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Research Activities

Dispersions of polymer colloids nowadays play an imminent role for the production and processing of plastics. They provide basic materials for the production of paints, protective coatings, adhesives and polymer films (more information at BASF AG) or are applied in the clothing industry for water and dirt repellent coatings(more information´at Clariant).
Beyond their important role in technical applications, colloidal dispersions have in the last twenty years evolved into an important topic for basic research [more Info]
Essentially unnoticed by standard text books of chemistry and physics there has been an development towards the establishment of an new, independent research area - called "soft condensed matter" (as compared to the conventional field of solid state research) - which due to its intrinsically interdisciplinary character reflects and promotes a continuous growing together of polymer science, colloid science, bioscience (e.g. protein = biocolloids; food colloids as milk, joghurt, butter etc.) and material science.
This recent development is based on three important advances:
  1. The possibility to design by tailor-made synthesis model colloids with well defined particle interactions and very narrow particle size distributions.
  2.  
  3. The establishment of light scattering techniques which allow to study particle structures, particle arrangements and follow particle motions on the length and time scales that are relevant for colloids (approx. 10 nm to 100 (m; 100 ns to 1000 s - corresponding to diffusion coefficients from 10-8 cm2s-1 to 10-18 cm2s-1) [more Info]
  4. The development of an formal analogy between colloidal dispersions and atomic fluids (colloids = macro atoms) [more Info]
The research activities of our group aim at combining two basic approaches to colloidal dispersions: their use as model systems in order to tackle fundamental problems of condensed matter science and the "borrowing" of theoretical concepts developed for simple (atomic) fluids in order to better understand the connections between colloid architecture (e.g. the surface properties), colloidal interactions and the behaviour of concentrated dispersions.
Current research topics:
  • Use of colloidal dispersions as model systems for understanding the fundamental processes involved in glass formation and the properties of the glassy state of matter
  • Study of the crystallisation kinetics of microgel colloids (cooperation with T. Palberg, Department of Physics, University of Mainz)
  • Connection between colloidal diffusion and rheological properties of concentrated dispersions (cooperation with N.Willenbacher and the Institute for Mechanische Verfahrenstechnik, University of Karlsruhe)
  • Phase behaviour, short range order and dynamics (diffusion, rheology) of binary mixtures of colloidal microgel particles
    • modeling of binary atomic systems
    • understanding how the particle size distribution
    • induction of tunable attractive interactions between colloidal particles (modeling of Lennard-Jones atoms)
    • study of new gel states and their relation to the glass state
 
Synthesis and Characterization of Model Colloids
Using colloidal particles as model atoms and trying to understand the properties of colloidal dispersions by comparison with atomic fluids depends on the availability of well-defined model colloids, where the interparticle interaction potential is known. Thus, part of the research activities of the group is devoted to the synthesis of colloidal particles with tailormade particle interactions. Accordingly, colloidal particles are characterized not only for particle size and size distribution. Looking at phase behaviour, short-range order and rheological behaviour, effective pair interaction potentials are established for colloidal particles.
Some examples are
 
Polystyrene microgel particles: in a good solvent these colloids interact with an effective pair potential which can be tuned in interaction range and "softness" from short-ranged hard sphere like to intermediate-ranged softly repulsive by changing the degree of internal crosslinking. To achieve narrow size distribution (to allow for crystal formation in order to determine the phase diagram) surfactant-free emulsion polmerization techniques are employed.
Core-shell microgel particles: in some case it is of interest to be able to selectively follow the motion of individual colloidal particles in a concentrated dispersion. This can be done by setting up a host-tracer dispersion, where the host colloids are invisible in a dynamic light scattering experiment (polymer material isorefractive to the solvent) and the tracer colloids made are of another polymer which strongly scatters light. In order to guarantee that the interaction potential of the tracer is identical to that of the host, the scattering polymer is covered by a shell composed of the host polymer ( as interactions are determined essentiall by surface properties). So far synthesized: polystyrene/poly(t-butylacrlate) core/shell microgel particles, fluoroacrylate/polystyrene microgels, Teflon + polystyrene microgel shell.
The colloidal particles are characterized with respect to Particle size and size distribution using static (SLS) and dynamic (DLS) light scattering and electron microscopy (TEM, SEM) Internal particle architecture using SLS, and small angle neutron scattering (SANS; cooperation with P. Lindner, ILL Grenoble) Colloidal interaction potentials using SLS and rheological methods (cooperation with Ch. Friedrich, Freiburger Materialforschungszentrum (FMF)) and by determination of phase diagrams.
 
Experimental Methods
At home:
     
  • 3D-DLS (to characterise  turbid systems)
  • Licht-Echo DLS (to characterise non ergotischic systems)
In cooperation:


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