Slow dynamics in thermal expansion of polyvinyl acetate thin film with interface layer

Abstract

The experimental observation of an unusual thickness change in polyvinyl acetate (PVAc) thin films during the temperature cycle across the glass transition temperature (Tg, ~31 °C) is reported. After sufficient thermal treatment at the high-temperature side of Tg, an unexpected lack of thickness change can be observed during cooling in the rubbery state; with further cooling beyond Tg, the thickness of the PVAc thin film can increase abruptly. However, the thermally responsive behaviors recover to normal during the heating process. The thickness does not change in the glassy state and then increases in the rubbery state. In the present research, these asymmetric thermally responsive behaviors of PVAc thin films were confirmed by an X-ray reflectivity technique in repeated experiments. To clarify the mechanism, the relationship between the unusual thickness change and the fine structural changes in PVAc thin films was considered. It is proposed that an interface layer might exist and vary with the conditions of the polymer thin-film system, causing an unusual thickness change in the polymer thin film with the variation in temperature. 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.