Cooling rate dependent glass transition in thin polymer films and in bulk

Abstract

The glass transition is a well-known phenomenon marking the crossover from a liquid in metastable equilibrium to an out-of-equilibrium glass. In this chapter employing fast scanning calorimetry (FSC), we investigate the glass transition of bulk polystyrene (PS) and the corresponding films spin-coated on silicon oxide with thickness as low as ~ 10 nm. To do so, we employ a procedure based on determining the glass transition temperature (T g) as that where non-equilibrium effects begin to be calorimetrically detectable. This provides a measure of the onset of the glass transition, that is the Tg(on). This method allows obtaining the Tg(on) in a wide range of cooling rates and with a precision generally not achievable by means of conventional methods. The main results of the study are that: (1) the cooling rate dependent Tg(on) follows the Vogel-Fulcher-Tammann law for all thicknesses; (2) Tg(on) decrease from bulk behaviour is observed for the smallest employed thickness (~ 10 nm). For the investigated film configuration (supported on silicon oxide), the decrease in Tg(on) becomes progressively large as the cooling rate is decreased. In particular, at the highest investigated cooling rates (~ 1000 K/s) a decrease of several Kelvin is observed, whereas the Tg(on) is depressed by ~ 15 K at a cooling rate of 0.1 K/s. This result is discussed in relation to the thickness dependence of the rate of spontaneous fluctuations in PS. This appears to be thickness independent for films configuration analogous to that of the present study. Hence, it is unequivocally shown how relaxational arguments, that is, those based on the effect of the rate of spontaneous fluctuations on the thermal glass transition, are insufficient to catch the Tg(on) decrease from bulk behaviour.