The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films

Abstract

To elucidate the mechanism underlying the effect of polymer/solid interfacial interactions on the dynamics of thin polymer films, the glass transition of thin end-functionalized polystyrene films supported on SiO₂–Si, such as proton-terminated PS (PS-H), α,ω-dicarboxy-terminated PS (PS-COOH), and α,ω-dihydroxyl-terminated PS (PS-OH), was investigated. All the PS films exhibited a substantial depression in T_g with decreasing film thickness, while the extent of such depression was strongly dependent on the chemical structure of the end groups and molecular weights. It was found that T^film_g − T^bulk_g of the various PS films increased linearly with increasing h_ads/R_g, in which h_ads is the thickness of the interfacial adsorbed layer and R_g is the radius of gyration of PS. The h_ads/R_g is a direct reflection of the macromolecular chain conformation within the adsorbed layer which was affected by its end groups and molecular weights. These findings are in line with the work of Napolitano, and present direct experimental evidence.