Rühreffekte bei der Vernetzungspolymerisation-Popcornpolymer-und Gelbildung

Stirring vinyl acetate—(1 Mol%) divinyl adipate mixtures during polymerization influences the gel formation. Popcornpolymer polymerization influences the gel formation. Popcorn-polymer formation is highly favored. To the contrary the growth of popcorn polymer particles in the stirred system is slowed down. An explanation of this behavior is offered.

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Mit 3 Abbildungen     

Observation of spin-orbit splitting in lambda single-particle states

The spin-orbit splitting of Lambda single-particle states in (13)(Lambda)C was measured. The 13C(K-,pi(-))(13)(Lambda)C reaction was used to excite both the 1/2(-) and 3/2(-) states simultaneously, which have predominantly 12C(0(+)) x p(Lambda) configuration. gamma rays from the states to the ground state were measured in coincidence with the pi(-)'s, by which ls splitting was found to be 152+/-54(stat)+/-36(syst) keV. The value is 20-30 times smaller than exhibited by the ls splitting in the nuclear shell model. This value gives us new insight into the YN interaction.     

Chromium(VI) binding to functionalized silica/water interfaces studied by nonlinear optical spectroscopy

Organic adlayers can significantly alter the interactions of environmentally relevant surfaces with their surroundings. We present the first second harmonic and broadband sum frequency generation (SHG and BBSFG) study that illustrates how organic surface functional groups can control the mobility of the priority pollutant chromium(VI) in soil: Cr(VI) binds to ester- and acid-functionalized surfaces but not to alkane-functionalized surfaces. The implications with respect to toxic metal transport across organic adlayers at liquid-solid interfaces are that aqueous Cr(VI) can be retained by polar groups common in biopolymers but not by hydrophobic groups common in surfactants.     

Transport and reaction in microscale segmented gas-liquid flow

We use micro particle image velocimetry (microPIV) and fluorescence microscopy techniques to characterize microscale segmented gas-liquid flow at low superficial velocities relevant for chemical reactions with residence times of up to several minutes. Different gas-liquid microfluidic channel networks of rectangular cross section are fabricated in poly(dimethylsiloxane) (PDMS) using soft lithography techniques. The recirculation motion in the liquid segments associated with gas-liquid flows as well as the symmetry characteristics of the recirculations are quantified for straight and meandering channel networks. Even minor surface roughness effects and the compressibility of the gas phase induce loss of symmetry and enhance mixing across the centerline in straight channels. Mixing is further accelerated in meandering channels by the periodic switching of recirculation patterns across the channel center. We demonstrate a new, piezoelectrically activated flow injection technique for determining residence time distributions (RTDs) of fluid elements in multiphase microfluidic systems. The results confirm a narrowed liquid phase RTD in segmented flows in comparison to their single-phase counterparts. The enhanced mixing and narrow RTD characteristics of segmented gas-liquid flows are applied to liquid mixing and in sol-gel synthesis of colloidal nanoparticles.     

Stereocontrol within confined spaces: enantioselective photooxidation of enecarbamates inside zeolite supercages

Dye-exchanged Y zeolite is shown to be an effective medium to control the stereoselectivity in the photooxygenation of chiral oxazolidinone-functionalized Z/E-1 enecarbamates. An enantioselectivity (ee) as high as 80% was observed in the methyldesoxybenzoin (MDB) product, obtained in the methylene-blue-exchanged NaY zeolite at room temperature. The efficacy of the asymmetric induction in the MDB product depends on the Z/E geometry of the alkene, the Z-isomer being more effective than the corresponding E-isomer. The stereoselectivity is rationalized in terms of conformational effects through cationic interactions between the zeolite and the substrate.