On Thursday, August 9, 2007, at 10:00 am CDT, Ph.D. candidate Hui Luo will defend her thesis: "Microstructure Development in Particulate Composite Coatings by Cryo-SEM." Hui is advised by Lorraine Francis. An abstract of her presentation follows:
Understanding microstructure development starting from dispersion to a final coating during drying is critical to achieve desirable microstructures and coating properties. Cryo-SEM technique is used to study the microstructure evolutions of three particulate composite coating systems, including aqueous latex/ceramic nanoparticle coatings, aqueous monodispersed latex/ceramic nanoparticle coatings, and non-aqueous magnetic coatings.
In the first study, cryo-SEM images reveal different colloidal states of two latex/ceramic nanoparticle dispersions, which confirm the DLVO theory-based calculations of total interaction energies from past research. Moreover, images at different drying stages show how nanoparticles segregate in interstitial spaces among large latex particles during drying. As water evaporates, the latex particles consolidate, and the nanoparticles concentrate in interstitial spaces between the latex particles. With continued drying, the latex particles compact, and the nanoparticles are forced to pack more closely in the interstitial spaces. Finally, the latex particles partially coalesce to form a coherent coating.
Ordered latex/nanoparticle coatings with a nanoparticle-rich surface and a latex-rich body were developed by drying dispersions of monodispersed latex and nanosized ceramic particles. The nanoparticles uniformly occupy the interstitial spaces among the orderly packed latex particles near the surface, but are absent from the compacted latex structure beneath.
Cryo-SEM images captured at successive drying times document two important sequences to form this unique structure. Latex particles consolidate at the air-water interface at an early drying stage, and the curved menisci among them create a pressure difference to drive a convective flow. This vertical flow then transports nanoparticles to the evaporating surface. The mechanism is supported by other evidence.
The cryo-SEM technique was also applied to a non-aqueous system consisting of a solvent mixture of toluene, cyclohexanone and methyl ethyl ketone, two polymer binders of poly(vinyl chloride) and polyurethane, and magnetic nanoparticles. This is the first known cryo-SEM study on a solvent-borne coating. Cryo-TEM results show the solvent mixture can be vitrified in liquid nitrogen, suggesting a low critical cooling rate for this system.
Cryo-SEM images show a uniform and random distribution of magnetic particles in the dispersion, and the particles rotate into planes parallel to the coating surface during drying. Moreover, the particles align to one direction in those planes when early drying is in a magnetic field.