arizona and lowest in Kagoshima among the five exposure sites. First, prediction curves were constructed from the degradation rate function (variables: UV irradiation, temperature, and humidity), and these curves were found to agree well with the measured aging behaviors. Second, the exposure data in Arizona demonstrated strong temperature dependence, while those in Okinawa and Florida had stronger dependence on UV irradiation compared to other sites. Lastly, the synchronicity between UV irradiation and temperature was the highest in Arizona and lowest in Kagoshima, which can explain the significantly faster deterioration in Arizona and the slow deterioration in Kagoshima.

DOI： 10.3390/polym13050820

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：CIMNE  

DOI： 10.23967/dbmc.2020.087

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

CiNii Books

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Publishing type：Research paper (international conference proceedings)  

Polyurethane composite coatings are frequently applied to strengthen the barrier properties of various construction materials such as wood, metal and concrete. The changes of color and glossiness of the composite coatings for building materials proceeds more slowly than that of polyurethane because of containing fillers and several types of additives. However, the most critical barrier properties have not been studied directly based on the difference from the degradation mechanism of polyurethane coatings. Additionally, the difference in thermal and UV degradation mechanisms of coatings is not clear in the existing studies dealing with conventional accelerated weathering tests and outdoor exposure tests. Therefore, the behavior of the filler contained in the coatings was observed using an oven to accelerate thermal degradation. The chemical analysis of the coating surface by XPS and the analysis of the images and elemental mappings by SEM-EDS were carried out, and the mechanism of degradation progress was investigated. In addition, the temperature dependency of the degradation was studied in changing the heating temperature. It was suggested that polymer and fillers interact in promoting mass transfer at the same time as the thermal degradation reaction consequently.

DOI： 10.23967/dbmc.2020.100

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Authorship：Lead author   Language：Japanese   Publishing type：Master’s thesis  

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Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)  

CiNii Books

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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J-GLOBAL

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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J-GLOBAL

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