Thickness Dependence of the Solvent-Induced Glass Transition in Polymer Brushes

We have investigated the sorption behavior of polystyrene brushes with thicknesses of 10−200 nm in toluene vapor. The sorption curve displays a kink, which is attributed to the solvent-induced glass transition. The derivative of the sorption curve with respect to solvent activity (“osmotic capacity”) shows a glass step at a certain characteristic activity a g . This is the analogue of the glass step observed in the heat capacity at T g with differential scanning calorimetry. Both a g and the height of the glass step decrease with decreasing film thickness. The glass step is most pronounced after annealing the films at elevated temperatures in a vacuum. Due to accumulation of solvent in the film, the step disappears after repeated swelling/drying cycles.