Calorimetric glass transition of single polystyrene ultrathin films

The calorimetric glass transition (T_g) is measured for single polystyrene ultrathin films using a commercial rapid-scanning chip calorimeter as a function of cooling rate and film thickness. Films have been prepared in two ways: spin-cast films placed on a layer of inert oil or grease and films directly spin-cast on the back of the calorimetric chip. For the films on oil or on grease, the 160 nm thick films show results consistent with those of a bulk sample measured by conventional DSC. On the other hand, the 47 nm thick film on oil and 71 nm thick films both on oil and on grease show a T_g depression which decreases with increasing cooling rate; the magnitude of the T_g depression is similar to results reported in the literature for the most mobile substrate-supported films. For films directly spin-cast onto the sensor, a T_g depression is not observed for 47 and 71 nm thick films but is observed for a 16 nm thick film. These results are also within the range of the data on supported films in the literature but show a smaller depression than films on oil or grease. The effect of annealing is also investigated. For thick films and those directly spin-cast onto the sensor, annealing at 160 C has no influence on heat flow curves; hence, T_g values remain unchanged. For the 47 and 71 nm thick films on either oil or grease, the depressed T _gs revert to the bulk values over the course of a day at 160 C. Atomic force microscope (AFM) images show that annealing results in dewetting of the films with hole growth and thickening of the film to 200 nm, the latter of which is presumed to be the reason that T_gs revert to bulk values.