(721c) Ultra-Stable Amorphous Teflon: Extreme Fictive Temperature Reduction As a Means to Probe Sub-Tg Dynamics

Authors: 
McKenna, G. B., Texas Tech University
Yoon, H., Texas Tech University
Simon, S. L., Texas Tech University
Koh, Y. P., Texas Tech University
We have succeeded in making ultra-stable films through physical vapor deposition of amorphous Teflon® on to substrates held at temperatures from 30.0 oC (303.2 K) to 116.7 oC (389.9 K) corresponding to approximately 0.75 to 0.97 times the glass temperature Tg of the vapor deposited materials. We find a maximum fictive temperature Tf reduction (compared to the Tg) of 57 oC and this opens a window for experimental investigation of deep glassy state behavior that is even greater than the 43.6 oC reduction seen in 20 million year old amber [1]. In that work samples could be tested using macroscopic rheometric methods to establish that the upper bounds to the relaxation times below the glass transition temperature deviate strongly from the notional Vogel-Fulcher type of super-Arrhenius behavior expected for glass-forming materials. These results remain to be reproduced. The present success in creating a low fictive temperature glass of amorphous Teflon® provides an opportunity to expand the window of investigation to even further below the glass temperature. However, unlike the amber samples which were large enough to use normal rheological measurements, the vapor deposition process produces samples of mass in the nano-to micro-gram range. Therefore nano-mechanical measurement methods are required to interrogate the viscoelastic response of the ultra-dense amorphous Teflon®. In the present investigation we are using the TTU nano-bubble inflation method [2,3] to determine the viscoelastic response of the ultra-dense material. By making measurements from the reduced Tf to above the Tg we are able to explore the deviations from super-Arrhenius behavior seen in the ancient amber, but now with a synthetic material that is readily reproduced, albeit in small quantities. Our current state of knowledge is that we have produced the ultra-stable amorphous Teflon® and carried out the first viscoelastic measurements of the material response in the temperature regime of interest. The full set of results and comparisons with the amber results will be presented and discussed.

[1] J. Zhao, S.L. Simon, and G.B. McKenna, "Using 20-million-year-old amber to test the super-Arrhenius behaviour of glass-forming systems," Nat. Comm., 4, 1783, (2013). DOI: 10.1038/ncomms2809

[2] P.A. O’Connell and G.B. McKenna, "Rheological Measurements of the Thermoviscoelastic Response of Ultrathin Polymer Films," Science, 307, 1760-1763 (2005).

[3] 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).