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Molecular dynamics simulations of energy accommodation between gases and polymers for ultra-low thermal conductivity insulation

by Tianli Feng, Amit Rai, Diana E Hun, Som S Shrestha
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International Journal of Heat and Mass Transfer
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Determining the energy accommodation between gases and solids is essential to developing high-resistivity thermal insulation materials for energy saving, greenhouse gases emission reduction, and fossil fuels saving. The energy accommodation coefficients of most gases, however, are still unclear, especially with the surface of thermal insulation materials, polymers. In this work, by using all-atom nonequilibrium molecular dynamics simulations, we reveal the accommodation coefficients of He, Ar, N2, and O2 on polymers, mainly polystyrene. We find that their values are around 0.51, 0.72. 0.79, and 0.90, respectively, providing a critical revisit to the commonly used theoretical maximum value, 1. Such change can lead to up to 70%, 55%, 40%, and 25% reduction to the thermal transport through He, Ar, N2, and O2 gases in small polymer pores or at low pressure. We find that, by reducing pore diameter to 10 nm, even at ambient temperature and pressure, the gas thermal conductivity of He, Ar, N2, O2, and CO2 in porous polystyrene can reach as low as 9.4×10-4, 2.7×10-4, 7.1×10-4, and 8.5×10-4 W·m-1·K-1, respectively, which are 2-3 orders of magnitude lower than their bulk values, promising for thermal insulation. This work not only reveals the fundamental energy exchange physics between gases and polymers but also provides important guidance for designing high-performance thermal insulation materials for energy saving.