Effect of Nanoscale Confinement on the Glass Transition Temperature of Free-Standing Polymer Films: Novel, Self-Referencing Fluorescence Method

The effect of nanoscale confinement on the glass transition temperature, T(g), of freely standing polystyrene (PS) films was determined using the temperature dependence of a fluorescence intensity ratio associated with pyrene dye labeled to the polymer. The ratio of the intensity of the third fluorescence peak to that of the first fluorescence peak in 1-pyrenylmethyl methacrylate-labeled PS (MApyrene-labeled PS) decreased with decreasing temperature, and the intersection of the linear temperature dependences in the rubbery and glassy states yielded the measurement of T(g). The sensitivity of this method to T(g) was also shown in bulk, supported PS and poly(isobutyl methacrylate) films. With free-standing PS films, a strong effect of confinement on T(g) was evident at thicknesses less than 80-90 nm. For MApyrene-labeled PS with M(n) = 701 kg mol(-1), a 41-nm-thick film exhibited a 47 K reduction in T(g) relative to bulk PS. A strong molecular weight dependence of the T(g)-confinement effect was also observed, with a 65-nm-thick free-standing film exhibiting a reduction in T(g) relative to bulk PS of 19 K with M(n) = 701 kg mol(-1) and 31 K with M(n) = 1460 kg mol(-1). The data are in reasonable agreement with results of Forrest, Dalnoki-Veress, and Dutcher who performed the seminal studies on T(g)-confinement effects in free-standing PS films. The utility of self-referencing fluorescence for novel studies of confinement effects in free-standing films is discussed. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2754-2764, 2008