Hyunseong Shin

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.

Multiscale analysis of polymer nanocomposites considering hyperelasto-plastic behavior

ecently, polymer nanocomposites receive attention in various fields such as academic field and manufacturing field [1]. Merits of polymer nanocomposites come from the interface effect between nanoparticle and polymer matrix. As the particle radius decreases, the interface effect becomes remarkable due to surface-to-volume ratio of nanoparticles. The mechanical behaviors of polymer nanocomposites are enhanced by interface effect. In order to characterize the mechanical properties of interphase, sequential multiscale bridging method is studied by many researchers. There are some studies on the plastic deformation of polymer nanocomposites through the molecular dynamics simulation. Yield point and hardening parameters could be obtained through molecular dynamics simulation, and interphase characteristics could be also obtained through multiscale framework with micromechanics based on Eshelby’s solution [2]. For carbon materials, weakened interface effect could be considered in multiscale model as well. However, a study on the hyperelasto-plastic behavior of polymer nanocomposites is not achieved enough. Nonlinear elastic behavior and plastic behavior of polymer nanocomposites with respect to various filler size are critical factor in view of mechanical design. Especially, in order to handle mechanics of structures with periodic microstructures subjected to large deformation with rotation, macroscopic constitutive equation obtained from twoscale homogenization method should be preceded. In this study, hyperelasto-plastic behavior of polymer nanocomposites are investigated through molecular dynamics simulation. Authors try to elucidate plastic mechanism of polymer matrix and polymer nanocomposites explicitly. With this motivation, unloading simulations and dihedral angle distribution change are conducted as well. Many molecular dynamics results show that change of dihedral conformation of molecules cause the plastic strain of thermoplastic structures [3]. For example, D. Hossain states that dihedral angle change from Gauche state to Trans state is main reason of plastic deformation of amorphous polyethylene in free volume [4]. However, there is no research considering both unloading simulation and plastic mechanism in molecular dynamics simulation. In this study, irreversible characteristics of dihedral angle change under loading and unloading process are identified. More studies to consider plastic mechanism in microscopic viewpoint will be included in future work. The hyperelasto-plastic model about polymer matrix and interphase are not completed in this study. In hyperelastic range, the generalized Mooney-Rivlin model is employed to describe the hyperelastic behavior of Nylon6 polymer matrix. In order to define plastic behavior of polymer matrix, hardening function is constructed by

An interphase design strategy for multifunctional polymer nanocomposites using multiscale method

In this study, a multiscale model which integrates molecular dynamics (MD) simulation and finite element (FE) analysis has been developed to design multifunctional polymer nanocomposites and their effective interphase. Both the global stiffness of the polymer nanocomposite model and the internal stress distribution on the nanofiller surface during mechanical loadings were quantitatively characterized. Through MD simulations, crosslinked epoxy resin (crosslinking ratio: 0.45) and nano-sized filler (spherical SiC and zigzag single walled carbon nanotube) embedded epoxy nanocomposite models were prepared with full atomistic detail. For each model, uniaxial tensile tests were carried out to obtain the elastic behavior of the nanocomposites and the strain energy distribution in the vicinity of a nanofiller surface. Meanwhile, a three-dimensional FE model of a three-phase was prepared, consisting of a nanofiller, polymer networks adsorbed on the nanofiller surface (interphase), and polymer networks non-adsorbed on the nanofiller surface (bulk matrix). The unknown mechanical response and thickness of the interphase were numerically characterized through homogenization and deformation energy matching to that of the full atomic molecular model, respectively. The present multiscale method, therefore, yields an effective region of the interphase as well as its mechanical properties. The suggested multiscale model accurately predicts virial local stresses at both the interphase and bulk matrix regions of the full-atomic model and explains the reinforcing mechanism at the interphase region.