School of Physics Soft Condensed Matter & Biophysics Seminar: Prof. Roland Winkler, Forschungszentrum Juliech, Germany

Nano- and microgels are nanometer to micrometer size crosslinked polymer networks often comprised of polyelectrolytes. Their ability to undergo reversible volume phase-transitions in response to environmental stimuli, such as pH, temperature, the ionic strength of the surrounding medium, or the quality of solvent renders them potential candidates for a broad-range of applications in drug delivery, sensing, template-based synthesis of inorganic nanoparticles, and separation and purification technologies to name a few. Numerous theoretical and simulation studies of the swelling behavior of polyelectrolyte networks have been performed, in order to arrive at a microscopic understanding of the underlying mechanisms. These studies typically focus on defect-free macrogels applying periodic boundary conditions, i.e., only the bulk properties of a gel are considered.

Comparably little is known about finite-size crosslinked polyelectrolyte nano-and microgels. Their finite size gives rise to phenomena, which are not present in bulk systems. Specifically, the permeability of a gel particles allows charges, e.g., counterions to freely penetrate or leave the gel particle in response to the actual charge distribution and environmental conditions. For weak electrostatic interactions, no longer all counterions are contained inside the gel particles, as for a bulk system, but rather a large fraction is distributed in its vicinity. In contrast, strong Coulomb attractions lead to counterion condensation and capturing of counterions inside a gel particle, comparable to macroscopic gels. The change of the counterion density associated with the repulsive Coulomb interaction between monomers leads to an interesting interplay, which significantly affects the nano- and microgel structural properties.

In the talk results of large-scale computer simulations of the structural and dynamical properties of nanogel particles will be presented. Specifically, the ion distribution and the associated degree of gel swelling will be addressed. A novel scaling regime is discussed, which captures the dependence of swelling on the Bjerrum length, number of crosslinks, and polymers comprising a nanogel particle. Moreover, the specific features of the dynamics of the polymers in core-shell nanogles will be outlined. Here, the network structure is clearly reflected in the dynamics structure factor.