Nonequilibrium Molecular Dynamics Calculation of the Thermal Conductivity of Amorphous Polyamide-6,6

Abstract

The thermal conductivity of the amorphous phase of polyamide-6,6 is investigated by nonequilibrium molecular dynamics simulations. Two different algorithms are used, reverse nonequilibrium molecular dynamics and the dual-thermostat method. Particular attention is paid to the force field used. Four different models are tested, flexible and rigid bonds and all-atom and united-atom descriptions. They mainly differ in the number of high-frequency degrees of freedom retained. The calculated thermal conductivity depends systematically on the number of degrees of freedom of the model. This dependence is traced to the quantum nature of the fast molecular vibrations, which are incorrectly described by classical mechanics. The best agreement with experiment is achieved for a united-atom model with all bonds kept rigid. It could be shown that both the thermal conductivity and the heat capacity of a model show a similar (but not equal) dependence on its number of degrees of freedom. Hence, the computationally more convenient heat capacity can be used for force field optimization. A side result is the anisotropy of the thermal conductivity in stretched polyamide; heat conduction is faster parallel to the drawing direction than perpendicular to it.

Anisotropic heat transport in nanoconfined polyamide-6,6 oligomers: Atomistic reverse nonequilibrium molecular dynamics simulation

“…22 As a comparison of the effect of chain anisotropy on the thermal conductivity anisotropy, observed in this work, with previous literature we may address to the experimental reports by Balasubramanian et al 41,42 on the increase in the thermal conductivity of bulk liquid polyisobutylene under shear and to the theoretical findings on the anisotropies in the thermal conductivities of stretched bulk polymer melts and bulk crystalline polymers. [26][27][28][29][30] …”

“…As part of the heat flow in polymers occurs through bonds and progresses along the chain via skeletal vibrations or phonons, it is the purpose of this work to give a quantitative measure of the rate of heat flow in the parallel direction, compared to the perpendicular direction and to the bulk polymer. Although there are simulation methods in the literature to describe the anisotropic heat flow in bulk polymers, [26][27][28][29][30] to our knowledge there is no report in the literature on the simulation of anisotropic heat flow in the nanoconfined polymers.…”

Correlation between thermal conductivity and bond length alternation in carbon nanotubes: A combined reverse nonequilibrium molecular dynamics—Crystal orbital analysis

“…67 In analogy to the broad application of the INDO CO Hamiltonian, the RNEMD approach has been also used to study the transport quantities of many materials. [68][69][70][71][72] Originally this approach had been developed for atomic liquids. Later it had been extended to molecular fluids as well as to crystalline and amorphous polymers.…”

Molecular Dynamics Calculations of the Thermal Conductivity of Molecular Liquids, Polymers, and Carbon Nanotubes

“…Starting with the simulation of simple Lennard-Jones liquids (1)(2)(3), theoretical calculations have progressed in the complexity of the materials (4)(5)(6)(7)(8)(9)(10)(11)(12) as well as in the accuracy of the prediction (13,14). This significant development is important from two points of view.…”

“…Calculations become more complicated when one moves from molecular liquids to polymers. The chosen force field plays a bigger role, and small modifications in molecular structure and molecular orientation can lead to uncertainties in thermal conductivity (4,9,12). In contrast, the calculation of thermal conductivities for single-walled carbon nanotubes seems to be robust against the force field and its potential form (27,28).…”