Confinement Effects on Dye Diffusivity in Polymer Films Depend on Polymer Molecular Weight: Relation to Fragility-Confinement Effects

We studied the translational diffusivity of 9,10-bis(phenylethynyl)anthracene (D_dye) in supported, polystyrene (PS) films. 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. These results are associated with fragility-confinement effects, with fragility decreasing in films with decreasing thickness below ~ 200 nm. Fragility reflects the breadth of the cooperative segmental relaxation distribution: that breadth narrows with confinement. The thickness dependence of D_dye reflects the time scales associated with the fast relaxation tail of that distribution. At thickness below ~200 nm, the distribution narrows, with the shortest relaxation times shifting to longer times, leading to a reduction in D_dye. We also studied D_dye in low MW PS (6 kg/mol). Because 6 kg/mol PS exhibits much lower bulk fragility than high MW PS, confinement has a much-reduced effect on low MW PS fragility: both fragility and D_dye in 100-nm-thick low MW PS films are unchanged from bulk. Thus, for thicknesses where fragility-confinement effects are observed (high MW PS), D_dyealso exhibits confinement effects. By suppressing fragility-confinement effects by using very low MW PS, confinement effects on D_dye are also suppressed.