Plasticization in Ultrathin Polymer Films: The Role of Supporting Substrate and Annealing

The effects of supporting substrate and thermalannealing on plasticization of poly(vinyl acetate) ultrathin filmsby absorption of ambient moisture have been studied by localdielectric spectroscopy. Upon exposure to ambient moisture, therelaxation rate of the α-process increases to a different extent atvariance of film thicknesses and supporting substrates. Namely,on hydrophobic gold substrates the speeding up is slightlyreduced by decrease of film thickness down to about 20 nm.Moreover, the increase of the relaxation rate measured on a 21nm thick film supported on gold is smaller compared to thatmeasured on a film with similar thickness supported on the relatively hydrophilic aluminum. We interpret such results in terms ofthe presence of an interfacial polymer layer at the substrate surface with different contents of water in dependence of thesubstrate. Furthermore, plasticization effect in ultrathin films significantly decreases after annealing at high temperature (∼ Tg +60 K) and for times longer than usual time scales of relaxation processes. Such results are consistent with the formation of a highdensityirreversibly adsorbed polymer layer at the interface, able to hinder the effect of the substrate.