Nonequilibrium molecular dynamics calculation of the thermal conductivity of amorphous polyamide-6,6.

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.

Nonequilibrium molecular dynamics methods for computing the thermal conductivity: application to amorphous polymers.

We develop two nonequilibrium simulation methods which are suitable for calculation of thermal conductivity with good accuracy. These methods are based on simple algorithms, and it will be very easy to extend their range of application. In particular, there are no restrictions (from, e.g., the force field) to apply them to a variety of systems. Here, they are applied to the calculation of the thermal conductivity of amorphous polyamide-6,6 systems. We treat two models of the polymer with different degrees of freedoms constrained. The results suggest that the methods are quite efficient, and that thermal conductivity strongly depends on the number of degrees of freedom in the model.

Thermal conductivities of molecular liquids by reverse nonequilibrium molecular dynamics.

The reverse nonequilibrium molecular dynamics method for thermal conductivities is adapted to the investigation of molecular fluids. The method generates a heat flux through the system by suitably exchanging velocities of particles located in different regions. From the resulting temperature gradient, the thermal conductivity is then calculated. Different variants of the algorithm and their combinations with other system parameters are tested: exchange of atomic velocities versus exchange of molecular center-of-mass velocities, different exchange frequencies, molecular models with bond constraints versus models with flexible bonds, united-atom versus all-atom models, and presence versus absence of a thermostat. To help establish the range of applicability, the algorithm is tested on different models of benzene, cyclohexane, water, and n-hexane. We find that the algorithm is robust and that the calculated thermal conductivities are insensitive to variations in its control parameters. The force field, in contrast, has a major influence on the value of the thermal conductivity. While calculated and experimental thermal conductivities fall into the same order of magnitude, in most cases the calculated values are systematically larger. United-atom force fields seem to do better than all-atom force fields, possibly because they remove high-frequency degrees of freedom from the simulation, which, in nature, are quantum-mechanical oscillators in their ground state and do not contribute to heat conduction.