Visual observation of flow irregularities in polymer solutions at theta-conditions

Summary Polystyrene/decalin solutions exhibit flow irregularities in a cone-and-plate rheometer at theta-conditions. The onset of flow irregularities was found at low shear rates. The elastic component (normal stress) is shown to be responsible for the anomalous flow. The onset of flow irregularities has been predicted from measurements of recoverable strain as a function of shear stress. We suggest that this analysis should be suitable to find the best coating fluid leading to the highest possible speed without exhibiting flow irregularities.

---

Zusammenfassung Polystyrol/Dekalin-Lösungen zeigen in einem Kegel-Platte-Rheometer unter Theta-Bedingungen Fließanomalien. Es wurde gefunden, daß diese Fließanomalien bei niedrigen Schergeschwindigkeiten einsetzen, und es wird gezeigt, daß die elastische Komponente (Normalspannung) für das ungewöhnliche Fließverhalten verantwortlich ist. Der Beginn der Fließanomalien konnte aus Messungen der reversiblen Scherung (recoverable strain) als Funktion der Schubspannung vorhergesagt werden. Wir schlagen vor, daß diese Art Analyse ein geeignetes Mittel sein sollte, um dasjenige Beschichtungsfluid zu ermitteln, das die höchstmögliche Beschichtungsgeschwindigkeit erlaubt, ohne daß Fließanomalien auftreten.

---

---

Paper presented at the Annual Meeting of the Deutsche Rheologische Gesellschaft in Aachen, March 5–7, 1979.

---

With 5 figures and 2 tables     

Characterization of hydroxypropyl- methylcellulose-stabilized emulsions

?Emulsions stabilized with hydroxypropylmethylcellulose were characterized with respect to their particle size and their flow behavior. This part of the study focuses on the former.

Active laser scanning revealed that the particle size is independent of the hydroxymethylcellulose content. The critical density of the surface film was determined to be ≤0.01?g/m². This is the basis for the interpretation of the flow behavior, which is described in detail in part II of this contribution.

     

Chiral boron-bridged bisoxazoline (borabox) ligands: structures and reactivities of Pd and Cu complexes

Anionic boron-bridged bisoxazolines (borabox ligands) have been synthesized and characterized in their protonated forms. The ligands are tuneable over a wide range, allowing either alkyl or aryl substituents at the oxazoline rings and the central bridging boron atom. The structural parameters of this new ligand type have been investigated by X-ray analyses of palladium and copper complexes. Electronic properties have been studied by (13)C NMR spectroscopy and by DFT calculations on palladium allyl complexes and compared to those of analogous bisoxazoline (box) complexes. Borabox complexes are more electron-rich at the metal center than their neutral box congeners, and as a consequence of the longer bonds between the bridging atom and the oxazoline rings, their bite angles are larger. Palladium(II) complexes bearing an unsubstituted allyl ligand and homoleptic copper(II) complexes each possess an almost flat chelate ring. NMR analysis of a (1,3-diphenylallyl)(borabox)palladium complex showed a 92:8 mixture of (syn,syn) and (anti,syn) allyl isomers, in contrast with a previously reported box analogue that existed exclusively in the (syn,syn) form. Comparison of the corresponding crystal structures revealed that the distance between the bisoxazoline and the allyl ligand in the borabox complex is shorter. In the copper-catalyzed allylic oxidation of cyclohexene and cyclopentene with tert-butyl perbenzoate, borabox ligands gave results similar-and in some cases superior-to those obtained with analogous box ligands.     

Chiral Diselenides in the Total Synthesis of (+)-Samin

Chiral selenium compounds are applied to stoichiometric as well as to catalytic reactions in the synthesis of substituted tetrahydrofuran derivatives: The selenium compound 1 was used in catalytic amounts for a rapid access to chiral diselenide 3. The efficient stereoselective addition to alkene 5 yields product 8 with a selenium functionality as a precursor for an intramolecular radical cyclization. In this way a short total synthesis of (+)-samin (11), a naturally occurring furofuran lignan, was achieved.     

Characterization of Water-Soluble Cellulose Derivatives in Terms of the Molar Mass and Particle Size as well as Their Distribution

The property profile of cellulose derivatives dissolved in aqueous solvents is not only dependent on the chemical composition (average-, molar- or regiospecific degree of substitution, as well as the substitution along the chain), solvent, temperature and concentration but also on the molar mass and the particle size. All this information can be obtained from the Mark-Houwink-Sakurada-relationship ([;gh]-M-) or the R_G-M-relationship, if these are at hand. These relationships are suitable for a specific degree of substitution. The R_G-M-relationship has only been determined and published for a few water-soluble cellulose derivatives. The prerequisite is the availability of a homologous series of samples with the same chemical composition. In this paper it is shown that only the ultrasonic degradation is able to create such a series. Due to the ability of coupled methods of analysis to acquiring absolute data, molar mass and particle size distributions have been compiled in recent years. Using such methods it was possible to determine molar mass and particle size distributions of several aqueous cellulose derivative solutions by combining a fractionation unit (size exclusion chromatography (SEC) or flow field-flow fractionation (FFFF)) with multi angle laser light scattering (MALLS) for the detection of Mw and R_G and concentration detection (DRI). Results for nonionic cellulose ethers, mixed cellulose ethers, ionic carboxymethyl cellulose, sulfoethyl cellulose, hydrophobically modified hydroxyethyl cellulose were obtained and are partially discussed with focus on the recovery of cellulose derivates after fractionation and the impact on the distribution functions.     

Unusual instability effects observed in ionic and non-ionic water soluble polymers

The solution instability of anionic, cationic and non-ionic water soluble polymers was observed and examined. This instability can be easily observed by measuring the change in the solution viscosity. A viscosity increase with storage time was observed in the case of aqueous polyvinylalcohol solutions. A viscosity decrease with storage time was observed in the case of aqueous polyacrylamide solutions and derivates of polyacrylamide, also in the case of aqueous solutions of polystyrene sulfonate, starch-acrylamide graft copolymers, amylose and poly(L-glutamic) acid. This unusual solution instability was observed when the solutions were stored in the dark, at room temperature, and without mechanical influence. This instability effect is of scientific (molecular origin) and of technical (application) interest. The intention of this paper is to present an overall view of polymers which show this unusual instability effect, to report on the experiments done so far and to discuss possible reasons for this.