Probing Glass Transition of Ultrathin Polymer Films at a Time Scale of Seconds Using Fast Differential Scanning Calorimetry

Abstract

Ultrasensitive, thin-film, differential scanning calorimetry is used to determine the glasstransition temperature of 3-400 nm thick, spin-cast films of polystyrene, poly (2-vinyl pyridine), and poly (methyl methacrylate) on a platinum surface. The technique used here is modified to characterize the glass transition over a time scale of seconds rather than milliseconds. No appreciable dependence of the glass-transition temperature on thickness is observed over the entire range used. The results are discussed in comparison with previous calorimetric measurements, characterized by the glass-transition time scale of milliseconds.

Calorimetric glass transition temperature and absolute heat capacity of polystyrene ultrathin films

“…They are consistent, though, with the earlier work of Forrest et al 26 on supported films, in which no significant differences between PS thin films on SiO x with one free surface and those on SiO x capped with an SiO x layer were found, although the artifacts potentially introduced by vapor deposition were recently discussed by Sharp and Forrest. 19 We also note that the T g and DC p depressions that we observe with decreasing film thickness by calorimetry differ from the results of recent nanocalorimetric measurements [43][44][45] in which no changes were observed (although in ref. 45, a change in C p was found, and the issue of DC p is difficult to address because of the increasing breadth of the transition).…”

“…The differences may be attributed to the timescales relevant to the different measurement techniques. In our work, 2 K temperature jumps are performed every minute, whereas the heating rate is on the order of 1-2 K/ms for cooling through T g in Allen and coworkers' nanocalorimetric studies, 43,44 and the measurement frequency in the ac-nanocalorimeter is 40 Hz; 45 hence, the nanocalorimetric works in the literature employ timescales 3 to 5 orders of magnitude shorter than that used in this work. In fact, Fakhraai and Forrest 21 recently reported that the T g depression in thin PS films depends on the cooling rate, thereby explaining the lack of agreement between the majority of the literature and the recent nanocalorimetry studies performed at much higher cooling rates or measurement frequencies.…”

“…In their work, Wang and Zhou examined stacked and microtomed epoxy ultrathin films and observed a 15 8C depression in T g without any significant change in the step change in the heat capacity at T g (DC p ). Recent nanocalorimetry measurements, 43,44 on the other hand, have shown neither a T g depression nor a change in DC p even for PS films as thin as 3 nm, presumably because small changes in T g and DC p are difficult to discern on account of the very high heating rates involved in this technique coupled with the fact that the T g depression may decrease at high cooling rates. 21 Similarly, recent ac-nanocalorimetry measurements made at 40 Hz show no discernable T g depression for films as thin as 4 nm; whether or not changes in DC p occur with decreasing film thickness is less clear, but a distinct broadening of the transition can be observed as the film thickness decreases.…”

“…45 In this study, the absolute heat capacity (C p ), which has not previously been reported for ultrathin polymer films, and T g of PS thin films are measured with DSC with the step-scan method in an effort to further examine the thermodynamic behavior of glass formers on the nanoscale. The step-scan method is analogous to temperature-modulated DSC applied in the time domain; the timescale for the measurements is thus much longer than that of recent nanocalorimety 43,44 and ac-nanocalorimety 45 studies. A primary advantage of the step-scan methodology is that the instrumental baseline is unimportant for the measurement of C p because the incremental heat flow response is measured for small temperature steps.…”

Scanning AC Nanocalorimetry and Its Applications

“…At slower heating rates, however, the temperature uniformity is significantly degraded mainly because of heat loss to the Si substrate via the tungsten heating element. These results are consistent with prior simulations of strip devices in the steady state [41]. The serpentine design, on the other hand, shows improved temperature uniformity over the entire temperature range and for all heating rates, with J around 3% at 200 o C and decreasing to approximately 1% at 1000 o C. We note that the temperature uniformity of the strip design improves significantly as the average temperature increases, because the increased radiative heat loss provides negative feedback for temperature non-uniformity.…”