# Perylene Dye as a Measure of Dynamic Free Volume, Fragility, and Dynamic Glass Transition in Thin Polystyrene Films

## TL;DR

This paper shows how perylene dye can measure dynamic properties of thin polystyrene films, revealing faster relaxation dynamics in thinner films.

## Contribution

The study introduces perylene dye as a novel dynamic probe for measuring glass transition and fragility in thin polymer films.

## Key findings

- Perylene dye captures GHz vibrational fluctuations in thin polystyrene films.
- Dynamic glass transition temperature Tgperyl is independent of film thickness down to 15 nm.
- Relaxation dynamics in thin films are significantly faster than in bulk.

## Abstract

We demonstrate the
ability of perylene dye to capture
the high
frequency GHz vibrational density fluctuations of polymer matrices
in thin polystyrene (PS) films with decreasing film thickness. From
perylene’s temperature dependent fluorescence characteristics
that are observed to be proportional to the polymer’s relaxation
time τ­(T), a perylene determined glass transition
temperature
Tgperyl
is defined by the change in temperature
dependence on cooling from a non-Arrhenius to Arrhenius behavior at
the polymer’s T
g. We find
Tgperyl
behaves like a dynamic measure of T
g that is independent of film thickness h down to
15 nm. Fits to a Williams–Landel–Ferry
(WLF) functional form above T
g quantify
a decrease in fragility with decreasing film thickness and demonstrate
that the relaxation dynamics are significantly faster in thin films,
with the average relaxation time at T
g decreasing by many orders of magnitude. These results are contrasted
with those from pyrene dye that exhibit T
g(h) decreases consistent with thermodynamic measures
of T
g like ellipsometry.