Publikationen

We present a comprehensive study on the influence of the synthesis atmosphere on the structure and properties of nanodiamond-derived carbon onions. Carbon onions were synthesized at 1300 and 1700 °C in high vacuum or argon flow, using rapid dynamic heating and cooling. High vacuum annealing yielded carbon onions with nearly perfect spherical shape. An increase in surface area was caused by a decrease in particle density when transitioning from sp3 to sp2 hybridization and negligible amounts of disordered carbon were produced. In contrast, carbon onions from annealing nanodiamonds in flowing argon are highly interconnected by few-layer graphene nanoribbons. The presence of the latter improves the electrical conductivity, which is reflected by an enhanced power handling ability of supercapacitor electrodes operated in an organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile). Carbon onions synthesized in argon flow at 1700 °C show a specific capacitance of 20 F/g at 20 A/g current density and 2.7 V cell voltage which is an improvement of more than 40% compared to vacuum annealing. The same effect was measured for a synthesis temperature of 1300 °C, with a 140% higher capacitance at 20 A/g for argon flow compared to vacuum annealing.

The energy performance of carbon onions can be significantly enhanced by introducing pseudocapacitive materials, but this is commonly at the cost of power handling. In this study, a novel synergistic electrode preparation method was developed by using carbon-fiber substrates loaded with quinone-decorated carbon onions. The electrodes are free standing, binder free, extremely conductive, and the interfiber space filling overcomes the severely low apparent density commonly found for electrospun fibers. Electrochemical measurements were performed in organic and aqueous electrolytes. For both systems, a high electrochemical stability after 10 000 cycles was measured, as well as a long-term voltage floating test for the organic electrolyte. The capacitance in 1 M H2SO4 was 288 F g−1 for the highest loading of quinones, which is similar to literature values, but with a very high power handling, showing more than 100 F g−1 at a scan rate of 2 Vs−1.

Octafunctional spherosilicates were used to prepare self-healing hybrid materials. The hydrosilation of the octakis(hydridodimethylsiloxy)-substituted spherosilicate with furfuryl allyl ether generates an inorganic nano-building-block that is used to formulate various self-healing hybrid materials based on a reversible Diels-Alder reaction. Curing with a molecular bismaleimide results in a hard, glassy but reversibly cross-linkable hybrid material. The reversibility of the curing mechanism allows the preparation of films with a heated press, which also opens the possibility to process the materials by injection molding. Substitution of the molecular cross-linker with an oligomeric poly(dimethylsiloxane) bismaleimide results in an elastomeric material. The kinetics of the Diels-Alder reaction upon cooling after a retro-Diels-Alder reaction are mainly controlled by the mobility of the cross-linker within the system.

In our preliminary work we were able to demonstrate habituation by analyzing attention correlates in singletrial sequences of auditory eventrelated responses (ERPs). Despite dierent quantitative studies of instantaneous phase of ERPs in longterm habituation, there has been no former studies in generative process underlying the distribution of instantaneous phase information in the context of longterm habituation and its relation to attentional binding. For this means we used a von Mises model, representing the phase information over a set of single trial responses. Additionally we use a quantitative neurofunctional model to predict the dynamics of the instantaneous phase in singletrial ERP data during the longterm habituation. Measured habituation data is used to crossvalidate the model's prediction. We conclude that the described method allows for an assessment of dynamic changes in the course of longterm habituation. The results also reinforce our neurofunctional multiscale model of longterm habituation and show the applicability of the described method for the experimental/clinical neurodiagnostic assessment of attentional binding.

Self-healing nanocomposites were studied consisting of surface-functionalized silica nanoparticles and polymers with different glass transition temperatures. The silica nanoparticles were synthesized using the Stöber process and afterwards surface-functionalized with the coupling agents 2-furyl-(undecenyl)-11-triethoxysilane or 3-maleimidopropyltriethoxysilane. The obtained nanoparticles were studied as cross-linking agents in thermally triggered self-healing nanocomposites based on Diels-Alder (DA) chemistry. Cross-linking of the nanoparticles with modified poly(butyl methacrylates) containing DA groups revealed that the rigid thermoplasts show only low tendency to undergo the cross-linking reactions. Contrary, similarly modified polysiloxanes display thermally reversible cross-linking in DSC as well as in rheology measurements. The results reveal that a specific extent of mobility and flexibility of the polymer backbones is crucial for a reasonable fast self-healing process if Diels-Alder groups are used as crosslinking agents.

ConspectusSynthetic polymer chemistry has undergone two major developments in the last two decades. About 20 years ago, reversible-deactivation radical polymerization processes started to give access to a wide range of polymeric architectures made from an almost infinite reservoir of functional building blocks. A few years later, the concept of click chemistry revolutionized the way polymer chemists approached synthetic routes. Among the few reactions that could qualify as click, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) initially stood out. Soon, many old and new reactions, including cycloadditions, would further enrich the synthetic macromolecular chemistry toolbox. Whether click or not, cycloadditions are in any case powerful tools for designing polymeric materials in a modular fashion, with a high level of functionality and, sometimes, responsiveness.Here, we wish to describe cycloaddition methodologies that have been reported in the last 10 years in the context of macromolecular engineering, with a focus on those developed in our laboratories. The overarching structure of this Account is based on the three most commonly encountered cycloaddition subclasses in organic and macromolecular chemistry: 1,3-dipolar cycloadditions, (hetero-)Diels?Alder cycloadditions ((H)DAC), and [2+2] cycloadditions. Our goal is to briefly describe the relevant reaction conditions, the advantages and disadvantages, and the realized polymer applications. Furthermore, the orthogonality of most of these reactions is highlighted because it has proven highly beneficial for generating unique, multifunctional polymers in a one-pot reaction.The overview on 1,3-dipolar cycloadditions is mostly centered on the application of CuAAC as the most travelled route, by far. Besides illustrating the capacity of CuAAC to generate complex polymeric architectures, alternative 1,3-dipolar cycloadditions operating without the need for a catalyst are described. In the area of (H)DA cycloadditions, beyond the popular maleimide/furan couple, we present chemistries based on more reactive species, such as cyclopentadienyl or thiocarbonylthio moieties, particularly stressing the reversibility of these systems. In these two greater families, as well as in the last section on [2+2] cycloadditions, we highlight phototriggered chemistries as a powerful tool for spatially and temporally controlled materials synthesis.Clearly, cycloaddition chemistry already has and will continue to transform the field of polymer chemistry in the years to come. Applying this chemistry enables better control over polymer composition, the development of more complicated polymer architectures, the simplification of polymer library production, and the discovery of novel applications for all of these new polymers.

The thermal transport properties of self-organized Ge nanostructures on Si were studied by means of ultrafast surface sensitive time-resolved electron diffraction. The thermal boundary resistance was determined from the temperature response of the Ge nanostructures upon impulsive heating by fs-laser pulses. The transient temperature was determined through the Debye–Waller effect. Epitaxial growth of Ge hut and dome clusters was achieved by in-situ deposition of 8 monolayers of Ge on Si(001) at 550 °C under ultra-high vacuum conditions. Time-resolved spot profile analysis of different orders of diffraction spots was used to distinguish between the thermal response of hut and dome clusters. Dome clusters of 6 nm height and 50 nm width cool with a time constant of tau = 150ps which agrees well with numerical simulations calculated in the framework of the diffuse mismatch model. The much smaller hut clusters with a height of 2.3 nm and width of 23 nm exhibit a cooling time of tau = 55 ps , which is a factor of 2 slower than predicted by theory.

Unlike liquids, for crystalline solids the surface tension is known to be different from the surface energy. However, the same cannot be said conclusively for amorphous materials like soft cross-linked elastomers. To resolve this issue we have introduced here a direct method for measuring solid-liquid interfacial tension by using the curved surface of a solid. In essence, we have used the inner surface of tiny cylindrical channels embedded inside a soft elastomeric film for sensing the effect of the interfacial tension. When a liquid is inserted into the channel, because of wetting-induced alteration in interfacial tension, its thin wall deflects considerably; the deflection is measured with an optical profilometer and analyzed using the Föppl–von Kármán equation. We have used several liquids and cross-linked poly(dimethylsiloxane) as the solid to show that the estimated values of the solid-liquid interfacial tension matches with the corresponding solid–liquid interfacial energy reasonably well.

Bacterial repellence in suture materials is a desirable property that can potentially improve the healing process by preventing infection. We describe a method for generating nanostructures at the surface of commercial sutures of different composition, and their potential for preventing biofilm formation. We show how bacteria attachment is altered in the presence of nanosized topographies and identify optimum designs for preventing it without compromising biocompatibility and applicability in terms of nanostructure robustness or tissue friction. These studies open new possibilities for flexible and cost-effective realization of topography-based antibacterial coatings for absorbable biomedical textiles.

Fusion of exocytotic vesicles with the plasma membrane gives rise to an increase in membrane surface area, whereas the surface area is decreased, when vesicles are internalized during endocytosis. Changes in membrane surface area, resulting from fusion and fission of membrane vesicles, can be followed by monitoring the corresponding proportional changes in membrane capacitance. Using the cell-attached configuration of the patch-clamp techniques, we were able to resolve the elementary processes of endocytosis and exocytosis in yeast protoplasts at high temporal and spatial resolution. Spontaneous capacitance changes were predominantly in the range of 0.2-1 fF, which translates to vesicles diameters of 90–200 nm. The size distribution revealed that endocytotic vesicles with a median at about 132 nm were smaller than exocytotic vesicles with a median at 155 nm. In energized and metabolizing protoplasts, endocytotic and exocytotic events occurred at frequencies of 1.6 and 2.7 events per minute, respectively. Even though these numbers appear very low, they are in good agreement with the observed growth rate of yeast cells and protoplasts.