Nonmonotonic glass transition behavior of polystyrene film in contact with polystyrene brushes

The interplay between a polymer melt the substrate grafted with ‘chemically identical’ polymer brushes has attracted attention due to the possible applicability to slip, adhesion, wettability, and lubrication. In this study, we controlled grafting density (σ) and chain length of polystyrene (PS) brushes on substrates to investigate their effect on the glass transition temperature (T_g) of overlaying PS films. To our surprise, we observed a nonmonotonic change of the T_g of PS film as a function of σ with the maximum value that exceeds the T_g of bulk PS. A computer simulation supports our experimental results in terms of the local segmental dynamics in overlaying PS film, which is related to the enhanced friction by interpenetration between the melt (PS film) and the grafted layer.