Pressure Dependence of Viscosity of Glassforming Melts

We suggest a model in which viscosity η is related to entropy of the system. The model successfully describes the dependence of viscosity on pressure. A master equation is developed that allows a comparison of data obtained at different temperatures. Earlier, in the framework of the same model we described the dependence of viscosity on temperature. The present treatment gives a simple interpretation of Tg , the glass transition temperature. Systems vitrify when the disorder decreases to an extent that the dispersion σ , of the height of the barriers that control motion, is only 3.5 % of maximum value of the activation energy Emax. Introduction There has been much discussion [1-5] of the behaviour of glass forming liquids and polymers in terms of activation energy and its dependence on some important parameters like temperature T, pressure P, etc. One of the main purposes of any theory is to create a model with fewer restricting assumptions. The aim of the present paper is to discuss the pressure dependence of viscosity of undercooled melts. The only hypothesis we made here is that the motion of the molecules is a thermally activated process with a jump frequency: J. Non-Cryst. Sol. 262 (2000) 258-263 260 ν i i i E E RT ( ) ~ exp −     (1) The average jump frequency is simply related to the viscosity η by a Maxwell relation