Glass Transition in Ultrathin Polymer Films: Calorimetric Study

The ultrasensitive differential scanning calorimetry is used to observe the glass transition in thin (1–400 nm) spin-cast films of polystyrene, poly (2-vinyl pyridine) and poly (methyl methacrylate) on a platinum surface. A pronounced glass transition is observed even at a thickness as small as 1–3 nm. Using the high heating ($20–200\mathrm{K}/\mathrm{m}\mathrm{s}$) and cooling ($1–2\mathrm{K}/\mathrm{m}\mathrm{s}$ in glass transition region) rates which are typical for this technique, we do not observe appreciable dependence of the glass transition temperature over the thickness range from hundreds of nanometers down to 3 nm thick films. The evolution of calorimetric data with film thickness is discussed in terms of broadening of transition dynamics and loss of transition contrast.

Figure 1

$C_P(T)$ curves for 1–3 nm thick films. Straight lines denote glass and liquid $C_P(T)$ functions. The heating rate is $30–40\mathrm{K}/\mathrm{m}\mathrm{s}$ (at the glass transition region $-100–160°\mathrm{C}$). ${\mathrm{}}^{\mathrm{a}}$—control experiment using a pure solvent for the film deposition. Inset: nanocalorimetry sensor, cross-section view (not to scale): 1—polymer film, 2—Pt heater/thermometer, 3—SiNx membrane, and 4—Si frame.

Figure 2

$T_f^{\prime }$ as a function of film thickness. ∘ for PS; ◇ for PS-H; □ for PVP; ▵ for PMMA. Error bars denote the standard deviation of eight measurements with different heating rates.

Figure 3

Normalized $C_P(T)$ curves for PS films of different thicknesses. Heating rate—see Fig. 1 caption.

Figure 4

Logarithm $\beta$ as a function of reciprocal film thickness. Lines represent a linear fit of data. ∘ for PS; □ for PVP; ▵ for PMMA.

Figure 5

Logarithm $\delta$ as a function of reciprocal film thickness. Lines represent a linear fit of data. ∘ for PS; □ for PVP; ▵ for PMMA. Error bars—see the Fig. 2 caption. Bulk values for $\delta$ are calculated using $C_P(T)$ [6] and bulk $T_f^{\prime }$ [9]. The upper inset shows $\delta$ as a function of thickness for PS.