Filament stretching of model elastic liquids

The elongational behavior of viscoelastic polymer solutions is investigated using a filament stretching extensional rheometer. Understanding the extensional behavior of polymeric liquids allows insight into physical mechanisms driving phenomena observed in complex processing flows such as fiber spinning and extrusion.;The design and calibration of the filament stretching device is discussed along with practical considerations for its operation. Two original control techniques, a closed-loop and an open-loop scheme, are developed for realizing the correct kinematics in a filament stretching experiment.;Three model elastic liquids are used for benchmark extensional rheology experiments. The dilute polymer solutions consist of a fixed concentration (0.05 wt.%) of high molecular weight monodisperse polystyrene (2, 7, and 20 million g/mol, respectively) dissolved in oligomeric styrene. The dynamic material functions and viscometric properties are well-described by the Rouse-Zimm bead-spring model. These three fluids form the basis of an interlaboratory comparison of filament stretching devices at Harvard/M.I.T., University of Toronto, and Monash University. Excellent agreement is observed between the results obtained from these devices under similar test conditions, suggesting that filament stretching has matured into a reliable method of quantifying the extensional behavior of viscoelastic liquids.;The dynamics of the elastocapillary breakup of themodel elastic liquids are also investigated using the filament stretching device. The measured diameter profiles of viscoelastic filaments necking under the action of capillary pressure agree well with predictions from a nonlinear viscoelastic constitutive model. A direct comparison between filament stretching and breakup experiments is made by balancing the forces acting on the breaking filament. Quantitative agreement is observed between the steady state extensional viscosities obtained from each experiment and capillary breakup observations offer a simple way to extend the operating range of the filament stretching device.;Finally, the effect of a known pre-deformation on the transient extensional viscosity of dilute polymer solutions is investigated. Two modes of pre-deformation are imposed immediately prior to filament stretching. For the test fluids considered, oscillatory squeeze flow parallel to stretching accelerates strain hardening; however, steady shear flow perpendicular to stretching delays strain hardening. The observed trends are supported by numerical calculations.