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.
Highlights
Thermal management in mechanical and electric devices is important to obtain high performance of such devices
The thermal conductivities λ for polyacrylic acid (PAA) with each of the three polymer chain lengths were calculated by Eq (1) and are shown in Fig. 4 together with the average density for the three polymer chain lengths
Higher density PAA has more heat transfer paths per unit volume compared to lower density PAA, many paths in PAA with high degree of polymerization have fast thermal transport along the covalent bonds, longer PAA has the higher thermal conductivity in the range of the polymer chain length simulated here
Summary
Thermal management in mechanical and electric devices is important to obtain high performance of such devices. Forming continuous heat transfer path via fillers in the TIM is not sufficient to meet the industrial high requirements Under such circumstances, search for novel materials for TIM has been continuing. LBL membranes made by polymer molecules with hydrophilic groups are expected to exhibit high thermal conductivity helped by intermolecular energy transfer due to electrostatic forces. Prior to the examination of the LBL membrane, thermophysical properties and molecular mechanism of thermal energy transfer in the bulk of the two materials were analyzed. In the present paper molecular dynamics simulation of bulk PAA is performed to examine molecular mechanism of thermal energy transfer. The heat flux decomposition analysis, which has been usefully applied to various liquids and soft matters, e.g., bulk liquid, binary mixture, solid-liquid interface, polymer and lipid bilayer membrane, helped us to understand how the various molecular mechanisms contribute to heat conduction in PAA. Atactic polymers were used and dissociation of PAA was not considered
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