Tailored morphology and highly enhanced phonon transport in polymer fibers: a multiscale computational framework

Abstract

Although tremendous efforts have been devoted to enhance thermal conductivity in polymer fibers, correlation between the thermal-drawing conditions and the resulting chain alignment, crystallinity, and phonon transport properties have remained obscure. Using a carefully trained coarse-grained force field, we systematically interrogate the thermal-drawing conditions of bulk polyethylene samples using large-scale molecular dynamics simulations. An optimal combination of moderate drawing temperature and strain rate is found to achieve highest degrees of chain alignment, crystallinity, and the resulting thermal conductivity. Such combination is rationalized by competing effects in viscoelastic relaxation and condensed to the Deborah number, a predictive metric for the thermal-drawing protocols, showing a delicate balance between stress localizations and chain diffusions. Upon tensile deformation, the thermal conductivity of amorphous polyethylene is enhanced to 80% of the theoretical limit, that is, its pure crystalline counterpart. An effective-medium-theory model, based on the serial-parallel heat conducting nature of semicrystalline polymers, is developed here to predict the impacts from both chain alignment and crystallinity on thermal conductivity. The enhancement in thermal conductivity is mainly attributed to the increases in the intrinsic phonon mean free path and the longitudinal group velocity. This work provides fundamental insights into the polymer thermal-drawing process and establishes a complete process-structure-property relationship for enhanced phonon transport in all-organic electronic devices and efficiency of polymeric heat dissipaters.

The Impact of Foaming Effect on the Physical and Mechanical Properties of Foam Glasses with Molecular-Level Insights

“…MD simulations are effective tools [ 18 , 19 , 20 , 21 ] for probing the material structures and behaviors in an especially small spatiotemporal scale that is beyond the reach of conventional experimental facilities. Considering that carbon as a foaming agent is still superior to dextrin and the mixture of carbon and dextrin, in terms of product strength [ 17 ], we performed MD simulations (details in Materials and Methods) to elucidate our chosen optimal amount of carbon (i.e., 1 wt.%) [ 17 ] with molecular-level insights into the mechanism.…”

Enhancing the Thermo-Mechanical Property of Polymer by Weaving and Mixing High Length–Diameter Ratio Filler

“…Mechanical modulus is directly related to sound speed of material. For low-frequency phonons close to point, the phonon group velocity can be approximated by sound speed [ 59 ]. Classical theoretical models often concentrate on thermal conductivity of filler and polymer matrix but pay little attention to mechanical modulus of systems.…”

Thermal Transport in Polymers: A Review

“…Both all-atom and united atom models have been used for polymer thermal conductivity calculations. 33,46,50,54 The major difference between these two types of force fields is whether hydrogen atoms are explicitly represented. Henry et al argued that the explicit simulation of hydrogen atoms is necessary to capture all degrees of freedom, which is important to thermal conductivity.…”

“…84 During drawing, when strain on polymer fibers increases, contribution to thermal conductivity from bonded interaction dominates, and nonbonded interaction like vdW becomes a minor factor. 54 Thermal transport is much more efficient along the covalent-bonded backbone than through vdW interaction. 88 Stretching the polymer fiber by a higher ratio makes chain more orientated along the drawing direction, taking more advantage of the strong covalent polymer backbone for thermal transport.…”