The Adsorption of Poly (vinyl-pyrrolidone) onto Silica. II. The Fraction of Bound Segments, Measured by a Variety of Techniques

The conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on pyrogenic silica was studied by measuring the fraction of segments bound directly to the solid/liquid interface. Two solvents, water and 1,4-dioxane, were used. Results obtained with infrared spectroscopy and with microcalorimetry are compared with IR, NMR, and EPR data from the literature. The experimental results are critically discussed in the context of recent theoretical predictions. It turns out that bound fractions derived from infrared spectroscopy agree rather poorly with theoretical data, whereas magnetic spectroscopy (NMR, EPR) seems a rather reliable way to determine the amount of polymer in trains. Microcalorimetry also yields reasonable results, but only for relatively short polymer chains. Explanations for the observed discrepancies are presented. With respect to the most probable conformation of the adsorbed molecules, it is concluded that at low adsorbed amounts the PVP molecules are in both solvents nearly flatly adsorbed with a high fraction of segments immobilized on the surface. As the adsorbed amount increases, loops and tails develop, and bound fractions of about 0.50 are reached at high adsorbed amount. The theoretical prediction that the bound fraction for a given polymer—solvent substrate combination is almost a unique function of the adsorbed amount, independent of molecular weight and concentration, is also supported by the present observations