Degradation of Polymer Solutions in Extensional Flows

Elongational flow techniques are applied to the examination of flow-induced chain scission of macromolecules in solution. An opposed-jets apparatus is used both to produce mechanical scission and to monitor the molecular weight distribution of the scission products. We have explored the combined effects of elongational flow and elevated temperatures upon degradation of almost monodisperse atactic polystyrene solutions. Between 25 and 150 °C degradation occurs as closely central scission of the molecules beyond a critical strain rate (ϵ_f). ϵ_f is found to be a decreasing function of temperature. At 150 °C we present also results for thermal degradation alone. These results correlate well with predictions based upon a thermally activated barrier to scission (TABS) model. We also present results on the strain-rate dependence of the scission rate beyond ϵ_f at room temperature. These results clearly indicate that in dilute solution only those molecules that are virtually fully stretched can undergo central scission. Degradation in real flow situations (for instance, flow through GPC columns) seems to parallel our idealized experiments. Our results have serious implications for the latest theories of polymer dynamics. Finally, we speculate that, contrary to common belief, simple laminar shear flows may be almost incapable of degrading polymer solutions and that degradation is only encountered when the flow contains an appreciable elongational component, commonly arising as a result of flow instabilities or turbulence.