Effect of Confinement on the Translational Diffusivity of Small Dye Molecules in Thin Polystyrene Films and Its Connection to Tg-Confinement and Fragility-Confinement Effects

Using fluorescence, we study the impact of nanoscale confinement on the translational diffusivity (D) of trace levels of a small-molecule dye, 9,10-bis(phenylethynyl)anthracene (BPEA), in supported polystyrene (PS) films via Förster resonance energy transfer (FRET). Reductions in BPEA diffusivity are observed in films thinner than ~200 nm, with D decreasing by 80–90% in 100-nm-thick films compared to bulk. The activation energy of BPEA diffusivity increases from ~210 kJ/mol in bulk films to ~370 kJ/mol in 130-nm-thick films. BPEA exhibits a greater diffusivity-confinement effect than a larger dye, decacyclene, in terms of both the length scale at which confinement effects become evident and the percentage reduction in diffusivity. For both BPEA and decacyclene, the diffusivity-confinement effect in PS films occurs at a length scale much larger than that for the glass transition temperature (Tg)-confinement effect and somewhat larger than that for the fragility-confinement effect. This difference in confinement-effect length scales can be rationalized as follows: small-molecule dye diffusivity relates predominantly to short times in the α-relaxation distribution, whereas Tg relates to long times in the α-relaxation distribution, and fragility reflects the overall breadth of this relaxation time distribution. If confinement results in a narrower relaxation time distribution in PS with the short-time relaxations being shifted to higher times and the longest-time relaxation regimes being shifted to lower times, then Tg, diffusivity, and fragility all decrease at sufficient levels of confinement. If the narrowing with confinement begins with the shortest relaxation time regimes, then fragility and small-molecule dye diffusivity are influenced by confinement at larger length scales than Tg.