Molecular dynamics study on thermal energy transfer in bulk polyacrylic acid

Abstract

We performed non-equilibrium molecular dynamics (NEMD) simulations on bulk amorphous polyacrylic acid (PAA) with three polymer chain lengths to investigate molecular mechanism of thermal energy transfer in heat conduction. Thermal conductivity obtained by NEMD simulations increased as the polymer chain length of PAA increased, and its dependence on polymer chain length exhibited a saturation behavior. By decomposing heat flux into each contribution of molecular interactions, it was found that dominant mechanism of the thermal energy transfer in PAA was intramolecular interaction, and contribution of the intramolecular interaction to thermal conductivity increased as the polymer chain length increased, and resulted in increase in total thermal conductivity. On the other hand, coiled conformation of PAA advanced in response to elongation of the polymer chain length; and this coiled conformation inhibited further increase of thermal conductivity due to the polymer chain elongation. Consequently, the elongation of the polymer chain length had two conflicting effects: increasing and suppression of thermal conductivity, due to increase in intramolecular interaction and change in conformation, respectively. This is the reason of the saturation tendency of thermal conductivity as a function of the polymer chain length. Detailed understanding of the molecular mechanism of thermal energy transfer obtained in the present study provided the in-depth knowledge to clarify the thermal energy transfer mechanism and will lead to the characterization of thermal energy transfer in more complicated materials such as a layer-by-layer membrane.