TEMPERATURE-DEPENDENT FREE-ENERGY OF CRYSTALLIZATION AND VISCOSITY OFGLASSY MATERIALS

Elementary thermodynamic procedures have been used to calculate the Gi bbs free energy difference DELTAG from a knowledge of the specific hea t difference DELTAC(p) between the undercooled melt and solid phase. H owever, the metastability of the liquid makes the experimental determi nation of DELTAC(p) very difficult over a wide range of temperatures. In the case of glass-forming materials, DELTAG can be best measured be tween the melting temperature T(F) and the glass transition temperatur e T(g). Several investigators have attempted to propose an appropriate expression for DELTAG in terms of more easily measured parameters. Th e present work incorporates a new expression for the difference in Gib bs free energy proposed in the power law form DELTAG = DELTAS(m) (DELT AT)x. This relation contains a relatively more physical parameter x an d is able to predict non-linearity between DELTAG and DELTAT and hence the deviation from the Turnbull formula. The calculated values using this approach agree fairly well with experimental results in liquids a s well as binary and ternary glasses over the temperature range of int erest. The temperature dependence of the viscosity can also be arrived at from the C(p) data up to the glass transition temperature. The var iation in viscosity above T(g) has been ver-v well accounted for by tw o different proposed expressions in the case of O-terphenyl and Na0.32 K0.68 glassy systems.