Structure and scaling of polymer brushes near the Theta condition

We have examined the forces of interaction between opposing polystyrene bru shes of various molecular weights in cyclohexane near the Theta temperature . The force-distance profiles and structural information obtained from surf ace forces experiments are compared to those of brushes in the good solvent , toluene. In both solvents, the chains are stretched a few times their fre e solution radii of gyration, though we find that when the solvent quality is decreased from good to near-Theta, the brush chains contract more than w ould free coils in solution undergoing the same solvent change. This shrink age of the brushes measured from surface forces experiments agrees with hyd rodynamic measurements of the layer thickness and theoretical predictions. A mean-field model successfully coalesces the surface forces data of differ ent brushes in the near-Theta solvent to a single universal profile; howeve r, this master curve is distinct from the universal profile formed by brush es bathed in a good solvent. We also show that in near-Theta cyclohexane th e osmotic free energy that swells the tethered layer is more than an order of magnitude higher than the corresponding. homopolymer solution of the sam e concentration. Although such a difference was seen for well-solvated brus hes, here the magnitude for near-Theta conditions is considerably greater. We assert that; the extra repulsion arises because the cyclohexane is a mar ginal solvent for the tethered chains (similar to the situation for branche d chains in solution) and show, by superposing the force-distance data from brushes in both toluene and near-Theta cyclohexane, that v > 0.5, where v is the exponent relating the radius of gyration to molecular weight. Additi onally, we report the results of experiments using a bimodal brush and disc uss how its arms of different length affect its structural behavior.