Thermal conductivity of commodity polymers under high pressures

Abstract

Understanding the thermal conductivity of polymers under high-pressure conditions is essential for a range of applications, from aerospace and deep-sea engineering to common lubricants. However, the complex relationship between pressure, $P$, the thermal transport coefficient, $\kappa$, and polymer architecture poses substantial challenges to both experimental and theoretical investigations. In this work, we study the pressur-dependent thermal transport properties of a widely used commodity polymer -- poly(methyl methacrylate) (PMMA) -- using a combination of all-atom molecular dynamics simulations and semi-analytical approaches. While we report both classical and quantum-corrected estimates of $\kappa$, the latter approach reveals that as the pressure increases from 1 atm to 10 GPa, $\kappa$ rises by up to a factor of four -- from 0.21 W m$^{-1}$ K$^{-1}$ to 0.80 W m$^{-1}$ K$^{-1}$. To better understand the mechanisms behind this increase, we disentangle the contributions from bonded and nonbonded monomer interactions. Our analysis shows that nonbonded energy-transfer rates increase by a factor of six over the pressure range, while bonded interactions show a more modest increase -- about a factor of three. This observation further consolidates the fact that the nonbonded interactions play the dominant role in dictating the microscopic heat flow in polymers. These individual energy-transfer rates are also incorporated into a simplified heat diffusion model to predict $\kappa$. The results obtained from different approaches show internal consistency and align well with available experimental data. Additionally, some data for polylactic acid (PLA) are presented.