2, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Metal oxide nanoparticles are important for a variety of applications including catalysis, photocatalysis, gas filtration, gas sensing, and improving the physical properties of materials. The development of methodologies to fabricate textiles covered with metal oxide nanoparticles has potential benefits for Soldier protection. Previous investigations using bi-component melt extrusion were conducted to explore the ability to incorporate metal oxide nanoparticles within polypropylene (PP) fibers. That effort showed that only relatively small concentrations of metal oxide nanoparticles are on the surface of the PP fibers and that these particles are inactive for adsorption and photocatalysis. Based on those findings, a new effort is underway which focuses on the fundamental research to understand and optimize surface segregation and diffusion of metal oxide nanoparticles within polymer films and fibers. Success of this research has the potential to lead to methods to extrude blends of polymers and metal oxide particles such that the extruded fibers have surfaces that are decorated with high concentrations of active particles. Scanning electron microscopy and surface science techniques are being used to understand how parameters such as the nature of the polymer, the type and shape of the nanoparticles, and temperature affect surface segregation of metal oxide nanoparticles in polymeric fibers and films. Annealing techniques were used in an attempt to affect surface segregation in PP fibers containing ca. 3 wt. % ZnO nanorods and nanospheres. Annealing at temperatures as high as 140°C did not result in appreciable migration of the nanoparticles to the periphery of the fiber. The reason for this lack of surface segregation may likely be due to the high molecular weight of the polymer, with the long chains impeding particle mobility. Based on these findings, surface segregation studies of ZnO nanospheres in low molecular weight PP films is currently being investigated. The amount of surface segregation, as determined by X-ray photoelectron spectroscopy (XPS), was found to depend on the annealing atmosphere, with greater oxygen content leading to greater surface segregation. XPS revealed that the PP surface was partially oxidized upon heating in oxygen-containing atmospheres, resulting in carboxylate groups that helped drive the segregation.