RESUMO
We have studied the glass transition temperature (T(g)) and molecular mobility of polystyrene (PS) thin films near the interface between the polymer thin film and substrate with bilayer thin films consisting of surface hydrogenated PS (h-PS) and bottom deuterated PS (d-PS) using neutron reflectivity. With decreasing the thickness of the bottom d-PS layer, T(g) near the interface between the polymer thin film and substrate increased compared to bulk T(g) and a drastic increase of T(g) was observed for the bottom d-PS layer <155 Å thick. The orientation of polymer chains at the interface is supposed to be related to the increase of T(g) near the interface between the polymer and substrate. The polymer chain mobility decreased with thickness even for the bottom d-PS layer with no discernible change of T(g). It is considered that the numerous contacts between polymer chains and substrate are related to the decrease of mobility near the interface between the polymer thin film and substrate.
RESUMO
We performed neutron reflectivity measurements on multilayered polymer thin films consisting of alternatively stacked deuterated polystyrene (d-PS) and hydrogenated polystyrene (h-PS) layers â¼200 Å thick as a function of temperature covering the glass-transition temperature T(g), and we found a wide distribution of T(g) as well as a distribution of the thermal expansivity α within the thin films, implying the dynamic heterogeneity of the thin films along the depth direction. The reported anomalous film thickness dependences of T(g) and α were reasonably understood in terms of the distributions, showing that the surface mobile layer and the bottom hard interfacial layer are, respectively, responsible for the depressions of T(g) and α with decreasing film thickness. The molecular mobility in each layer is also discussed in relation to the distribution of T(g), based on the results on mutual diffusion at the layer interface.
