Modifying thermal transport in electrically conducting polymers: Effects of stretching and combining polymer chains

Abstract

If their thermal conductivity can be lowered, polyacetylene (PA) and polyaniline(PANI) offer examples of electrically conducting polymers that can have potential use as thermoelectrics. Thermal transport in such polymers is primarily influenced by bonded interactions and chain orientations relative to the direction of heat transfer. We employ molecular dynamics simulations to investigate two mechanisms to control the phonon thermal transport in PANI and PA, namely, (1) mechanical strain and (2) polymer combinations. The molecular configurations of PA and PANI have a significant influence on their thermal transport characteristics. The axial thermal conductivity increases when a polymer is axially stretched but decreases under transverse tension. Since the strain dependence of the thermal conductivity is related to the phonon scattering among neighboring polymer chains, this behavior is examined through Herman's orientation factor that quantifies the degree of chain alignment in a given direction. The conductivity is enhanced as adjacent chains become more aligned along the direction of heat conduction but diminishes when they are orthogonally oriented to it. Physically combining these polymers reduces the thermal conductivity, which reaches a minimum value for a 2:3 PANI/PA chain ratio. If their thermal conductivity can be lowered, polyacetylene (PA) and polyaniline (PANI) offer examples of electrically conducting polymers that can have potential use as thermoelectrics. Thermal transport in such polymers is primarily influenced by bonded interactions and chain orientations relative to the direction of heat transfer. We employ molecular dynamics simulations to investigate two mechanisms to control the phonon thermal transport in PANI and PA, namely, (1) mechanical strain and (2) polymer combinations. The molecular configurations of PA and PANI have a significant influence on their thermal transport characteristics. The axial thermal conductivity increases when a polymer is axially stretched but decreases under transverse tension. Since the strain dependence of the thermal conductivity is related to the phonon scattering among neighboring polymer chains, this behavior is examined through Herman's orientation factor that quantifies the degree of chain alignment in a given direction. The conductivity is enhanced as adjacent chains become more aligned along the direction of heat conduction but diminishes when they are orthogonally oriented to it. Physically combining these polymers reduces the thermal conductivity, which reaches a minimum value for a 2:3 PANI/PA chain ratio.

Reducing thermal transport in electrically conducting polymers: Effects of ordered mixing of polymer chains

“…As S/V increases, j pn decreases. While a typical mixture rule predicts a single value of j p , 9,20 it is instead evident from Fig. 2 that the thermal transport and the conductivity also depend upon the polymer mixture configurations and the magnitudes of their interfacial contact areas.…”

“…8,9 Unlike crystalline materials, polymer chains exhibit nonlinear anharmonic bonded interactions 8 so that phonon transport through them is strongly dependent on the orientations of their backbone relative to the direction of heat transfer. 6,9 Polymer chains that are more aligned along this direction offer a smaller resistance to phonon transport. Destroying such an ordered orientation by imposing mechanical strain or mixing dissimilar polymers increases phonon scattering and atomic collisions, and limits the phonon mean free paths, all of which reduce j p .…”

“…Destroying such an ordered orientation by imposing mechanical strain or mixing dissimilar polymers increases phonon scattering and atomic collisions, and limits the phonon mean free paths, all of which reduce j p . 9 Longitudinal phonon transport is coupled with transport in the transverse direction. 10 In this Letter, we investigate the effect of this coupling in EC polymers and explore its applicability for reducing j p .…”

“…14,15 Details of the forcefield functions are described elsewhere. 9,16 For all simulations, periodic boundary conditions are imposed along all three coordinate (2013) directions. The VMD package 17 is used to visualize the simulated structures.…”

Understanding unusual thermal transport behavior in soft materials under mechanical strain – A molecular dynamics study

“…Besides, polymer molecules can be entangled or disentangled, and unaligned or aligned with the direction of heat transfer. Thus, the dissimilar structures, and intra-and intermolecular interactions in soft [1][2][3] and hard [4][5][6] materials produce different thermal transport behaviors.…”