Stephan Förster

Amphiphilic Block Copolymers in Structure‐Controlled Nanomaterial Hybrids

Amphiphilic block copolymers (ABCs) represent a new class of functional polymers that are unique building blocks serving a number of applications mainly related to the energetic and structural control of materials interfaces. The chemical structure of ABCs can be programmed such that interfaces between materials with very different chemical nature, polarity, and cohesion energy can be controlled to a much broader extent than currently possible with low molecular weight surfactants. This is the physical basis for the construction of thermodynamically stable materials hybrids with nanoscale structure and order that consist of a polymer on one side and metal or ceramic nanoparticles or nanophases on the other. These hybrids are particularly interesting since they inherit some of the properties of both the polymer and the inorganic materials, such as mechanical performance and magnetic and optical characteristics.

From self-organizing polymers to nanohybrid and biomaterials.

Block copolymers form a large number of superlattices with characteristic dimensions in the range of a few nanometers up to several micrometers by self-organization. The interplay of supramolecular physics and chemistry opens up new approaches to the production of inorganic, organic, and biological structures and to their integration into functional units. Possible applications in the fields of materials science and molecular biology are being investigated. Block copolymers find numerous applications from the production of inorganic nanoparticles (metals, semiconductors, magnets) and mesoporous materials up to take-up/release systems in chemo- and gene therapy.

Micellization of strongly segregated block copolymers

Series of diblock copolymers of the type poly(styrene‐b‐4‐vinylpyridine) (PS‐P4VP) are synthesized via anionic polymerization. In toluene, a selective solvent for the PS block, well‐defined micelles with narrow molecular weight distribution are formed. Size and shape of the micelles are characterized by static (SLS) and dynamic light scattering (DLS), and electron microscopy (EM). The aggregation number Z exhibits a scaling relation Z∝N1.93P4VPN−0.79PS, where N is the degree of polymerization of each block. We find this behavior close to a Z∼N2AN−0.8B limiting scaling law which appears to be a characteristic of strongly segregated diblock and triblock copolymer systems as well as low molecular weight cationic, anionic, and nonionic surfactants. A simple micellization model is developed that predicts aggregation numbers of surfactants, diblock, triblock, and graft copolymers. The corona dimension Dh as measured by DLS scales as Dh∝Z0.21N0.63PS in agreement with the theoretical prediction D∼Z1/5N3/5B for st...

Polyelectrolytes in solution

This review should demonstrate that polyelectrolyte solutions are currently advancing to a fascinating topic in polymer science. The increasing effort in analytical theory, simulations and experiments is about to result in a deeper, fundamental understanding of the interacting macroions and of the conformational properties of flexible polyions. As these “simple” problems have still not been completely solved, new challenges have already emerged, such as the effect of polyion chain architecture (polyelectrolyte stars, combs, rings, μ-gels), the effect of bivalent or multivalent metal ions bound to the polyelectrolyte chain (intramolecular cross-linking, polyion coil collapse), and the complex field of surfactant-polyion and polyanion-polycation complexes. In comparison with the growing technical importance of such complex ionic systems, the fundamental academic research in this area has hardly started yet.

Micelle and Vesicle Formation of Amphiphilic Nanoparticles

Nanoparticle brushes: Complex nanostructures can be formed by self assembly of amphiphilic CdSe/CdS core-shell nanoparticles that bear a brushlike layer of poly(ethylene oxide) chains. This route is based on controlling the volume fractions of hydrophilic and hydrophobic moieties within the particles and allows the formation of micellar, cylindrical, or vesicular nanoobjects (see picture).

Static and dynamic light scattering by aqueous polyelectrolyte solutions: effect of molecular weight, charge density and added salt

Abstract Aqueous solutions of quaternized poly(2-vinylpyridine) were investigated by static (SLS) and dynamic (DLS) light scattering over a wide range of polyelectrolyte, c pe , and salt concentrations, c s (10 −3 ≤ c pe ≤ 10 2 gl −1 , 10 −5.5 ≤ c s ≤ 10 −1 moll −1 ). Using DLS the cooperative diffusion coefficient D was measured as a function of c pe and c s . D exhibits a characteristic behaviour in each of three different concentration regimes. In the ‘dilute lattice’ regime, where λ = c pe c s ⪡ 1 , one diffusion coefficient is observed. In the transition regime, where λ ≈ 1, D increases with increasing polyelectrolyte concentration and a slow diffusive mode gradually appears. For λ ⪢ 1 (‘semidilute lattice regime’) a fast diffusive process, which is interpreted as the gel mode of a transient network, and a slow diffusive process, which is associated with long-range concentration fluctuations, are observed. For the dynamic behaviour, the absolute ionic strength of the solution is only of minor importance.

Evidence for the preservation of the particle identity in miniemulsion polymerization

A combination of small-angle neutron scattering (SANS), conductivity, and surface tension measurements is used to show that the primary droplets in miniemulsion polymerization have the same size and the same size distribution as the final particles. It is also shown that hexadecane employed as a cosurfactant is homogeneously dispersed in the droplets and does not possess any interface activity. The presented data support that a 1 : 1 copy from monomer droplets to polymer particles is achieved.

Colloidal quasicrystals with 12-fold and 18-fold diffraction symmetry

Micelles are the simplest example of self-assembly found in nature. As many other colloids, they can self-assemble in aqueous solution to form ordered periodic structures. These structures so far all exhibited classical crystallographic symmetries. Here we report that micelles in solution can self-assemble into quasicrystalline phases. We observe phases with 12-fold and 18-fold diffraction symmetry. Colloidal water-based quasicrystals are physically and chemically very simple systems. Macroscopic monodomain samples of centimeter dimension can be easily prepared. Phase transitions between the fcc phase and the two quasicrystalline phases can be easily followed in situ by time-resolved diffraction experiments. The discovery of quasicrystalline colloidal solutions advances the theoretical understanding of quasicrystals considerably, as for these systems the stability of quasicrystalline states has been theoretically predicted for the concentration and temperature range, where they are experimentally observed. Also for the use of quasicrystals in advanced materials this discovery is of particular importance, as it opens the route to quasicrystalline photonic band gap materials via established water-based colloidal self-assembly techniques.

From self-organizing polymers to nano- and biomaterials

By self-organization polymers form a large number of superstructures with characteristic dimensions in the range of a few nanometers up to several micrometers. They can be used to produce in a controlled way a wide range of nano- and microstructured materials such as nanoparticles, nanoporous materials and drug carriers. Applications in the fields of materials science, electronics, molecular biology and pharmacy are being investigated.

Nanoparticle-Loaded Magnetophoretic Vesicles

Magnetic nanoparticles have been assembled into the bilayer membrane of block copolymer vesicles. The nanoparticles decorate the hydrophobic/hydrophilic interface, which leads to bridging of adjacent bilayers and the formation of oligo-lamellar vesicles. The nanoparticle uptake of the vesicles is sufficiently high to become magnetophoretic in external magnetic fields as shown by video microscopy.