RANDOM-WALK APPROACH TO DYNAMIC AND THERMODYNAMIC PROPERTIES OF SUPERCOOLED MELTS .1. VISCOSITY AND AVERAGE RELAXATION-TIMES IN STRONG AND FRAGILE LIQUIDS

A random-walk concept of structural units in liquids is used to descri be mechanical and thermodynamic properties of supercooled melts. Coope rative nonelastic excitations are considered as jumps of structural un its in configurational space implying that the energetic distribution of metastable states that can be occupied by the structural units (den sity of possible metastable states (DPMS) function) is the principal c haracteristic feature of a particular liquid. Specific DPMS functions are proposed for both ''strong'' and ''fragile'' liquids. Over-barrier jumps of structural units involving bond breaking are assumed to be r ate-limiting in the case of strong liquids while in weakly bonded frag ile liquids jumps of structural units may occur without bond breaking. The model is able to explain (i) the non-Arrhenius and Arrhenius-like temperature dependences of viscosity for fragile and strong liquids, respectively, (ii) the dependence of the glass-transition temperature upon the cooling rate, and (iii) the temperature dependences of thermo dynamic functions (free energy, entropy, etc.) for both strong and fra gile liquids in a quantitative fashion.