Interfacial adsorption competing with thermal mixing in confined hydrogen-bonded polymer bilayers

In confined polymer thin films, interfacial chain adsorption can fundamentally compete with thermally driven interdiffusion, leading to mixing behavior distinct from bulk systems. Here, we investigated ultrathin hydrogen-bonded PMMA/PVPh bilayers on silicon substrates using complementary X-ray reflectivity, in situ neutron reflectivity, and near-edge X-ray absorption fine structure spectroscopy. Thermal annealing induced progressive interdiffusion and eventual miscibility across the bilayer. However, even after apparent homogenization, a nanometer-thick PVPh-rich layer persisted at the substrate interface and remained resistant to dissolution in a good solvent. Neutron scattering length density profiles revealed that this low-SLD interfacial region survived at elevated temperatures, while surface-sensitive spectroscopy confirmed preferential enrichment and strongly stabilized adsorption of PVPh chains at the oxide interface. These findings demonstrate that strong polymer-substrate interactions can arrest complete interfacial homogenization under confinement, generating asymmetric composition profiles despite bulk miscibility. The results highlight the fundamental role of surface-induced stabilization of adsorbed chains in governing mixing and structural evolution in ultrathin polymer films.