Chemical exchange in KHSeO4 and NH4HSeO4 studied by two-dimensional NMR

The microscopic mechanism of proton transport in partially deuterated potassium hydrogen selenate (KHSe) and in partially deuterated ammonium hydrogen selenate (AHSe) were studied by means of one-dimensional Fourier transform²H nuclear magnetic resonance (NMR), two-dimensional²H NMR and dielectric measurements over a wide temperature range. In both systems, KHSe and AHSe, the slow chemical exchange processes of deuterons between different hydrogen bridges occur. It was established that the rates of exchange between deuteron sites, which are involved in infinite chains of hydrogen bonds, are approximately the same for both crystals. The rates of exchange between these positions and the deuterons in the dimer groups of KHSe are approximately hundred times more slowly. On the basis of our findings, we discuss the models of the microscopic mechanism of hydrogen transport for both substances.     

Condensation of steam on the surface of hard coated copper discs

At atmospheric pressure filmwise (FWC) and dropwise (DWC) condensation have been studied on the surface of copper discs which were coated by silicon-modified amorpheous hydrogenated carbon (a-C?:?H-Si) films of different thickness. On vertically oriented surfaces the DWC heat transfer coefficients were found to be larger by a factor of about 10 than the FWC coefficients which follow as function of surface subcooling temperature quite well Nusselt’s theory. Varying the angle of surface inclination, the DWC coefficient decreased down to about 40% of the vertical-surface values for 180° (face down orientation). The mean value for all inclination angles between 30° and 180° was calculated to be 87.6% of the maximum value for the 90°-orientation. Partly coating of the copper surface indicates a strong heat transfer enhancement of DWC over FWC even for relative small coated parts (e.g., 19%-coating yields an enhancement by a factor 2.3 for a cooling water flow rate of 4?m³/h). The diamond like properties of the a-C?:?H-coatings promise long stand times and thus application also in real technical condensation systems.     

Influence of the crosslink network structure on stress‐relaxation behavior: Viscoelastic modeling of the compression set experiment

A viscoelastic approach of the compression set test is addressed in this work. This test measures the ability of rubber compounds to retain elastic properties after prolonged action of compressive stresses. Elastic properties were tested by recording the normal stress under a constant deformation of 25% with a laboratory rheometer. Considering the Boltzmann superposition principle, compression set data were modeled from the relaxation of Young's modulus, described by a Maxwell spectrum plus a constant E_∞ defining the elastic properties at the long times. This approach was developed with the copolymer of ethylene and vinyl acetate (EVA) networks crosslinked by radical chemistry and by an exchange reaction between acetate groups and silane compounds as crosslinking agents. Regarding the recovery of the elastic properties, radical chemistry provided better results than the exchange reaction for the identical crosslinking density of the network. Then, the Curro–Pincus molecular approach was developed to understand the influence of the microstructure of the EVA network on the elastic properties. The difference of the elastic properties between the two networks crosslinked by two different chemistry means was accounted for by considering the probability of having a dangling end of n units for a random crosslinking process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1779–1790, 2003     

Explicit rational forms for the Poincaré series of the trace rings of generic matrices

In this paper we derive explicit rational forms for the Poincaré series of the commutative and the non-commutative trace rings (5.3 and 5.5). To this end, we use the Molien-Weyl formula to reduce the question to a problem about flows in a particular graph. The author is supported by an NFWO grant.     

Hydroxylamine-functionalised silica surfaces for biochip applications

The syntheses of triethoxy and trimethoxy silanes possessing an unprotected hydroxylamine group are described. The grafting of these coupling agents at the surface of oxidised silicon wafers was studied. Accessibility of the hydroxylamine group at the surface was demonstrated with chemical reagents, and the surface proved efficient for covalent immobilisation of peptides possessing the COCHO function.