NEUTRON REFLECTION FROM POLYSTYRENE ADSORBED ON SILICA FROM CYCLOHEXANE AT TEMPERATURES AT AND BELOW THE THETA TEMPERATURE

Using neutron reflection, we have studied the adsorption of polystyren e onto amorphous silica from a cyclohexane solution with changing temp erature, thus varying the solvent conditions, from better than theta c onditions to below the cloud point, T(cloud), where phase separation o ccurs. For this system, T(cloud) was determined by light scattering to be 21.9-degrees-C. We observe a dramatic increase in adsorption near the cloud point. The adsorption is reversible and the rate of establis hment of equilibrium is fast. At 22-degrees-C, equilibrium is establis hed in 25 min. Our measurements are very sensitive to adsorption at te mperatures below 22.5-degrees-C where the layer is sufficiently thick and dense to give total reflection. Our reflectivity profiles are not sensitive to the shape of the tail of the decay into solution, but the thickness of the main part of the adsorbed layer is much larger than R(g), of the order of 1000 angstrom in comparison with an R(g) of 160 angstrom. The surface excess at 22.5-degrees-C is 2.2 +/- 0.8 mg m-2. As the temperature is reduced, further adsorption occurs and at 15-deg rees-C the surface excess is 6.5 +/- 0.8 mg M-2.