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

There is considerable interest in data reported by Cangialosi et al. [Cangialosi, D., Boucher, V. M., Alegria, A., & Colmenero, J. (2013). 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 Tg 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 Tg. 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.

Automated summary

Cangialosi et al.'s data challenges the general view of smooth dynamics of glass structure recovery, as observed by Kovacs. The reported irregular responses and two mechanism recovery at temperatures below Tg have sparked controversy. This study suggests that poor temperature control in a vacuum oven could explain the anomalous results, and reproduces the experimental results through simulation.