Gary Wayne Scheiffele

Incorporation of polymerizable surfactants in hydroxyethyl methacrylate lenses for improving wettability and lubricity

Dryness and discomfort are the main reasons for dropouts in contact lens wearers. Incorporating surfactants in lens formulations could improve wettability and lubricity, which can improve comfort. We have focused on incorporating polymerizable surfactants in hydroxyethyl methacrylate lenses to improve comfort, while minimizing the potential for surfactant release into the tears. The surfactants were added to the polymerization mixture, followed by UV curing and extraction of leachables in hot water. Wettability and lubricity were characterized by measuring the contact angle and coefficient of friction. Lenses were also characterized by measuring transmittance, loss and storage moduli and ion permeability. Incorporation of surfactants significantly reduced contact angle from 90° for p-HEMA gels to about 10° for 2.43% (w/w) surfactant loading in hydrated gel. The coefficient of friction also decreased from about 0.16 for HEMA gels to 0.05 for the gels with 2.43% surfactant loading. There was a good correlation between the contact angle and coefficient of friction suggesting that both effects can be related to the stretching of the surfactant tails near the surface into the aqueous phase. The water content was also correlated with the surfactant loading but the contact angle was more sensitive suggesting that the observed improvements in wettability and lubricity arise from the protrusion of the surfactant tails in into the liquid, and not purely from the increase in the water content. The gels were clear and certain compositions also have the capability to block UVC and UVB radiation. The results suggest that incorporation of polymerizable surfactants could be useful in improving surface properties without significantly impacting any bulk property.

Mullitization Behavior of Alpha Alumina/Silica Microcomposite Powders

The mechanism of mullite formation was investigated using submicrometer composite particles which consisted of alpha alumina cores and amorphous silica coatings. Mullitization behavior was monitored using X-ray diffraction analysis, differential thermal analysis, and scanning electron microscopy. The transformation occurred with an incubation period which was followed by stages of rapid mullite growth (up to ∼70% conversion) and slower mullite growth. The first growth stage occurred primarily by nueleation and growth within the siliceous matrix. Available evidence indicates that the growth rate was controlled by dissolution of alumina in the siliceous phase. The second stage of mullitization occurred primarily by interdiffusion of alumina and silica through the mullite grains formed during the first stage. The transformation rate in the second stage was increased significantly by using mullite seed particles which produced smaller grain sizes during the first stage (and, thereby, decreased the interdiffusion distances needed to complete the reaction). This seeding approach allowed fabrication of bulk mullite samples with nearly 100% relative density and fine grain size (∼0.4 μm) after heat treatment at only 1400°C (2 h).