Calorimetric evidence for a mobile surface layer in ultrathin polymeric films: poly(2-vinyl pyridine)

Specific heat spectroscopy was used to study the dynamic glass transition of ultrathin poly(2-vinyl pyridine) films (thicknesses: 405-10 nm). The amplitude and the phase angle of the differential voltage were obtained as a measure of the complex heat capacity. In a traditional data analysis, the dynamic glass transition temperature T-g is estimated from the phase angle. These data showed no thickness dependency on T-g down to 22 nm (error of the measurement of +/- 3 K). A derivative-based method was established, evidencing a decrease in T-g with decreasing thickness up to 7 K, which can be explained by a surface layer. For ultrathin films, data showed broadening at the lower temperature side of the spectra, supporting the existence of a surface layer. Finally, temperature dependence of the heat capacity in the glassy and liquid states changes with film thickness, which can be considered as a confinement effect.