Two calorimetrically distinct parts of the dynamic glass transition

Although known for more than thirty years, the physical nature of the glass-transition crossover region is still an open question. Heat capacity spectroscopy experiments in the crossover region of poly(n-hexyl methacrylate) are presented which indicate, for the first time, that the dynamic glass transition consists of two separate parts in series: A thermorheologically rather simple high-temperature part (called a-process) and a low-temperature part with increasing molecular cooperativity on the nanometer scale (alpha-process). The calorimetric experiments show, surprisingly, a decline of the aprocess and a separate onset of the alpha-process.