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Polyelectrolyte brushes consisting of polystyrene−poly(acrylic acid) (PS−PAA) diblock copolymers were investigated experimentally using surface pressure isotherms and ellipsometry. The surface pressure π of the block copolymers at the air/water interface was measured as a function of the grafting density σ at various salt concentrations and pH. It is concluded that the scaling behavior of π(σ) of long PAA chains at high ionic strengths and low pH agrees with predictions of analytical mean-field models. The theoretical predicted scaling behavior of π(σ) for annealed brushes at low ionic strength and low pH is not observed because of adsorption of the polyacid chains to the air/water interface. The thickness of PAA brushes on hydrophobically modified Si wafers was measured with ellipsometry as a function of pH, total ionic strength I, and σ. It is observed that at a given pH the brush thickness behaves nonmonotonically as a function of I (i.e., it initially increases and subsequently decreases with increasing I). This nonmonotonic behavior agrees with theoretical predictions for annealed brushes. The experimentally observed scaling exponent α in the power law H ∼ I α is ∼0.1, which is less than that predicted theoretically (1/3).

We present a new technique for preparing and characterizing brushes consisting of weak acidic monomers. The brushes consist of polystyrene-polyacrylic acid block copolymers. The block copolymers are deposited from the air/water interface on a hydrophobically modified Si wafer via the Langmuir-Blodgett technique. The titration of such brushes at low ionic strength is measured with reflectometry. It is shown that the titration curve of the grafted PAA is shifted toward higher pH with increasing grafting density, as predicted by theoretical models for weak acidic brushes.

Grafted adsorbing polymers are investigated with the Scheutjens-Fleer self-consistent field model. The surface pressure of such systems is calculated numerically and semiquantitative agreement is found with experimental surface pressure isotherms of PS-PEO diblock copolymers at the air/water interface. Scaling relationships of mean-field models predict the surface pressure π and the height H of neutral brushes to scale as π ∼ σ 5/3 and H ∼ σ 1/3 , respectively, as a function of the grafting density σ. These scaling relationships for the surface pressure and the thickness are corroborated experimentally for long PEO chains, provided contributions to π due to adsorption to the air-water interface are taken into account. In the SCF model the pancake-brush transition in a good solvent is found to be continuous for all chain lengths and adsorption energies. At high adsorption energies the transition is abrupt and resembles a continuous phase transition close to a critical point, a so-called λ-transition.

Abstract:The interaction between polymer brushes and mesoscopic particles is investigated both theoretically and experimentally. We present an analytical mean-field theory for a polymer brush (a layer of long polymer chains end-grafted to a substrate) with varying excluded volume interactions between monomer units. This system mimics the reversible adsorption of mesoscopic particles, such as surfactant micelles or proteins, on the grafted chains. The equilibrium structural properties of the brush (the brush thickness and overall degree of complexation) as well as the number of adsorbed particles per unit area, ⌫, are analysed as functions of the affinity between particle and chain, grafting density and excluded volume interactions. In our model ⌫ is found to have a maximum as a function of . Experimentally the adsorption of BSA on a hydrophobic substrate with grafted PEO chains is measured with reflectometry. In the case of short grafted chains the adsorbed amount of BSA, ⌫, decreases continuously with increasing , which agrees with previous results and model calculations in the literature. In the case of long PEO chains, however, ⌫ is found to have a maximum as a function of . Qualitatively the experimental dependence of ⌫ on is found to agree with the results of our mean-field model. PEO chains show no affinity for BSA in the bulk, whereas in a grafted conformation an effective attraction is found. Some comments are made on the nature of this affinity, which is not yet fully understood.