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Forschungsarbeit

Colloidal Molecules – From Preparation to their Dynamics in Solution

by Martin Hoffmann (12.07.2010)

In the field of colloidal chemistry we have a clear idea of the preparation, solution behavior and application of spherical (nano-) particles. For example, "soft" particles made of a solid core and a thermoresponsive shell can be used to investigate the glass transition of "colloidal molecules" in solution. Such systems may be used in optical switches or photonic devices. In the nano and micrometer-range, "colloidal molecules" are the counterpart of simple chemical compounds, for example due to a similar symmetry (tetrahedrons and methane, CH4).

In my PhD, I focus on non-spherical, well defined particles as ideal model systems for colloidal molecules. On the one hand, I am interested in the synthesis of the new materials and on the other hand in a fundamental understanding of the solution behaviour (diffusion, assembly). An example of non-spherical colloids is given in Figure 1. The particles are in the range of 300 nm and have a solid polymer core (appearing dark) carrying long chains of poly(styrene sulfonate). The attached chains shrink (and stretch) when the ambient ionic strength in solution is increased (decreased).

Figure 1: Cryo-TEM micrograph of dumbbell-shaped polyelectrolyte brushes with a solid polymer core  carrying a flexible polyelectrolyte layer. Figure 2: Scheme for the scattering experiment with geometric or optic anisotropic colloidal particles.[Bildunterschrift / Subline]: Figure 1 (left): Cryo-TEM micrograph of dumbbell-shaped polyelectrolyte brushes with a solid polymer core (dark) carrying a flexible polyelectrolyte layer (thin grey "lines" perpendicular to the core). The inset shows a TEM image of the particles in the dried state. Figure 2 (right): Scheme for the scattering experiment with geometric or optic anisotropic colloidal particles. Anisotropy leads to a partial depolarization of light indicated with V (vertical) and H (horizontal).

Since these objects have both optic and geometric anisotropy, they will depolarize light (Figure 2). A certain amount of the vertically polarized light in the beginning (V) is horizontally polarized (H) after the scattering process. In a dynamic light scattering experiment this gives access to the translational- and the rotational diffusion coefficient. First is a measure of particle speed due to Brownian motion and latter how often the particle relaxes around a main symmetry axis. Both values allow the calculation of particle size (length, diameter…).

The experimental diffusion coefficients can be modelled for certain particle configurations. In cooperation colloidal clusters with 1 to 4 constituent spheres were prepared and investigated as described above. The left hand side of Figure 3 shows representative FESEM images of the clusters ranging between 150 and 300 nm, and the experimental diffusion coefficients for rotation (upper value) and translation (lower value), respectively. As a comparison of experimental data and predictions (Shell Model) reveals, the colloids can be modelled as hard objects underlying stick boundary conditions.

Figure 3: Comparison of the translational (DT) and rotational diffusion coefficient (DR) as obtained by depolarized dynamic light scattering together with the theoretical results using the shell model.[Bildunterschrift / Subline]: Figure 3: Comparison of the translational (DT) and rotational diffusion coefficient (DR) as obtained by depolarized dynamic light scattering together with the theoretical results using the shell model. For the particle doublets and triplets, the rotational diffusion coefficients perpendicular to the main symmetry axis are measured. In the left column, the particle clusters are oriented with their main body parallel to the plane of the figure.

Martin Hoffmann
Martin Hoffmann
* 1981, Marktredwitz/Deutschland

Stationen
  • Seit Okt. 2007
  • Promotion in der Arbeitsgruppe von Prof. Dr. M. Ballauff, Lehrstuhl PC I, Universität Bayreuth und F-I2 Soft Matter and Functional Materials, Helmholtz Zentrum Berlin
  • Okt. 2002 – Aug. 2007
  • Studium der Polymer- und Kolloidchemie (Diplom) an der Universität Bayreuth
  • seit 2005
  • Elitestudiengang Macromolecular Sciences an der Universität Bayreuth

Veröffentlichungen
  • 1. Dumbbell-Shaped Polyelectrolyte Brushes Studied by Depolarized Dynamic Light Scattering. J. Phys. Chem. B. 2008, 112, 14843-14850.
  • 2. Coupling of the rotational motion and the shape fluctuations of the tunable core-shell microgels. Macromolecules, 2009, 42, 1264-1269.
  • 3. Surface potential of spherical polyelectrolyte brushes in the presence of trivalent counterions J. Coll. Interf. Sci. 2009, 338, 566-572.
  • 4. Well-defined crystalline TiO2-Nanoparticles Generated and Immobilized on a Colloidal Nanoreactor. Macromol. Chem. Phys. 2009, 210, 377-386.
  • 5. 3D Brownian Diffusion of Submicron-Sized Particle Clusters. ACS NANO, 2009, 3, 3326–3334.