Based on the free volume concept and the equation by Doolittle, an empirical equation is offered for the flow activation energy, E*, for polymer melts for the range of over 150ยฐC above glass transition temperature, Tg. This E* represents the temperature coefficient of viscosity for the Newtonian region which is also equal to the value measured at constant shear stress for non-Newtonian flow. Data show that the E* of linear polymers approaches a constant value for a temperature range above Tg+150ยฐC. Data on 17 polymers are correlated. The proposed equation for this region predicts the E* of polymer melts from the volume expansion coefficient, a t, above Tg and also from the Tg.
Correlations have also been developed between E* and az and between E* and Tg by simplifying the equation by use of the Simha-Boyer expression. A polymer having a lower at or higher Tg generally has a higher E*. However, more satisfactory results are obtained by calculating E* from both at and Tg. The E* calculated is found to agree with measurements within the experimental precision of about + 1 Kcal/mole.
The effects of polymer composition, molecular weight, branching and microstructure on E* are also discussed. These factors influence E* in the way in which they effect al 'and Tg.