화학공학소재연구정보센터
Polymer, Vol.99, 417-426, 2016
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 I-1/I-3 is sensitive to molecular caging, which increases with stiffness. At 140 degrees 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 Tg at 100 degrees C and in the glassy state at 60 degrees 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 degrees C and 60 degrees C, stiffness-gradient length scales extend similar to 45-85 nm from the substrate and similar to 35-85 nm from the free surface. At 140 degrees C, the stiffness-gradient length scales extend similar to 85-200 nm from the substrate and <= 20 nm from the free surface. (C) 2016 Elsevier Ltd. All rights reserved.