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.
In this paper, we studied the kinetics of the adsorption of poly(methyl methacrylate), PMMA, onto native aluminum oxide surfaces by X-ray photoelectron spectroscopy and reflection-absorption infrared spectroscopy, with the intent of tracking the various changes observed in the infrared spectrum of the adsorbed polymer layer as a function of adsorption time. Specifically, we utilized the relative changes in the absorption bands of the carbonyl, carboxylic acid, and carboxylate groups to determine the sequence of events that culminate in the formation of bonds between carboxylate groups on hydrolyzed PMMA and specific sites on the aluminum oxide surface. We have shown that the adsorption process involves the hydrolysis of a fraction of the methoxy groups of the PMMA to generate COOH groups. Unlike previous assumptions, the formation of COOH groups on the PMMA chains does not constitute a sufficient condition for the actual chemisorption of the polymer chains onto the metal oxide surface. To promote bonding, the acid groups must undergo dissociation to form the carboxylate groups, followed subsequently by actual bond formation, that is, active anchoring, on the surface. This process is mediated by the aluminum oxide sites on the surface in the presence of water. Hence, the adsorption process occurs via a two-step mechanism, in which the first step, that is, the hydrolysis step, is a necessary but insufficient condition and the second step, that is, the anchoring step, is largely dependent on the type of interfacial chemistry possible for a particular polymer-metal oxide surface, the polymer conformation, the molecular weight, and the flexibility of the adsorbing molecules.
Comprehensive two-dimensional liquid chromatography (2DLC) offers a number of practical advantages over optimized one-dimensional LC in peak capacity and thus in resolving power. The traditional "product rule" for overall peak capacity for a 2DLC system significantly overestimates peak capacity because it neglects under-sampling of the first dimension separation. Here we expand on previous work by more closely examining the effects of the first dimension peak capacity and gradient time, and the second dimension cycle times on the overall peak capacity of the 2DLC system. We also examine the effects of re-equilibration time on under-sampling as measured by the under-sampling factor and the influence of molecular type (peptide vs. small molecule) on peak capacity. We show that in fast 2D separations (less than one hour), the second dimension is more important than the first dimension in determining overall peak capacity and conclude that extreme measures to enhance the first dimension peak capacity are usually unwarranted. We also examine the influence of sample types (small molecules vs. peptides) on second dimension peak capacity and peak capacity production rates, and how the sample type influences optimum second dimension gradient and re-equilibration times.
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