Measuring Anisotropic Thermal Conductivity of High-Density Polyethylene Fibers Using Time-Domain Thermoreflectance Method

Low thermal conductivity of bulk amorphous polymers limits their heat dissipation capability, which affects the reliability and lifetime of devices where amorphous polymers are applied. When polymer chains are aligned, such as in polymer fibers, the thermal conductivity in the chain direction is much higher than their amorphous counterpart. Various theoretical simulations have proved that the thermal conductivity of aligned polymer chains is high along the chain direction, which can be larger than 50 W m^-1K^-1. Experimental measurements also support that the thermal conductivity of polymers can be high in the chain direction. Recently, Shen et al. reported that the thermal conductivity of polyethylene (PE) nanofibers can be on the order of 100 W m^-1 K^-1. Due to the alignment of polymer chains, the polymer fibers will become an anisotropic materials system where the phonon transport could be different from isotropic materials systems. For example, there might be phonon focusing effects in anisotropic materials systems and phonon mean free paths could be different than an isotropic system. Indeed, He and Liu et al. modeled the phonon mean free paths distribution in the radial direction of polyethylene single crystal and found that the phonon mean free paths can be much longer than a few nanometers, the value in the amorphous polyethylene. Even though the thermal conductivity in the axial direction of polymer fibers has been previously measured, the thermal conductivity in the radial direction has rarely been reported. Wang et al. reported that the axial thermal conductivity of commercial high-density polyethylene (HDPE) fibers is 12 - 16 W m^-1 K^-1. Lu and Liu et al. have measured thermal conductivity in the radial direction of deformed polymer fibers. They found that the radial thermal conductivity of the fibers decreases with increasing pressing strains, but the thermal conductivity without any deformation is still unknown. In this work, we are interested in measuring the thermal conductivity of polymer fibers in the radial direction without any deformation. We measured both the thermal conductivity and the interfacial thermal conductance between aluminum and high-density polymer fiber at 60 K to 300 K using the time-domain thermoreflectance (TDTR) method with various modulation frequencies. We found that thermal conductivity in the radial direction is higher than the amorphous counterpart, which agrees with the theoretical prediction. The dependence of the thermal conductivity on the modulation frequency in TDTR and temperature suggests that not all the phonons have very short mean free paths. Our results can shed some light on the thermal transport in anisotropic materials.