Gregory McKenna, May 2018
One of the major challenges in the study of the behavior of glass-forming materials, including polymers, is to know the equilibrium behavior below the glass transition temperature Tg. This is due to the need for a fundamental understanding of the equilibrium behavior in order to have a baseline to the non-equilibrium response inherent in the deep glassy state where materials are used[1]. In the present work we describe experiments in which we have used vapor deposition methods to make extremely stable amorphous fluorocarbon polymer films which have fictive temperature reductions relative to the Tg of nearly 60 K[2]. Hence, they are deep into the energy landscape[3]. By making viscoelastic measurements on films of these materials one can establish upper bounds to the equilibrium relaxation times and, consequently test theories of the glass transition and potentially distinguish among different theories. In particular, one can compare the dynamics with those expected by the theories, but now far below Tg. Initial work suggests that, consistent with prior results [4] on an ancient amber material, theories that predict a divergence of time-scales at a finite temperature are not consistent with the present upper bound results. Rather it appears that there is a strong deviation of the response from, e.g., WLF behavior, towards an Arrhenius-like response, albeit with a high activation energy. [1] G.B. McKenna and S.L. Simon, "50th Anniversary Perspective: Challenges in the Dynamics and Kinetics of Glass-Forming Polymers," Macromolecules, 50, 6333-6361 (2017).[2] H. Yoon, Y.P. Koh, S.L. Simon and G.B. McKenna, "An Ultra-Stable Polymeric Glass: Amorphous Fluoropolymer with Extreme Fictive Temperature Reduction by Vacuum Pyrolysis," Macromolecules, 50, 4562-4574 (2017).[3] C.P. Royall, F. Turci, S. Tatsumi, J. Russo and J. Robinson, "The race to the bottom: approaching the ideal glass?" arXiv:171104739v1 [cond-mat.soft] 13 Nov 2017.[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).