Effect of CO₂ on a mobility gradient of polymer chains near an impenetrable solid

We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO₂) gas. A series of bilayers composed of bottom hydrogenated polystyrene (h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3R_g (R_g is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO₂ and subsequent solvent leaching, we reveal the presence of the CO₂-induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vicinity of the solid substrate even in the presence of the effective plasticizer.