Structural dynamics in glass forming liquids from Brillouin scattering

The Brillouin light scattering technique was used to measure the complex mechanical modulus of glass forming liquids of variable fragility, at GHz frequencies, allowing one to characterize structural dynamics on a molecular scale. The data invalidates the assumption of thermo-rheological simplicity (time-temperature superposition) over extended temperature ranges. A model has been developed which consistently describes the temperature dependence of the complex moduli for all investigated substances. This model assumes that two types of structural relaxations occur. One is purely visco-elastic, and the other involves transitions between thermodynamically distinct structural states. The latter is responsible for the transition between glassy and liquid structures, and controls the static modulus. The structural transitions are locally abrupt, but affect the entire system only gradually, over a range of temperatures. By combining the complex modulus data with findings from molecular dynamic simulations, and with the knowledge from conventional structural analysis, mechanisms for the transition between liquid and glass structures have been identified.