The viscoelastic signature underpinning polymer deformation under shear flow

TL;DR

This paper introduces a new analytical model to interpret small angle neutron scattering data of entangled polymers under shear, revealing their viscoelastic deformation behavior and estimating mechanical stress.

Contribution

The authors develop a piecewise straight segment model to approximate polymer structure and derive an analytical form factor for better data fitting.

Findings

Polymer deformation under shear is intermediate between viscous and elastic behaviors.

The fitted structure allows estimation of mechanical stress aligning with rheology data.

The model enhances understanding of molecular deformation in entangled polymers.

Abstract

Entangled polymers are deformed by a strong shear flow. The shape of the polymer, called the form factor, is measured by small angle neutron scattering. However, the real-space molecular structure is not directly available from the reciprocal-space data, due to the phase problem. Instead, the data has to be fitted with a theoretical model of the molecule. We approximate the unknown structure using piecewise straight segments, from which we derive an analytical form factor. We fit it to our data on a semi-dilute entangled polystyrene solution under in situ shear flow. The character of the deformation is shown to lie between that of a single ideal chain (viscous) and a cross-linked network (elastic rubber). Furthermore, we use the fitted structure to estimate the mechanical stress, and find a fairly good agreement with rheology literature.