Discovering Size Effect of Thermal Conductivity of Amorphous Polymers

Polymers have shown great potential in various modern applications such as thermal insulation, soft robotics, organic electronics, 3D printing, artificial skin, flexible and wearable electronics, battery electrolytes, and thermoelectrics. They have many advantages compared to traditionally used metals and ceramics such as light weight, low cost, high corrosion resistance, nontoxicity, and good manufacturability. Among the physical properties, thermal conductivity is a crucial component that determines the energy efficiency of building enclosures, thermal management effectiveness of robotics and electronics, the safety of batteries, and the efficiency of thermoelectrics. The ability of manipulating thermal transport in polymers is key to many future technology revolutions, e.g., the ultra-low thermal conductivity materials for building insulations.

Recent studies have led to significant advances in the understanding of thermal transport in amorphous polymers. It has been realized that the thermal transport is dominated by the intra-chain axial conduction via covalent bonds while the inter-chain van der Waals conduction is marginal. The thermal conductivity can be significantly affected by the chain conformation (i.e., spatial extent of chains), chain length, inter-chain cross-links, spatial confinement or density, strain, angular bending, branching, kinks, etc. Despite these advances, several fundamental questions remain elusive: Can the thermal conductivity of amorphous polymers be further reduced below their common amorphous limit by size effect that has been extensively found in crystalline materials in past decades? What affects the propagating distance of heat carriers in amorphous polymers? Aside from size effect, how can we tune the thermal conductivity of amorphous polymers that usually consist of covalent bonds? Since the thermal conductivity of crystals was found to be subject to strong size effects, the utilization of nanoengineering has flourished in the past two decades to tune thermal conductivity by several orders of magnitude for various applications. The ability to tune the thermal conductivity of amorphous polymer will be promising as well. Generally, the thermal transport in amorphous materials is regarded as the diffusion of unlocalized waves and the hopping of localized vibrations .  Cahill’s model assumes that the thermal transport in amorphous materials is dominated by the random walk of localized oscillators between nearest neighbors (~Å) and does not have a size effect beyond ~1 nm. The phonon mean free path (MFP) is usually estimated at about 0.1-1 nm by classical kinetic theory, which indicates no size effect as well. These models, however, do not have insight into the spectrum of thermal transport and thus are not able to give exact answers. Therefore, experimental attempts have been made to explore the possible size effect of amorphous polymers, but the unknown interfacial resistance has prevented reaching a firm conclusion.

In this work, for the first time, we find that the thermal conductivities of amorphous polymers can be further reduced below their amorphous limit by size effect. Surprisingly, the phonon mean free path in amorphous polymers can extend beyond ~100 nm. A two-channel model is proposed to account for the size-dependent and size-independent components of the thermal conductivity. Remarkably, we also find that the presence of charged molecules in polymers can significantly tune the thermal conductivity as well as its size effects due to electrostatic interactions. This work provides a critical revisit to the thermal conductivity of amorphous polymers that will have broad impact in the nano and chemical engineering of polymers for various energy-related applications.