Homopolymer adsorption at the liquid-air interface by XEWIF
Abstract
The adsorption of poly(dimethylsiloxane) to the air surface from solution in bromoheptane
(good solvent) and bromocyclohexane ( = 29 °C) has been studied by X-ray evanescent wave induced
fluorescence (XEWIF). The sensitivity of the technique to the first ~40 A 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 (~40 A) 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
(~90 ± 10%) which decays to under 20% within the first ~20 A in the dilute and semidilute regimes. For
near- conditions, we find the adsorbed amount in dilute solution to be ~ 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 ~ 40 A of the surface. The
results in good and near- 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 < Tc).