| We investigate the influence of molecular weight and concentration on the transient extensional properties of a wide range of polystyrene solutions. In addition to monitoring the evolution of the tensile stress in a filament extensional rheometer (FiSER), the temporal evolution of the filament's spatial profile can be used to differentiate between the extensional response of dilute and concentrated polymer solutions. We summarize the results of filament stretching experiments on dilute, semi-dilute and concentrated entangled fluids over a spectrum of strain rates (0.1 to 7 s-1) and Hencky strains (up to 6). The correlation between the linear viscoelastic behavior and the composition of polymer solutions can be summarized in a Graessley diagram; i.e. a plot of molecular weight versus polymer concentration that shows criteria such as chain overlap (c*) and onset of molecular entanglement (Me). By varying the physical composition of solutions tested in the extensional rheometer we can probe the role of different molecular mechanisms in the growth of tensile stresses for each of the five regions identified in the Graessley diagram. The high viscosity of concentrated entangled fluids results in a close-to-ideal filament stretching behavior that is almost unaffected by gravitational sagging. However, little strain-hardening is obtained and observations show that the filament fails at moderate strains by a necking instability. By contrast, chain stretching and pronounced strain-hardening are observed in all the unentangled fluids which prevents filament necking, but ultimately leads to an elastic endplate instability. The onset of chain stretching in semi-dilute and concentrated entangled polymer solutions occurs at very high deformation rates and can be identified by the onset of strain-hardening and by the visual detection of transition from necking failure to elastic end-plate instability. These experimental trends are in agreement with the predictions of the Doi-Edwards-Marrucci-Grizzuti model. |
