Center for Functional Nanomaterials Seminar

Despite decades of study, the microscopic mechanism governing the glass-forming process is still unknown. The main challenge is to understand the non-Arrhenius temperature dependence of the structural relaxation time τα of the super cooled liquid upon approaching the glass transition temperature Tg. Most theories relate the mechanism to the cooperativity in molecular motion. Various experimental results and simulations indeed demonstrated cooperative molecular motion associated with the structural relaxation and even estimated the characteristic length scale to be ~1-4 nanometers. The connection between molecular cooperativity and the steepness of temperature dependence of τα in the glass-forming process is still not clear. On the other hand, the collective vibrations in the GHz-THz frequency range, the so-called boson peak, are also considered as a cooperative process with a characteristic length scale (ξ) about a few nanometers. Some researchers speculate that the length scale associated with the boson peak is related to the cooperativity length scale of the main structural relaxation. We have performed light scattering studies of a large number of glass-forming materials to estimate the characteristic length scale ξ from the boson peak spectra. Analysis shows that ξ estimated from boson peak agrees well with the dynamic heterogeneity length scale of the main structural relaxation obtained by 4-dimensional NMR. This indicates that the collective vibrations and the structural relaxation involve similar molecular cooperativity. There are two contributions slowing down the dynamics in a super cooled liquid when temperature decreases: reduction of volume and decrease of thermal energy. We demonstrate that only the volume contribution to the variation of τα directly correlate with ξ among different materials, whereas the thermal energy contribution does not have a direct connection with molecular cooperativity.