Shear-Induced One-Dimensional Alignment of Alumina and Silica Nanoparticles in Coatings

Abstract

One-dimensional (1-D) strings of alumina and silica nanoparticles, formed in an extended pearl-necklace fashion were observed near the surface of cured polyurethane films. The nanoparticles have been shown to arrange in perfectly straight lines in the direction of shear. As shear increases, the abundance and length of the arrangements increases. The mechanism of alignment is hypothesized to be a combination of shear-induced patterning and oriented attachment. The alignments have been imaged with atomic force microscopy as well as scanning electron microscopy. Energy dispersive x-ray spectroscopy elemental analysis has confirmed the presence of colloidal nanoparticles in arrangements of smaller particles, while similar arrangements of larger particles that can be seen with an optical microscope have been shown to contain mostly silicon element, presumably a phase-separated polysiloxane containing additive present in all formulations. These findings help advance the knowledge on preparing 1-D aligned particles through template-free approaches.