Differential AC‐chip calorimeter for glass transition measurements in ultrathin films

A differential AC‐chip calorimeter capable of measuring the step in heat capacity at the glass transition in nanometer‐thin films is described. Because of the differential setup, pJ/K sensitivity is achieved. Heat capacity can be measured for sample masses below 1 ng in broad temperature range as needed for the study of the glass transition in nanometer‐thin polymeric films. Relative accuracy is sufficient to investigate the changes in heat capacity as the step at the glass transition of polystyrene. The step is about 25% of the total heat capacity of polystyrene. The calorimeter allows for the frequency dependent measurement of complex heat capacity in the frequency range from 1 Hz to 1 kHz. The glass transition in thin polystyrene films (50–4 nm) was determined at well‐defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits (±3 K)—neither at constant frequency (40 Hz) nor for the trace in the activation diagram (1 Hz–1 kHz).