Some properties of aqueous xanthan gels formed under the action of aluminium(III) ions

Aqueous xanthan solutions form gels when aluminium salts are added and the solutions are heated above 45 °C. The gelation process was followed by dynamic light scattering. Characterization was based on the heterodyne and nonergodic approaches. Both techniques gave the same fast relaxation times, but for the slow motion much larger values were found in the heterodyne than in the nonergodic approach. The heterodyne fraction 1-X was found to correlate closely with the plateau height of the time correlation function (TCF) at large delay times in the nonergodic experiments. Three methods of gel point determination are demonstrated: (i) onset of heterodyne/nonergodic behavior, (ii) observation of a sharp maximum for the fast relaxation time at the gel point, (iii) observation of power-law behavior of the TCF. The statistics of nonergodic fluctuations were examined and evaluated. The potential of this procedure for detailed structure evaluation of inhomogeneities in the gel is emphasized.     

Polymer characterization: Quasi-elastic and elastic light scattering

Dynamic or quasi-elastic light scattering (LS) from polymers in solution arises from concentration fluctuations. With the aid of modern photomultipliers these can be followed as a function of time. A proper evaluation allows to study the center of mass motion and the dynamics of individual chains. The relevance of simultaneous recording of static and dynamic LS is emphazised. Various aspects are discussed in three main sections. In the first part basic relationships are reviewed. The particle scattering factor P(8), structure factor S(q,c) and osmotic compressibility RT(∂c/∂T) occuring in static LS are defined. The time correlation functions (TCF) in dynamic LS are described. The TCF of the scattered electric field contains the time dependent structure factor S(q,t) and the static structure factor S(q). The initial part of the TCF (short delay times) can be approximated by a cumulant expansion; the first cumulant is related to the translational diffusion coefficient D. The concentration dependence of D contains a thermodynamic and a hydrodynamic contribution where the thermodynamic part is identical with the osmotic compressibility in static LS. The second part deals with the behaviour of various polymeric architectures in dilute solutions. Two new structure sensitive parameters, C and = Rg/Rh, are introduced. Chain stiffness and branching are extensively discussed. In the third part properties of different macromolecular architectures in semi-dilute solution are considered. The inverse osmotic compressibility = osmotic modulus and the concentration dependence of the translational diffusion coefficient are discussed in the light of re-normalization group and scaling theories.     

The significance of transfer reactions in pulsed laser polymerization experiments

In this work simulation calculations are presented, which carefully analyze pulse initiated polymerization experiments of butyl acrylate in bulk in an extended temperature range published in the recent literature. Taking into account entire data sets of experimental results, a model has been developed which describes the experiments using a single parameter set. Especially the inadequacies in experiments are also captured by the model, that occur evaluating propagation rate coefficients using the pulsed laser polymerization technique at low laser repetition rates and elevated temperatures. Enhanced chain transfer to small species is identified to be responsible for these effects and transfer rate constants are derived from the simulations. The model is then used to test experimental strategies in order to expand the k_p determination towards temperatures higher than 35°C, the maximum temperature for which k_p values of butyl acrylate are available so far. Performing pulsed laser experiments at high laser repetition rates (200 Hz) and initial radical concentrations (1 · 10⁻⁴ mol/L) should prevent the formation of the characteristic structure in the molecular weight distribution to be suppressed by this competing process.     

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