Understanding Perylene's Sensitivity to Local Polymer Dynamics at Short Nanosecond Relaxation Times

Interfacial interactions lead to dynamical perturbations and gradients in local properties like the glass transition temperature Tg. The mechanisms by which these interfaces alter dynamics over extended length scales are still open questions. Dye labeled measurements that can be localized near these interfaces at different depths have played important roles in characterizing local property gradients in these systems. For years, the workhorse for such studies has been pyrene dye, where its long excited-state fluorescence lifetime of 450 ns leads to Tg values that agree well with volumetric thermal expansion measures like ellipsometry. In contrast, we explore here perylene dye’s ability to capture polymer vibrational dynamics at GHz frequencies given its short excited-state lifetime of 6 ns. Using a perylene-labeled polystyrene (PS) we synthesized, we compare and contrast the behavior and sensitivity of perylene to that previously measured by pyrene-labeled PS in various scenarios. From our existing work on perylene doped PS films, we have demonstrated perylene’s sensitivity to the short nanosecond relaxation times of the polymer that appears to capture dynamic free volume. From temperature-dependent fluorescence measurements, we have characterized changes in polymer fragility with decreasing film thickness. Perylene-labeled PS provides a new local probe that can serve as an alternate way to understanding polymer dynamics near interfaces.