Infrared Dichroism, Chain Flattening, and the Bound Fraction Histogram in Adsorbed Poly(methyl methacrylate) Layers

We examine the distribution of adsorption substates, in contrast to their average. The distribution of infrared dichroism and bound fraction were determined in a model system (poly(methyl methacrylate) adsorbed chiefly by hydrogen bonding onto oxidized silicon from CCl₄). The method was infrared spectroscopy in attenuated total reflection (FTIR-ATR). From the surface excess (Γ), dichroic ratio (D_zx), and fraction of carbonyl groups bound to the surface (p), a broad distribution of conformations was found to prevail, from severely flattened (p ≈ 0.5 and D_zx ≪ 1 for the asymmetric methyl stretch) to nearly solution-like (p ≈ 0.1 and D_zx ≈ 1 for the asymmetric methyl stretch). The extent of flattening depended mainly on the number of adsorption sites available to each chain at the time it deposited to the surface; chains that adsorbed later, finding fewer and fewer surface sites available, then became attached by fewer and fewer segments. This in turn implied that chains which arrived initially had the center-of-mass closer to the solid surface than those which arrived later. Conformational rearrangements were not observed on the experimental time scale of 3 h; this allowed determination of the distribution of conformational substates by subtraction of sequentially-acquired infrared spectra. The broad and asymmetric distribution of conformational substates appeared to originate, predictably, from the piecemeal process by which the surface was coated. It presents analogies with issues of random sequential adsorption.