Rheology of polyelectrolyte multilayer modified suspension

In the last two decades, the tuning of soft materials’ properties has reached more and more technological and scientific significance. A wealth of new colloidal systems emerged from the need of soft materials with well-defined, and adjustable, rheological properties. Prominent examples are multiarm star polymers, microgels, thermosensitive colloids, and depletion gels. These systems are tailored to correlate microscopic interactions with macroscopic behavior, which is poorly understood for complex systems. The interparticle interactions are controlled at the synthesis level or strongly coupled to the background fluid. This is a serious drawback of the established colloidal systems. What lacks is a colloidal system that combines high flexibility in changing the tuning parameters with fast and cost effective production. Self-assembled polyelectrolyte multilayer modified colloids are a relatively new and promising colloidal system that is expected to overcome the previousmentioned drawbacks of the established systems. For the first time, this thesis explores themicro-macro interactions of polyelectrolytemutlilayer (PEM) modified colloids. Thereby, the focus lies on dense colloidal suspensions for which the individual motion of single particles is strongly restricted by the neighboring particles. Then, the suspensions show simultaneously elastic and viscous properties with dominating solid-like behavior at rest, and yield and flow under large applied stresses. The first experiments of this thesis investigate the effect of the layer number on rheology. At low layer numbers, the rheology suffers from an inhomogeneous PEM surface which introduces localized attractions to the suspensions. This results in a more brittle material as the local attractions are strong in radial direction, but easily break down under lateral shear – a mechanism that is similar to a fridge magnet sticker. In rheology, this behavior becomes apparent when the yield stress, as the measure for the equilibrium stress scale, is compared to the elasticity of the equilibrium microstructure. The ratio of yield stress to elasticity decreases with increasing layer number and reaches a plateau for high layer numbers where the PEM surface is well-defined. This finding gives a first guideline for the estimation of the precursor regime with macroscopic tools. Another remarkable result was the finding that the rheology is governed by the kind of terminating polyelectrolyte of the PEM film. The role of the layer number and the terminating polyelectrolyte were investigated in a second set of experiments, thereby focusing on the rheology at high shear stresses. At high shear stresses, dense suspensions are expected to shear thicken. That is, their viscosity increases with shear stress. The transition from Newtonian flow at medium shear stress to shear thickening is a very sensitive measure for the relevant forces that govern the rheology, and hence the experiments identify that at high shear stresses hydrodynamic forces dominate the interparticle interactions. Moreover, the strength of the hydrodynamic force depends again on the terminating polyelectrolyte and is due to the specific porosity of the PEM film. Following experiments investigated the role of the terminating polyelectrolyte in more detail. Systematic variation of the polyelectrolyte conformation and the ionic strength of the background fluid showed that the terminating polyelectrolyte behaves similar to polyelectrolyte brushes. This finding gives rise to the assumption that terminating polyelectrolyte acts like a hairy layer and the particles can be seen as hairy core-shell particles, which share some properties with multiarm star polymers.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Hess, Andreas

Contributors dc:contributor

• Aksel, Nuri

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/57/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:57