ON THE VISCOSITY-TEMPERATURE BEHAVIOR OF POLYMER MELTS

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 150 degrees C above glass transition temperature, T-g. 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 T-g+150 degrees C. Data on 17 polymers are correlated. The proposed equation for this region predicts the E* of polymer melts from the volume expansion coefficient, a(l), above T-g and also from the T-g. Correlations have also been developed between E* and a(l) and between E* and T-g by simplifying the equation by use of the Simha-Boyer expression. A polymer having a lower a(l) or higher T-g generally has a higher E*. However, more satisfactory results are obtained by calculating E* from both a(l) and T-g. The E* calculated is found to agree with measurements within the experimental precision of about +/-1Kcal/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 a(l) and T-g.