THERMODYNAMIC STRENGTH OF THE GLASSY STATE AND THE MAXIMUM ENTHALPY STORED IN THE PROPYLENE GLYCOL-D2O SYSTEM

The glassy state of several concentrations of propylene glycol in D2O is presently characterized by enthalpy relaxation experiments. The exi stence of a lowest glass transition temperature T-s has defined an utm ost excess enthalpy that can be stored by the glass during cooling. Si milar to the thermal behavior of the shear viscosity for liquids, thes e T-s values are maximum for the binary system with compositions in th e range 78-82% (w/w) propylene glycol in D2O, suggesting that these co ncentrations correspond to the strongest glasses, and provide a higher thermodynamic strength for these glasses. These glasses correspond al so to the maximum excess enthalpy that can be stored by the glassy sta te during cooling. These concentrations present, however, an apparent minimum threshold stress to induce fracture nucleation in the glasses with the definition of a maximum mechanical fragility. The present res ults underline a problem in the definition of the thermodynamic and me chanical strength of the glassy state as an extension of the fragility definition for liquids.