Accessing the Unexplored Regions of the Glassy State to Test Paradigms of the Glass Transition*

A major challenge in glass physics is the determination of the temperature dependence of the relaxation times in the equilibrium state well below the laboratory glass temperature T_g. This is because the times required to achieve equilibrium in this regime become of geological/astronomical scale [1,2]. To finesse this problem, we are using materials with extremely low fictive temperature T_f relative to T_g, hence unlocking an unexplored region of the glassy state to investigation. First, we measured the viscoelastic response of a 20 million year old amber with T_f ∼ 43.6 K below T_g. The relaxation times deviated strongly from the expected VFT or WLF-behaviors turning towards an Arrhenius-response, albeit with a high activation energy. Though convincing as evidence that the dynamics of the glass do not diverge at a finite temperature, often linked to the Kauzmann [3] temperature T_K, the amber work is complicated because the natural origins of amber make reproducing the experiments difficult. We also built on the ultra-stable glasses exploited by Ediger and co-workers [4] and made an amorphous Teflon material withT_f ∼55 K below T_g and close to the putative T_K. Made only in microgram quantities, we needed the TTU bubble inflation [5] method to measure the creep response in the range between T_f and T_g, expanding the amber work to T_K. The observed relaxation times deviate from the extrapolated WLF-line challenging the view that there is an "ideal" glass transition as posited by multiple theories and commonly considered an important aspect of glass-formation and glassy behavior.

[1] J. Zhao, S.L. Simon and G.B. McKenna, Nat. Commun., 4:1783, 1 (2013).
[2] J. Dudowicz, K.F. Freed and J.F. Douglas, J. Chem. Phys., 124, 064901 (2006).
[3] W. Kauzmann, Chem. Rev. 43, 219 (1948).
[4] M. D. Ediger, J. Chem. Phys., 147, 210901 (2017).
[5] P.A. O'Connell and G.B. McKenna, Science, 307, 1760 (2005).