AFM dilatometry measurements on ultra-stable fluoropolymer glasses: Further evidence of extreme fictive temperature reduction

Ultra-stable amorphous fluoropolymers glasses were created using vacuum pyrolysis deposition (VPD). Glass films with thickness ranging from 90 to 160 nm were grown at a substrate temperature of 0.86 Tg, where Tg is the glass transition temperature of the virgin polymer and is in units of K. Atomic force microscope (AFM) dilatometry measurements were conducted to investigate density behavior of the ultra-stable glasses. Thickness measurements were made in stepwise fashion over a range of temperatures from ambient to above the Tg. Results show that the intersections of the line for the equilibrium liquid and those for the rejuvenated and stable glasses identifying the fictive temperature Tf result in Tf, rejuvenated = 347.3 K and Tf, stable = 269.5 K, that is, nearly 80 K below the Tg of the rejuvenated material and well below the notional Kauzmann temperature as estimated from the Vogel-Fultcher-Tammann (VFT) analysis of the cooling rate dependence of the calorimetric glass transition temperature reported previously. The results corroborate the published calorimetric results on the same ultra-stable fluoropolymer glasses that witnessed Tf reductions of up to 62.6 K below the Tg of the rejuvenated system. In addition, to demonstrate the versatility of the AFM dilatometry methodology for the thin film response, isothermal de-aging experiments were carried out to illustrate the devitrification kinetics. We also carried out one of the Kovacs' signature key experiments, the asymmetry of approach, to further illustrate the method.