Neutron reflectivity of end grafted polymers in poor solvent: Kinetic effects

Kinetics of adsorption of polymers on a solid surface in poor solvent and the competition between long and short chains were studied using neutron reflectivity. A brush which consists of a 375 K deuterated polystyrene terminated by a zwitterionic end group was grafted on a single crystal silicon wafer from a protonated cyclohexane solution. We found that the long deuterated polymer forms a density profile qualitatively similar to a 50K protonated polymer studied previously. As for the low molecular weight polymer, the profile consists of a denser layer at the solid surface, a step-like profile convoluted with a Gaussian roughness and an exponential tail which stretches out almost to the length of the same molecular weight in good solvent. Our results also suggest that the brush, formed in poor solvent, grows in islands and eventually covers the entire surface. After equilibrium was reached, the brush formed by the 375 K polymer, was exposed to a low molecular weight, 26 K protonated polymer. The profile of the long deuterated layer was studied as a function of time. In poor solvent, a new equilibrium is formed between the long and the short polymers where the exact composition of the long and short chains depends on the temperature and the ratio of the molecular weights.