Thermodynamical properties of glass forming systems: A Nuclear Magnetic Resonance analysis

We present a Nuclear Magnetic Resonance (NMR) study of the thermal evolution of the magnetic properties of three different glass formers: glycerol, o-terphenyl and salol. In particular, we analyze how the response of these liquids to the applied magnetic field changes with temperature. We focus on the total magnetization and on the chemical shift of each protonated group. By means of these quantities we account that the dynamics of the glass forming materials, on decreasing the temperature, is dominated by the onset of well defined local inhomogeneities due to precise microscopic cooperative processes. Just these “dynamical heterogeneities” and their energetic topology determine the dynamic crossover from fragile (super Arrhenius) to strong (pure Arrhenius) glass forming behavior. The specific heat changes evaluable from the measured NMR chemical shift associate this phenomenon, and all the related ones, to local configurational changes.