Normal Mode Analysis of The Thermal conductivity in Amorphous Polymers: The Importance of Localized Modes

Polymers are a unique class of materials from the perspective of normal mode analysis. Polymers consist of individual chains with repeating units and strong intra-chain covalent bonds, and amorphous arrangements among chains with weak inter-chain van der Waals and for some polymers also electrostatic interactions. Intuitively, this strong heterogeneity in bond strength can give rise to interesting features in the constituent phonons, but such effects have not been studied deeply before. Here, we use lattice dynamics and molecular dynamics to perform modal analysis of the thermal conductivity in amorphous polymers for the first time. We find an abnormally large population of localized modes in amorphous polymers, which is dramatically different from amorphous inorganic materials. Contrary to the common picture of thermal transport, localized modes in amorphous polymers are found to be the dominant contributors to thermal conductivity. We find that a significant portion of the localization happens within individual chains, but heat is dominantly conducted when localized modes involve two chains. These results suggest that even though each polymer is different, localized modes play a key role. The results provide new perspective on why polymer thermal conductivity is generally quite low and gives insight into how to potentially change it.