Decoupling between molecular mobility and glass transition in polymer nanospheres by fast scanning calorimetry

In the present contribution we investigate glass dynamics of polystyrene (PS) nanospheres with diameter ranging from 200-600 nm, obtained by means of nanoprecipitation. To do so we have employed fast scanning calorimetry (FSC), which allows exploring cooling/heating rates as large as several thousands Kelvin per second. The cooling rate dependent limiting fictive temperature (Tf), determined by the Moynihan method, which defines the thermodynamic state achieved by the glass after a certain cooling protocol, exhibits clear decrease with the nanospheres diameter. Similar results, agreeing with those of the Tf , were found for the liquid to glass transition temperature (Tg), obtained by a step response method. Conversely, the so-called “dynamic” Tg, obtained by the step response method and in the same experiment as the Tg by Fourier transforming heat flow and cooling rate signals, revealed essentially bulk-like dynamics. These results provide compelling arguments that the thermal Tg – related to the way the glass former vitrifies – and the dynamic Tg – providing information on the intrinsic molecular mobility – are decoupled for glasses subjected to geometrical confinement.