Substrate effect on mechanical relaxation of polystyrene in ultrathin films

Abstract

Mechanical relaxation behavior in ultrathin polystyrene (PS) films supported on silicon oxide (SiOx) and gold (Au) substrates has been studied by dynamic viscoelastic measurement. Based on the method, effects of free surface and substrate interface on the segmental dynamics were discussed. In the case of thin PS films with a thickness of approximately 200 nm, αa-relaxation process corresponding to the segmental motion did not show any deviation from the bulk behavior. In contrast, for the films thinner than about 50 nm, the relaxation time distribution for the αa-process became broader, probably due to a mobility gradient in the surface and interfacial regions. When we sandwiched an ultrathin PS film between SiOx layers, another relaxation process, in addition to the original αa-process, appeared at a higher temperature side that we assigned to the interfacial αa-relaxation process. However, this was never seen for an ultrathin PS film between Au layers, implying that restriction from the substrate interface might be weak in this case.