MOLECULAR MOTIONS NEAR THE GLASS-TRANSITION RANGE AND THE FRAGILITY OF GLASS-FORMING SYSTEMS

The viscoelastic properties of glass-forming systems exhibiting different nature of cohesive forces were measured near their glass temperature by means of high resolution low frequency mechanical spectroscopy. The dynamic behaviour thus observed was analyzed at a microscopic level through a modelling based on the physical concepts of microfluctuations of density and constrained dynamics of disordered matter. It is shown that the ratio between strong and weak bonds per atom, C-SW, can be regarded as a pertinent parameter for the description of molecular mobility in disordered matter. The approach presented here shows that the temperature dependence of relaxation properties observed through the liquid-glass transition is related to the ratio T-HS between the enthalpy and entropy of weak sites formation. The investigation was carried out on different systems, namely polymeric, molecular and network glass formers. In all cases, the decrease of the ratio C-SW is related to (i) a decrease of the quantity T-HS, (ii) a delocalization of the weak sites, i.e., a larger number of structural units implied in molecular correlated motions and (iii) a tendency of the liquid to become more fragile.