Evidence of post-polymerization of 110-million-year-old Spanish amber and its consequences for rejuvenation-based interpretation

Heating erases the history of a physically aged glass and cooling thereafter produces the original, unaged glass, i.e., a low-enthalpy glass is said to be rejuvenated by one heat-cool cycle. The knowledge is used in interpreting some properties of millions-year-old fossil amber [Nat. Commun. 4 (2013) 1783; J. Phys.: Condens. Matter, 25 (2013) 295402; Phys. Rev. Lett. 112 (2014) 165901], a naturally-occurring organic glass produced by free radical polymerization and a multiplicity of chemical reactions in the exuded resin of certain trees. Here we analyze the available heat capacity data of 110-million-year-old Spanish fossil amber that was used as a model for investigating the aging effect on physical properties of a glass and show that it post-polymerizes on heating its melt state. This causes the crosslinked structure of the melt-cooled amber to slightly differ from that of fossil amber and therefore there is no rejuvenation. Discussion of the aging effect and Boson peak height in terms of potential energy landscape need reconsideration, especially when the unaged state of amber is not available for comparison.