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

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.

Pinch‐off dynamics and extensional relaxation times of intrinsically semi‐dilute polymer solutions characterized by dripping‐onto‐substrate rheometry

“…However, the transition becomes less sharp as the polymer concentration is increased, implying that the coil‐stretch transition becomes milder. Experiments with the filament stretching extensional rheometer and recent Brownian dynamics simulations carried out at Monash also demonstrate that both coil‐stretch transition, and coil‐stretch hysteresis are milder above the overlap concentration. Recent experiments on extensional flows of fluorescently labeled DNA chains in a semi‐dilute solution, visualized in cross‐slot geometry, also show a milder coil‐stretch transition .…”

“…We find that for the aforementioned three molecular weights, the estimates for Zimm time obtained by using monomeric friction values extracted from DoS rheometry measurements are of the same order of magnitude as those obtained using the formula . However, other factors like amplified concentration fluctuations, possible role of topological constraints, conformational anisotropy and the resulting influence on the overall drag coefficient and on the relaxation dynamics, are not captured by eq , and these warrant further investigation …”

“…Near the overlap concentration ( ), measured extensional relaxation time values show a power law in their concentration dependence, that is distinct from the linear concentration‐dependence , where k H is the Huggins constant, anticipated by Muthukumar and Freed . However, even if polymer coils are initially nonoverlapping (as expected in dilute solution), as stretched chains can overlap, several authors invoke the blob theory or composite Rouse‐Zimm model developed for intrinsically unentangled semi‐dilute solutions for describing the dynamics of partially overlapping stretched polymer coils, as is discussed next.…”

“…It is well‐established that in comparison with Newtonian fluids of the same viscosity and surface tension, additional viscoelastic stresses contributed by the presence of polymers result in a measurable delay in pinch‐off that can be used for obtaining a measure of both extensional viscosity (transient as well as steady terminal extensional viscosity, ) and an extensional relaxation time, that correlates with the characteristic time needed for chain relaxation after an extensional or stretching deformation . Strain‐rate dependent extensional viscosity of polymer solutions can be measured using stagnation point flows realized in four roll mills, cross‐slot rheometers, and opposed jet extensional rheometers, as extensional rate can be controlled independently, and either resulting pressure drop, or birefringence, can be measured.…”

“…At the molecular level, the question that remains unanswered is how such chain of Pincus blobs relaxes or behaves when surrounded by a sea of similarly stretched chains in a good solvent. Prabhakar and coworkers outlined related theoretical challenges and open questions quite recently for unentangled polymer solutions in a theta solvent . Their new constitutive model probes the influence of stretching on polymer dynamics by combining the blob picture with an assessment of conformation‐dependent drag, and infers that stretching causes intermolecular interactions to become stronger in the dilute regime and weaker in the semi‐dilute regime.…”