Effect of CO$_{2}$ on a polystyrene adsorbed nanolayer*

We report the role of compressed carbon dioxide (CO$_{2}$) in a mobility gradient of polymer chains near a planar solid substrate. A series of bilayers of bottom hydrogenated polystyrene and top deuterated polystyrene layers were prepared on Si substrates, and high pressure neutron reflectivity (NR) was used to study diffusive motion at the polymer/polymer interface. The interdiffusion is hindered when the distance is less than 3Rg (Rg, radius of polymer gyration). This reduced chain diffusivity is attributed to CO$_{2}$-induced polymer adsorption on the substrate, transforming the 0.6Rg thick region from the substrate interface into an irreversibly adsorbed polymer layer. The cohesion strength is attributed to loops in the adsorbed chains with which the neighboring chains can entangle.