Dynamics below the glass transition temperature and viscoelastic and calorimetric investigation of different fossil resins



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The dynamics of glass-forming systems below the glass transition temperature (Tg) have become of special interest. The classic Vogel-Fulcher-Tammann (VFT) and Williams, Landel, and Ferry (WLF) equations predict the dynamic divergence behavior that the equilibrium relaxation time and viscosity of glass-forming liquid become infinite at a finite temperature above the absolute zero. However, such VFT behavior is challenged by an increasing amount of evidence from both theoretical and experimental works. In order to determine the dynamic response of glass-forming liquids, both dielectric and mechanical experiments were performed in poly(vinyl acetate) (PVAc) glass former in this work. The temperature dependence of the equilibrium shift factors at below Tg shows an apparent Arrhenius behavior rather than the classic VFT response. Besides, the dielectric and mechanical responses of PVAc seem to be probing the glass state differently as the former response reaches its equilibrium behavior much faster than the latter. In addition, a 20 million old Dominican amber was used to investigate the dynamics of glass-forming systems far below Tg. The upper bound to the equilibrium relaxation time shows that the dynamic response deviates dramatically from the VFT extrapolation as much as 44 K below Tg. Finally, 12 pieces of fossil resin from different locations and ages ranging from approximately 100 years to 230 million years have been studied using differential scanning calorimetry. The results show that different resins have different glass transition temperatures and thermal stabilities, though the Tg did not correlate with the age of the amber. Furthermore, both the apparent activation energy and dynamic fragility of amber increase as glass transition temperature increases. The Tg dependence of fragility for amber shows a similar trend with temperature as do the metallic glass formers and aromatic polymers with phenyl rings on the side chain.



Dynamic divergence, Glass-forming system, Amber