RESUMO
A light-weight, flexible electromagnetic interference (EMI) shield was prepared by creating a layer-structured metal-polymer composite film consisting of electrospun nylon 66 nanofibers with silver films. The EMI shielding effectiveness (SE), specific SE, and absolute SE of the composite were as high as 60.6 dB, 67.9 dB cm3/g, and 6792 dB cm2/g in the X- and Ku-bands, respectively. Numerical and analytical calculations suggest that the energy of EM waves is predominantly absorbed by inter-layer multiple reflections. Because the absorbed EM energy is dissipated as heat, the thermal conductivity of absorption-dominant EMI shields is highly significant. Measured thermal conductivity of the composite was found to be 4.17 Wm-1K-1 at room temperature, which is higher than that of bulk nylon 66 by a factor of 16.7. The morphology and crystallinity of the composite were examined using scanning electron microscopy and differential scanning calorimetry, respectively. The enhancement of thermal conductivity was attributed to an increase in crystallinity of the nanofibers, which occurred during the electrospinning and subsequent hot pressing, and to the high thermal conductivity of the deposited silver films. The contribution of each fabrication process to the increase in thermal conductivity was investigated by measuring the thermal conductivity values after each fabrication process.
RESUMO
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.
RESUMO
The thermal conductivity enhancement of neat poly(vinyl alcohol) and poly(vinyl alcohol) (PVA)/cellulose nanocrystal (CNC) composite was attempted via electrospinning. The suspended microdevice technique was applied to measure the thermal conductivity of electrospun nanofibers (NFs). Neat PVA NFs and PVA/CNC NFs with a diameter of approximately 200 nm showed thermal conductivities of 1.23 and 0.74 W/m-K, respectively, at room temperature, which are higher than that of bulk PVA by factors of 6 and 3.5, respectively. Material characterization by Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis confirmed that the thermal conductivity of the PVA/CNC NFs was enhanced by the reinforcement of their backbone rigidity, while that of the neat PVA NFs was attributed to the increase in their crystallinity that occurred during the electrospinning.
