Transient shear banding in entangled polymers: a study using the Rolie-Poly model

TL;DR

This study investigates transient shear banding in entangled polymers using the Rolie-Poly model, revealing that early-time instabilities can lead to temporary shear bands even in systems with monotonic constitutive behavior.

Contribution

It demonstrates that transient shear banding can occur in entangled polymers modeled by the Rolie-Poly model with monotonic behavior, linked to early-time flow instabilities.

Findings

Transient shear bands arise from early-time instabilities.

Instability correlates with negative slope of the constitutive curve.

Perturbations can trigger transient shear banding.

Abstract

Spatially inhomogeneous shear flow occurs in entangled polymer solutions, both as steady state shear banding and transiently after a large step strain or during start up to a steady uniform shear rate. Steady state shear banding is a hallmark of models with a non-monotonic constitutive relation between total shear stress and applied shear rate, but transient banding is sometimes seen in fluids that do not shear band at steady state. We model this behavior using the diffusive Rolie-Poly model in a Newtonian solvent, whose constitutive behavior can be monotonic or non-monotonic depending on the degree of convected constraint release (CCR). We study monotonic constitutive behaviour. Linear stability analysis of start up to a sufficiently high shear rate shows that spatial fluctuations are unstable at early times. There is a strong correlation between this instability and the negative slope of the (time dependent) constitutive curve. If the time integral of the most unstable eigenvalue is sufficiently large then the system exhibits transient shear bands that later vanish in steady state. We show how perturbations, due to fluctuations or the inhomogeneous stresses, can trigger this instability. This transient behavior is similar to recent observations in entangled polymer solutions