Janus Particles at Interfaces

This thesis describes the synthesis and the characterization of both polymeric and hybrid Janus particles of well-defined size, shape and functionality and their high potential for applications in colloidal and material science. Soft Janus particles, based on polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymers, represent a fascinating group of polymeric materials because their size, shape and functionality directly influences their adsorption behavior at liquid-liquid interfaces. The adsorption behavior of Janus cylinders at liquid-liquid interfaces was studied using the pendant drop technique. The interfacial tension decreases with increasing Janus cylinder length and concentration. From the time evolution of the interfacial tension the characteristics of early and late stages of the Janus cylinder adsorption were specified. A series of TEM images of the liquid-liquid interface taken during the cylinder adsorption confirm these observations. Janus cylinders behave differently at the interfaces as compared to the block terpolymer precursor SBM and to cylinders of comparable sizes with a polybutadiene core and a homogeneous polystyrene shell. Understanding the influence of particle size and architecture on the adsorption process is a very important criterion for an efficient industrial use of the Janus particles. To establish the effect of the Janus character together with the effect of particle shape on the interfacial activity and orientation of the Janus particles at an liquid-liquid interface, we present a combination of experimental and simulation data together with detailed studies elucidating the mechanisms governing the adsorption process of Janus spheres, Janus cylinders and Janus discs. These studies demonstrate that changes in the geometry of the particles strongly influence the stabilization of the liquid-liquid interface. As the shape changes from spheres to discs and cylinders, different adsorption kinetics, different packing behavior, different energy barriers and finally different equilibrium values for the interfacial tension can be found. Another main point of this thesis was the synthesis of functional and/or stimuli-responsive hybrid core-shell-corona Janus particles based on inorganic colloids and the characterization of their unique properties and fascinating self-assembly behavior. The first step towards these Janus particles was to understand in detail the formation of core-shell-corona particles with a homogeneous corona, and then in a second step, to use our new knowledge to create hybrid core-shell-corona Janus particles. We developed an easy and completely reproducible preparation and characterization of the solution behavior and functional properties of superparamagnetic and/or fluorescent, thermo-responsive inorganic/organic hybrid nanogels with an intermediate protective silica shell and an interactive polymer layer. These well-defined multifunctional nanogels were prepared via two consecutive encapsulation processes of superparamagnetic and/or fluorescent semiconductor nanocrystals with a silica layer and a crosslinked and responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) corona. The precise adjustment of the conditions allows to achieve a reliable encapsulation and to either entrap several particles or single ones and to precisely tailor the thickness of the silica shell. Full functionality of the encapsulated nanocrystals is retained, but excellent wettability, biocompatibility, flexible surface chemistry, increased chemical stability are implemented together with a thermo-responsive polymer corona. On the basis of our well-characterized core-shell particles we took advantage of the variable surface chemistry of the silica shell to combine the properties of the superparamagnetic core-shell nanoparticles with the catalytic character of nickel complexes in hybrid core-shell-corona nanoparticles forming heterogeneous nanocatalysts. In that way a heterogeneous catalyst was created for facile product separation in the catalytic conversion of olefins. In the next level, an efficient strategy for the large-scale synthesis of well-defined hybrid Janus particles with a silica core (˂˂ 100 nm) and a stimuli-responsive PDMAEMA hemicorona was developed. The synthesis is based on a modified version of the Pickering emulsion technique in combination with surface-initiated atom transfer radical polymerization (ATRP) in a “grafting from” approach. First, 30 nm silica nanoparticles are immobilized at the interface of sub-micrometer sized droplets of poly(vinyl acetate). Since the nanoparticles are partially embedded, one hemisphere is protected. After the modification with an ATRP-initiator and the detachment of the modified silica particles, PDMAEMA was grafted from one hemisphere via ATRP. The obtained Janus nanoparticles are well-defined in size and shape and show stimuli-responsive structural changes depending on pH and temperature.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Ruhland, Thomas M.

Contributors dc:contributor

• Müller, Axel

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/104/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:104