Polymer composites, with both high thermal conductivity and high electrical insulation strength, are desirable for power equipment and electronic devices, to sustain increasingly high power density and heat flux. However, conventional methods to synthesize polymer composites with high thermal conductivity often degrade their insulation strength, or cause a significant increase in dielectric properties. In this work, we demonstrate epoxy nanocomposites embedded with silver nanoparticles (AgNPs), and modified boron nitride nanosheets (BNNSs), which have high thermal conductivity, high insulation strength, low permittivity, and low dielectric loss. Compared with neat epoxy, the composite with 25 vol% of binary nanofillers has a significant enhancement (~10x) in thermal conductivity, which is twice of that filled with BNNSs only (~5x), owing to the continuous heat transfer path among BNNSs enabled by AgNPs. An increase in the breakdown voltage is observed, which is attributed to BNNSs-restricted formation of AgNPs conducting channels that result in a lengthening of the breakdown path. Moreover, the effects of nanofillers on dielectric properties, and thermal simulated current of nanocomposites, are discussed.

Thermal transport in the axial direction of polymers has been extensively studied, while the strain effect on the thermal conductivity, especially in the radial direction, remains unknown. In this work, we calculated the thermal conductivity in the radial direction of a crystalline polyethylene model and simulated the uniaxial strain effect on the thermal conductivity tensor by molecular dynamics simulations. We found a strong size effect of the thermal transport in the radial direction and estimated that the phonon mean free path can be much larger than the prediction from the classic kinetic theory. We also found that the thermal conductivity in the axial direction increases dramatically with strain, while the thermal conductivity in the radial direction decreases with uniaxial strain. We attribute the reduction of thermal conductivity in the radial direction to the decreases in inter-chain van der Waals forces with strains. The facts that the chains in the crystalline polyethylene became stiffer and more ordered along the chain direction could be the reasons for the increasing thermal conductivity in the axial direction during stretching. Besides, we observed longer phonon lifetime in acoustic branches and higher group velocity in optical branches after uniaxial stretching. Our work provides fundamental understandings on the phonon transport in crystalline polymers, the structure-property relationship in crystalline polymers, and the strain effect in highly anisotropic materials.

Thermal conductivities (TCs) of the vast majority of amorphous polymers are in a very narrow range, 0.1 ∼ 0.5 Wm −1 K −1 , although single polymer chains possess TC of orders-of-magnitude higher. Entanglement of polymer chains plays an important role in determining the TC of bulk polymers. We propose a thermal resistance network (TRN) model for TC in amorphous polymers taking into account the entanglement of molecular chains. Our model explains well the physical origin of universally low TC observed in amorphous polymers. The empirical formulae of pressure and temperature dependence of TC can be successfully reproduced from our model not only in solid polymers but also in polymer melts. We further quantitatively explain the anisotropic TC in oriented polymers.

The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time. There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman’s formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers. Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones. The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data. Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.

With potential thermal management applications, such as plastic heat exchangers and thermal interface materials, thermally conductive polymers have gained renewed interest in the past decade. Ultradrawn polyethylene fibers and films have been experimentally shown to have thermal conductivities at least two orders of magnitude of these in their amorphous counterparts. However, the theoretical molecular-level understanding of strain effects on the thermal transport in drawn semicrystalline polymers, such as polyethylene, especially the roles of different interlamellar chain topologies in the crystalline-amorphous interphase region, remains elusive. Using molecular dynamics simulations, we investigated the strain effects on the thermal conductivity and vibrational transport in a simplified sandwich semicrystalline structure. We found that the topology of the interlamellar chains determines the dependence of thermal conductivity on strains. Comparing thermal resistances at different regions in the interlamellar structure, thermal resistance at the amorphous region is not necessarily the highest; the interphase region with the transition from the crystalline to amorphous state can have a much higher resistance. We conducted the frequency domain analysis to obtain the heat flux spectrum in the crystalline-amorphous interphase region and found that the vibrational modes at intermediate and high frequencies can contribute more than these at relatively low frequencies to the total heat flux because of the complex interlamellar chain topologies (e.g., loop chains). Our work provides molecular-level understandings of the structural-property relationship in semicrystalline polymers with strains, which could assist the design and development of thermally conductive polymers for thermal management applications.