Molecular weight and interfacial effect on the kinetic stabilization of ultrathin polystyrene films

Abstract

In this article, the kinetic stabilization processes of ultrathin polystyrene (PS) films with different molecular weights (Mw) and interfacial conditions were monitored by controlling the cooling rate. We found that, for the high Mw PS ultrathin film, the fictive temperature (Tf) of liquid-cooled sample shows large change in a broad range of cooling rates while the kinetic stability proves no obvious variation. By using the low Mw PS ultrathin film, the kinetic stability begins to show cooling rate dependence, which indicates the modification of kinetic stability of shorter polymer chains is easier. With the aid of fluorescence resonance energy transfer (FRET) method, the variations of inter-chain proximity of polymer chains with the cooling rate were investigated for both oligomer and high Mw PS thin films, and the former is more remarkable. Given that the further the inter-chain proximity, the higher the degree of excess free volume, there are more space for the rearrangement of chains of low Mw polymer ultrathin films to reach energy-minimum configurations during cooling. Additionally, the shift of Tg,dyn after cooling can be enlarged or removed by introducing a mobile interface or burying the free surface. These results suggest that the kinetic stability of liquid-cooled ultrathin films may stay unchanged when fictive temperature varies obviously, and it can be modified by controlling the molecular weight of polymer and the interfacial condition.