Effect of Processing on the Microstructure and Electrical Conductivity of Hot Pressed PMMA/ITO Bulk Nanocomposites
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Effect of Processing on the Microstructure and Electrical Conductivity of Hot Pressed PMMA/ITO Bulk Nanocomposites Charles J. Capozzi and Rosario A. Gerhardt School of Materials Science & Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA, 30332-0245 INTRODUCTION There are few studies that discuss the effect of the fabrication conditions and bulk thickness on the electrical conductivity of hot pressed polymer-matrix composites. For polymer-matrix composites that possess a segregated-network microstructure, the processing parameters can significantly impact the electrical properties and microstructure of the composite material. Our group has recently fabricated novel polymer-matrix nanocomposites, which possess a segregated network microstructure containing regular, polyhedral-shaped polymer matrix particles12 . This paper investigates the effect of processing pressure and specimen thickness on the electrical properties and microstructure of hot pressed poly(methyl methacrylate) (PMMA) containing segregated networks of indium tin oxide (ITO) nanopowders.
EXPERIMENTAL PMMA / ITO nanocomposites were fabricated with Buehler© transoptic powder (PMMA) and ITO powders obtained from Aldrich©. The PMMA powders have a wide particle size distribution of 5-100 µm, and the ITO particles possess a particle size distribution between 50-100 nm. The raw materials were mechanically mixed in order to deposit the ITO nanoparticles onto the surface of the PMMA; similar to the method described by Turner and co-workers3. After mixing, the ITO-coated PMMA powders were hot pressed for 15 mins. above the glass-transition temperature of the polymer. Compositions of 5 wt.%, 9 wt.%, and 13 wt.% ITO were used to study the effect of pressure and thickness on the composite specimens. In order to study the pressure effect, the force was varied between 5-40 kN during hot pressing for each composition. To determine the influence of the specimen thickness, the filler content was held constant at 13 wt.% ITO and the amount of powder used during pressing was varied to yield ~0.5 mm, ~1.0 mm, and ~2.0 mm thick composites. AC Impedance Spectroscopy was used to measure the electrical resistance of the specimens. Prior to the measurements, SEM high-purity silver paint was applied to both sides of the composite to act as a current collector. A Solartron© Impedance-Gain Phase Analyzer was utilized to acquire impedance data between 107-0.01 Hz at 0.1 Vrms. The value of the Dc resistance was taken from the intercept between the imaginary data and the real axis on the complex-plane impedance plot and converted to conductivity. In preparation for scanning electron microscopy (SEM), the specimens were fractured at room temperature in order to expose the cross-section of the composites and gain information about the microstructure. The fracture surfaces were gold-coated before examination under a LEO 1550 SEM. The images were acquired using an accelerating voltage of 10 kV under a magnification of 150X.
RESULTS Figure 1 shows
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