Kinematics of filament stretching in dilute and concentrated polymer solutions (original) (raw)

Extensional stress growth and stress relaxation in entangled polymer solutions

Journal of Rheology, 2003

We report an evaluation of the double constraint release model with chain stretch ͑DCR-CS͒ suggested by Ianniruberto and Marrucci ͓J. Rheol. 45, 1305-1318 ͑2001͔͒, in predicting the transient stress growth and stress relaxation behavior of two well-characterized entangled polymer solutions undergoing homogeneous uniaxial extensional flow. The experiments are conducted using a filament stretching rheometer. The DCR-CS model belongs to a family of simplified single-segment models that incorporate constraint release, double reptation, and segmental stretching into the basic reptation mechanism proposed in the original Doi-Edwards theory and seeks to extend the predictive capacity of the theory to more complex flow fields. We show that the single-mode DCR-CS differential model performs well in predicting the transient extensional stress growth and steady-state extensional viscosity over a range of stretch rates. The model also predicts the observed stress relaxation following cessation of stretching satisfactorily. We further show that the model predicts shear thickening even in steady shear flow.

Extensional Step Strain Rate Experiments on an Entangled Polymer Solution

Macromolecules, 2017

The dynamics of chain entanglement/disentanglement in concentrated polymer solutions are matters of current debate. We report the results of step strain rate experiments in uniaxial extensional flow on a well-characterized polymer solution containing about 22 entanglements per chain which allows these dynamics to be probed. In these experiments the polymer solution is subjected to homogeneous stretching at a given strain rate up until a predetermined value of strain. After this strain is reached, the strain rate is changed to a new value, and stretching is continued until steady state is acquired in the extensional stress. The strain rates are increased in step-up experiments and decreased in stepdown experiments. The strain rates are adjusted so that both the orientation and the stretching dynamics can be probed. Additionally, the predictions of two recent single-mode molecular models are evaluated against the experimental data. These models include the effects of entanglement dynamics in their predictions in an ad hoc manner. This leads to only a qualitative improvement in the predictive capacity of one of the models and reduces that of the other. Slip-link simulations using the primitive chain network model are also included for further insight into the underlying dynamics. The models are found to yield qualitative predictions of the experimental observations.

Extensional Rheometry of Entangled Solutions

Macromolecules, 2002

The seminal ideas of de Gennes and Doi and Edwards have provided the theoretical framework for much of the recent effort to model the rheological behavior of entangled polymer melts and solutions. Recent theoretical work has incorporated a number of important additions to the basic Doi-Edwards theory, including an explicit description of chain stretch and additional relaxation mechanisms such as contour length fluctuations (CLF) and convective constraint release (CCR). However, very little quantitative data has been published on the rheological behavior of entangled systems in strong flows. Hence, a comprehensive examination of the theoretical developments has not been possible. The experiments described in this paper use the filament stretching rheometer to obtain transient extensional stress growth data and steady state uniaxial extensional viscosity data for a number of entangled, narrow molecular weight distribution polystyrene solutions in the strain-rate regime characterized by a significant degree of both chain alignment and stretch. These results are then compared with theoretical predictions for a number of the current generation of reptation-based models, including mechanisms for chain stretching, contour length fluctuations, and convective constraint release. These comparisons demonstrate that when the model parameters are properly obtained from linear viscoelastic measurements, the recent model due to Mead, Larson, and Doi (Macromolecules 1998, 31, 7895) provides quantitative predictions for this class of flows for solutions spanning the complete range from very lightly to highly entangled solutions.

Rupture of entangled polymeric liquids in elongational flow

Journal of Rheology, 2003

Polymer melts and concentrated solutions rupture at high rates of elongation in a manner that is reminiscent of the cohesive failure of solids. We propose a simple molecular picture of rupture of a polymer filament, in which catastrophic failure occurs when the frictional force on an entangled chain can no longer balance the tension in the chain. The model, which is fully predictive and contains no adjustable parameters, captures the rupture characteristics of the available data sets and agrees quantitatively with critical stress-critical strain data and the dependence of critical strain on the Weissenberg number.

New Experiments for Improved Theoretical Description of Nonlinear Rheology of Entangled Polymers

Macromolecules, 2013

The present work discusses four types of new experiments that can improve the current theoretical description of nonlinear rheology of entangled polymers. First, a slowly imposed strain is found to result in nonmonotonic evolution of the state of chain entanglement during quiescent relaxation, consistent with the idea of chain disentanglement after step shear. Second, the stress relaxation upon a sizable step strain is found to be identical to that for small step strain, consistent with a molecular scenario that a strained entangled melt has an entropic barrier to resist chain retraction. Third, the ability of a step-strained polymer to undergo elastic recovery is found to be the same up to strain amplitude of unity, and a sample sheared for a period much longer than the Rouse time is shown to still undergo nearly full elastic recovery. Fourth, an entangled melt, stretched at a rate significantly lower than the Rouse relaxation rate, undergoes full elastic recovery until the point of tensile force maximum. We have discussed an alternative conceptual framework to describe these nonlinear responses of entangled polymers despite the possibility that the tube model might be further remedied to characterize the new rheometric measurements presented in this work.

Rupture of entangled polymeric liquids in elongational flow with dissipation

Journal of Rheology, 2004

Shear and extensional rheology of entangled polymer melts: Similarities and differences

2012

This work extends our previous understanding concerning the nonlinear responses of entangled polymer solutions and melts to large external deformation in both simple shear and uniaxial extension. Many similarities have recently been identified for both step strain and startup continuous deformation, including elastic yielding, i.e., chain disentanglement after cessation of shear or extension, and emergence of a yield point during startup deformation that involves a deformation rate in excess of the dominant molecular relaxation rate. At a sufficiently high constant Hencky rate, uniaxial extension of an entangled melt is known to produce window-glass-like rupture. The present study provides evidence against the speculation that chain entanglements tie up into "dead knots" in constant-rate extension because of the exponentially growing chain stretching with time. In particular, it is shown that even Instron-style tensile stretching, i.e., extending a specimen by applying a constant velocity on both ends, results in rupture. Yet, in the same rate range, the same entangled melt only yields in simple shear, and the resulting shear banding is clearly not a characteristic of rupture. Thus, we conclude that chain entanglements respond to simple shear in the manner of yielding whereas uniaxial extension is rather effective in causing some entanglements to lock up, making it impossible for the entanglement network to yield at high rates.

Dynamics of linear, entangled polymeric liquids in shear flows

We study predictions in transient and steady shearing flows of a previously proposed self-consistent reptation model, which includes chain stretching, chain-length fluctuations, segment connectivity and constraint release. In an earlier paper it was established that the model is able to capture all trends observed experimentally for viscometric flows allowing focus of the present work on the model. That is, we study in detail the physics and underlying dynamics of the model to explain the macroscopically observed rheological properties in terms of chain behavior and dynamics on the molecular level. More specifically, we discuss the effects of chain tumbling, molecular chain stretching and constraint release and their influence on the macroscopic stress as well as the extinction angle under various flow conditions. In particular, we find that chain tumbling causes the undershoot in extinction angle during inception of shear; chain tumbling is itself suppressed by the presence of molecular stretching; and the anticipated strong correlation between normal stress and molecular stretching is confirmed; following cessation of steady shear, it is observed that chain stretching undershoots, and a mechanism involving constraint release is suggested to explain the phenomenon, and; from stresses following cessation, a previously proposed technique for estimating stretching during steady shear flows-involving a generalized damping function-is shown to be inaccurate. Also investigated is the monomer density along the chain contour which reveals information about the local chain stretching and orientation. Here, it is found that the distribution of monomers along the contour becomes non-uniform when the shear rate exceeds the inverse Rouse relaxation time. Finally, we discuss a possible violation of the stress-optic rule during start up of steady shear flow at high shear rates.

Exploring stress overshoot phenomenon upon startup deformation of entangled linear polymeric liquids

Journal of Rheology, 2009

This work explores the picture associated with stress overshoot during sudden continual ͑i.e., startup͒ external deformation of entangled polymeric liquids and proposes a specific scaling form to depict the intermolecular interactions responsible for chain deformation. Following a previously proposed idea that the stress overshoot in startup deformation is a signature of yielding, we search for ingredients that should go into the description of the force imbalance at the yield point and show that the expression for the intermolecular locking force f iml , derived from the characteristics associated with the yield point, can be tested against experiment. New rate-switching experiments support the proposed formula for f iml .

Kinematics of filament stretching in dilute and concentrated polymer solutions (original) (raw)

Related papers