A comparison of extensional viscosity measurements from various RME rheometers

The transient uniaxial extensional viscosity η _e of linear low density polyethylene (LLDPE) has been measured using the commercial Rheometric Scientific RME and the Münstedt Tensile Rheometer in an effort to compare the performance of available extensional rheometers. The RME indicated a significant strain hardening of the LLDPE, especially at a strain rate of 1 s⁻¹. In contrast, the Münstedt rheometer showed the LLDPE to be only slightly strain hardening. This artificial strain hardening effect in the RME resulted from the strain rate applied to the sample, determined from the sample deformation, being up to 20% less than the set strain rate. These results initiated a round-robin experiment in which the same LLDPE was tested on several RMEs in various locations around the world. All but one of the RMEs indicated a deviation between set and applied strain rates of at least 10%, especially at strain rates above 0.1 s⁻¹. The strain rate deviation was found to depend strongly on the value of the basis length L ₀ , and may result from the upper pair of belts not properly gripping the sample during extension. Thus visual inspection of the sample deformation is necessary to determine the applied strain rate. The most accurate measurements of η _e with respect to the strain rate deviation were obtained when the correct L ₀ value and belt arrangement were used. A list of recommendations for running an RME test is provided. Future work focusing on the fluid mechanics during the test may identify fully the cause of the strain rate deviation, but from a practical point of view the problem can be corrected for in the determination of η _e . Received: 27 September 2000/Accepted: 5 February 2001     

Speeding up Viedma Deracemization through Water-catalyzed and Reactant Self-catalyzed Racemization

Viedma deracemization is based on solution phase racemization, dissolution of racemic or scalemic conglomerates and crystal growth through autocatalytic cluster formation. With rate limiting racemization, its acceleration by appropriate catalysts may result in speeding up deracemization. A conglomerate-forming chiral compound may principally racemize directly, or via reverse of its formation reaction. For a hydrazine derivative, we investigated available racemization pathways in presence of pyrrolidine or thiourea amine as base catalysts: via Mannich or aza-Michael reaction steps and their reverse, or by enolization. Racemization by enolization was computationally found to dominate, both under water-free conditions and in presence of water, involving a multitude of different pathways. Faster racemization in presence of water resulted indeed in more rapid deracemization, when the base was pyrrolidine. Under water-free conditions, the role of water as enolization catalyst is assumed by chiral hydrazine itself – in autocatalytic racemization and in which both reactant and product are catalysts.     

The origin dependence of the material constants: the permittivity and the inverse permeability

New derivations of origin-independent expressions for the electric permittivity are presented, starting either from the response function of the current density that defines the absorption coefficient, or from the off-resonance single-photon scattering amplitude that leads to the Kramers–Heisenberg dispersion formula. The resulting expression for the permittivity is compared with earlier work on the origin dependence of the material constants. Different origin-independent expressions for the permittivity, the inverse permeability and the magnetisability are calculated and discussed. By considering electromagnetic plane waves in the absence of external sources, the macroscopic Maxwell equations are used to describe the response of matter to external fields. In combination with the constitutive relations, a wave equation expressed in terms of the material constants is derived. It is shown that the different definitions of the material constants lead to the same wave equation. The non-uniqueness of the definitions of the material constants is discussed in this context. Finally, based on the discussions, we propose a possible unique, origin-independent definition of the material constants.     

Structure stability and crystallization behavior of water in oil in water (WOW) double emulsions during their characterization by differential scanning calorimetry (DSC)

Journal of Thermal Analysis and Calorimetry - Water in oil in water (WOW) double emulsions are of great interest for the encapsulation of valuable substances. Nevertheless, the stabilization...     

Regioselective Cleavage of Thioether Linkages in Microcystin Conjugates

Microcystins are cyanobacterial toxins that can be found in fresh and coastal waters during algal blooms. Microcystin contamination of water can cause severe poisoning of animals and humans. Quantification of these toxins in biological samples is complicated because a major proportion of microcystins is covalently linked to proteins through thioether bonds formed through a Michael‐type addition of cysteine residues of proteins to an N‐methyldehydroalanine residue in the microcystins. We investigated chemical methods that can be used to cleave such thioether bonds by means of an elimination reaction that leaves the microcystin backbone intact for subsequent analysis. The known reagent O‐mesitylenesulfonylhydroxylamine (MSH) led to regioselective thioether cleavage, but a large excess of reagent was needed, thus making purification challenging. An unexpected side reaction observed during the investigation of the base‐induced elimination inspired us to develop a new thioether‐cleavage methodology based on the addition of propargylamine as a nucleophile that can trap the elimination product. This methodology could be successfully applied to the quantitative cleavage of a microcystin‐LF–glutathione conjugate. The alkyne moiety introduced by this procedure offers the possibility for further reactions with azides by using click chemistry, which might be useful for the derivatization or isolation of microcystins.