Segment Level Chemistry and Chain Conformation in the Reactive Adsorption of Poly(methyl methacrylate) on Aluminum Oxide Surfaces

We have used X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS), and solid-state nuclear magnetic resonance spectroscopy (NMR) to study the interface between poly(methyl methacrylate) (PMMA) and amorphous aluminum oxide. For the first two techniques, the aluminum oxide was a native oxide grown in ambient on vapor-deposited aluminum metal films, while for the third, high surface area porous aluminum oxide was used. In all cases the polymer was adsorbed from solution. Our results have shown that when PMMA adsorbs on the aluminum oxide surface, the surface hydroxyl groups hydrolyze the ester bond in the side chain of the polymer. As a result of the reaction, a side chain carboxylate ion is formed which bonds ionically with the surface, while methanol is released as a byproduct. In the IR spectra the formation of carboxylate ions is indicated by the appearance of a new peak at 1670 cm^-1, while in the NMR spectra the loss of the methoxy carbons is indicated by a significant decrease in the methoxy carbon peak. While XPS spectra are less specific, they consistently show a new peak in the 0(2p) region of the adsorbed polymer spectrum. The strength and the specificity of this interaction drive conformational changes of the polymer molecules upon adsorption, which show up as shifts in peak positions in the NMR spectra. These changes suggest a relatively flat configuration of the extensively hydrolyzed molecules on the oxide surface.