Stratification and two glass-like thermal transitions in aged polymer films

Abstract

Using X-ray reflectivity, spectroscopic ellipsometry and Raman spectroscopy, we have studied the stratified structure and the two glass-like thermal transitions in sufficiently aged glassy polystyrene films. We find that favorable interaction between the solid substrate and the polymer film induces stratification within the film resulting in different densities across the film thickness. Existence of two glass-like thermal transitions (one at 70 °C and the other at 95 °C) is independently confirmed by temperature dependent spectroscopic ellipsometry and Raman spectroscopy measurements. Interestingly, the thermal coefficient of expansion of the polymer film displays anomalous behavior with temperature and is found to have the lowest value over the temperature range 70-95 °C, i.e. between the two observed glass-like thermal transition temperatures.

Vitrification decoupling from α-relaxation in a metallic glass

“…Different mechanisms of devitrification were also observed in other metallic glasses aged over prolonged time (19,50,51), where the fast mechanisms, providing early devitrification, were associated to fluctuations of local regions (19). Similar results were observed for nonmetallic glass-forming systems as well, such as a plastic crystal aged over 7 years (18), polystyrene aged for months (20), and several polymers aged over about 30 years (21). Our study provides, in addition, the observation that, in a bulk glass (that is, unrestricted by geometrical confinement) and for time scales as short as a few seconds (i.e., without any specific annealing), vitrification can take place according to the atomic mobility of processes different from the -relaxation.…”

Physical Aging Behavior of a Glassy Polyether

“…Specifically, in these cases, specific heat scans show the presence of an excess endotherm of the aged sample with respect to the unaged one. This outcome appears to be general, as it was found in glasses of different nature [28,29], including polymers [27,30,31], metallic glasses [29,32], and a plastic crystal [28]. Importantly, this behavior is magnified in polymer glasses exhibiting large free interfacial area [33,34], as a result of the acceleration of physical aging in these systems [35][36][37][38][39], which amplifies the separation among different mechanisms of equilibrium recovery [36].…”

Thermodynamic Ultrastability of a Polymer Glass Confined at the Micrometer Length Scale

“…This is the case of a liquid crystal [45], a metallic glass [46], and several high-T g polymers [47] aged at room temperature over several years. A low-temperature overshoot in the coefficient of thermal expansion was recently observed in polystyrene aged at room temperature over six months [48]. The presence of a prepeak or second glass transition in different glass formers-including water [49], polymers [50], and metallic glasses [51]-may be ascribed to a second mechanism of equilibration.…”