| 20 Million Year Old Amber and Ultra-stable Amorphous Teflon and the Unexplored Region of Glassy Behavior Gregory Mckenna1; 1TEXAS TECH UNIVERSITY, Lubbock, United States; PAPER: 49/Molten/Plenary (Oral) SCHEDULED: 11:45/Thu. 24 Oct. 2019/Ambrosia A (77/RF) ABSTRACT: A major question related to the glass transition event is whether there exists an ideal glass temperature <i>T<sub>g,ideal</sub></i> [1,2]. Because the laboratory glass transition <i>T<sub>g</sub></i> is some 40 to 100 K above the putative <i>T<sub>g,ideal</sub></i> it is virtually impossible to perform direct measurements that even approach the true equilibrium state at this temperature. Therefore, it is important to develop methods to finesse the problem and to work in the so-called "unexplored" region of glassy behavior [3] where the non-equilibrium response should be an upper bound to the dynamical behavior of the glass [4,5]. The framework for the study is that of the fictive temperature originally proposed by Tool [6] and that creates a solid framework for understanding the volume or enthalpy vs. temperature behaviors of glass-forming liquids as well as the evolution of the glassy properties during arbitrary thermal histories. In this framework, the fictive temperature <i>T<sub>F</sub></i> of the glass defines a point of intersection of the glass-like behavior with that of the equilibrium liquid. When the temperature <i>T</i> is below <i>T<sub>F</sub></i>, the dynamics are faster than those of the material in the equilibrium state. When <i>T</i>><i>T<sub>F</sub></i>, the dynamics are slower, i.e., provide an upper bound to the equilibrium relaxation times at <i>T</i>. At <i>T = T<sub>F</sub></i> the equilibrium response should, in principle, be obtained. Therefore, to test concepts such as the possibility that the relaxation times or viscosity diverge at a temperature above absolute zero (possibly at <i>T<sub>g,ideal</sub></i>), as seen in multiple models of glass-forming liquids, the goal became to find or to create glasses with fictive temperatures as far as possible below the glass transition temperature <i>T<sub>g</sub></i>. Then, if one can work in the ''window'' between <i>T<sub>F</sub></i> and <i>T<sub>g</sub></i>, the theoretical predictions or extrapolations from the known equilibrium behavior above <i>T<sub>g</sub></i> can be tested down to <i>T<sub>F</sub></i>. We have addressed this challenge by using a 20 million year old amber [4] having a fictive temperature some 43.6 K below the conventionally measured <i>T<sub>g</sub></i> and were able to determine upper bounds to the relaxation times in the relevant temperature window. We subsequently were able to create an ultra-stable amorphous Teflon through a vapor deposition process that had a <i>T<sub>F</sub></i> some 55 K below the <i>T<sub>g</sub></i> of the same material and very close to the nominal <i>T<sub>g,ideal</sub></i>. In this case, and unlike the amber for which the dynamics could be measured by macroscopic rheological methods, there was an additional challenge. Here the vapor deposition process made only micro-gram quantities of material, at least in a reasonable time of multiple hours. Therefore the challenge was to make dynamic measurements on these ultra-small quantities of material. This we did by using the TTU bubble inflation method [7] of viscoelastic measurements to determine the creep response in the temperature range from just below <i>T<sub>F</sub></i> to <i>T<sub>g</sub></i> and applying time-temperature superposition to estimate the temperature dependence of the relaxation times, again in the upper bound condition. Two important results came from these investigations. The first is that the temperature dependence of the dynamics was found to deviate significantly from the finite temperature divergence given by extrapolation of the equilibrium response obtained for temperatures greater than <i>T<sub>g</sub></i>, thus challenging the idea of an ideal glass transition, at least as seen in the temperature dependence of the dynamics. The second is that the data are good enough to permit the evaluation of more modern theories that do not predict diverging time scales at finite temperature. The comparison with several of these will be shown. It is also of interest that, in spite of the challenge to ideas of an ideal glass transition, the activation energies of these upper bound relaxation times are still extremely high, thus the ''turn over'' from super-Arrhenius to Arrhenius-like behavior does not resolve the conundrum of the high apparent activation energies of the relaxation processes in glass-forming liquids, one of the original motivating factors in the ongoing study of complex fluids [8-11]. References: [1] C.A. Angell and J. Donnella, "Mechanical collapse vs ideal glass formation in slowly vitrified solutions: A plausibility test," J. Chem. Phys., 67, 4560-4563 (1977); https://doi.org/10.1063/1.434597 [2] C.A. Angell, "On the uncertain distinction between fast landscape exploration and second amorphous phase (ideal glass) interpretations of the ultrastable glass phenomenon," Journal of Non-Crystalline Solids, 407, 246-255 (2015); http://dx.doi.org/10.1016/j.jnoncrysol.2014.08.044 [3] G.B. McKenna and S.L. Simon, "50th Anniversary Perspective: Challenges in the Dynamics and Kinetics of Glass-Forming Polymers," Macromolecules, <b>50</b>, 6333-6361 (2017); DOI: 10.1021/acs.macromol.7b01014. [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). DOI: 10.1038/ncomms2809 [5] A.J. Kovacs, "Transition Vitreuse dans les Polymeres Amorphes. Etude Phenomenologique," Fortschr. Hochpolym. Forsch., <b>3</b>, 394-507 (1963). [6] A.Q. Tool, "Relation Between Inelastic Deformability and Thermal Expansion of Glass in Its Annealing Range," J. Am. Ceram. Soc., 29, 240-253 (1946); A.Q. Tool, "Viscosity and the Extraordinary Heat Effects in Glass," J. Research National Bureau of Standards (USA), <b>37</b>, 73-90 (1946). [7]P.A. O'Connell and G.B. McKenna, "Novel Nanobubble Inflation Method for Determining the Viscoelastic Properties of Ultrathin Polymer Films," Rev. Sci. Inst., 78, 013901-1 - 013901-12 (2007). [8] H. Vogel, "Das TemperaaturabhAngigkeitsgesetz der Viskositat Flussigkeiten," Phys. Z., 22, 645-646 (1921) [9] G.S. Fulcher, "Analysis of recent measurements of the viscosity of glasses," J. Am.Ceram. Soc., 8, 339-355 (1925). [10] G. Tammann, "Glasses as supercooled liquids," J. Soc. Glass Technol., 9 166-185 (1925). [11] H. Le Chatelier, "Sur la viscosite du verre," Compt. Rendus, 179, 517-521 (1924); H. Le Chatelier, "Sur l'allotropie du verre," Compt. Rendus, 179, 718-723 (1924). |
