title: Thermal Conductivity of Ultrahigh Molecular Weight Polyethylene Crystal: Defect Effect Uncovered by 0 K Limit Phonon Diffusion.
abstract: Crystalline ultrahigh molecular weight polyethylene (UHMWPE) has the highest reported thermal conductivity at room temperature: 104 W/(m·K), while theoretical predictions proposed an even higher value of 300 W/(m·K). Defects and amorphous fraction in practical UHMWPE fibers significantly reduces the thermal conductivity from the ideal value. Although the amorphous effect can be readily analyzed based on the effective medium theory, the defect effects are poorly understood. This work reports on the temperature-dependent behavior (down to 22 K) of thermal diffusivity and conductivity of UHMWPE fibers in anticipation of observing the reduction in phonon density and scattering rate against temperature and of freezing out high-momentum phonons to clearly observe the defect effects. By studying the temperature-dependent behavior of thermal reffusivity (Θ, inverse of thermal diffusivity) of UHMWPE fibers, we are able to quantify the defect effects on thermal conductivity. After taking out the amorphous region's effect, the residual thermal reffusivities (Θ0) for the studied two samples at the 0 K limit are determined as 3.45 × 10(4) and 2.95 × 10(4) s/m(2), respectively. For rare-/no-defects crystalline materials, Θ0 should be close to zero at the 0 K limit. The defect-induced low-momentum phonon mean free paths are determined as 8.06 and 9.42 nm for the two samples. They are smaller than the crystallite size in the (002) direction (19.7 nm) determined by X-ray diffraction. This strongly demonstrates the diffuse phonon scattering at the grain boundaries. The grain boundary thermal conductance (G) can be evaluated as G ≈ βρc(p)v with sound accuracy. At room temperature, G is around 3.73 GW/(m(2)·K) for S2, comparable to that of interfaces with tight atomic bonding.
authors.affiliation: Department of Mechanical Engineering, Iowa State University , 2010 Black Engineering Building, Ames, Iowa 50011, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Liu
authors.affiliation: Department of Mechanical Engineering, Iowa State University , 2010 Black Engineering Building, Ames, Iowa 50011, United States.
authors.forename: Zaoli
authors.initials: Z
authors.lastname: Xu
authors.affiliation: Department of Mechanical Engineering, Iowa State University , 2010 Black Engineering Building, Ames, Iowa 50011, United States.
authors.forename: Zhe
authors.initials: Z
authors.lastname: Cheng
authors.affiliation: Department of Mechanical Engineering, Iowa State University , 2010 Black Engineering Building, Ames, Iowa 50011, United States.
authors.forename: Shen
authors.initials: S
authors.lastname: Xu
authors.affiliation: Department of Mechanical Engineering, Iowa State University , 2010 Black Engineering Building, Ames, Iowa 50011, United States.|School of Environmental and Municipal Engineering, Qingdao Technological University , Qingdao, Shandong 266033, People's Republic of China.
authors.forename: Xinwei
authors.initials: X
authors.lastname: Wang
keywords: heat capacity; interface thermal conductance; phonon scattering; polyethylene; thermal reffusivity