Suyoung Yu

Multiscale modeling of cross-linked epoxy nanocomposites to characterize the effect of particle size on thermal conductivity

A sequential multiscale model to characterize the size effects of nanoparticles on the effective thermal conductivity of SiC/epoxy nanocomposites is developed through non-equilibrium molecular dynamics (NEMD) simulations and continuum micromechanics. Even at the fixed volume fraction condition of a spherical nanoparticle, a significant particle size effect on the thermal conductivity of SiC/epoxy nanocomposites has been demonstrated using NEMD simulations. The main contributions of the particle size dependency are Kapitza thermal resistance at the interface between the particle and matrix, and the formation of highly densified polymer sheathing (adsorption layer) near the particle. To account for these two effects in a continuum regime, both the Kapitza interface and the effective interphase are defined in a micromechanics model, and a four-phase multiscale bridging method is suggested. The thermal conductivity of the effective interphase is implicitly obtained from the four-phase micromechanics model. Th...

Sequential thermoelastic multiscale analysis of nanoparticulate composites

The thermoelastic properties of SiC/epoxy nanocomposites are investigated through a molecular dynamics (MDs) simulation and micromechanics bridging method. One major finding from the MD simulation is that not only the elastic modulus but also the thermal expansion coefficient of the nanocomposites exhibits particle-size dependency at fixed volume fractions. In order to describe such effects that are observed from atomistic simulations, a micromechanics-based scale bridging method is suggested that handles both the elastic and residual fields of the nanocomposites with the help of the effective interface concept and sequential information transfer.

Analysis of thermal conductivity of polymeric nanocomposites under mechanical loading

When the plastic deformation is applied to neat polymer, the polymer chains are aligned and the thermal conductivity of neat polymer increases linearly along the loading direction. However, the thermal conductivity change of nanocomposites consisting of polymer matrix and nanofillers during plastic deformation is not simple. The volume fraction and size of nanofillers scarcely affect the structural change of polymer chains during the plastic deformation. In this study, the structural change of polymeric materials according to the mechanical loading and its effect on the thermal transport properties are investigated through a molecular dynamics simulation. To investigate the effects of nanofiller, its volume fraction, and size on the thermal transport properties, the unit cells of neat amorphous nylon 6 and nanocomposites consisting of amorphous nylon 6 matrix and spherical silica particles are prepared. The molecular unit cells are uniaxially stretched by applying constant strain along the loading directi...

Molecular Dynamics Study to Identify Mold Geometry Effect on the Pattern Transfer in Thermal Nanoimprint Lithography

Molecular dynamics simulations are performed to investigate the mold geometry effect on the pattern transfer in thermal nano imprint lithography (NIL). Layered structures composed of single crystalline diamond molds with different taper angles of 0, 15, 30, and 45°, an amorphous poly(methyl methacrylate) (PMMA) thin film, and a rigid silicon substrate were investigated, and the sequential movement of the mold followed by NVT (isothermal ensemble) simulation was implemented in accordance with a real NIL process. Both van der Waals interaction and electrostatic potentials were considered in all non-bond interactions between each layer. From the relative atomic concentration profile of the PMMA resist, the springback effects of the different mold geometries were compared. As a result, mold-PMMA interaction energy, the potential energy variation of the PMMA resist and lateral springback length were determined to be affected by the mold geometry.

A Study on the Prediction of Elastoplastic Behavior of Carbon Nanotube/Polymer Composites

In this research, a paramteric study to account for the effect of interfacial strength and nanotube agglomeration on the elastoplastic behavior of carbon nanotube reinforced polypropylene composites is performed. At first, the elastoplastic behavior of nanocomposites is predicted from molecular dynamics(MD) simulations. By combining the MD simulation results with the nonlinear micromechanics model based on the Mori-Tanaka model, a two-step domain decomposition method is applied to inversely identify the elastoplastic behavior of adsorption interphase zone inside nanocomposites. In nonlinear micromechanics model, the secant moduli method combined with field fluctuation method is used to predict the elastoplastic behavior of nanocomposites. To account for the imperfect material interface between nanotube and matrix polymer, displacement discontinuity condition is applied to the micromechanics model. Using the elastoplastic behavior of the adsorption interphase zone obtained from the present study, stress-strain relation of nanocomposites at various interfacial bonding condition and local nanotube agglomeration is predicted from nonlinear micromechanics model with and without the adsorption interphase zone. As a result, it has been found that local nanotube agglomeration is the most important design factor to maximize reinforcing effect of nanotube in elastic and plastic behavior.

A Study on the Sequential Multiscale Homogenization Method to Predict the Thermal Conductivity of Polymer Nanocomposites with Kapitza Thermal Resistance

In this study, a sequential multiscale homogenization method to characterize the effective thermal conductivity of nano particulate polymer nanocomposites is proposed through a molecular dynamics(MD) simulations and a finite element-based homogenization method. The thermal conductivity of the nanocomposites embedding different-sized nanoparticles at a fixed volume fraction of 5.8% are obtained from MD simulations. Due to the Kapitza thermal resistance, the thermal conductivity of the nanocomposites decreases as the size of the embedded nanoparticle decreases. In order to describe the nanoparticle size effect using the homogenization method with accuracy, the Kapitza interface in which the temperature discontinuity condition appears and the effective interphase zone formed by highly densified matrix polymer are modeled as independent phases that constitutes the nanocomposites microstructure, thus, the overall nanocomposites domain is modeled as a four-phase structure consists of the nanoparticle, Kapitza interface, effective interphase, and polymer matrix. The thermal conductivity of the effective interphase is inversely predicted from the thermal conductivity of the nanocomposites through the multiscale homogenization method, then, exponentially fitted to a function of the particle radius. Using the multiscale homogenization method, the thermal conductivities of the nanocomposites at various particle radii and volume fractions are obtained, and parametric studies are conducted to examine the effect of the effective interphase on the overall thermal conductivity of the nanocomposites.

Thermal transport properties of nanoparticulate composites under mechanical loading

In this study, the effect of mechanical loading on the thermal transport properties of polymer and polymer nanocomposites is investigated through molecular dynamics simulations. Amorphous nylon6 matrix with highly conductive inorganic fillers is considered and non-equilibrium molecular dynamics simulations are implemented to estimate the thermal conductivity from Fourier’s law. The Kapitza thermal resistance at the interface between nanoparticle and matrix is measured by observing the temperature drop at the interface. In order to survey the effect of mechanical loading and resultant anisotropy of thermal conductivity, molecular unit cells are uniaxially stretched by applying constant strain along the loading directions. Using the pre-stretched unit cells as initial configurations, thermal conductivities at different strain states are evaluated. The structural alignment of matrix molecules is analyzed by tracing the orientation correlation function of each matrix molecule. As a result, with the addition of nano particles, the thermal conductivity of nanocomposite increase without mechanical loading. In addition, the thermal conductivity of polymer along the loading direction increases as the mechanical loading is applied. As the magnitude of mechanical loading increases, polymer molecules tend to be aligned parallel to the loading direction and anisotropic thermal transport phenomena is observed. In nanocomposites, not only the arrangement of the matrix molecules but also the formation of void is found to be the dominant factors that affect the thermal conductivity under mechanical loading.

Influence of crosslink ratio on the thermomechanical properties of polymer nanocomposites and interphase: a molecular dynamics simulation

The effect of different crosslink ratios on the thermal and mechanical properties of thermoset epoxy-based nanocomposites are investigated by molecular dynamics (MD) simulations and a sequential scale bridging method. For establishing molecular models crosslinked epoxy structures composed of epoxy resin EPON 862 and curing agent TETA with a wide range of crosslink ratios are considered. Silica (SiO2) nanoparticles having different radii are introduced as a filler material in order to analyze the characteristics of interphase regions regarding the particle size effect. The elastic modulus and thermal expansion coefficient of various unit cells with different crosslink ratios and particle sizes are investigated using MD simulations. The results illustrate that the degree of bulk property changes of nanocomposites with respect to crosslink ratios is lower than the cases of pure epoxy structures. For the quantification of the properties of interphase regions, the interaction energy densities are investigated and a sequential scale bridging method is applied. The interaction between the particle and matrix is weakened as the crosslink ratio increases. Moreover, the elastic modulus of interphase is reduced and the size effects on thermal expansion of interphase are hindered with the presence of more crosslinked networks. The results indicate that the interphase behaviors are significantly diminished as crosslink ratio increases.

Multiscale model for polymer-based nanocomposites considering phase transition behavior

In this study, the particle size effect of polymer-based nanocomposites is estimated quantitatively below and above the glass transition temperature through molecular dynamics (MD) simulations. The thermoelasticity and thermal expansion coefficients (CTE) at each state are calculated for various particle radii under same particle volume fraction in order to characterize the size effect on thermomechanical properties. As a simulation result, the particle size effect is clearly observed in stiffness and CTE both in the glassy and rubbery state of nanocomposites because of the interphase formed by condensed matrix around the embedded particle. In order to develop the sequential bridging model which can explain the particle size effect and thermoelastic behavior in nanocomposites, the MD results are transferred into the multi inclusion micromechanics model which consists of particle, interphase, and matrix. The effective thermal strain is also considered in current micromechanics model to define the thermomechanical properties and the volume fraction of interphase as a function of the embedded particle radius and temperature. It is demonstrated that the current scale bridging model reproduce and predict the thermomechanical properties of the nanocomposites at a wide range of temperatures with reliable accuracy.