We performed molecular dynamics (MD) simulation that includes multidisciplinary characteristics from synthesis to mechanical properties of epoxy resin. First, to reproduce the actual chemical reaction between matrix and curing agents, we conducted curing simulation wherein the activation energy and heat of formation are considered for the chemical reaction. Subsequently, we performed MD simulations using cross-linked structure obtained from curing simulation to derive density and Young's modulus. Results indicated that crosslinked structures involving both activation energy and heat of formation could reproduce experiment results that are evaluated using differential scanning calorimetry (DSC) measurements and mechanical tests. The simulated results imply that electrostatic interaction plays an important role in Young's modulus. The density of the hydrogen bond between the oxygen of the hydroxyl group and the hydrogen atom is a key factor for the difference in Young's modulus for each base resin. These findings confirm that MD simulation is a potential alternative to experiments for the appropriate material selection of epoxy resin.

We performed molecular dynamics (MD) simulations on amorphous polyethylene (PE) and polystyrene (PS) in order to elucidate the effect of crosslinks between polymer chains on heat conduction. In each polymer system, thermal conductivities were measured for a range of crosslink concentration by using nonequilibrium MD techniques. PE comprised of 50 carbon atom long chains exhibited slightly higher conductivity than that of 250 carbon atom long chains at the standard state. In both cases for PE, crosslinking significantly increased conductivity and the increase was more or less proportional to the crosslink density. On the other hand, in the PS case, although the thermal conductivity increased with the crosslinking, the magnitude of change in thermal conductivity was relatively small. We attribute this difference to highly heterogeneous PS based network including phenyl side groups. In order to elucidate the mechanism for the increase of thermal conductivity with the crosslink concentration, we decomposed energy transfer into modes associated with various bonded and non-bonded interactions. V

Although the computation of heat flux and thermal conductivity either via Fourier's law or the Green-Kubo relation has become a common task in molecular dynamics simulation, contributions of three-body and larger many-body interactions have always proved problematic to compute. In recent years, due to the success when applying to pressure tensor computation, atomic stress approximation has been widely used to calculate heat flux, where the LAMMPS molecular dynamics package is the most prominent propagator. We demonstrated that the atomic stress approximation, while adequate for obtaining pressure, produces erroneous results in the case of heat flux when applied to systems with many-body interactions, such as angle, torsion, or improper potentials. This also produces incorrect thermal conductivity values. To remedy this deficiency, by starting from a strict formulation of heat flux with many-body interactions, we reworked the atomic stress definition which resulted in only a simple modification. We modified the LAMMPS package accordingly to demonstrate that the new atomic stress approximation produces excellent results close to that of a rigid formulation.

The thermal boundary conductance between water and self-assembled monolayer was studied using nonequilibrium molecular dynamics simulations. Different thermal transport behaviors were observed for hydrophobic and hydrophilic self-assembled monolayers. In the temperature range between 280 and 340 K, the thermal boundary conductance was found to depend on the temperature for hydrophobic self-assembled monolayers. On the contrary, the difference in thermal boundary conductance at different temperatures was slight for hydrophilic self-assembled monolayers. The correlations in velocity and density between terminal atoms of self-assembled monolayer and water molecules within the interface region were analyzed to understand the mechanism of thermal transport across the interface. The vibrational density of states calculation indicated that the temperature dependence does not originate from the overlap of phonon spectrum. The analysis of radial density distribution revealed that the temperature dependence is mainly attributed to the number of water molecules surrounding the terminal atoms of self-assembled monolayers.

In this study, we carried out molecular dynamics simulations of a cylindrical Lennard-Jones droplet on a flat and smooth solid surface and showed that Young’s equation as the relation among solid-liquid, solid-vapor, and liquid-vapor interfacial tensions γSL, γSV, and γLV, respectively, was applicable only under a very restricted condition. Using the fluid stress-tensor distribution, we examined the force balance in the surface-lateral direction exerted on a rectangular control volume set around the contact line. As the mechanical route, the fluid stress integrals along the two control surfaces normal to the solid-fluid interface were theoretically connected with γSL and γSV relative to the solid-vacuum interfacial tension γS0 by Bakker’s equation extended to solid-related interfaces via a thought experiment, for which the position of the solid-fluid interface plane was defined at the limit that the fluid molecules could reach. On the other hand, the fluid stress integral along the control surface lateral to the solid-fluid interface was connected with γLV by the Young-Laplace equation. Through this connection, we showed that Young’s equation was valid for a system in which the net lateral force exerted on the fluid molecules from the solid surface was zero around the contact line. Furthermore, we compared γSL − γS0 and γSV − γS0 obtained by the mechanical route with the solid-liquid and solid-vapor works of adhesion obtained by the dry-surface method as one of the thermodynamic routes and showed that both routes resulted in a good agreement. In addition, the contact angle predicted by Young’s equation with these interfacial tensions corresponded well to the apparent droplet contact angle determined by using the previously defined position of the solid-fluid interface plane; however, our theoretical derivation indicated that this correspondence was achieved because the zero-lateral force condition was satisfied in the present system with a flat and smooth solid surface. These results indicated that the contact angle should be predicted not only by the interfacial tensions but also by the pinning force exerted around the contact line.