Many colloidal dispersions of technological importance are stabilized against aggregation by a surface layer of end-grafted polymers. In a new twist on an old problem, our collaborators have been grafting polymers to the surface of virus particles used as vectors for Gene Therapy. We have used small angle neutron scattering (SANS) to study the structure of polymer layers grafted to the surface of these viruses and to model gold colloids, in order to understand the influence of grafting and the particle environment (electrolyte concentration) on the polymer distribution.

As a model grafted polyelectrolyte we have prepared poly(styrene sulfonate) coated gold nanoparticles, employing a two-step, soft-sulfonation reaction. We find that the SANS from the coated gold nanoparticles is well described by a simple core-shell model, corresponding to a step-function density profile, consistent with analytical models for weak polyelectrolyte brushes. The change in the structure of the polymer layer in response to changes in electrolyte concentration has also been explored using SANS, and we observe a collapse of the layer structure, when the added electrolyte concentration reaches the concentration of counterions confined within the brush layer.

We have also investigated the effect of particle curvature on the structure of grafted polymer layers. In particular we have used SANS to study the structures formed by grafting poly(ethylene glycol) (PEG) of different molecular weights, to particles of different sizes. We find a greater departure from the simple step-function density profile, that scaling theory predicts for a flat surface, as the chain length increases. We attribute this to particle curvature exerting a greater influence as the dimensions of the polymer coil approach those of the particle radius.

To our knowledge these are the first successful SANS measurements of the structure of polymer layers at gold nanoparticle surfaces, providing a new characterization technique for coated gold nanoparticles. The effects observed for coated virus particles are more subtle and we have benefited from the insights provided by the gold nanoparticle studies in interpreting these results.