HOMOPOLYMER ADSORPTION AT THE LIQUID AIR INTERFACE BY XEWIF

The adsorption of poly(dimethylsiloxane) to the air surface from solution in bromoheptane (good solvent) and bromocyclohexane (T(theta) = 29-degrees-C) has been studied by X-ray evanescent wave induced fluorescence (XEWIF). The sensitivity of the technique to the first approximately 40 angstrom of the surface region allows the determination of important qualitative features of the adsorbed concentration profiles. The effects of molecular weight, concentration, and solvent quality are examined in detail. In a good solvent, we find that the region of the profile nearest to the surface (approximately 40 angstrom) is roughly independent of molecular weight in dilute solution, in agreement with theoretical predictions. Additional information about the adsorbed profile is obtained by varying the bulk polymer concentration. From these data we infer a profile with a high surface concentration (approximately 90 +/- 10%) which decays to under 20% within the first approximately 20 angstrom in the dilute and semidilute regimes. For near-theta conditions, we find the adsorbed amount in dilute solution to be approximately 4 times larger than in a good solvent, with the profile decaying much more slowly with depth than in the good solvent. In addition, there appears to be a higher dependence on molecular weight for the region of the profile within approximately 40 angstrom of the surface. The results in good and near-theta conditions are well described by both mean-field and scaling theories, while, on the contrary, the profile becomes qualitatively different in the proximity of the bulk coexistence curve (i.e., T < T(c)).