Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene

Abstract

Thermally conductive polymer crystals are of both fundamental and practical interest for their high thermal conductivity that exceeds that of many metals. In particular, polyethylene fibers and oriented films with uniaxial thermal conductivity exceeding 50 W⋅m−1⋅K−1 have been reported recently, stimulating interest into the underlying microscopic thermal transport processes. While ab initio calculations have provided insight into microscopic phonon properties for perfect crystals, such properties of actual samples have remained experimentally inaccessible. Here, we report the direct observation of thermal phonons with mean free paths up to 200 nm in semicrystalline polyethylene films using transient grating spectroscopy. Many of the mean free paths substantially exceed the crystalline domain sizes measured using small-angle X-ray scattering, indicating that thermal phonons propagate ballistically within and across the nanocrystalline domains; those transmitting across domain boundaries contribute nearly one-third of the thermal conductivity. Our work provides a direct determination of thermal phonon propagation lengths in molecular solids, yielding insights into the microscopic origins of their high thermal conductivity.

Origin of micrometer-scale propagation lengths of heat-carrying acoustic excitations in amorphous silicon

“…We calculated the MFP accumulation function that is consistent with these measurements using the method of Ref. [42]. More precisely, the posterior probability distribution of the thermal conductivity accumulation function percentage of the heat carried by excitations with MFPs larger than 1 μm is ∼31 ± 18%, qualitatively agreeing with the recent observation that more than 50% of the heat is carried by MFPs exceeding 100 nm [30].…”

“…1(a); a complete description of the TG spectroscopy experiment employed in this work can be found in Refs. [41,42]. Briefly, two pump laser pulses (wavelength 515 nm, pulse duration ≈1 ns, repetition rate 200 Hz) are focused on the sample (1/e 2 diameter 520 μm); the optical interference creates a spatially periodic heating profile with tunable grating period (L ∼ 0.75-15.7 μm) and wave vector (q = 2π/L).…”

Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams

“…7,8 The thermal conductivity of cellulose Ib is several times higher along the covalently bonded chain direction (c-axis), compared with the transverse direction where weaker interactions connect the adjacent chains. 9,10 The alignment of cellulose in wood 11 results in materials with anisotropic heat transport properties, similar to other aligned 1D and 2D nanomaterials such as bulk carbon allotropes, 12 carbon nanotubes, 13 single-layer black phosphorene, 14 drawn oriented polyethylene, 15 and graphene. 12,16,17 Anisotropic heat transport properties are of interest in applications where directional heat management is…”

Nanocomposites of Au/Disentangled UHMWPE: A Combined Optical and Structural Study

“…Uniaxial orientation was found to significantly increase the tensile modulus and strength [ 22 , 23 , 24 ], and more recently, the thermal conductivity of UHMWPE samples, achieving metal-like thermal conductivities in the stretched direction [ 25 , 26 ]. A considerable increase in the thermal conductivity was observed even at relatively low draw ratios, where ballistic phonons contribute to the increased thermal conductivity over distances as far as 200 nm, according to transient grating spectroscopy [ 27 ]. In the presence of suitable fillers (metallic nanoparticles or conjugated molecules), orientation brings anisotropy to the optical properties as well.…”