Brushes and soap : grafted polymers and their interactions with nanocolloids

Layers of polymer chains end-attached to a grafting plane at high densities, so-called brushes, are a curious state of matter. The (average) monomer density within the brush is as high as in a semi-dilute polymer solution, resulting in a high osmotic pressure in the brush. Due to the grafting, however, this isotropic osmotic pressure results in an anisotropic stretching of the chains normal to the surface. This degree of stretching can be quite extensive; in this thesis PEO-chains of 700 monomers are considered which are stretched up to 20% of their total contour length, i.e. form a brush with a thickness of 50 nm, merely by the presence of similar grafted chains.

It is evident that such extended polymer layers may strongly modify the properties of the grafting surface. To this end brushes are applied as, for instance, adsorption inhibitors or colloidal stabilisators. In this thesis we focus on the thermodynamic and structural properties of polymer brushes, both neutral and charged, and on their interactions with nanocolloids. A mean-field model is developed that describes the effect of complexes formed by polymer (or polyelectrolyte) chains and nanocolloids on the polymer conformation, and the phase behaviour of such mixtures. These two modes of investigation converge in the theoretical and experimental investigation of the interaction between neutral brushes and nanocolloids which may form complexes with the polymer chains in a bulk solution.

A general introduction to brushes and polymer-nanocolloid complexes is presented in Chapter 1. The concepts underlying scaling and analyticalself-consistent-field (aSCF) models of brushes are briefly discussed, as are a number of technological applications of grafted polymers. The difficulties encountered in the preparation of a brush of a controlled chain length and grafting density are also considered.

In Chapter 2 surface pressure isotherms of neutral, end-grafted chains that can adsorb to the grafting plane are modelled with the numerical Scheutjens-Fleer self-consistent-field (nSCF) model. These numerical results are compared to experimental isotherms of PS-PEO block copolymers irreversibly adsorbed at the air/water interface. Semi-quantitative agreement between the numerical and experimental isotherms is found. It is shown that for long chains the experimental and numerical isotherms obey the power law for the brush surface pressure as a function of the grafting density predicted by aSCF models.

The predicted power law for the brush thickness is only obeyed when the experimental surface pressure isotherms also follow the aSCF power law. The adsorption/desorption transition of grafted polymers upon increasing grafting density is investigated numerically by considering the chemical potential of the grafted chains and its derivative with respect to the grafting density. It is shown that this adsorption/desorption transition is continuous, irrespective of the chain length and the adsorption strength. The behaviour of the chemical potential at large adsorption energies is reminiscent to that of a (mean-field) magnetic system approaching its critical point.

The monomer density profiles of monodisperse and bimodal PEO-brushes are determined with neutron reflectivity and compared to profiles predicted by the nSCF model in Chapter 3. The monomer density distribution predicted by aSCF-models, namely a parabolic profile, is only found at a relatively high grafting density. At lower densities the contribution of a `tail' region at the edge of the brush to the reflectivity spectra is considerable. In this distal region, which originates from fluctuations of the extended chains, the density smoothly drops to zero. Good agreement is found between the experimental and nSCF density profiles. When short and long PEO-chains are mixed at relatively high grafting densitites a bimodal brush is formed. This biomodal density distribution is enhanced by unequal chain length ratio's and mixing ratio's at high grafting densities of such mixed layers. As expected on the basisof theoretical predictions, the long chains in the bimodal brush are additionally stretched by the presence of the shorter ones.

In Chapter 4 the properties of annealed polyeclectrolyte brushes, consisting of grafted polyacrylic-acid (PAA) chains in contact with aqueous solution, are examined with surface pressure measurements, optical reflectivity and ellipsometry. When the ionic strength of the subphase is high and the pH relatively low, the predicted power law for the surface pressure as a function of the grafting density in the salted brush (SB) regime is found. At low ionic strength and pH, however, the PAA-chains are found to adsorb at the air/water interface.

Due to such adsorption the predicted osmotic brush regime is not observed at the air/water interface. A novel manner to prepare brushes on a solid substrate, namely Langmuir-Blodgett deposition of PS-PAA block copolymers from an air/water interface on a hydrophobic modified silicon wafer and subsequent thermal annealing, is developed. Using this technique the average degree of dissociation of grafted PAA chains as a function of pH is measured with reflectometry. It is shown that dense grafting of the PAA-chains shifts the titration curves significantly to higher pH, as predicted by scaling models and numerical studies.

The thickness of the PAA brushes on hydrophobic modified silicon wafers is measured with ellipsometry as a function of pH, ionic strength and grafting density. At a pH not far from the monomeric pKa, the brush thickness is theoretically predicted to initially increase with increasing ionic strength and to decrease again at high ionic strength. This non-monotonic behaviour of the brush thickness is now observed experimentally for the first time.

The initial increase in brush thickness with increasing ionic strength is, however, experimentally less pronounced than predicted by theory.

An analytical mean-field theory for long polymer chains that form complexes with nanocolloids is developed in the following chapters. In Chapter 5 the complexation between single polymer chains in a good solvent and surfactants in micellar aggregates is considered, using a Flory-like approach. It is shown that the number of complexed micelles on a polymer chain continuously increases with increasing surfactant concentration, in agreement with experimental evidence. The size of the coil can monotonously increase, decrease, or have a maximum as a function of the surfactant concentration. Comparison with experimental data for PEO-gels complexed with SDS shows a reasonable agreement between the predicted dependence of the gel volume on the ionic strength and experiments.

In Chapter 6 semi-dilute solutions of complexed chains are considered. Osmotic interactions are found to strongly influence the degree of complexation in a semi-dilute solution. The degree of loading of the chains by nanocolloids decreases with increasing monomer density when the osmotic interactions between complexed particles are strong compared to those between bare monomers. If, however, the complex-monomer osmotic interactions are strong compared to both the complex-complex and monomer-monomer, phase separation into a relatively dilute phase consisting of highly loaded chains coexisting with a relatively dense phase of bare chains may occur. Such phase separation is promoted when the solvent quality decreases. If the solution is below the Theta-temperature of the bare polymer, a first-order phase transition from a bare, collapsed globule to a swollen coil with increasing particle density is predicted.

Such a first-order phase transition is reported experimentally for collapsed polymer globules with increasing surfactant concentration. An analytical self-consistent-field theory for polymer brushes, in the presence of particles capable of complexation is presented in Chapter 7. As a monomer density gradient is present in a brush, the density of complexed particles is also predicted to vary across the brush. Roughly speaking, the complexes are predominantly located in the distal region of the brush, where the average monomer density is low. In the proximal region of the brush, close to the grafting plane, the density of complexed particles is low. Microphase separation may occur in the brush under the same conditions for which macroscopic phase separation occurs in a bulk solution.

The overall number of complexed particles is predicted to have a maximum as a function of the grafting density. The height of the brush is found to either increase monotonously with increasing grafting density, or have a local maximum and minimum. The adsorption of the protein BSA on hydrophobic silicon wafers covered with grafted PS-PEO-chains is experimentally examined in Chapter 8. The amount of adsorbed BSA is measured with reflectometry at several grafting densities and different PEO chain lengths.

Conventional models for the interaction between a brush and adsorbing proteins predict the adsorbed amount to decrease with increasing grafting density and chain length as the interaction between PEO and BSA in the bulk is purely repulsive. However, it is observed that the adsorbed amount has a maximum as a function of the grafting density for long chains, whereas it decreases monotonously in the case of short chains. This maximum is qualitatively understood with our aSCF model presented in Chapter 7 and indicates that some (unknown) attraction between grafted PEO and BSA may exist.

Finally, in Chapter 9, our theoretical model is extended to complexation of polyelectrolyte chains with oppositely charged nanocolloids. In a given system (particle size, charge densities of the chain and particle) the ionic strength is the main parameter which controls complexation. At high ionic strength the attractive electrostatic interactions are suppressed and the degree of complexation is negligible. As the ionic strength decreases the attractive electrostatic interactions induce complexation. The transition from a bare polyelectrolyte to a complexed chain is predicted to be either continuous or abrupt, depending on the ratio of the charge densities and the Hamaker constant of the particles. In the former case the complex remains soluble, in the latter a non-soluble coacervate is formed. Both kinds of loading processes have been reported in the literature.