Temperature dependent perylene fluorescence as a probe of local polymer glass transition dynamics

We demonstrate how the temperature dependence of perylene's fluorescence emission spectrum doped in bulk polymer matrices is sensitive to the local glass transition dynamics of the surrounding polymer segments. Focusing on the first fluorescence peak, we show that the intensity ratio I-Ratio (T) = I-peak(7)/I-SRR between the first peak and a self referencing region (SRR) has a temperature dependence resulting from the temperature-dependent nonradiative decay pathway of the excited perylene dye that is influenced by its intermolecular collisions with the surrounding polymers segments. For different polymer matrices, poly(methyl methacrylate) (PMMA), polystyrene (PS), poly(2-vinyl pyridine) (P2VP), and polycarbonate (PC), we demonstrate that I-Ratio (T) exhibits a transition from a non-Arrhenius behavior above the glass transition temperature T-g of the polymer to an Arrhenius temperature dependence with constant activation energy E below the T-g of the polymer matrix, indicating perylene's sensitivity to cooperative alpha-relaxation dynamics of the polymer matrix. This transition in temperature dependence allows us to identify a perylene defined local T-g(peylene) of the surrounding polymer matrix that agrees well with the known T-g values of the polymers. We define a fluorescence intensity shift factor c(f) log (I-Ratio(T)/I-Radio (T-ref)) log (a(T)) in analogy with the Williams-Landel-Ferry (WLF) equation and use literature WLF parameters for the polymer matrix to quantify the calibration factor c(f) needed to convert the fluorescence intensity ratio to the effective time scale ratio described by the conventional WLF shift factor. This work opens up a new characterization method that could be used to map the local dynamical response of the glass transition in nanoscale polymer materials using appropriate covalent attachment of perylene to polymer chains.