ADSORPTION OF A BIDISPERSE POLYMER MIXTURE ONTO A FLAT WALL

This paper studies the thermodynamic properties of the competitive ads orption between two polymer species differing only in length onto a fl at wall from dilute solution by both mean-field and scaling theories. The employed mean-field approach is based on a recently developed theo ry for monodisperse solutions, which goes beyond the usual ground-stat e approximation to account for the contribution of the tails. As in th e monodisperse case we also find a crossover length z which separates a loop-dominated region adjacent to the wall from a tail-dominated re gion farther away from the wall. This length scale strongly depends on the length of both chains and is the relevant scaling variable for th e loop and tail concentration profiles. The space variation of these p rofiles and the adsorbances are discussed in detail. We find a strong adsorption preference for the long chains, which is very pronounced in dilute solution. This result parallels those of experiments and, in i ts mean-field version, can be compared quantitatively with the numeric al calculations of the Scheutjens-Fleer theory [G. J. Fleer et al., Po lymers at Interfaces (Chapman and Hall, London, 1993)]. However, mean- field theory predicts that the adsorbances depend only on the ratio of the two chain lengths, whereas an absolute dependence on chain length is found by scaling theory. Since the scaling theory extends the mean -field treatment to good solvent conditions, this qualitative differen ce between both theoretical approaches should be observable in experim ents.