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Abstract The hygroelasticity of oxygen-functionalized graphene (GO)-epoxy nanocomposites is studied. Two different transversely isotropic nanocomposite molecular models are constructed: with uniformly distributed and interface-concentrated water molecules, respectively. The stress-strain curves and coefficients of moisture expansion (CMEs) are determined according to moisture content. The degradation of the GO/epoxy interface due to the infiltrated moisture is characterized by interfacial decohesion tests. The micromechanics model is used to derive new closed-form solutions for the effective elastic stiffness and CME of multi-phase composites with interfacial imperfections. Regardless of the moisture distribution, the overall hygroelastic behavior of nanocomposites clearly degrades upon moisture absorption. [ABSTRACT FROM AUTHOR]

In this study, we revealed the interfacial strengthening mechanism between a Stone- Thrower-Wales (STW) defective single layer graphene and polypropylene (PP), through a density functional theory (DFT) simulation and atomistic molecular dynamics simulations. In quantum mechanical simulation, the adhesion energy of propylene monomer on STW defective graphene is calculated with van der Waals interaction. An improved adsorption characteristic of propylene to the STW defective graphene is clearly observed, compared with a pristine counterpart. For deeper understanding of the adsorption, the electronic structure calculation and geometrical analysis of the adsorbed structures are also performed. In molecular dynamics simulation, three transversely isotropic nanocomposite unit cell structures consisting of PP and single layer graphene having a different number of STW defects are constructed. The stress-strain curves of nanocomposites according to the density of STW defects are obtained from uniaxial tension and shear tests. Since the properties of graphene itself are degraded by the STW defects, the overall stress-strain characteristics of nanocomposites involving the deformation of graphene are degraded by the addressed STW defects. However, in longitudinal shearing, where interfacial shearing between graphene and PP is involved, the STW defect can critically improve the shear load bearing capability. The increased interfacial shear load transfer is mostly attributed to the rippling of graphene at the STW defective sites, and the resultant surface roughness of graphene. [ABSTRACT FROM AUTHOR]

A multiscale continuum model is presented for the mechanics of hyperelastic nanocomposites reinforced with randomly oriented fibers and subjected to finite plane elastostatics. The hyperelastic response of the matrix material is characterized by using the Mooney Rivlin model and the kinematics of the embedded fibers are formulated via the first and second gradient of continuum deformations. In particular, we employ the shear leg theory and Krenchel orientation parameters through which the size and orientation effects of the short fibers are computed and subsequently integrated into the models of continuum deformations. Within the framework of variational principles and a virtual work statement, the Euler equation and the admissible boundary conditions are derived. Molecular dynamic simulations are also performed to obtain the microscopic responses of the graphene-reinforced composites with three distinct configurations of graphene sheets which are then incorporated into the proposed continuum model. To this end, model implementation has been made to the deformation analysis of hyperextension of nanocomposite and the continuum damage mechanics of nanocomposite induced by the interfacial debonding. The obtained results are found to be in good agreement with the existing experimental results in the literature including the extension of Ecoflex-0030 composite up to 1000% stretch. The practical utility of the proposed model may be expected in the design and analysis of hyperelastic nanocomposites exhibiting nonlinear stress–strain responses (strain-stiffening/softening). [Display omitted] • Multiscale continuum model is presented for hyperelastic nanocomposites. • Size effect of nanofibers on the Young's modulus of the composite is predicted. • Interfacial debonding induced damage mechanics is assimilated for nanocomposite. • Large deformation is assimilated for hyperelastic nanocomposites. [ABSTRACT FROM AUTHOR]

A multiscale statistical homogenization method based on a finite element analysis is proposed to predict the embedded filler size-dependent thermoelastic properties of nanoparticulate polymer composites. A molecular dynamics simulation is used to predict the particle size-dependent elastic constants and coefficient of thermal expansion (CTE) of nanocomposites. Because of the densified interphase zones formed in the vicinity of nanoparticles, and their relative dominance according to particle size, the thermoelastic properties are more prominently increased by smaller nanoparticles. The equivalent continuum microstructure of a nanocomposite is modeled as a three-phase periodic unit cell consisting of the nanoparticle, matrix, and effective interphase zone. An inverse numerical scheme is proposed to predict the thermoelastic properties of the effective interphase zone from the known properties of nanocomposites. In order to account for more realistic variation of the nanoparticle radius and spatial distribution in nanocomposites, statistical homogenization is performed by assigning randomness to the particle size using a beta distribution and to the spatial distribution through larger many-particle embedded representative volume elements (RVEs). Compared with the result from a regular homogenization method using a mono-particle RVE, the mean value of the elastic moduli increases, while that of the CTE decreases in the many-particle RVEs whose mean particle radius corresponds to that of the mono-particle RVE at the same volume fraction of nanoparticles. [ABSTRACT FROM AUTHOR]

In this study, we investigate the influence of the intrinsic defects of carbon nanotube (CNT) on the interfacial shear strength (IFSS) of the interface between CNT and a polypropylene (PP) matrix using molecular mechanics (MM) and molecular dynamics (MD) simulations. Three different inherent defects of the single void (Void) defect, adsorbed atom (Adatom) defect, and crystallographic Thrower–Stone–Wales (TSW) defect are considered. To quantitatively and qualitatively evaluate the IFSS according to the type and number of defects, quasi static CNT pull-out tests are performed through MM simulations. Among three defects, the single Void defect is found to decrease the IFSS while both the TSW and Adatom defects are found to promote interfacial shear load transfer. Moreover, self-assembly of PP molecules onto the surface of the defected CNT and resultant interfacial strength during the adsorption process are studied through MD simulations at the glassy state of the PP molecules. Consistent results on the interfacial strength between the defected CNT and PP molecules from the pull-out simulation and a defect-dependent adsorption time rate of PP in self-assembly are obtained. [ABSTRACT FROM AUTHOR]

The degradation mechanism of mechanical properties of a polymer nanocomposite consisting of agglomerating fillers is elucidated. It is found that overall elastic moduli of nanocomposites obtained through molecular dynamics simulation decreases according to agglomeration of the two embedded nanoparticles, which prevent efficient formation of interphase zone. Meanwhile, no prominent local field fluctuation by the agglomeration is observed. A percolation-related interphase model based on multiscale mathematical homogenization method is thus proposed to describe the degradation of nanocomposites in terms of the properties and overlap of interphase zone. Extensibility of the proposed model to a polydisperse nanoparticulate composites model is also confirmed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Influence of mold and substrate materials combinations on the adhesion characteristics in nanoimprint lithography is studied. [•] Both silica and Ni have higher adhesion with PMMA resist, thus, suitable for substrate. [•] Si and diamond are rather suitable for mold than substrate for their weak adhesion to PMMA. [•] Simple bi-layered molecular model analysis is proposed to predict local adhesion characteristics. [•] Both full NIL simulation and simple layer model analysis generate coincident result on adhesion characteristics. [ABSTRACT FROM AUTHOR]

• Reciprocity of interface, interphase, and elasticity of CNT in mechanical behaviour of nanocomposites was for the first time revealed by hierarchical multiscale modelling. • Covalent grafting of CNTs to PET matrix degrades mechanical properties of CNTs. • Shear load transfer at the interface in CNT/PET nanocomposites is promoted by covalent grafting and makes interphase zone actively participate in load bearing. • Two-step inverse micromechanical model was proposed to characterize the mechanical properties of interphase zone grafted to the CNT • Effect of interface/interphase at non-dilute condition of CNTs could be accurately described by the proposed multiscale model Covalent grafting between carbon nanotubes (CNTs) and a polymer matrix is the most efficient way to tailor the intrinsically weak interface in nanocomposites. To understand the grafted structure-to-improved property relationship of nanocomposites, however, the degradation of grafted CNT and the properties of surrounding interphase zone should be accounted for in constitutive model. In this study, the reciprocity of the interface, interphase, and elasticity of CNTs depending on the covalent grafting between the CNT and a polyethylene terephthalate (PET) matrix were studied through molecular dynamics simulations and a mean-field micromechanical interface/interphase model. The replacement stiffness method was used to determine the effect of the tailored interface on the overall elasticity of a nanocomposite in micromechanics. The elasticity of the CNTs and the nanocomposites was determined from molecular mechanics and molecular dynamics simulations, respectively. Despite the degraded elasticity of the nanotubes, clear improvements in the transverse modulus and shear moduli were observed in the covalently grafted nanocomposites. The elasticity of the interphase was determined in terms of the number of covalent grafting and the interfacial compliance using a two-step inverse micromechanical analysis. Regardless of the number of covalent grafting addressed, the elastic moduli of the interphase were always larger than those of a neat PET matrix. Furthermore, the accuracy of the proposed multiscale micromechanical interface/interphase model at various volume fractions of CNTs was validated. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: A molecular dynamics (MD) simulations study is performed on Thrower–Stone–Wales (TSW) defected carbon nanotube (CNT)/polypropylene (PP) composites. We identify the degradation of the CNT and the improvement of the interfacial adhesion between the defected CNTs and PP molecules considering different CNTs with different numbers of TSW defects. By embedding the CNTs into a PP matrix, the effect of the TSW defects on the transversely isotropic elastic stiffness of polymer composites is calculated by MD simulations. Even if the TSW defects degrade the elastic properties of the CNTs, the transverse Young’s modulus and the transverse and longitudinal shear moduli of the composites increase due to the stronger interfacial adhesion between the defected CNTs and matrix, whereas the longitudinal Young’s modulus of the composites decreases. To elucidate the improved interfacial load transfer between the CNTs and the matrix, random polymer chain crystallization onto the surface of CNTs is simulated. The simulation shows that PP chains are wrapped more uniformly onto the surfaces of defected CNTs than onto the pristine CNT. The non-bond adhesion energy between the PP chains and the defected CNTs is greater than that between the PP chains and the pristine CNT. [Copyright &y& Elsevier]

Abstract: In this study, multiscale homogenization modeling to characterize the thermal conductivity of polymer nanocomposites is proposed to account for the Kapitza thermal resistance at the interface and the polymer immobilized interphase. Molecular dynamics simulations revealed that the thermal conductivity dependent on the embedded particle size originated from the structurally altered interphase zone of surrounding matrix polymer in the vicinity of nanoparticles, and clearly indicate strong dominance of interfacial phonon scattering and dispersion. To account for both the thermal resistance and the immobilized interphase, a four-phase equivalent continuum model composed of spherical nanoparticles, Kapitza thermal interface, effective interphase, and bulk matrix phase is introduced in a finite element-based homogenization method. From the given thermal conductivity of the nanocomposites obtained from MD simulations, the thermal conductivity of the interphase is inversely and numerically obtained. Compared with the micromechanics-based multiscale model, the thermal conductivity of the interphase can be obtained more accurately from the proposed homogenization method. Using the thermal conductivity of the interphase, the random distribution and radius of nanoparticles to describe real nanocomposite microstructure are considered, and the results confirm the applicability of the proposed multiscale homogenization model for further stochastic approaches to account for geometric uncertainties in nanocomposites. [Copyright &y& Elsevier]

Abstract: A hierarchical multiscale modeling approach to characterize the elastic and plastic behavior of carbon nanotube (CNT)-reinforced polymer nanocomposites is proposed via molecular dynamics simulations and a continuum nonlinear micromechanics based on the secant moduli method. Even though the importance of the densified interphase zone formed between the CNT and polymer matrix has been demonstrated by many related studies for elastic properties, studies on how to identify the behavior and contribution of the interfacial condition and interphase zone in the overall elastoplastic behavior of nanocomposites is still an open issue. Different from conventional micromechanics approaches that homogenize overall elastoplastic behavior of heterogeneous structures from known behaviors of its constituent phases, the present study focuses on the identification of local elastoplastic behavior of the interphase region from the known elastoplastic behavior of nanocomposites through a hierarchical domain decomposition method. Firstly, the overall elastoplastic behavior of the CNT-reinforced nanocomposites is obtained from molecular dynamics (MD) simulations which are based on an ab initio force field. Due to a weak van der Waals interaction between the pristine CNT and the matrix polymer, the elastoplastic behavior of the nanocomposites clearly shows a weakened interface condition, while the matrix molecular structure in the vicinity of the CNT confirms the existence of the interphase zone. In upper level analysis, an effective matrix concept is adopted, and its elastoplastic behavior is inversely identified by equating the MD simulation result to a two-phase nonlinear micromechanics model that can consider imperfect interfacial condition. Then, the effective matrix domain is again decomposed into the interphase and pure matrix polymer regions in lower level analysis, and the elastoplastic behavior of the interphase is again identified through the same method. Using the constitutive relation of the interphase obtained from the proposed multiscale model, the overall elastoplastic behavior of the nanocomposites is obtained and compared with some available experimental results and an additional MD simulation result to validate the applicability and physical rigorousness of the proposed nonlinear multiscale approach. [Copyright &y& Elsevier]

Abstract: In this study, we propose a sequential multiscale modeling approach to describe the thermoelastic behavior of cross-linked epoxy/silicon carbide (SiC) nanocomposites embedding different radii of nanofiller through molecular dynamics (MD) simulations and a continuum micromechanics constitutive model. In MD simulations, the coefficients of thermal expansion (CTEs), the elastic moduli, and the glass transition temperatures of different nanocomposites having different particle sizes are obtained at temperatures from 250 K to 550 K, the range in which cross-linked epoxy polymers generally experience the glassy-to-rubbery phase transition and consequently, their CTEs and elastic moduli change dramatically. In the equivalent continuum model, an interphase is defined between the particle and the matrix to account for the contribution of the polymer densification in the vicinity of the nanoparticle to the size-dependent CTE and elastic modulus at each temperature. Based on the thermal strain field defined in the micromechanics constitutive model, a physically meaningful description of the thermal expansion behavior of the interphase is obtained to reproduce the MD simulation results from fully continuum-based approaches for nanocomposites in rubbery state as well as in glassy state. Finally, the accuracy of the proposed multiscale approach is confirmed from finite element analysis. [Copyright &y& Elsevier]

Abstract: We developed an efficient and extensible multiscale analysis to consider the carbon nanotube (CNT) size effect and weakened bonding effect at the interface on the effective elastic stiffness of CNT/polymer nanocomposites using molecular dynamics (MD) simulations and continuum micromechanics. Under the assumption that the CNT molecular structure is an equivalent solid cylinder, molecular mechanics calculation results for transversely isotropic elastic stiffness were found to decrease as the radius of the CNT increased. Similarly, the transversely isotropic elastic moduli of aligned pristine CNT-reinforced polypropylene composites obtained from molecular dynamics simulations exhibited the same CNT size dependency. However, a weakened interface effect was observed from the transverse Young’s modulus and two shear moduli. To account for the size effect and the weakened interface in the micromechanics-based multiscale model, a modified multi-inclusion model is derived with an effective particle scheme. Also, an effective matrix concept is suggested to account for the formation of an interphase near the surface of the CNT, and the elastic stiffness of the CNT and the effective matrix is defined as a function of the CNT radius to describe size-dependent elastic stiffness in the micromechanics regime. [Copyright &y& Elsevier]

Abstract: The effect of different sized alumina (Al2O3) nanoparticles on the mechanical properties of thermoset epoxy-based nanocomposites is investigated using molecular dynamic (MD) simulations combined with sequential scale bridging methods. In molecular structures, the cross-linked networking effect of the pure EPON862®–TETA® polymer has been independently considered and validated by MD simulations. Based on the validation of pure epoxy structures, nanocomposites'' unit cells, consisting of spherical Al2O3 particles and epoxy, have been constructed. In order to investigate the particle size effects, various unit cells having different particle radii but the same volume fraction have been considered and simulated. The mechanical properties of the nanocomposites are calculated using the Parrinello–Rahman fluctuation method to give an enhanced reinforcing effect in smaller particle reinforced cases. Based on the MD simulation results, the sequential bridging method is adopted for efficient estimation of the particle size and epoxy networking effects. An effective interface concept is incorporated as a characteristic phase which can describe the particle size effects. The values calculated from the micromechanics model are in good agreement with those of the molecular dynamics simulations. [Copyright &y& Elsevier]

• Epoxide and hydroxyl groups degrade mechanical properties of single-layer graphene. • Oxygen functional groups enhance surface roughness to boost interfacial shear load transfer between graphene and polymer matrix in composites. • Local evolution of the shear stress in matrix and oxygen functionalized graphene is sensitively affected by the oxygen functional groups. • Interfacial compliance for the equivalent continuum modelling of nanocomposites varies according to the oxygen functionalization of embedded graphene. • The transverse Young's modulus and longitudinal shear modulus of graphene were conjectured to be as large as the longitudinal Young's modulus and in-plane shear modulus. Based on the results of molecular dynamics (MD) simulations and a mean-field micromechanics model, we report on some positive contributions of the oxygen functional groups in single-layer graphene oxide (GO) to the mechanical and interfacial properties of polyethylene (PE)/graphene nanocomposites. As the epoxide and hydroxyl group degrade the mechanical properties of single-layer graphene, clear degradations in the longitudinal Young's and in-plane shear moduli are observed when the deformation of graphene is involved in the loading of the nanocomposite unit cells. However, a significant improvement in the longitudinal shear modulus of nanocomposites is predicted. By comparing the MD simulation results with double-inclusion (D-I) model predictions, contributions of the interphase zone and the interfacial stiffening effect to the elasticity of nanocomposites are again confirmed. Finally, we demonstrate a novel evolution of the out-of-plane normal stress and longitudinal shear stress in single-layer GO arising from its interaction with the surrounding PE matrix via atomic virial stress. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A multiscale modeling approach is proposed to characterize the interfacial behavior and the interphase properties of epoxy nanocomposites. The interfacial characteristics between the filler and matrix are investigated using molecular dynamics (MD) and molecular mechanics (MM) simulations. With increasing crosslink conversions, the interfacial adhesion between the filler and matrix is reduced which is attributed to the changes of inherent non-bond interaction characteristics at the interface, resulting in retarded reinforcing effect on the stiffness and thermal stability of epoxy nanocomposites. Moreover, to understand the structural change in the interphase region of nanocomposites with crosslinking, the radial density profile, the local crosslinks distribution, and the free volume at the filler surface are further examined. The results of structural features consistently demonstrate that the structural conformation of the interphase is substantially influenced by the reduction of interfacial communication with increasing crosslink conversion. In order to take into account the variations of interfacial compliance and the thermomechanical property of the interphase region, the effective interphase concept is implemented. Further, the micromechanics-based multi-inclusion model provides a reasonable prediction for the thermomechanical property of composites using the effective interphase concept. [ABSTRACT FROM AUTHOR]

• Hyperthermal erosion of nanocomposites by atomic oxygen and nitrogen molecule was compared for the first time. • Optimum reactive simulation condition to mimic disintegration under low earth orbital environment was provided. • Atomic oxygen bombardment showed reaction-assisted disintegration and desorption of nanocomposites. • Attack of nitrogen molecules showed comparable surface erosion to the atomic oxygen bombardment without active reaction. • Erosion yield of nanocomposites comparable to the in-flight experiment could be obtained. Against the hyperthermal atomic oxygen (AO) bombardment, mitigation of the surface recession has been the main issue in designing surface shielding materials in low earth orbital (LEO) environment. Meanwhile, the role of nitrogen molecules (N 2) in LEO and sub-LEO on disintegration of the shielding materials has rarely been studied despite their abundance and non-negligible colliding energy. This study for the first time compared the surface chemistry and disintegration of Kapton polyimide-based nanocomposites under AO and N 2 bombardment conditions using a reactive molecular dynamics (MD) simulation. The heat transfer from the surface to the behind layer and the associated boundary condition for the bombardment simulation were primarily discussed to provide the optimum bombardment simulation setup. Polyhedral oligomeric silsesquioxane (POSS), carbon nanotube (CNT) and graphene were respectively blended with Kapton to evaluate the damage mitigation efficacy. The addition of all types of additives to Kapton induced a notable thermo-protection effect. AO bombardment resulted in a sequential disintegration of oxidation and desorption, whereas the surface recession of nanocomposites by exposure to N 2 collision was mainly caused by physical desorption. It was also noteworthy that the surface recession caused by N 2 bombardment was comparable to that by AO attack at the same fluence. [Display omitted]. [ABSTRACT FROM AUTHOR]

Low-dimensional nanocarbon reinforcements in multifunctional composites have neither defect-free conjugate structures nor efficient interfacial properties in general. Moreover, synthesizing polymer nanocomposites involves intrinsic and extrinsic structural defects, which undoubtedly degrade the properties of the nanocarbons. However, several advantages and even useful applications of such structural defects have pioneered the defect engineering of nanocarbon structures. Here, we assessed the reciprocity of the degradation of single-layer graphene and the tailoring of the interfacial load transfer by structural defects. For this purpose, we used a molecular dynamics simulation and its hierarchical bridging to a mean-field micro-mechanics model for linear elastic behavior. Crystallographic defects and oxygen functionalization were considered the representative intrinsic and extrinsic defects, respectively. Despite the notably degraded elasticity of the nanocomposites involving direct load bearing by the defected graphene, those readily governed by the interfacial load transfer were significantly improved. The contributions of both types of defects to the interface were verified using the local stress concentration on the embedded graphene during the mechanical loading of nanocomposites and an additional mode I and II decohesion test. Lastly, the critical aspect ratio of graphene that maximizes the effects of the defects on the interface stiffness of the nanocomposites was derived for defect engineering. [Display omitted] • Oxygen functional group and TSW defect degrade mechanical properties of graphenes. • Interfacial load transfer and associated elastic modulus of nanocomposites were promoted by structural defect. • Cohesive law between defected graphene and PET matrix was correlated to the surface roughness of graphenes. • Reciprocity of interface and elasticity of defected graphene was studied by micromechanics model. • Critical aspect ratio of graphene to take advantage of defect was defined for defect engineering. [ABSTRACT FROM AUTHOR]

A statistical multiscale homogenization strategy of polymer nanocomposites is proposed to account for the inherent uncertainties of molecular dynamics (MD) simulations. The proposed statistical multiscale homogenization scheme includes a discrete MD simulation system, a continuum theory of micromechanics of Eshelby's solution and two-scale homogenization, and Monte-Carlo simulations. The means and standard deviations of the elastic properties of the nanocomposites are quantified and discussed through statistical analyses that show the interphase effect. The elastic properties of the matrix, interphase, and composites are assumed to follow a lognormal distribution. An iterative inverse algorithm for obtaining the probability density distribution of the interphase is proposed and validated. [ABSTRACT FROM AUTHOR]

Shape-memory polyurethanes (SMPUs) are promising materials that change shape in response to external heat. These polymers have a dual-segment structure: a hard segment for netpoint and a soft segment for molecular switch. Understanding the molecular behavior of each segment and microphase-separated morphology is crucial for comprehending the shape-memory mechanism. This study aimed to understand the shape-memory behavior by observing the phase separation of SMPU using mesoscale models based on dissipative particle dynamics (DPD) simulations. The SMPU copolymer was modeled using 4,4′-diphenylmethane diisocyanate (MDI, hard segment) and poly(ethylene oxide) (PEO, soft segment). By calculating segment solubility and repulsion parameters, we found that the hard-segment domain changes from isolated form to a lamellar and interconnected structure and eventually to a continuous form as its content increases. Combining these insights with shape-memory performance models can enhance our understanding of better SMPU design and contribute significantly to the optimization of smart stimuli-responsive materials. [Display omitted] • Phase separation behavior of shape-memory polyurethane has been reproduced by dissipative particle dynamics simulations. • The repulsion parameter between hard and soft segments was calculated by using the difference in solubility of each segment. • As the hard-segment content increase, the hard-segment domain changes from an isolated state to a continuous state. • The shape-memory performance of semi-crystalline SMPU can be adjusted by optimizing the phase separation. [ABSTRACT FROM AUTHOR]

The thermo-mechanical characteristics of thermoset epoxy based nanocomposites are investigated with molecular dynamics (MD) simulations. For establishing molecular models, spherical silica (SiO 2 ) nanoparticles and crosslinked epoxy structures (EPON 862 ® -TETA) are considered as a filler and matrix phase material, respectively. The reinforcing effect of stiffness and thermal stability by addressing the spherical silica nanofillers is clearly observed: increase in elastic modulus and decrease in thermal expansion coefficient. Meanwhile, the degree of enhancement decreases with increasing crosslink density. This phenomenon is attributed to the reduction of interfacial interactions between the filler and epoxy matrix with the valence changes of atoms which involve crosslinking reactions. To investigate the interphase property, a multiscale bridging method, combined with the multi-inclusion model and MD simulation, is introduced. Furthermore, the effective interphase concept is addressed to account for the inherent interfacial characteristics with the formation of crosslinks. [ABSTRACT FROM AUTHOR]

The characterization of the effective mechanical properties and geometry of the interphase in a heterogeneous material is a key issue for the design of polymer nanocomposites due to the dominant effect of the interphase on the overall behavior of the composites. In this study, molecular dynamics (MD) simulation and finite element (FE) analysis were integrated to develop a mechanics-based multiscale approach that can derive both the global stiffness and the local load transfer on the filler surface of the particulate nanocomposites. The unknown mechanical response and geometrical boundaries of the interphase (polymer networks adsorbed on the particle surface) are numerically obtained from a continuum model through the matching of homogenization and deformation energy to a full atomic molecular model. The equivalent continuum models given from the present multiscale method successfully represent the virial local stresses at both the interphase and matrix regions of the full-atomic model, as well as the particle size dependent stiffness of the nanocomposites. The proposed method is used to characterize the internal mechanical behavior of the intermediate media in terms of the nanoparticle size, and the nanophysics of the intermediate media are discussed in detail. [ABSTRACT FROM AUTHOR]

Though all-atomistic (AA) molecular dynamics (MD) simulations have been effectively employed to develop structure-to-property relationships in thermal transport phenomena, the modeling of structures with millions of atoms pertinent to the real microstructures of highly conductive condensed matter is rarely attempted. Herein, we present a novel coarse-grained (CG) modeling scheme for predicting the thermal conductivity of an amorphous polymer using nylon 6 as a representative thermoplastic, based on non-equilibrium molecular dynamics (NEMD) simulations. To accurately describe the thermal transport through the primary and secondary bonds in any structural conformation of nylon 6, the CG potential parameters were systematically optimized using a particle swarm optimization (PSO) algorithm to reproduce the thermal conduction of a single chain as well as bulk amorphous state. The validity and performance of the newly developed CG potential parameters were primarily confirmed from the compatibility with the reference AA model in terms of two basic structure-property relationships: the domain-size effect on thermal conductivity of a single chain and strain-dependent thermal conductivity of bulk amorphous state. Finally, we calculated the vibrational properties of the amorphous state and found that the CG model could describe the low-frequency vibrational motion that was primarily observed in the AA model. • A novel coarse-grained (CG) modeling scheme for predicting thermal transport properties of thermoplastics. • PSO were used for CG parameterization to describe phonon transfer via primary and secondary bonds. • CG model reproduced chain-level thermal conduction and strain-dependent thermal conductivity of bulk state. • Applicability to microscopic heat conduction and material heterogeneity was discussed. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the glass transition and thermoelastic properties of cross-linked epoxy-based nanocomposites and their filler-size dependency are investigated through molecular dynamics simulations. In order to verify the size effect of nanoparticles, five different unit cells with different-sized silicon carbide (SiC) nanoparticles are considered under the same volume fraction. By considering a wide range of temperatures in isobaric ensemble simulations, the glass transition temperature is obtained from the specific volume–temperature relationship from the cooling-down simulation. In addition, the coefficient of thermal expansion (CTE) and the elastic stiffness of the nanocomposites at each temperature are predicted and compared with one another. As a result, the glass transition and thermoelastic properties of pure epoxy are found to be improved by embedding the SiC nanoparticles. Especially regarding the CTE and elastic moduli of nanocomposites, the particle-size dependency is clearly observed below and above the glass transition temperature. [Copyright &y& Elsevier]

The moisture- and salt water-induced degradation of asphalt concrete molecular systems was investigated via molecular dynamics simulations. To establish the microstructure-to-property relationship of asphalt concrete mixture under hygroscopic aging, water molecules and sodium chloride (NaCl) ionic solutes were uniformly distributed in the binder or concentrated at the interface. The mechanical properties of the binder were slightly degraded by moisture and NaCl solutes. The viscosity of the binder decreased with distributed water, whereas it increased as the content of NaCl solute in the salt water increased. Moreover, the NaCl solutes promoted the hygroscopic eigenstrain of the asphalt binder. The interfacial properties between the binder and aggregate were not noticeably affected when moisture and NaCl solutes were uniformly distributed in the binder. While the moisture penetration into the interface was thermodynamically spontaneous when the amount of water was sufficient, the NaCl solutes in the water hindered the diffusion of moisture into the interface. Nonetheless, salt water penetrated into the interface seriously reduced the interfacial adhesion and the cohesive law in mode I and mode II decohesion than pure water. The condensation of penetrated salt water and the associated peeling mechanism of the interface were discussed to clarify the hygroelastic damage mechanism of the asphalt concrete mixture. • Degradation of hygroscopically aged asphalt concrete mixture was studied. • Moisture and Salt water resulted in hygroscopic swelling and remarkable change in viscosity of AAA-1 binder. • Penetration of water molecules in asphalt mixture was thermodynamically spontaneous and degraded the interface. • Dissolved NaCl mitigated the penetration of salt water while seriously deteriorated the interface in asphalt mixture. • Hygroelastic properties of binder and interface essential to constitutive modelling of asphalt mixture were determined. [ABSTRACT FROM AUTHOR]

We propose an efficient simulation-based methodology to characterize the quasi-static (experimental low strain rate) yield stress of an amorphous thermoset polymer, which has generally been considered a limitation of molecular dynamics (MD) simulations owing to the extremely short time steps involved. In an effort to overcome this limitation, the temperature-accelerated method – in which temperature is treated as being equivalent to time in deformation kinetics – is employed to explore the experimental strain rate conditions. The mechanical tensile behavior of a highly crosslinked polymer is then investigated with MD simulations by considering different strain rates and temperatures below the glass transition temperature. The derived yield stress represents the time- and temperature-dependent characteristics, showing that the yield stress decreases with increasing temperature and decreasing strain rate. Changeable vertical and horizontal shift factors are introduced for the first time to reflect nonlinear characteristics of the yield stress across a broad range of strain rates and to quantify the correlation between increasing temperatures and decreasing strain rates. With the proposed method, the Eyring plot, which describes the rate effect on yield from quasi-static to high-rate conditions, is predicted from MD simulations, and agrees well with macroscopic experimental results. From the constructed Eyring plot, the experimentally validated quasi-static stress-strain response is also estimated by using linear elastic model and Ludwick's hardening model. The proposed method provides new avenues for the design of glassy polymers using only fully atomistic MD simulations, thus overcoming the existing temporal scale limitations. [ABSTRACT FROM AUTHOR]

A multiscale model was developed to simultaneously investigate the effect of covalent grafting and clustering of silica nanoparticles on the mechanical behaviors of polyimide nanocomposites. The interphase percolation model was utilized to account for the enhanced interfacial load transfer efficiency by covalent grafting and weaken formation of the interphase zone by agglomeration. Through homogenization analysis of the degree of agglomeration for each grafting ratio, we concluded that the highly grafted interface offsets the negative effects of nanoparticle clustering. The proposed multiscale framework was validated against existing experimental data, which implies that this approach can be used effectively for the design of grafted nanoparticle-reinforced polymer matrix composites. Image 1 [ABSTRACT FROM AUTHOR]