Thickness changes in polyvinyl acetate thin films, during the temperature cycle across the glass transition temperature (Tg) has been studied in detail. Even after enough thermal treatment done at the high-temperature side of Tg, thickness increase during cooling (negative thermal expansion) is observed. On the other hand, slow thickness decrease is observed with aging at the high-temperature side of Tg. The above interesting phenomena can be well explained by the model considering the presence and changes of the interface layer.
