Perylene Dye as a New Measure of Dynamic Free Volume, Fragility, and Dynamic T_g in Thin Polystyrene Films

How the glass transition and associated properties are altered in thin films due to interfacial effects like the free surface is still an open question. As such new experimental measures providing additional characterization of these systems are needed. We present fluorescence results using perylene dye doped in polystyrene (PS) thin films that appear to provide a measure of dynamic free volume, fragility, and a dynamic glass transition temperature T_g through the dye's sensitivity to high frequency molecular vibrations of the surrounding polymer. Following our recent work on bulk films, we define an intensity ratio I_ratio(T) between the intensity of the first peak in the fluorescence spectrum and a temperature-independent location we dub the self-referencing region (SRR) [Han, Roth, Soft Matter 2022, 18, 6094]. The temperature dependence of log[ I_ratio(T) / I_ratio(T_ref) ] exhibits a non-Arrhenius Williams-Landel-Ferry (WLF) behavior above T_g and an Arrhenius temperature dependence below T_g. We find that in thin PS films, the temperature dependence of the WLF parameters above T_g weaken showing a decrease in fragility and the effective activation energy with decreasing film thickness, yet the location of the dynamic T_g and the activation energy below T_g are independent of film thickness. We compare and contrast these new perylene dye measurements with the thermodynamic T_g results identified from pyrene dye measurements previously published in the literature.