(324d) The Calorimetric Glass Transition and Structural Relaxation of Single Polystyrene Films

The behavior of materials confined at the nanoscale has been of considerable interest over the past two decades and considerable controversy has arisen.  In particular, calorimetric measurements on utlrathin polymeric films using fast scanning or nanocalorimetry have been interpretted as showing no depression in the glass transition temperature (T_g), whereas mulitple other techniques have generally shown decreases in T_g.  We attempt to resolve this discrepancy by using a commercial rapid-scanning chip calorimeter to measure T_g for single polystyrene thin films as a function of cooling rate and film thickness.  Films were prepared in two ways: spincast films placed on a layer of inert oil or grease and films directly spincast on the back of the calorimetric chip.  Results for thick films on oil or grease and directly spicast 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.  For films directly spincast onto the sensor, the magnitude of the T_g depression is smaller and is only observed for our thinnest film, 16 nm thick.  All of our data are within the range of the results reported in the literature on supported films and are consistent with prior findings that T_g decreases with decreasing film thickness and substrate surface energy.  The differences in our data and those of prior researchers using nanocalorimetry are attributed to rate effects and differences in data analysis.  Our results with respect to high temperature annealing also conclusively demonstrate that annealing results in film dewetting and thickening and is accompanied by a reversion of T_g to the bulk value.  Thus, the T_g depression observed is neither due to degradation nor to plasticization effects.  In addition to measurements of T_g as a function of cooling rate, the structural recovery of single thin films will be discussed.