Neutron reflectivity study on the nanostructure of PMMA chains near substrate interfaces based on contrast variation accompanied with small molecule sorption.

In the case of poly(methyl methacrylate) (PMMA) thin films on a Si substrate, thermal annealing induces the formation of a layer of PMMA chains tightly adsorbed near the substrate interface, and the strongly adsorbed PMMA remains on the substrate, even after washing with toluene (hereinafter called adsorbed sample). Neutron reflectometry revealed that the concerned structure consists of three layers: an inner layer (tightly bound on the substrate), a middle layer (bulk-like), and an outer layer (surface) in the adsorbed sample. When an adsorbed sample was exposed to toluene vapor, it became clear that, between the solid adsorption layer (which does not swell) and bulk-like swollen layer, there was a "buffer layer" that could sorb more toluene molecules than the bulk-like layer. This buffer layer was found not only in the adsorbed sample but also in the standard spin-cast PMMA thin films on the substrate. When the polymer chains were firmly adsorbed and immobilized on the Si substrate, the freedom of the possible structure right next to the tightly bound layer was reduced, which restricted the relaxation of the conformation of the polymer chain strongly. The "buffer layer" was manifested by the sorption of toluene with different scattering length density contrasts.