(173g) Dynamics of Glass-Forming Liquids: Is the Finite Temperature Divergence Real?

There is significant interest in the apparent divergence of the dynamics of glass-forming liquids at temperatures above absolute zero.  One considered signature of the complex liquid is the so-called Vogel-Fulcher-Tammann (VFT) or the Williams, Landel, Ferry (WLF) super-Arrhenius behavior of the dynamics or the viscosities.  Yet, because it is extremely difficult to make measurements in the equilibrium state far below the glass transition temperature range, the actual continuation of this apparent VFT or WLF divergence is little tested.  Here we examine the problem by making measurements on a poly(vinyl acetate) (PVAc) polymer in the vicinity of the glass transition using both mechanical measurements and dielectric measurements.  By aging the samples for long times we are able to establish a "plateau" in the aging behavior that we associate with the nominal equilibrium response.  Three findings of considerable interest are made.  First, the equilibrium dynamics for the dielectric response as much as 16 K below the nominal T_g are found to deviate from the VFT extrapolation and tend towards an Arrhenius behavior, albeit one with very high activation energy. On the other hand, the mechanical response only deviates mildly from the VFT extrapolation and description of these results as consistent with an Arrhenius type of temperature dependence would be problematic.  Finally, the dielectric and mechanical responses also age into equilibrium very differently with the dielectric response coming to a stationary response as much as an order of magnitude faster than the mechanical response.  The significance of such non-divergence of time scales as temperature is lowered becomes important in the evaluation of models of the glass transition itself.  Models in which divergence is predicted are precluded by the present dielectric findings, viz., simple free volume models and the configurational entropy based models from the middle of the 20th century as well as more modern models such as that of Lubchenko and Wolynes [1].  On the other hand, modern treatments such as that from Elmatad et al [2], from Mauro et al [3] or from Dyre [4] should be further investigated.  Furthermore, the present results also suggest the need for more concerted experiments in which very long time equilibrium is achieved in order to establish the detailed nature of the deviations from the VFT/WLF-types of response.

[1] V. Lubchenko, and P. Wolynes, Annual Review of Physical Chemistry, 58, 235-266, 2007.

[2] Y. Elmatad, D. Chandler, and J. Garrahan, J. Phys. Chem. B, 113, 5563–5567, 2009.

[3] J. Mauro, Y. Yue, A. Ellison, P. Gupta, and D. Allan, PNAS, 19780–19784, 2009.

[4] J. Dyre, Reviews of Modern Physics, 78, 953-972, 2006.