Neutron reflection from polystyrene adsorbed on silica from cyclohexane at temperatures at and below the theta temperature

Abstract

Using neutron reflection, we have studied the adsorption of polystyrene onto amorphous silica from a cyclohexane solution with changing temperature, thus varying the solvent conditions, from better than theta conditions to below the cloud point, T cloud, where phase separation occurs. For this system, T cloud was determined by light scattering to be 21.9°C. We observe a dramatic increase in adsorption near the cloud point. The adsorption is reversible and the rate of establishment of equilibrium is fast. At 22°C, equilibrium is established in 25 min. Our measurements are very sensitive to adsorption at temperatures below 22.5°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 Å in comparison with an R g of 160 Å. The surface excess at 22.5°C is 2.2 ± 0.8 mg m −2. As the temperature is reduced, further adsorption occurs and at 15°C the surface excess is 6.5 ± 0.8 mg m −2.