I. Controlled foaming of polystyrene using supercritical carbon dioxide. II. Polymer adsorption to silane-modified silica surfaces. III. Protein adsorption to silane-modified silica surfaces

Part I of this dissertation focuses on the use of supercritical CO ₂ as a blowing agent to produce microcellular foams. The goal of this research was to determine the origins of observed differences between the foaming behavior of commercial materials and those materials produced in our laboratory. It was shown that polymer molecular weight and polydispersity are not important factors in determining cell size and are not responsible for the disparity in cell sizes observed. This disparity was, however, a result of the presence of a very low molecular weight component (∼270 g/mol) found the commercial samples. Extraction of this component reduced the cell diameter of resulting foams to that of the NMW distribution samples. Addition of a styrene oligomer (285 g/mol) to a NMW distribution sample resulted in foams with larger cell diameters. Varying the concentration of this oligomer allows control of cell size in foams.; Part II concentrates on the adsorption behavior of end-functionalized polymers to monochlorosilane-modified silica surfaces. Chemically grafted tris(trimethylsiloxy)silyl (sub)monolayers (tris(TMS)) were prepared on the native oxide of silicon by the vapor phase reaction of the monochlorosilane at elevated temperatures. By exploiting the inherently sluggish kinetics of the silanization reaction, the grafting density of the tris(TMS) (sub)monolayer can be tuned. Unreacted silanol groups on the substrate surface were used as adsorption sites for carboxylic acid end-functionalized polystyrenes (PS-COOH). The thickness of the adsorbed layer could be controlled by the tris(TMS) surface coverage, adsorbing solvent, and polymer molecular weight. The topography of the adsorbed layers was investigated by AFM.; Part III probes the absorption behavior of a specific protein, albumin, to monochlorosilane-modified silica surfaces. Covalently attached monolayers were prepared on the native oxide of silicon by reaction of various chlorosilanes at room temperature. Both one-component (pure) surfaces and two-component (mixed) surfaces were investigated. The adsorption of albumin to these surfaces was carried out under physiological conditions, and ellipsometry was used to determine the total adsorbed amount of protein to these surfaces. The adsorption of albumin closely follows the surface energy of the substrate as seen by plotting adsorbance (Γ) versus the cosine of the water contact angle. The morphology of the adsorbed albumin layer was examined by AFM and was correlated to the mode of adsorption of the albumin molecule as directed by the surface chemistry.