Effect of polymerization on the boson peak, from liquid to glass

Reactive epoxy-amine mixtures during isothermal polymerization have been characterized by Raman scattering measurements in order to follow the modification of the vibrational density of states. In the initially liquid solutions an increasing number of Van der Waals bonds are replaced by stiffer covalent bonds. During the chemical reaction, the molecular diffusion slows down and ultimately the systems are frozen in a glassy structure. The transformation from reactive liquids to chemical glasses is accompanied by microscopic structural changes driven by the bonding process. During the reaction the samples density and the sound velocity increase, both of them contribute to significantly change the Debye level. By combining Raman with Brillouin inelastic X-ray scattering measurements, we compare the relative variations of the boson peak with that of the Debye level. We find that the shift and intensity variation of the boson peak are fully explained by the modification of the elastic properties of the medium and a boson peak master curve is obtained.