Quasi-elastic light scattering from polymethylmethacrylate in a good and a theta solvent

Summary Translational diffusion coefficients of 9 PMMA samples have been measured by quasi-elastic light scattering in acetone at 20 °C and butylchloride at 34.5 °C (theta solvent). The coil expansion due to excluded volume effects has been characterized by the expansion factor _h of the hydrodynamic radiusR _h . _h has been found to reach avalue well below 1.0 atM _w M _o (the monomer). By means of CowieBywater plots on the other hand the unperturbed chain dimensions are shown being virtually identical in both solvents. Thus, the _h < 1.0 values is the result of differences in the hydrodynamic radii of the segments in the two solvents. The ratioS/R =S ² =^1/2/R _b is found being lower than predicted by theory. Reasons for this behaviour are discussed. The concentration dependence ofD has been evaluated on the basis of a theory by Pyun and Fixman. A reasonably good agreement with the simple models of a hard and a soft sphere in a good and a poor solvent, respectively, is observed.

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Zusammenfassung Die translatorische Diffusionskoeffizienten von 9 verschiedenen PMMA Proben wurden in Aceton bei 20 °C und in Butylchlorid bei 34.5 °C (Thetatemperatur)|durch quasielastische Lichtstreuung bestimmt. Die durch das ausgeschlossene Volumen verursachte Knäuelexpansion wurde durch einen Expansionsfaktor ah des hydrodynamischen RadiusR _h beschrieben. _h erreicht beiM _w M _o (MonomerMolekulargewicht) Werte deutlich unterhalb von 1.0. Aus der Cowie-Bywater Auftragung wurden andererseits keine unterschiedlichen ungestörten Dimensionen in den beiden Lösungsmitteln erhalten. Hieraus wird der Schluß gezogen, daß _h < 1.0 eine Folge unterschiedlicher hydrodynamischer Radien der Segmente ist. Das VerhältnisS/R =S ²> ^1/2/R _h erwies sich nach den Messungen kleiner als die Theorie erwarten ließ. Mögliche Gründe für dieses Verhalten werden diskutiert. Die Konzentrationsabhängigkeit der Diffusionskoeffizienten wurde anhand der Theorie vonPyun undFixman ausgewertet. Es wurde eine zufriedenstellende Übereinstimmung mit den von den Autoren verwendeten einfachen Modellen der harten bzw. weichen Kugel in dem guten bzw. schlechten Lösungsmittel gefunden.

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Dedicated to Professor Dr. G. Rehage on the occasion of his 60th birthday.

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With 8 figures and 4 tables     

Measurement of the charged pion electromagnetic form factor

Separated longitudinal and transverse structure functions for the reaction 1H(e,e(')pi(+))n were measured in the momentum transfer region Q2 = 0.6--1.6 (GeV/c)(2) at a value of the invariant mass W = 1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parametrization fitted to very low Q2 elastic data.     

Competitive clustering in a bidisperse granular gas

A bidisperse granular gas in a compartmentalized system is experimentally found to cluster competitively: Depending on the shaking strength, the clustering can be directed either towards the compartment initially containing mainly small particles or to the one containing mainly large particles. The experimental observations are quantitatively explained within a flux model.     

Single-shot electron-beam bunch length measurements

We report subpicosecond electro-optic measurements of the length of individual relativistic electron bunches. The longitudinal electron-bunch shape is encoded electro-optically on to the spectrum of a chirped laser pulse. The electron-bunch length is determined by analyzing individual laser-pulse spectra obtained with and without the presence of an electron bunch. Since the length of the chirped laser pulse can be easily changed, the electron bunch can be visualized on different time scales. This single-shot imaging technique is a promising method for real-time electron-bunch diagnostics.     

Using regression analysis to model the performance of UK Coastguard centres

This paper combines the use of (binary) logistic regression and stochastic frontier analysis to assess the operational effectiveness of the UK Coastguard (Maritime Rescue) coordination centres over the period 1995–1998. In particular, the rationale for the Government's decision—confirmed in 1999—to close a number of coordination centres is scrutinized. We conclude that the regression models developed in this paper represent a performance measurement framework that is considerably more realistic and complex than the one apparently used by the UK Government. Furthermore, we have found that the coordination centres selected for closure were not necessarily the ones that were least effective in their primary purpose—that is, to save lives. In a related paper, we demonstrate how the regression models developed here can be used to inform the application of data envelopment analysis to this case.     

Hollow fiber ultrafiltration: The concept of partial backwashing

In dead-end hollow fiber filtration with humic acid solutions, we observed that the humic material accumulates in the final part of the fiber. We proved the hypothesis that backwashing of the last part of the fiber is sufficient to operate the filtration process in a sustainable manner. This strategy works very well for feed solutions that result in either local accumulation of the material at the end of the fiber, or that form a well-defined concentration polarization layer that does not lead to deposition. When deposition throughout the whole module occurs, the proposed cleaning strategy is unsuccessful. This overall deposition occurs when a force balance over the particle, results in net transport to the membrane surface over the entire length of the module. We simulated such conditions by the addition of calcium to the feed solutions. The latter results in large aggregates carried towards the whole membrane area by convective water flow, and an increased interaction between the aggregates and the membrane.     

A Dicobalt Complex with an Unsymmetrical Quinonoid Bridge Isolated in Three Units of Charge: A Combined Structural, (Spectro)electrochemical,Magnetic and Spectroscopic Study

Quinonoid ligands are excellent bridges for generating redox‐rich dinuclear assemblies. A large majority of these bridges are symmetrically substituted, with examples of unsymmetrically substituted quinonoid bridges being extremely rare. We present here a dicobalt complex in its various redox states with an unsymmetrically substituted quinonoid bridging ligand. Two homovalent forms and one mixed‐valent form have been isolated and characterized by single crystal X‐ray diffraction. The complex displays a large comproportionation constant for the mixed‐valent state which is three orders of magnitude higher than that observed for the analogous complex with a symmetrically substituted bridge. Results from electrochemistry, UV/Vis/NIR spectroelectrochemistry, SQUID magnetometry, multi‐frequency EPR spectroscopy and FIR spectroscopy are used to probe the electronic structures of these complexes. FIR provides direct evidence of exchange coupling. The results presented here display the advantages of using an unsymmetrically substituted bridge: site specific redox chemistry, high thermodynamic stabilization of the mixed‐valent form, isolation and crystallization of various redox forms of the complex. This work represents an important step on the way to generating heterodinuclear complexes for use in cooperative catalysis.     

Benchmarking of electro-optic monitors for femtosecond electron bunches

The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties.