Combined effects of polymer/substrate interaction and chain adsorption on the Tg-confinement effect of supported thin polystyrene films.

The thickness dependence of the glass transition temperature (i.e., the Tg-confinement effect) and its molecular weight (Mw) dependence in polystyrene (PS) thin films supported on attractive substrates are systematically investigated as a function of annealing time at 423 K. It is demonstrated that the shift in Tg (ΔTg= Tgfilm - Tgbulk) exhibits a consistent linear dependence on the degree of adsorption (hads/hadseq) across PS films with different Mw on a given substrate, described by ΔTg = khads/hadseq + b. The slope k is found to be independent of the substrate, whereas the intercept b increases with the increasing polymer/substrate interactions due to additional losses in interfacial free volume arising from more densely packed adsorbed conformations. In addition, the dependence of the Tg-confinement effect on Mw is observed under non-equilibrium adsorption conditions caused by Mw-dependent adsorption kinetics. These results provide physical insights into how chain adsorption, which is strongly modulated by polymer/substrate interactions, affects the Tg-confinement effect in polymer thin films.