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.
Highlights
Polymer fibers have been widely explored in many engineering applications due to the advantages of their anti-corrosion, hightoughness, and low-density features
The low thermal conductivities (TCs) in bulk polymers have been attributed to various fundamental mechanisms, including the disordered arrangement of polymer chains in the amorphous phase, the presence of grain boundaries between polycrystalline domains, inter-chain entanglements, nanovoids, and other defects and impurities, all of which result in significant phonon scatterings that hinder heat conduction.[5,6,7,8]
Using the effective-medium-theory model developed here based on serialparallel thermal resistors, we show that both chain alignment and crystallinity are important to TCs
Summary
Polymer fibers have been widely explored in many engineering applications due to the advantages of their anti-corrosion, hightoughness, and low-density features. This is achieved through target property matching and the multistate-iterative Boltzmann inversion (MS-IBI) method[36] with our AA-MD simulation results serving as the benchmark Utilizing this CG force field, we conduct thermaldrawing MD simulations and thermal-conductivity calculations on bulk PE fibers to study the effects of thermal-drawing protocols, including the stretching (drawing) temperature, tensile strain level, and strain rate, on the observed enhancement in roomtemperature TCs. PE chain alignment and crystallinity (global and local) are quantified to discover the correlation between PE morphology (Fig. 1b) and TC. We find that the greatly enhanced TC along the fiber-drawing direction is mainly attributed to the increased intrinsic phonon MFP, as well as the increased longitudinal phonon group velocities resulting from the enlarged elastic modulus along that direction
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