Thickness dependence of surface glass transition temperature of polymer supported films

The surface glass transition temperature (T_g^surf) of polystyrene (PS) films supported by silica was studied for film thickness, h, from 7 nm to 100 nm by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The width of the surface glass transition, ΔT_g^surf=(T_g^surf+-T_g^surf-) and T_g^surf=(T_g^surf++T_g^surf-)/2 were extracted from the end group intensity as a function of temperature. We found that T_g^surf decreases with decreases h and is ~20 K lower than the glass transition temperature (T_g) of the film for any given h. We attribute this observation to effects of the free surface. On the other hand, ΔT_g^surf increases with decreasing h starting from large h (> ~60nm). We explore possible origins for the noted broadening in surface glass transition of thin films.