Polymer glass/melt transformation kinetics

The formation of a glassy phase upon cooling (vitrification) and the gradual evolution of thermodynamic state of a glassy material towards the equilibrium (physical aging) are kinetically driven processes [1]. According to Onsager regression theory, this fact suggest that they are both governed by at least one molecular mechanism, which is commonly assumed to be the main α relaxation with a super-Arrhenius temperature dependence [2]. Consequently, one could logically infer that the destruction of a glassy phase upon heating (devitrification) and the resurgence of equilibrium of a deep glass that has been heated to a temperature Tdeag greater than the aging temperature Tag (deaging) should both occur within a timescale comparable to the main α process. To obtain a deeper understanding of the underlying molecular mechanisms that govern vitrification/aging and devitrification/deaging phenomena, we employed Differential (DSC) and Flash Scanning Calorimetry (FSC) to study the time-dependent evolution of the glass enthalpic state of amorphous poly-4-chlorostyrene (Polymer Source, Mw 24.000 g/mol, PdI 1,20) across a wide temperature range. The evolution of specific heat capacity (cp) curves with respect to aging and deaging time at 397 K is depicted in figure 1, which highlights the differences in equilibration time (τeq) of the two processes. To conclude, our study indicates that the process of eliminating thermal history in a deeply aged glass occurs at timescales that deviate from the expected values. The magnitude and direction of the deviation depend on the explored temperature range, with the timescales being either faster or slower than the alpha relaxation process. [1] Schmelzer, J. W. P.; Gutzow, I. S. Glasses and the glass transition; Wiley-VCH, Weinheim, 2011. [2] Onsager, L. Reciprocal relations in irreversible processes. I. Phys. Rev., 37, 405, 1931