Enthalpy relaxation upon glass transition and kinetic fragility of molecular liquids.

Relaxation enthalpies in the vicinity of glass transitions were recorded on the basis of the heat capacity curves measured in nine glass-forming molecular liquids and solutions in the well defined cooling and subsequent heating cycles. It is observed that enthalpy hysteresis in the cooling/heating cycles is less significant in liquids with increasing fragility. A notable correlation of the fragility index m with relaxation enthalpy DeltaH(R), glass transition temperature, and heat capacity jump from glasses to liquids is demonstrated. The introduction of DeltaH(R) favors the thermodynamic evaluation of fragility in the glass forming multicomponent systems such as solutions and alloys.