Poly(methyl methacrylate) (PMMA)/indium tin oxide (ITO) nanocomposites were prepared by mechanical mixing and compression molding in order to study the properties and microstructure of the composites. The composites were examined by optical and scanning electron microscopy, impedance spectroscopy, and UV‐VIS spectrophotometry. It was observed that upon compaction of the powders above the glass‐transition temperature of the matrix, the PMMA transforms from spherical to polyhedral‐shaped, and develops sharp edges and flat faces. The ITO nanoparticles do not penetrate the polymer particles, resulting in a novel segregated network microstructure. Excellent correlation between the electrical, optical, and microscopy data also provide good insight about the behavior of the filler as the content is increased in the nanocomposites. There is strong evidence that the ITO nanoparticles are extensively displaced during compaction as the PMMA powders become polyhedral‐shaped. Our results indicate that percolation occurs due to the ITO forming a continuous network along the edges of the faceted PMMA particles. The ITO nanoparticles do not appear on the faces of the PMMA particles until after a percolation path has formed and a marked increase in electrical conductivity has occurred. This behavior significantly diverges from previous models for segregated network microstructures which proposed that percolation occurred as the result of limited displacement of the filler during compaction of the mixed powders.
Highly crystalline nonagglomerated ITO colloidal nanoparticles form an optically clear solution in nonpolar solvents. Their ∼5 nm diameter and narrow size distribution is achieved through a fatty acid mediated reaction, that lacks the need for high‐temperature annealing.
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 fc
, 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.
Composite specimens possessing polyhedral segregated network microstructures require a very small amount of nanosize filler, <1 vol %, to reach percolation because percolation occurs by accumulation of the fillers along the edges of the deformed polymer matrix particles. In this paper, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) were used to confirm the location of the nanosize fillers and the corresponding percolating paths in polymethyl methacrylate/carbon black composites. The EFM and C-AFM images revealed that the polyhedral polymer particles were coated with filler, primarily on the edges as predicted by the geometric models provided.
This study describes the optical and ac electrical characterization of polymethyl methacrylate (PMMA)/indium tin oxide (ITO) nanocomposites containing a Voronoi-type segregated network microstructure. Optical transmission microscopy, scanning electron microscopy (SEM), and impedance spectroscopy were used to investigate the microstructure and electrical properties of the composites. The complex refractive index values of the composites were measured by using a modified version of the internal reflection intensity analysis (IRIA) method. SEM images of the composites reveal that the microstructure consists of polyhedral-shaped PMMA particles, where the ITO nanoparticles become concentrated along the edges of the PMMA to form a percolated network. Electrical measurements as a function of filler content indicate that the percolation threshold is <0.50 vol % ITO. The in-plane and through-plane refractive indices obtained by the modified IRIA method suggest that the microstructure is isotropic. The estimated extinction coefficients increase with higher ITO content, which is expected for a percolating system. A microstructural model, which takes into account the accumulation of the ITO nanoparticles at the edges of the polyhedron-shaped PMMA particles, is proposed that can predict the percolation threshold in composites that have Voronoi-type segregated network microstructures.
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