Correlation of the ac Electrical Conductivity and the Microstructure of PMMA/ITO Nanocomposites That Possess Phase-Segregated Microstructures

This paper investigates the effect of processing parameters on the microstructure and the ac electrical conductivity of poly(methyl methacrylate) (PMMA)/indium tin oxide (ITO) nanocomposites. The PMMA/ITO composites were fabricated by hot pressing ITO-coated PMMA powders and confining the ITO nanoparticles to the perimeter of the polymer phase. Correlations between the microstructure and the measured ac conductivity were determined by comparing scanning electron microscopy (SEM) images of fractured cross sections of the composites and the ac conductivity determined from impedance spectroscopy measurements. It was found that the microstructure of the PMMA/ITO composites consists of polyhedral-shaped matrix particles, which are affected by the starting amount of powder and compaction pressure used to form the composites. The critical frequency f c, which determines the transition from frequency-independent to frequency-dependent ac conductivity, was observed to increase as the steady state conductivity increased. When higher compaction pressures were used, the specimens had lower conductivities and the fractured surfaces showed deformed polyhedral shapes with combinations of rough and smooth regions. These changes in the microstructure of the composites were easily detected by the ac electrical conductivity measurements. The SEM images of the fractured cross-sections indicated that these changes in the ac conductivity were related to the local concentration of the ITO nanoparticles in the PMMA matrix. This study demonstrates that impedance spectroscopy is an excellent tool for examining structure−property relationships in polymer−matrix composites that possess phase-segregated microstructures.