Molecular alignment induced high thermal conductivity in amorphous/ low crystalline polyimide fibers

Abstract

It is of considerable scientific and technological importance to enhance the thermal conductivity (k) of polymers for the application in thermal management. In this work, we study the thermal transport in amorphous/low-crystalized polyimide (PI) composite fibers (crystallinity lower than 2%-10%) by using the transient electro-thermal (TET) technique and the steady-state electro-thermal (SET) technique from 320 K to 7 K. It is found that the amorphous/low-crystalized PI fibers can achieve a high k (2.13 W/m K) due to oriented chain alignment and inter-chain hydrogen bonds from the condensation polymerization, wet-spinning and low-ratio thermal drawing process. By using the thermal reffusivity (Θ) model, the phonon propagation in PI fibers is investigated. The structural domain size (SDZ) of the amorphous/low-crystalized PI fibers is determined based on the residual Θ at 0 K limit, which is calculated to be 3.85 nm and 9 nm for the undrawn and post-drawn PI fiber, respectively. The SDZ value is close to the radius of gyration (Rg) instead of the atomic scale characteristic length of PI. This result indicates that for amorphous/low crystalized PI fibers with hydrogen bonds, the Rg could have played a more important role in determining k.