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 ($8\mathrm{n}\mathrm{m}<h<290\mathrm{n}\mathrm{m}$) 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 $h\lesssim 40\mathrm{n}\mathrm{m}$ while all “properly” capped films exhibited a $T_g$ value the same as that of the bulk polymer ($370\pm 1\mathrm{K}$). 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.

Figure 1

Glass transition temperature ($T_g$) in PS films for simple supported films (○), PS films coated with a 5 nm evaporative Au layer (▲), and PS films that have been coated (evaporatively) with a 5 nm layer of Al (■). The inset shows raw ellipsometric data (●) as well as the temperature derivative (△) for an uncapped film of thickness 12.5 nm.

Figure 2

$T_g$ values for the $2(h/2)$ films (see text) before (◇) and after (●) removal of the Al capping layer. The solid line describes the thickness dependence of the $T_g$ in thin PS films obtained for the uncapped PS films in Fig. 1.