Confinement Effects on Dye Translational Diffusivity in Polystyrene Thin Films Depend on Polymer Molecular Weight: Connection to Fragility-Confinement Effects

The impact of confinement on the translational diffusivity, D_dye, of small-molecule dye 9,10-bis(phenylethynyl)anthracene is studied in supported, thin films by fluorescence. Relative to bulk films and near T_g, D_dye is reduced by 80 - 90% in 100-nm-thick, high molecular weight (MW) PS (400 kg/mol) films. In contrast to T_g, which reflects the slow portion of the cooperative segmental relaxation distribution, D_dye reflects the fast relaxation regions, which can be many orders of magnitude faster than slow regions. D_dye results are associated with fragility, which reflects the relaxation distribution breadth, and its confinement effects: with sufficient confinement, the relaxation distribution narrows (and high MW PS fragility decreases) with the fast relaxation tail becoming slower, leading to a decrease in D_dye. In yet thinner films, the slow relaxation regions become faster, decreasing T_g. Because low MW PS (6 kg/mol) exhibits a lower bulk fragility than high MW PS, confinement has a much-reduced effect on low MW PS fragility, and both fragility and D_dye in 100-nm-thick low MW PS films are unchanged from bulk.