Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments*

Abstract

Physical Review Letters, 111(9), 095701] that purportedly showed unusual irregular responses in the isothermal structural recovery behavior of glasses because the observation challenges the general view of the dynamics being smooth functions of time as observed in extensive dilatometric experiments by Kovacs [Kovacs, A. J. (1964). In Fortschritte der Hochpolymeren-Forschung, 3, 394-507. Springer, Berlin, Heidelberg.] in the 1960s. The reported data show an apparent two mechanism structural (enthalpy) recovery at aging temperatures ranging from 6 to 17 K below T g which is also controversial as it could not be reproduced in work from Koh and Simon [Koh, Y. P., & Simon, S. L. (2013). Macromolecules, 46(14), 5815-5821.] where a second plateau observed in the enthalpy loss curve was not reproduced in nearly 1-year-long experiments at 15 K below T g . Therefore, it is important to determine what might be possible reasons for the apparent non-smooth relaxations. Here, we examine the possibility that the poor temperature control of a typical vacuum oven could explain the anomalous results. We used the Tool-Narayanaswamy-Moynihan (TNM) model of structural recovery to calculate the effect of typical vacuum oven temperature variation on the structural recovery of polystyrene and find that we can reproduce the reported experimental results. The issue of temperature control using a vacuum oven is discussed, and data from thermocouple measurements of temperature at various locations inside a vacuum oven are shown.

Comment on “Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments”

“…A significant part of Jin and McKenna work [1] recalls renowned results by Kovacs [6] and more recent ones by Czerniecka-Kubicka et al [7] showing a single decay toward equilibrium. Regarding Kovacs aging data, these are volume recovery results never extending over 10 5 s aging time.…”

“…Hence, as a due premise, all the discussion reported in their manuscript must be considered as a mere hypothesis. Furthermore, in this comment, we provide compelling arguments that the issues raised by Jin and McKenna [1] are completely unjustified from both the viewpoint of the analysis of available literature data on equilibrium recovery of glasses and that of technical aspects of temperature control.…”

“…A recent paper by Jin and McKenna [1] discusses on the presence of multiple steps in the approach to equilibrium of glasses, often addressed as physical aging. [2,3] This scenario indicates the existence of mechanisms of equilibration beyond the main α relaxation, generally associated to the glass transition.…”

“…[2,3] This scenario indicates the existence of mechanisms of equilibration beyond the main α relaxation, generally associated to the glass transition. Specifically, Jin and McKenna [1] raise severe concerns on the validity of our seminal work addressing the presence of multiple steps, [4] in the light of apparently contradicting results by Koh and Simon [5] on a single polystyrene (PS) sample with one molecular weight at one single aging temperature showing a single step. In particular, according to Jin and McKenna, [1] the presence of multiple steps in the approach to equilibrium would be the result of an artifact originating from poor temperature control in the oven used to anneal glassy samples.…”

“…First of all, it is worthy of remark that a direct test of the correct functioning of our oven by Jin and McKenna [1] has never been carried out. Hence, as a due premise, all the discussion reported in their manuscript must be considered as a mere hypothesis.…”