Structure and scaling of polymer brushes near the Θ condition

We have examined the forces of interaction between opposing polystyrene brushes of various molecular weights in cyclohexane near the Θ temperature. The force-distance profiles and structural information obtained from surface forces experiments are compared to those of brushes in the good solvent, toluene. In both solvents, the chains are stretched a few times their free solution radii of gyration, though we find that when the solvent quality is decreased from good to near-Θ, the brush chains contract more than would free coils in solution undergoing the same solvent change. This shrinkage of the brushes measured from surface forces experiments agrees with hydrodynamic measurements of the layer thickness and theoretical predictions. A mean-field model successfully coalesces the surface forces data of different brushes in the near-Θ solvent to a single universal profile; however, this master curve is distinct from the universal profile formed by brushes bathed in a good solvent. We also show that in near-Θ cyclohexane the osmotic free energy that swells the tethered layer is more than an order of magnitude higher than the corresponding homopolymer solution of the same concentration. Although such a difference was seen for well-solvated brushes, here the magnitude for near-Θ conditions is considerably greater. We assert that the extra repulsion arises because the cyclohexane is a marginal solvent for the tethered chains (similar to the situation for branched chains in solution) and show, by Superposing the force-distance data from brushes in both toluene and near-Θ cyclohexane, that v > 0.5, where v is the exponent relating the radius of gyration to molecular weight. Additionally, we report the results of experiments using a bimodal brush and discuss how its arms of different length affect its structural behavior.