Haiko Didzoleit

One for all: cobalt-containing polymethacrylates for magnetic ceramics, block copolymerization, unexpected electrochemistry, and stimuli-responsiveness

Novel cobalt-containing homo- and diblock copolymers with poly(methyl methacrylate) (PMMA) are synthesized by atom transfer radical polymerization (ATRP) of a neutral cobalt-complex methacrylate. An efficient route for a single-step synthesis of the cobalt precursor based on easily-available starting materials followed by esterification with methacrylic acid is presented. The cobalt-methacrylate monomer is furthermore polymerized by thermal, free radical and statistical copolymerization with MMA and investigated with respect to (absolute) molar masses, polymer composition, and thermal properties. ATRP affords block copolymers as evidenced by 1H NMR spectroscopy, size exclusion chromatography (SEC) and differential scanning calorimetry (DSC). The cobalt-containing homopolymers are investigated and tailored with respect to their thermal conversion into magnetic cobalt oxides and elemental cobalt which is evidenced by X-ray diffraction (XRD), Raman spectroscopy, and superconducting quantum interference device (SQUID) magnetometry measurements. The (reversible) electrochemistry of the cobalt-containing polymethacrylates and block copolymers thereof are thoroughly addressed by cyclic voltammetry (CV) studies. Interestingly, the prepared metalloblock copolymers exhibit redox-responsiveness (both reduction and oxidation) and thus structure formation in the presence of a reduction or oxidation reagent are demonstrated by transmission electron microscopy (TEM).

Surface‐Initiated Anionic Polymerization of [1]Silaferrocenophanes for the Preparation of Colloidal Preceramic Materials

A novel strategy for the preparation of poly(ferrocenylsilane) (PFS) immobilized on the surface of cross-linked polystyrene (PS) nanoparticles is reported. The ferrocene-containing core/shell architectures are shown to be excellent candidates as preceramic polymers yielding spherical ceramic materials consisting of iron silicide (Fe3 Si) and metallic iron after thermal treatment. For this purpose, dimethyl- and hydromethyl[1]silaferrocenophane monomers are polymerized by surface-initiated ring-opening polymerization upon taking advantage of residual vinylic moieties at the PS particle surface. A strategy for selective chain growth from the particle surface is developed without the formation of free PFS homopolymer in solution. The grafted particles are characterized using transmission electron microscopy (TEM) and dynamic light scattering (DLS). These particles are excellent precursors for ceramics as studied by thermogravimetric analysis (TGA). The composition of the ceramics is studied using X-ray diffraction (XRD) measurements, while the morphology is probed by scanning electron microscopy (SEM) revealing the original spherical shape of the precursor particles. Obtained ceramic materials- predominantly based on iron silicides-show ferromagnetic behavior as investigated by superconducting quantum interference device (SQUID) magnetization measurements at different temperatures.

Controlling Polymerization Initiator Concentration in Mesoporous Silica Thin Films

We present a strategy toward controlled polymer density in mesopores by specifically adjusting the local amount of polymerization initiator at the pore wall. The polymerization initiator concentration as well as the polymer functionalization has a direct impact on mesoporous membrane properties such as ionic permselectivity. Mesoporous silica-based thin films were prepared with specifically adjusted amount of polymerization initiator (4-(3-triethoxysilyl)propoxybenzophenone (BPSilane)) or initiator binding functions ((3-aminopropyl)triethoxysilane (APTES)), directly and homogeneously incorporated into the silica wall pursuing a sol-gel-based co-condensation approach. The amount of polymerization initiator was adjusted by varying its concentration in the sol-gel precursor solution. The surface chemistry, porosity, pore accessibility, and reactivity of the surface functional groups were investigated by using infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray reflectometry, ellipsometry, atomic force microscopy, and transmission electron microscopy. We could gradually modify the amount of reactive polymerization initiators in these mesoporous membranes. Mesopores were maintained for APTES containing films for all tested ratios up to 25 mol % and for BPSilane containing films up to 15 mol %. These films showed accessible and charge-dependent ionic permselectivity and an increasing degree of functionalization with increasing precursor ratio. This approach can directly result in control of polymer grafting density in mesoporous films and thus has a direct impact on applications such as the control of ionic transport through mesoporous silica membranes.

On the supramacromolecular structure of core–shell amphiphilic macromolecules derived from hyperbranched polyethyleneimine

The supramacromolecular structure of core-shell amphiphilic macromolecules (CAMs) with hyperbranched polyethyleneimine (HPEI) cores and fatty acid chain shells (HPEI-Cn) for different chain lengths was investigated both, in colloidal suspension, solid phase and at the air-water interface using Small Angle X-ray Scattering (SAXS), Wide Angle X-ray Scattering (WAXS), X-ray Reflectometry (XRR) and Langmuir isotherms. At low temperatures colloidal toluene suspensions of the HPEI-Cn polymers form, as evidenced by peaks arising in the structure factor of the system showing mean particle-to-particle distances correlated with the length of the aliphatic chains forming the shells of HPEI-Cn unimicelles. The CAM sizes as found from the SAXS experiments also display a clear dependence on shell thickness suggesting that the aliphatic chains adopt a brush-like configuration. After solvent extraction, HPEI-Cn adopts ordered structures with hexagonal packing of the aliphatic chains. Submitted to lateral pressure Π at the air-water interface, HPEI-Cn undergoes a disorder-order transition with increasing transition pressure for increasing chain lengths. The CAMs show different behaviors in-plane and out-of-plane. While out-of-plane the aliphatic chains behave as a brush remaining almost fully unfolded, whereas parallel to the air-water interface the chains fold down in a mushroom way with increasing lateral pressure Π.