Controlled self-assembly of ITO nanoparticles into aggregate wire structures in PMMA-ITO nanocomposites

In this research, polymer-matrix nanocomposites were fabricated with the objective of designing and characterizing materials that have unique phase-segregated microstructures. Indium tin oxide (ITO) nanoparticles were embedded in poly(methylmethacrylate) (PMMA) to form the nanocomposites which are the subject of this research. ITO is a degenerate semiconductor primarily used as a transparent conducting electrode in electronic devices. Since the ITO nanoparticles had an electrical conductivity several orders of magnitude higher than the PMMA, the PMMA-ITO nanocomposites could be characterized using non-destructive electrical measurements. In turn, correlations were made between the electrical properties and features in the microstructures of the nanocomposites.;Compression molding ITO-coated PMMA particles at ∼157°C resulted in a transition in the morphology of the PMMA particles from spherical to polyhedral shapes in the specimens. Consequentially, the microstructure of the PMMA-ITO nanocomposites resembled a Voronoi arrangement. It was also observed under these conditions that the ITO nanoparticles on the surfaces of the PMMA particles experienced significant displacement during compression molding, as a function of their concentration. When the ITO concentration was near or below the percolation threshold for the PMMA-ITO nanocomposites, the ITO nanoparticles accumulated along the edges of the polyhedral-shaped PMMA particles and self-assembled into aggregate structures resembling nano- or microwires. These aggregate wire structures were responsible for 3-dimensional percolation in the PMMA-ITO nanocomposites, which occurred between 0.33-0.50 vol.% ITO. The ITO nanoparticles began to form conducting sheets across the flat faces of the polyhedral PMMA particles only after the percolation threshold concentration was exceeded.;The specimens with controlled microstructures were investigated using ac impedance spectroscopy, transmission optical and scanning electron microscopy (SEM), UV-Vis-IR transmission spectroscopy, internal reflection intensity analysis (IRIA), ultra-small angle x-ray scattering (USAXS), and stereological measurements. A geometrical model based on the volume-to-surface area ratios of the PMMA and ITO particles was also derived in order to predict the percolation threshold in the specimens. The model accounted for the polyhedral morphology of the PMMA particles in the microstructure. USAXS and stereological measurements were also used to characterize the dimensions of the self-assembled ITO aggregates in order calculate the amount of ITO nanoparticles that would be expected for percolation to occur in the PMMA-ITO nanocomposites. The experimental-based calculations showed reasonable agreement with the percolation threshold values detected by electrical measurements of the specimens.