Ultra-stable Glasses: Testing Paradigms of the Glass Transition
There are two important "signatures" related to glass formation that are commonly
related. The first is the apparent entropy catastrophe that occurs at the Kauzmann
temperature TK [1] where the extrapolated entropy seems to go below that of the crystal, hence violating
Nernst' third law of thermodynamics. A related phenomenon is that the dynamics (relaxation
times) of glass-forming systems seem to extrapolate to a finite temperature divergence
at the so-called VFT temperature TVFT, which is often found to be close to TK. Testing the equilibrium response near to these temperatures has become a major challenge
in glass physics. This is because the times required to achieve equilibrium in this
regime become of geological/ astronomical scale [2,3]. To finesse this problem, we
are using materials with extremely low fictive temperature TF relative to Tg , hence unlocking an unexplored region of the glassy state to investigation. First,
we used of a 20 million year old amber with TF ∼ 43.6 K below Tg. We found that 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,
the amber work is complicated because the natural origins of amber make reproducing
the experiments difficult. Therefore, we built on the ultra-stable glasses ideas exploited
by Ediger and co-workers [4] and developed a vapor deposition procedure to make an
amorphous Teflon material with TF ∼55 K below Tg and close to the putative TK. Made only in microgram quantities, we needed the TTU bubble inflation [5] method
to measure the creep response in the range between TF and Tg ,expanding the amber work to TK. The observed relaxation times deviate from the extrapolated VFT-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.
In addition, we[6] have examined the thermodynamics of the problem by using an athermal
mixture of a poly(a-methyl styrene) with its on pentamer and show that in this system
the equilibrium entropies continue smoothly without evidence of a second order transition
to at least 180 K below the Kauzmann temperature. Such results are consistent with
there not being an "ideal" glass transition and demand reconsideration of theories
that use or predict such a thermodynamic point in glass-forming systems.
Reference
[1] W. Kauzmann, Chem. Rev. 43, 219 (1948).
[2] J. Zhao, S.L. Simon and G.B. McKenna, Nat. Commun., 4:1783, 1 (2013).
[3] J. Dudowicz, K.F. Freed and J.F. Douglas, J. Chem. Phys., 124, 064901 (2006).
[4] M. D. Ediger, J. Chem. Phys., 147, 210901 (2017).
[5] P.A. O'Connell and G.B. McKenna, Science, 307, 1760 (2005).
[6] D. Huang, S.L. Simon and G.B. McKenna, J. Chem. Phys., 119, 3590-3593 (2003).
Some Publications
1. H. Yoon and G.B. McKenna*, "Testing the paradigm of an ideal glass transition:
Dynamics of an ultrastable polymeric glass," Science Advances, 4, eaau5423 (2018).
2. G.B. McKenna* and J. Zhao, "Accumulating Evidence for Non-Diverging Time-scales in Glass-forming Fluids," Journal of Non-Crystalline Solids, 407, 3-13 (2015).
3. J. Zhao and G.B. McKenna*, "Temperature Divergence of the Dynamics of a Poly(vinyl acetate) Glass: Dielectric
vs. Mechanical Behaviors," Journal of Chemical Physics, 136, 154901-1 – 154901-8 (2012).
4. J. Zhao, S.L. Simon, G. B. McKenna*, "Using 20-million-year-old amber to test the super-Arrhenius behavior of glass-forming
systems," Nature Communications, 4, 1783-1 - 1783-6 (2013).
5. J. Zhao and G.B. McKenna*, "Response to "Comment on 'Temperature divergence of the dynamics of a poly(vinyl acetate)
glass: Dielectric vs. mechanical behaviors'" [J. Chem. Phys. 139, 137101 (2013)]", J. Chem. Phys., 139, 137102 (2013).
6. G.B. McKenna, "Looking at the Glass Transition: Challenges of Extreme Time Scales and Other Interesting
Problems," Rubber Chemistry and Technology, 93, 79-120 (2020).
Funding
- National Science Foundation. Division of Materials Research (DMR), Polymers Program.
- John R. Bradford Endowment at Texas Tech University.
