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

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-(<i>T</i>), a perylene determined glass transition temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>g</mml:mi></mml:mrow> <mml:mrow><mml:mi>peryl</mml:mi></mml:mrow> </mml:msubsup> </mml:math> is defined by the change in temperature dependence on cooling from a non-Arrhenius to Arrhenius behavior at the polymer's <i>T</i> _g. We find <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>g</mml:mi></mml:mrow> <mml:mrow><mml:mi>peryl</mml:mi></mml:mrow> </mml:msubsup> </mml:math> behaves like a dynamic measure of <i>T</i> _g that is independent of film thickness <i>h</i> down to 15 nm. Fits to a Williams-Landel-Ferry (WLF) functional form above <i>T</i> _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 <i>T</i> _g decreasing by many orders of magnitude. These results are contrasted with those from pyrene dye that exhibit <i>T</i> _g(<i>h</i>) decreases consistent with thermodynamic measures of <i>T</i> _g like ellipsometry.