Andrea Prager

Ring‐Opening Metathesis Polymerization Based Pore‐Size‐Selective Functionalization of Glycidyl Methacrylate Based Monolithic Media: Access to Size‐Stable Nanoparticles for Ligand‐Free Metal Catalysis

Monolithic polymeric supports have been prepared by electron-beam-triggered free-radical polymerization using a mixture of glycidyl methacrylate and trimethylolpropane triacrylate in 2-propanol, 1-dodecanol, and toluene. Under appropriate conditions, phase separation occurred, which resulted in the formation of a porous monolithic matrix that was characterized by large (convective) pores in the 30 μm range as well as pores of <600 nm. The epoxy groups in pores of >7 nm were hydrolyzed by using poly(styrenesulfonic acid) (Mw = 69,400 g mol(-1), PDI=2.4). The remaining epoxy groups inside pores of <7 nm were subjected to aminolysis with norborn-5-en-2-ylmethylamine (2) and provided covalently bound norborn-2-ene (NBE) groups inside these pores. These NBE groups were then treated with the first-generation Grubbs initiator [RuCl2 (PCy3 )2 (CHPh)]. These immobilized Ru-alkylidenes were further used for the surface modification of the small pores by a grafting approach. A series of monomers, that is, 7-oxanorborn-5-ene-2,3-dicarboxylic anhydride (3), norborn-5-ene-2,3-dicarboxylic anhydride (4), N,N-di-2-pyridyl-7-oxanorborn-5-ene-2-carboxylic amide (5), N,N-di-2-pyridylnorborn-5-ene-2-carboxamide (6), N-[2-(dimethylamino)ethyl]bicyclo[2.2.1]hept-5-ene-2-carboxamide (7), and dimethyl bicyclo[2.2.1]hept-5-en-2-ylphosphonate (8), were used for this purpose. Finally, monoliths functionalized with poly-5 graft polymers were used to permanently immobilize Pd(2+) and Pt(4+), respectively, inside the pores. After reduction, metal nanoparticles 2 nm in diameter were formed. The palladium-nanoparticle-loaded monoliths were used in both Heck- and Suzuki-type coupling reactions achieving turnover numbers of up to 167,000 and 63,000, respectively.

Electron Beam-Based Functionalization of Poly(ethersulfone) Membranes

Poly(ethersulfone) membranes were surface modified in a one-step procedure. For this purpose, the membranes were soaked with aqueous solutions of different low-molecular weight molecules bearing diverse hydrophilic functionalities and subject to electron beam treatment. No catalysts, photoinitiators, organic solvents or other toxic reagents were used, and no additional synthetic or purification steps were required.

Biocompatible polysaccharide-based cryogels.

This study focuses on the development of novel biocompatible macroporous cryogels by electron-beam assisted free-radical crosslinking reaction of polymerizable dextran and hyaluronan derivatives. As a main advantage this straightforward approach provides highly pure materials of high porosity without using additional crosslinkers or initiators. The cryogels were characterized with regard to their morphology and their basic properties including thermal and mechanical characteristics, and swellability. It was found that the applied irradiation dose and the chemical composition strongly influence the material properties of the resulting cryogels. Preliminary cytotoxicity tests illustrate the excellent in vitro-cytocompatibility of the fabricated cryogels making them especially attractive as matrices in tissue regeneration procedures.

Vacuum‐UV Irradiation‐Based Formation of Methyl‐Si‐O‐Si Networks from Poly(1,1‐Dimethylsilazane‐co‐1‐methylsilazane)

The vacuum-UV (VUV)-induced conversion of commercially available poly(1,1-dimethylsilazane-co-1-methylsilazane) into methyl-Si-O-Si networks was studied using UV sources at wavelengths around 172, 185, and 222 nm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS), X-ray photo electron spectroscopy (XPS), and Fourier transform infrared (FTIR) measurements, as well as kinetic investigations, were carried out to elucidate the degradation process. First-order kinetics were found for the photolytically induced decomposition of the Si-NH-Si network, the subsequent formation of the methyl-Si-O-Si network and the concomitant degradation of the Si-CH(3) bond, which were additionally independent of the photon energy above a threshold of about 5.5 eV (225 nm). The kinetics of these processes were, however, dependent on the dose actually absorbed by the layer and, in the case of Si-O-Si formation, additionally on the oxygen concentration. The release of ammonia and methane accompanied the conversion process. Quantum-chemical calculations on methyl substituted cyclotetrasilazanes as model compounds substantiate the suggested reaction scheme. Layers <100 nm in thickness based on mixtures of poly(1,1-dimethylsilazane-co-1-methylsilazane) and perhydropolysilazane (PHPS) were coated onto polyethylene terephthalate (PET) foils by a continuous roll to roll process and cured by VUV irradiation by using wavelengths <200 nm and investigated for their O(2) and water vapor-barrier properties. It was found that the resulting layers displayed oxygen and water vapor transmission rates (OTR and WVTR, respectively) of <1 cm(3) m(-2) d(-1) bar(-1) and <4 g m(-2) d(-1), respectively.

Electron beam-based functionalization of polymer membranes

A new electron beam-based approach for the direct functionalization of polyethersulfone, polyvinylidene fluoride, polysulfone as well as polyacrylonitrile membranes in a one-step procedure is presented. Aqueous solutions of functional molecules were immobilized on the membrane surface by electron beam treatment. The resulting membranes show significantly increased flux and water wettability accompanied by decreased protein adsorption. Stability tests demonstrated the permanence of the modification. This new method neither requires any preceding surface functionalization nor the use of catalysts/photoinitiators or other toxic reagents. In addition, it avoids the synthesis of hydrophilic monomers/polymers, thus avoiding additional synthetic and purification steps as well as the use of organic solvents.

Electron-beam derived polymeric cryogels

In this study we publish a novel attempt to macroporous, polymeric cryogels (MPCs) by electron-beam (EB) initiated free radical crosslinking polymerization of (meth)acrylates in frozen aqueous media. The EB-process is an environmentally benign fast process (radiation and reaction time about 10–20 min) which allows the initiator-free polymerization of double-bond containing monomers. The MPCs were thoroughly characterized by swelling experiments, SEM, detailed XPS studies, DMA, Hg intrusion porosity and ESEM. The processes during the reaction were studied using ESR experiments. The influences of the reaction parameters like dose, radiation time, freezing conditions, chemical composition and (meth)acrylate concentration on the properties of the MPCs were investigated. The MPCs were synthesized in situ in capillaries or in tubes up to a diameter of 25 mm. The porous structure of the MPCs is mainly controlled by the freezing temperature and the macromonomer/crosslinker concentration and consists of large interconnected pores in the range of 10–70 μm. The crosslinker concentration directly affects the stiffness and the Tg of the materials. The MPCs were successfully modified with poly(allylamine hydrochloride). This straightforward method provides a toolbox for the synthesis of manifold MPCs from various unsaturated substances in large quantities and scale.

Bio-Inspired Polymer Membrane Surface Cleaning

To generate polyethersulfone membranes with a biocatalytically active surface, pancreatin was covalently immobilized. Pancreatin is a mixture of digestive enzymes such as protease, lipase, and amylase. The resulting membranes exhibit self-cleaning properties after “switching on” the respective enzyme by adjusting pH and temperature. Thus, the membrane surface can actively degrade a fouling layer on its surface and regain initial permeability. Fouling tests with solutions of protein, oil, and mixtures of both, were performed, and the membrane’s ability to self-clean the fouled surface was characterized. Membrane characterization was conducted by investigation of the immobilized enzyme concentration, enzyme activity, water permeation flux, fouling tests, porosimetry, X-ray photoelectron spectroscopy, and scanning electron microscopy.

Peptide-Mediated Specific Immobilization of Catalytically Active Cytochrome P450 BM3 Variant

Cytochrome P450 BM3 (CYP102A1) from Bacillus megaterium is an interesting target for biotechnological applications, because of its vast substrate variety combined with high P450 monooxygenase activity. The low stability in vitro could be overcome by immobilization on surfaces. Here we describe a novel method for immobilization on metal surfaces by using selectively binding peptides. A P450 BM3 triple mutant (3M-P450BM3: A74G, F87V, L188Q) was purified as protein thioester and ligated to indium tin oxide or gold binding peptides (BP) named HighSP-BP and Cys-BP, respectively. The ligation products were characterized by Western Blot and tryptic digestion combined with mass spectrometry, and displayed high affinity binding on the depicted surfaces. Next, we could demonstrate by benzyloxyresorufin O-dealkylation assay (BROD assay) that the activity of immobilized ligation products is higher than for the soluble form. The study provides a new tool for selective modification and immobilization of P450 variants.

Mechanistic aspects of the radiation-chemical reduction of graphene oxide to graphene-like materials.

Abstract Purpose: The aim of the work was to investigate mechanistic details of the preparation of graphene-like materials (GLM) via reduction of graphene oxide (GO) in aqueous dispersions by electron beam (EB) generated reducing free radicals. Materials and methods: A 10 MeV linear accelerator was employed to irradiate aqueous GO dispersions at ambient temperatures. The kinetics of GO reduction was followed using UV-Vis spectroscopy. The resulting GLM were characterized by X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), Raman spectroscopy and conductivity measurements. Results: The reduction of GO could be afforded with high efficiency within minutes at room temperature via the reaction of GO with reducing radicals generated by EB irradiation. The detailed investigation of the reduction mechanism allowed a selection of the best reducing free radicals in terms of both their efficiency and environmental impact of their precursors and final products. Conclusions: The EB-treatment of aqueous GO dispersions is a highly efficient, environmentally friendly, cost-effective and easily up-scalable method for the preparation of GLM. The efficiency of the new reduction approach is comparable with the best existing methods.

Membrane Functionalization with Hyperbranched Polymers

Polymer membranes have been modified with hyperbranched polymers with the aim to generate a high density of hydrophilic functional groups at the membrane surface. For this purpose hyperbranched polymers containing amino, alcohol, and carboxylic acid end groups were used for membrane modification, respectively. Thus, surface potential and charges were changed significantly to result in attractive or repulsive interactions towards three different proteins (albumin, lysozyme, myoglobin) that were used to indicate membrane fouling properties. Our studies demonstrated that hydrophilization alone is not effective for avoiding membrane fouling when charged proteins are present. In contrast, electrostatic repulsion seems to be a general key factor.