Significantly High Thermal Rectification in an Asymmetric Polymer Molecule Driven by Diffusive versus Ballistic Transport

Abstract

Tapered bottlebrush polymers have novel nanoscale polymer architecture. Using nonequilibrium molecular dynamics simulations, we showed that these polymers have the unique ability to generate thermal rectification in a single polymer molecule and offer an exceptional platform for unveiling different heat conduction regimes. In sharp contrast to all other reported asymmetric nanostructures, we observed that the heat current from the wide end to the narrow end (the forward direction) in tapered bottlebrush polymers is smaller than that in the opposite direction (the backward direction). We found that a more disordered to less disordered structural transition within tapered bottlebrush polymers is essential for generating nonlinearity in heat conduction for thermal rectification. Moreover, the thermal rectification ratio increased with device length, reaching as high as ∼70% with a device length of 28.5 nm. This large thermal rectification with strong length dependence uncovered an unprecedented phenomenon-diffusive thermal transport in the forward direction and ballistic thermal transport in the backward direction. This is the first observation of radically different transport mechanisms when heat flow direction changes in the same system. The fundamentally new knowledge gained from this study can guide exciting research into nanoscale organic thermal diodes.

Molecular Bridge Thermal Diode Enabled by Vibrational Mismatch

“…and Tian simulated tapered bottlebrush polymer by MD and obtained R with a maximum value of 0.7 [21]. Similar length dependent R was observed in their work, which was attributed to the ballistic transport.…”

“…demonstrated that in a superlattice K is not merely an intrinsic property of the interface but also positively correlated to the phonon mean free paths (MFPs) of materials in the interfaces [44]. In the ballistic thermal transport of 1D nanosystems, the effective MFPs are greatly determined by the lengths of CNT [21,45]. For example, based on the Matthiessen law for diffusive-ballistic transport [45,46], the effective MFP of CNT, λeff, can be expressed as , where λ0 is the intrinsic MFP and L is the length of CNT.…”

“…To fully realize these advantages, fundamentally understanding the rectification 3 mechanisms at the small scales becomes critical. There have been several proposed models, including those based on asymmetric geometry of nanostructures [17][18][19][20][21][22], asymmetric coupling with thermal contacts [23,24], and asymmetric onsite potential of particles [25], and a model of one-dimension (1D) particle chains with gradient mass distribution. This latter one has been proposed to achieve R of several hundred [1,[25][26][27].…”

Chain rotation significantly reduces thermal conductivity of single-chain polymers

“…PCFF, a class II potential which includes anharmonic bonding terms, 20,21 is intended for applications in polymers and organic material. 6,[22][23][24] The size of supercells used to calculate k x in this study also can be found in Table I. We intentionally set the supercells of these single-chain polymers to be large enough to rule out size effects.…”

Dual-mode solid-state thermal rectification

“…By rectifying heat currents, thermal rectification has substantial implications for thermal transport, for example, by allowing energy systems to dissipate heat to their surroundings but simultaneously protecting them from damage when the surrounding temperature is too high. Although several mechanisms based on thermal expansion 7 , asymmetric nanostructures and mass loading [8][9][10][11][12][13] , solid-liquid phase change 14,15 , solid-solid phase transition [16][17][18][19][20][21] and bi-material interface 22 were demonstrated for realizing thermal rectification, it has remained a significant challenge to achieve a large and tunable rectification effect, which generally requires either macroscale size or large temperature bias.…”