Due to their unique properties, polymers – typically thermal insulators – can open up opportunities for advanced thermal management when they are transformed into thermal conductors. Recent studies have shown polymers can achieve high thermal conductivity, but the transport mechanisms have yet to be elucidated. Here we report polyethylene films with a high thermal conductivity of 62 Wm⁻¹ K⁻¹, over two orders-of-magnitude greater than that of typical polymers (~0.1 Wm⁻¹ K⁻¹) and exceeding that of many metals and ceramics. Structural studies and thermal modeling reveal that the film consists of nanofibers with crystalline and amorphous regions, and the amorphous region has a remarkably high thermal conductivity, over ~16 Wm⁻¹ K⁻¹. This work lays the foundation for rational design and synthesis of thermally conductive polymers for thermal management, particularly when flexible, lightweight, chemically inert, and electrically insulating thermal conductors are required.

Molecular engineering of intra- and interchain interactions transforms polymers into good heat conductors.

To push upper boundaries of thermal conductivity in polymer composites, understanding of thermal transport mechanisms is crucial. Despite extensive simulations, systematic experimental investigation on thermal transport in polymer composites is limited. To better understand thermal transport processes, we design polymer composites with perfect fillers (graphite) and defective fillers (graphite oxide), using polyvinyl alcohol (PVA) as a matrix model. Measured thermal conductivities of ~1.38 ± 0.22 W m ⁻¹ K ⁻¹ in PVA/defective filler composites is higher than those of ~0.86 ± 0.21 W m ⁻¹ K ⁻¹ in PVA/perfect filler composites, while measured thermal conductivities in defective fillers are lower than those of perfect fillers. We identify how thermal transport occurs across heterogeneous interfaces. Thermal transport measurements, neutron scattering, quantum mechanical modeling, and molecular dynamics simulations reveal that vibrational coupling between PVA and defective fillers at PVA/filler interfaces enhances thermal conductivity, suggesting that defects in polymer composites improve thermal transport by promoting this vibrational coupling.

The ongoing research activities in the fields of bottom-up synthesis of thermally conductive polymers and thermal transport mechanisms in polymers are reviewed. Perspectives on developing polymers with high thermal conductivities are discussed.

Understanding thermal transport mechanisms in polymeric composites allows us to expand the boundaries of thermal conductivity in them, either increasing it for more efficient heat dissipation or decreasing it for better thermal insulation. But, these mechanisms are not fully understood. Systematic experimental investigations remain limited. Practical strategies to tune the interfacial thermal resistance (ITR) between fillers and polymers and the thermal conductivity of composites remain elusive. Here, we studied the thermal transport in representative polymer composites, using polyethylene (PE) or polyaniline (PANI) as matrices and graphite as fillers. PANI, with aromatic rings in its backbone, interacts with graphite through strong noncovalent p-p stacking interactions, whereas PE lacks such interactions. We can then quantify how p-p stacking interactions between graphite and polymers enhances thermal transport in composites. PE/graphite and PANI/graphite composites with the same 1.5% filler volume fractions show a ~22.82% and ~34.85% enhancement in thermal conductivity compared to pure polymers, respectively. Calculated ITR in PE/graphite and PANI/graphite are ~6 × 10^-8 m² KW^-1 and ~1 × 10^-8 m² KW^-1, respectively, highlighting how p-p stacking interactions reduce ITR. Molecular dynamics simulations suggest that p-p stacking interactions between PANI chains and graphite surfaces enhance alignments of PANI's aromatic rings with graphite surfaces. This allows more carbon atoms from PANI chains to interact with graphite surfaces at a shorter distance compared to PE chains. Our work indicates that tuning the p-p stacking interactions between polymers and fillers is an effective approach to reduce the ITR and enhance the thermal conductivity of composites.