Abstract
The thermal conductivity (k) of poly(acrylic acid) (PAA) nanofibers, which were electrospun at various electrospinning voltages, was measured using suspended microdevices. While the thermal conductivities of the as-spun PAA nanofibers varied depending on the electrospinning voltages, the most pronounced 3.1-fold increase in thermal conductivity in comparison to that of bulk PAA was observed at the electrospinning voltage of 14 kV. On the other hand, a reduction in the thermal conductivity of the nanofibers was observed when the as-spun nanofibers were either thermally annealed at the glass transition temperature of PAA or thermally crosslinked. It is notable that the thermal conductivity of crosslinked PAA nanofibers was comparable to that of crosslinked bulk PAA. Polarized Raman spectroscopy and Fourier transform infrared spectroscopy verified that the k enhancement via electrospinning and the k reduction by the thermal treatments could be attributed to the conformational changes between gauche and trans states, which may be further related to the orientation of molecular chains. In contrast, hydrogen bonds did not contribute significantly to the k enhancement. Additionally, the suppression of k observed for the crosslinked PAA nanofibers might result from the shortening of single molecular chains via crosslinking.
Highlights
Polymers are widely used due to their cost-effectiveness, light weight, high manufacturability, and chemical resistance
The measured thermal conductivity value of an ultra-drawn PE nanofiber was reported to be 104 W/m-K at room temperature [4]. This phenomenal increase in k had been predicted theoretically by the preceding molecular dynamic calculations, which reported the thermal conductivity of approximately 350 W/m-K in a single PE molecular chain [5]
The thermal conductivity of the same NFs decreased after the annealing treatments at Tg because of the conformational transition from the trans state to the gauche state
Summary
Polymers are widely used due to their cost-effectiveness, light weight, high manufacturability, and chemical resistance. The amorphous characteristic of polymers translates into low thermal conductivity (k), which is on the order of 0.1 W/m-K, limiting their applications for heat dissipation. To overcome this drawback, there have been many attempts to improve the thermal conductivity of polymers, such as the addition of fillers, the modification of chain alignment, and the enhancement of interchain bonds [1,2]. The measured thermal conductivity value of an ultra-drawn PE nanofiber was reported to be 104 W/m-K at room temperature [4]. This phenomenal increase in k had been predicted theoretically by the preceding molecular dynamic calculations, which reported the thermal conductivity of approximately 350 W/m-K in a single PE molecular chain [5]
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