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Paper des Monats
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Chemiluminescence (CL) is a sensitive method of investigating the thermo-oxidative stability of polymers in the early stages of degradation. Therefore, CL measurements were applied to different automotive coatings to evaluate their degradation behavior during artificial weathering. In this work, CL measurements were carried out for different automotive coatings to follow their degradation behavior during artificial weathering. The CL emission depends on sample-specific parameters like stabilizer, matrix, or hardener; experimental parameters like the oxygen partial pressure; and exposure parameters like the duration of weathering. The potential of CL could be demonstrated by classifying materials into categories of good, medium, and low degradation performance at a much earlier stage of weathering exposure than in combination with the usual visual detection of weathering effects.
Thermally stimulated luminescence (TL) can be used to detect damage in the early stages of degradation after artificial weathering, and to analyze the efficiency of stabilizers used in automotive coatings. TL is particularly suitable for this task due to its inherent high sensitivity to structural, morphological, and chemical changes in macromolecular chains. In this work, TL measurements were carried out for different automotive coatings to follow their degradation progression during artificial weathering. The TL results demonstrate high sensitivity to parameters like stabilizers, matrices, hardeners, and weathering time, and provide an opportunity to monitor the early stages of damage in polymers, which therefore reduce the time required for further outdoor weathering tests.
Results of a round-robin experiment in multiple-pulse LIDT measurement with ultrashort pulses
(2004)
In contrast to classical low temperature polymer electrolyte fuel cells (LT-PEFCs), the membrane conductivity in high temperature polymer electrolyte fuel cells (HT-PEFCs) (operating temperature ~ 160 °C) is based on proton transport within phosphorus-oxygen acids at different levels of hydration, orthophosphoric acid (H3PO4) being the simplest example. We present for the first time in-situ synchrotron X-ray radiography measurements applied to a HT-PEFC to gain insight into the local composition of the membrane electrode assembly (MEA) under dynamic operating conditions. Transmission changes during the radiographic measurements exhibit a clear influence of the formation of product water on the membrane composition.
Miniaturized pacemakers with a surface consisting of a Ti alloy may have to be removed after several years from their implantation site in the heart and shall, therefore, not be completely overgrown by cells or tissue. A method to avoid this may be to create at the surface by laser-ablation self-organized sharp conical spikes, which provide too little surface for cells (i.e., fibroblasts) to grow on. For this purpose, Ti-alloy substrates were irradiated in the air by 790 nm Ti:sapphire femtosecond laser pulses at fluences above the ablation threshold. The laser irradiation resulted in pronounced microstructure formation with hierarchical surface morphologies. Murine fibroblasts were seeded onto the laser-patterned surface and the coverage by cells was evaluated after 3–21 days of cultivation by means of scanning electron microscopy. Compared to flat surfaces, the cell density on the microstructures was significantly lower, the coverage was incomplete, and the cells had a clearly different morphology. The best results regarding suppression of cell growth were obtained on spike structures which were additionally electrochemically oxidized under acidic conditions. Cell cultivation with additional shear stress could reduce further the number of adherent cells.
Herein, Ti6Al4V alloy is surface modified by femtosecond laser ablation. The microstructure image obtained by secondary electron microscopy reveals a combination of micrometer spikes or cones superimposed by nanoripples (laser‐induced periodic surface structures). To make the surface hydrophilic, anodization is performed resulting in further smoothness of microstructure and a final thickness of 35 ± 4 nm is estimated for oxide produced after anodization at 10 V (scan rate = 0.1 V s−1) versus standard hydrogen electrode. The obtained electrochemically active surface area (ECSA) is approximately 8 times larger compared with flat mirror polished Ti6Al4V surface. Combined chemical analysis by Pourbaix diagram and X‐ray photoelectron spectroscopy (XPS) analyses reveal that titanium and aluminum are passivating into TiO2 and Al2O3, but the dissolution of aluminum in the form of solvated ion is inevitable. Finally, cell seeding experiments on anodized and laser‐treated titanium alloy samples show that the growth of murine fibroblast cells is significantly suppressed due to unique surface texture of the laser‐treated and anodized titanium alloy sample.
Struktur des basischen Zink-O,O'-ethylhexyldithiophosphats in unpolar, aprotischen Lösungsmitteln
(1995)
The role of 1,1-diphenylethylene (DPE) in radical emulsion polymerization of methyl methacrylate is investigated. The presence of DPE causes a strong decrease in both the rate of polymerization and the molecular weight. According to the results of structure analysis by means of MALDI-TOF mass spectrometry, UVvis spectroscopy, and 1HNMR spectroscopy, DPE is incorporated in the copolymer chain exclusively as a reactive recombinant ?,p-dimer, underlining the peculiar role of DPE in radical polymerizations.
On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses
(2005)
The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated.
The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that
depends quadratically on the pulse fluence and is attributed to twophoton excitation of self-trapped excitons that form after band-to-band
excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant
pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2 , HfO2,
Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.
A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 105 cm2, have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30100 m range from the excision edge at all fluences above the ablation threshold of F th60 J cm2 (I th9 GW cm2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F th1 J cm2 or I th3 TW cm2 (3×1012 W cm2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.