No abstract

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.

A mean-field analytical model is developed for end-grafted polymers in contact with a solution containing surfactants. At concentrations above the critical association concentration, the surfactants are capable of cooperative association in a micellar form with the polymer chains. For single-grafted coils it is found that the coverage of the polymer chain by micelles increases continuously with increasing adsorption strength or bulk surfactant concentration. The size of the coils can increase, decrease, or have a maximum with increasing micellar coverage, depending on the relative strength of the monomer−monomer, monomer−micelle, and micelle−micelle excluded volume interactions. The amount of surfactants adsorbed on densely grafted polymer chains (brushes) is found to increase either continuously or discontinuously with increasing adsorption strength or decreasing grafting density. The character of the transition is determined by the interplay of the adsorption energy and the excluded volume interactions. When the adsorbed amount decreases continuously with increasing grafting density, the brush height increases at small and large grafting densities and has a local maximum in the intermediate range. In the case of an abrupt transition in the adsorbed amount the coexistence of stretched and collapsed chains is predicted in the transition regime. Critical adsorption strengths and grafting densities are discussed. Where possible, the numerical results are compared to experimental data.

Introduction. The interactions between polymers at interfaces and the environment is of the utmost importance for numerous processes in biomedical applications, but also in the food industry and in paint applications. 1,2 The past two decades polymers end-grafted to interfaces at high densities, so-called polymer brushes, have been recognized as interesting objects from an academic point of view. [2][3][4][5][6] Recently, numerous studies have examined brushes as promising candidates to modify the adsorption properties of surfaces in contact with heterogeneous solutions. [7][8][9][10] For instance, the adsorption of proteins and bacteria on surfaces, also known as biofouling, may be suppressed via grafting of hydrophilic polymers. 11 However, advancement in the field of brushes in contact with heterogeneous media seems to be slow due to two reasons: (a) the difficulty of experimentally preparing well-characterized polymer brushes and (b) the current lack of model systems to study brush-particle interactions. In recent papers we reported a novel method of preparing polymer brushes on solid substrates, based on the Langmuir-Blodgett (LB) technique. [12][13][14] Also, in the past the interaction between silica surfaces and PEO-homopolymers has been examined in detail. 15 Using reflectometry to measure the adsorption of nanocolloidal SiO 2 particles onto brushes prepared via the LB-technique, we believe to have the first brushparticle system in which the polymer-polymer, polymerparticle, and particle-particle interactions can be varied independently via the pH, grafting density, and ionic strength, respectively. In this communication, we report the first results for this model system and compare the results with predictions of theoretical models.Materials and Methods. Diblock copolymers consisting of PS(38)-PEO(700) were received from Dr. G. Riess, Mulhouse, France (the numbers denote the number of monomers in the blocks). 16 The PS-PEO was dissolved in chloroform (1 g L -1 ) and deposited on the air/water interface of a Langmuir trough via a micropipet. The PS-PEO monolayer was compressed to the desired density, characterized by the surface pressure. 17,18 Silicon wafers were hydrophobised using styrene as described in refs 19 and 20. On top of the styrene monolayer a PS layer of thickness 60 to 70 nm was spincoated at 3000 min -1 from a 14 g L -1 solution in toluene. Subsequently, the PS-PEO monolayer was transferred onto the coated wafer via the well-known LB-technique. 21 The interaction between the PS-groups on the block copolymer and the spin-coated PS layer is strong enough to provide stable PEO-brushes, irrespective of the solution conditions.

The structure of monodisperse and bimodal brushes consisting of poly(ethylene oxide) (PEO) at the air/water interface is investigated with neutron reflectivity, and the results are compared with the structure predicted by the Scheutjens−Fleer self-consistent-field lattice model. The monomer density profile of a monodisperse PEO brush at low and intermediate grafting densities is block-like with an extended tail region in which the density smoothly decays to zero. At high grafting densities, however, the profile is predominantly parabolic, as predicted by analytical self-consistent-field models. Quantitative agreement is found between the experimentally measured profiles and those predicted by the Scheutjens−Fleer model. Bimodal brushes are investigated for three chain length ratios and mixing ratios at various grafting densities. It is concluded that at a given grafting density, the long chains are more extended in bimodal brushes than in monodisperse brushes at the same grafting density. This additional stretching increases with increasing length of the smaller block or increasing fraction of smaller blocks. The agreement between the Scheutjens−Fleer and measured density profiles is also good in the case of bimodal brushes.