Structure of strongly interacting polyelectrolyte diblock copolymer micelles

TL;DR

This study investigates the structure and rheology of spherical polyelectrolyte diblock copolymer micelles in water, revealing how corona interpenetration influences viscosity and gel formation at high concentrations.

Contribution

It provides detailed structural insights into PS-b-PA micelles using SANS and SAXS, highlighting corona behavior and interpenetration effects at varying concentrations and ionic conditions.

Findings

Corona shrinks with increasing packing fraction.

Interpenetration leads to a gel-like state with high viscosity.

Counterions are distributed within the corona segments.

Abstract

The structure of spherical micelles of the diblock poly(styrene-block-acrylic acid) [PS-b-PA] copolymer in water was investigated up to concentrations where the polyelectrolyte coronal layers have to shrink and/or interpenetrate in order to accommodate the micelles in the increasingly crowded volume. We obtained the partial structure factors pertaining to the core and corona density correlations with small angle neutron scattering (SANS) and contrast matching in the water. The counterion structure factor was obtained with small angle X-ray scattering (SAXS) with a synchrotron radiation source. Furthermore, we have measured the flow curves and dynamic visco-elastic moduli. The functionality of the micelles is fixed with a 9 nm diameter PS core and a corona formed by around 100 PA arms. As shown by the SAXS intensities, the counterions are distributed in the coronal layer with the same density profile as the corona forming segments. Irrespective ionic strength and micelle charge, the corona shrinks with increasing packing fraction. At high charge and minimal screening conditions, the polyelectrolyte chains remain almost fully stretched and they interdigitate once the volume fraction exceeds the critical value 0.53+/-0.02. Interpenetration of the polyelectrolyte brushes also controls the fluid rheology: the viscosity increases by 3 orders of magnitude and the parallel frequency scaling behavior of the dynamic moduli suggests the formation of a physical gel. In excess salt, the coronal layers are less extended and they do not interpenetrate in the present concentration range.