Time-Dependent Polymer Rheology under Constant Stress and under Constant Shear Conditions

Abstract

The kinetic rate theory previously presented for describing non-Newtonian phenomena has been further modified to predict the flow behavior of viscoelastic materials under constant stress conditions. The thixotropic shear stress or shear rate is predicted by the kinetic theory, and the experimental stress or shear rate is obtained by modifying the thixotropic value by a stress or shear rate retardation term. The retardation term stems from a Maxwellian approach for stress retardation. In order to test the validity of this approach, transient and steady-state data have been obtained for two solutions of polymethylmethacrylate in diethyl-phthalate. Both constant stress measurements and constant shear rate data have been taken over a broad range. In a systematic manner as suggested by the theory, the parameters were evaluated from constant stress data, and were in turn used to predict constant shear rate data as well as the constant stress measurements. It should be emphasized that the parameters were not obtained from the best empirical fit to the data but were evaluated in a manner suggested by the theory. The agreement between theory and data was good enough to ascertain that the approach is adequate for correlating polymer rheology data. The overall average absolute mean deviation ranged from 4.2% for the steady-state measurements to 11.2% for the constant stress transient measurements. It was further observed that stress overshoot at constant shear rate conditions normally occurred when the Deborah number was greater than unity. Gradual stress growth curves were observed when the number was less than unity.