Stiffness of thin, supported polystyrene films: Free-surface, substrate, and confinement effects characterized via self-referencing fluorescence

Stiffness-confinement effects are characterized via a non-contact, self-referencing fluorescence approach in polystyrene (PS) films labeled with trace levels of 1-pyrenylmethyl methacrylate. The pyrene fluorescence measurable I₁/I₃ is sensitive to molecular caging, which increases with stiffness. At 140 °C, molecular caging and hence stiffness in single-layer PS films supported on silica is independent of thickness down to 240 nm and increases with decreasing thickness at 165 nm and below. In contrast, near T_g at 100 °C and in the glassy state at 60 °C, molecular caging and hence stiffness in single-layer films is independent of thickness down to 63 nm and increases with decreasing thickness at 36 nm and below. In bulk bilayer films, perturbations originating at the substrate interface (free-surface interface) cause major increases (decreases) in caging and hence stiffness in 20-nm-thick substrate-adjacent (free-surface-adjacent) layers. In contrast, in 40-nm-thick bilayer films, the 20-nm-thick substrate-adjacent and free-surface-adjacent layers exhibit little difference in caging and stiffness. Thus, the gradient in stiffness from a film interface depends significantly on confinement, which we hypothesize begins to occur when thickness becomes comparable to the combined length scales over which free-surface and substrate perturbations propagate inside the film. Bulk bilayer films were used to investigate the length scales associated with interfacial perturbations. At 100 °C and 60 °C, stiffness-gradient length scales extend ∼45–85 nm from the substrate and ∼35–85 nm from the free surface. At 140 °C, the stiffness-gradient length scales extend ∼85–200 nm from the substrate and ≲ 20 nm from the free surface.