Cooperative and non-cooperative dynamics in ultra-thin films of polystyrene studied by dielectric spectroscopy and capacitive dilatometry

The effect of thickness reductions on the glass transition dynamics in ultrathin films of polystyrene has been studied by dielectric spectroscopy (DS) and capacitive dilatometry (CD). Upon reduction of the film thickness, a systematic decrease in the dilatometric glass transition temperatures, Tg (dil), was observed via CD, while DS revealed a continuous speed-up and broadening of the α-process, accompanied by only minor reductions in the fragility index. A good agreement between ‘spectroscopic’ and the dilatometric glass transition temperatures was found for films thicker than 20 nm, while for thinner films both quantities diverge increasingly. A likely explanation for this discrepancy is the presence of another dynamic process showing Arrhenius-behavior (Ea ∼ 72 kJ/mol) with a pre-exponential factor of 10−12 s being indicative for non-cooperative dynamics. Such a new process might be assigned to distinct surface dynamics in polystyrene films as suggested in recent papers.