Free Surfaces Cause Reductions in the Glass Transition Temperature of Thin Polystyrene Films

The effect of free surfaces on the glass transition temperature (T_g) of thin polystyrene films was studied. Measurements were performed on films (8   nm<h<290   nm) with one free surface and on films capped with a 5 nm thick metal layer (no free surfaces). Potential problems with evaporative deposition were eliminated by studying samples made of two supported films placed with their free surfaces in contact and annealed. Uncapped films displayed reduced T_g values for h≲40   nm while all “properly” capped films exhibited a T_g value the same as that of the bulk polymer (370±1  K). When the free surface was restored, the measured T_g values the same as those of uncapped films of the same thickness. These results show that free surfaces are crucial for observing T_g reductions in thin polymer films and address the role of the sample preparation history.