Effect of molecular weight on dynamics of linear isotactic polypropylene melt at very high shear rates

Abstract

In this work, three isotactic linear polypropylenes, with weight average molecular weights between 56 250 - 75 850 g/mol, have been characterized at 230 oC over a very wide shear rate range. A low shear rate primary Newtonian plateau, a pseudoplastic region and a well developed secondary Newtonian plateau were identified for all the polypropylene melts. Flow activation energy at low (E0) and high (E∞) shear rates was found to be 56.590 kJ/mol and 25.204 kJ/mol, respectively. For the first time, it has been discovered that the secondary Newtonian viscosity, η∞, depends linearly on the weight average molecular weight, Mw, in log-log scale as η∞ = 1.19. 10-6 M 1.084 . The observed slope close to 1 w between η∞ and Mw suggests that polymer chains in the melt are disentangled at the secondary Newtonian plateau region. This conclusion is supported by the experimental observation that the high shear rate flow activation energy E∞ for given PP melts is comparable with the flow activation energy of PP like oligomer (squalane, C30H62; 2,6,10,15,19,23-hexamethyltetracosane). © 2018 Society of Plastics Engineers. All rights reserved.