Polymer solutions under elongational flow: 2. An evaluation of models of polymer dynamics for transient and stagnation point flows

Abstract

Models for the dynamics of dilute polymer solutions under elongational flow have been examined in the light of new experimental data. In particular birefringence measurements on the polystyrene-decalin and poly(ethylene oxide)—water systems in both stagnation point and transient elongational flows are the most revealing as to the nature of polymer chain uncoiling and fracture. Both the deformation and fracture behaviour have been observed to depend on the type of elongational flow (transient, stagnation point) and the chemistry of the polymer chain. The data on the critical strain rate for chain fracture along with observations of precise midchain scission (as estimated from size exclusion chromatography), a limited overall chain deformation, and the role of accumulated energy (strain) in mechanochemical degradation, tend to imply that a hybrid model of polymer dynamics in elongational flows may be realistic. One such model, a ‘multi-stranded yo-yo with surface energy dissipation and solid like stress response’ has been developed as a general hypothesis to account for the deformation, stretching and fracture behaviour of polymer chains in both transient and stagnation point elongation flows. This modified yo-yo is found to be an energetically favourable alternative to an affinely deforming chain over certain degrees of elongation. It has been shown to qualitatively explain the apparent difference in behaviour of polymer chains in both stagnation point and transient elongational flows. The viscous coupling between the solvent and the polymer chain is also used as a qualitative parameter to account for the deformation and fracture behaviour of polymer chains under transient and stagnation point elongation flows. Based on these analyses, a set of recommendations as to future experiments are presented.