Mobility and glass transition temperature of polymer nanospheres

Recent studies have illustrated a decoupling between cooperative segmental mobility and the glass transition temperature (T-g) of thin polymer films and nanocomposites. Here, we use dielectric spectroscopy to probe the cooperative segmental mobility and capacitive dilatometry to determine the T-g of films of polystyrene nanospheres with diameters (d) less than 400 nm. We find that both capacitive dilatometry and calorimetry revealed nearly identical suppressions in T-g as the size of the nanospheres was reduced. While T-g was impacted by confinement, in the range 130 nm <= d <= 400 nm, in stark contrast, the cooperative segmental mobility, i.e., the peak position of the alpha-relaxation process was not. Furthermore, when d <= 200 nm, an additional molecular relaxation process, not observed in bulk, was present. We interpret these findings as evidence of a decoupling between T-g and cooperative segmental mobility in nanospheres. That is, the latter may be impacted by confinement under conditions in which the former is not. (C) 2012 Elsevier Ltd. All rights reserved.