Connected Nanoconfinement Effects on Glass Transition Temperature, Fragility, and Dye Translational Diffusivity in Polystyrene Thin Films Depend on Molecular Weight

Nanoconfined polymer films often display significantly different glass transition temperature (Tg) values, fragilities, and small-molecule diffusivities compared to their bulk counterparts. Here, we probe these effects in supported polystyrene (PS) films of very low molecular weight (MW, ~6 kg/mol), motivated by our prior hypothesis linking confinement effects on fragility to those on Tg and dye diffusivity. Using ellipsometry and nonradiative energy transfer (NRET)-based fluorescence measurements, we show that fragility and dye diffusivity of low-MW PS films remain constant with confinement down to ~100 nm thickness, differing from high-MW PS films that exhibit significant reductions. This implies no narrowing of the α-relaxation time distribution of low-MW PS with confinement; further, Tg reductions appear only upon confinement to thicknesses ≲50 nm, consistent with previous reports and independent of molecular weight. These results validate the proposed framework connecting confinement effects and demonstrate that fragility governs the susceptibility of polymer dynamics to nanoscale confinement.