Further improvement of lattice thermal conductivity from bulk crystalline to 1-D-chain polyethylene: A high-yet-finite thermal conductivity using first-principles calculation
Abstract
We calculate the thermal conductivity (appa) of both bulk crystalline and single-chain polyethylene (PE) using the first-principles-based anharmonic lattice dynamics. Despite its low appa in amorphous state, the predicted bulk crystal has high axial appa (237 W/m-K) at room temperature. The much lower measured appa is attributed to the small size of nanocrystallites (~10 nm) in synthesized semi-crystalline PE. For the 1-D chain, the predicted appa is much larger and yet finite (1400 W/m-K at room temperature). The reduction of scattering phase space caused by the diminished interchain van der Waals interactions explains this larger appa. It is also found that the transverse phonon branches with quadratic dispersion make minor contribution to this, due to their vanishing group velocity in the long-wavelength limit. Moreover, the low-frequency bending and twisting phonon modes are strongly coupled and dominate anharmonic phonon scatterings, leading to the finite appa. The predicted high appa of bulk and chain PE crystals enable polymer usage in thermal management and the above phonon scatterings provide guide for their nano-designs.