Dynamic Response of Block Copolymer Wormlike Micelles to Shear Flow

TL;DR

This study investigates how giant wormlike micelles made of Pb-Peo block copolymers respond dynamically to oscillatory shear flow, revealing shear-thinning and shear banding phenomena near the isotropic-nematic phase transition.

Contribution

It provides a combined rheological and microscopic analysis of micelle dynamics near phase transition, introducing a theoretical model for the order-parameter tensor and stress response.

Findings

Strong shear-thinning observed near phase transition

Gradient shear banding occurs due to critical slowing down

Scaling laws identified for orientational order and stress

Abstract

The linear and non-linear dynamic response to an oscillatory shear flow of giant wormlike micelles consisting of Pb-Peo block copolymers is studied by means of Fourier transform rheology. Experiments are performed in the vicinity of the isotropic-nematic phase transition concentration, where the location of isotropic-nematic phase transition lines is determined independently. Strong shear-thinning behaviour is observed due to critical slowing down of orientational diffusion as a result of the vicinity of the isotropic- nematic spinodal. This severe shear-thinning behaviour is shown to result in gradient shear banding. Time-resolved Small angle neutron scattering experiments are used to obtain insight in the microscopic phenomena that underly the observed rheological response. An equation of motion for the order-parameter tensor and an expression of the stress tensor in terms of the order-parameter tensor are used to interpret the experimental data, both in the linear and non-linear regime. Scaling of the dynamic behaviour of the orientational order parameter and the stress is found when critical slowing down due to the vicinity of the isotropic-nematic spinodal is accounted for.