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

The viscoelastic properties of glass-forming systems exhibiting differ ent nature of cohesive forces were measured near their glass temperatu re by means of high resolution low frequency mechanical spectroscopy. The dynamic behaviour thus observed was analyzed at a microscopic leve l through a modelling based on the physical concepts of microfluctuati ons of density and constrained dynamics of disordered matter. It is sh own that the ratio between strong and weak bonds per atom, C-SW, can b e regarded as a pertinent parameter for the description of molecular m obility in disordered matter. The approach presented here shows that t he temperature dependence of relaxation properties observed through th e liquid-glass transition is related to the ratio T-HS between the ent halpy and entropy of weak sites formation. The investigation was carri ed out on different systems, namely polymeric, molecular and network g lass 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 w eak sites, i.e., a larger number of structural units implied in molecu lar correlated motions and (iii) a tendency of the liquid to become mo re fragile.