Effect of stretching-induced changes in hydrodynamic screening on coil-stretch hysteresis of unentangled polymer solutions

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Effect of stretching-induced changes in hydrodynamic screening on coil-stretch hysteresis of unentangled polymer solutions

Ranganathan Prabhakar, Chandi Sasmal, Duc At Nguyen, Tam Sridhar, and J. Ravi Prakash

Phys. Rev. Fluids 2, 011301(R) – Published 5 January 2017

Abstract

Extensional rheometry and Brownian dynamics simulations of flexible polymer solutions confirm predictions based on blob concepts that coil-stretch hysteresis in extensional flows increases with concentration, reaching a maximum at the critical overlap concentration $c^{*}$ before progressively vanishing in the semidilute regime. These observations demonstrate that chain stretching strengthens intermolecular hydrodynamic screening in dilute solutions, but weakens it in semidilute solutions. Flow can thus strongly modify the concentration dependence of viscoelastic properties of polymer solutions.

Received 7 September 2016

DOI:https://doi.org/10.1103/PhysRevFluids.2.011301

Physics Subject Headings (PhySH)

Brownian motion Polymer conformation changes Rheology Viscoelasticity

Polymer solutions Polymers

Bead-spring models Ideal-chain models Rheology techniques

Polymers & Soft Matter

Authors & Affiliations

Ranganathan Prabhakar^1,*, Chandi Sasmal², Duc At Nguyen², Tam Sridhar², and J. Ravi Prakash²

¹Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia
²Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia

^*prabhakar.ranganathan@monash.edu

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Vol. 2, Iss. 1 — January 2017

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Images

Figure 1
Transient evolution of $\bar{η}$ normalized by the zero-shear rate solution viscosity $η_{0}$ to steady state at $Wi = 2$ (filled circles) and following strain-rate quenches (open circles) to lower values of $Wi$ : the stretch-to-coil transition is identified by noting the quench $Wi$ below which $\bar{η}$ relaxes continuously without reaching a steady state within the maximum Hencky strain achievable of about 7.
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Figure 2
Concentration dependence of coil-stretch hysteresis in ${\bar{η}}_{p}$ , the polymer contribution to extensional viscosity, in (a) FiSER experiments (top panel) and (b) multichain Brownian dynamics simulations (bottom panel); gray lines indicate the maximum width of the hysteresis windows observed in each case at $c / c^{*} = 0.5$ . Coiled and stretched states are represented by filled and open symbols respectively. Concentration increases from left to right. No hysteresis is observed in the simulations at the lowest and highest concentrations. The number of Kuhn segments $N_{k} = L^{2} / R_{0}^{2} \sim 1300$ in both experiments and simulations [20].
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Figure 3
Concentration dependence of the width of the hysteresis window observed in FiSER experiments (circles) and BD simulations (squares): The continuous and broken curves are predictions of $ζ_{s} / ζ_{0}$ obtained with the blob model for $R = L / 2$ and for the hydrodynamic interaction parameters $h_{k}^{*} = 0.02$ and 0.05, respectively; all data are for the number of Kuhn segments, $N_{k} \sim 1300$ .
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Figure 4
Stretch–concentration state space for polymer solutions showing the dilute regime and the semidilute regimes with weak and strong hydrodynamic screening. (a) Isolated, equilibrium coils described by Zimm hydrodynamics: Each coil is a single tension blob. (b) Equilibrium coils at critical overlap: Each coil is a single correlation blob. (c) Semidilute coils with correlation blobs smaller than coil size. (d) Concentrated solution with correlation blobs of the same size as a Kuhn segment. (e) Isolated, stretched chains with tension blobs. (f) Chains of tension blobs begin to interact hydrodynamically with a screening length approximately equal to chain length. (g) Critical transverse overlap with anisotropic correlation blobs larger than tension blobs. (h) Transition of stretched chains from weak to strong hydrodynamic screening with correlation and tension blobs of same size. (i) Concentrated solution of stretched chains with correlation blobs of the same size as a Kuhn segment. (j) Fully stretched chains in the dilute limit with tension blobs as small as a single Kuhn segment. (k) Concentrated solution of fully stretched chains.
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