Dynamics of terrace formation in a nanostructured thin block copolymer film

We have used dynamic self-consistent field (DSCF) theory to investigate the structural evolution of an ABA block copolymer thin film placed between a solid substrate and a free surface. In line with the few existing theoretical studies for pure homopolymers and mixtures, the free interface is introduced by a void component. In our calculations, the free surface experiences surface roughening and eventually the formation of terraces, as in the experiments. The kinetic pathway of the microstructures was compared to findings of an existing detailed experimental study (Knoll, A.; Lyakhova, K. S.; Horvat, A.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Nat. Mater. 2004, 3, 886) and was found to be equivalent in detail. This corroborates our assumption in this earlier work that the pathway due to changing film thickness is similar to a pathway due to changing surface energetics. Moreover, our calculations show for the first time that microstructural transitions are a driving force of polymer/air interface curving and the formation of terraces.     

Analysis of optodynamic sources generated during laser-induced breakdown in water

We propose a method for evaluating the size of the laser-induced breakdown region in water based on the detection and analysis of optodynamic waves. The breakdown region is an optodynamic source of pressure waves that propagate into the surrounding liquid as an ultrasonic pulse. In the experiment the optical breakdown was generated by a standard ophthalmic Nd:YAG laser with a pulse duration of 10 ns and a maximum energy per pulse of 10 mJ. The pulses were detected inside the liquid with a laser-beam deflection probe. The waveforms were captured in the far-field and analyzed. The analysis provides information about the apparent size of the optodynamic source, which is directly related to the size of the breakdown region. The proposed method can be adapted for online monitoring.     

Interactions of multicationic bis(guanidiniocarbonylpyrrole) receptors with double-stranded nucleic acids: syntheses, binding studies, and atomic force microscopy imaging

Compounds 1-3, composed of two guanidiniocarbonylpyrrole moieties linked by oligoamide bridges and differing in number and type of basic groups, were prepared. The sites and degree of protonation of 1-3 depend strongly on the pH value. The interactions of these compounds with several double-stranded (ds) DNA and dsRNA were investigated by means of UV/Vis and CD spectroscopy as well as isothermal titration microcalorimetry (ITC). These studies revealed that the binding of 1-3 to the polynucleotides is driven by three factors, the presence of aliphatic amino groups, the protonation state of the compounds, and the steric properties of the polynucleotide binding site, that is, the shape and structure of their grooves. The results obtained by all applied methods consistently indicated that receptors 1-3 bind to the minor groove of DNA, but, by contrast, to the major groove of RNA. Additionally, it was shown by atomic force microscopy (AFM) imaging that upon interaction of compound 2 with calf thymus (ct) DNA induced aggregation of the DNA occurs, leading to pronounced changes in its secondary structure.     

A bound on the scale of spacetime noncommutativity from the reheating phase after inflation

In an approach to noncommutative gauge theories, where the full noncommutative behavior is delimited by the presence of the UV and IR cutoffs, we consider the possibility of describing a system at a temperature T in a box of size L. Employing a specific form of UV/IR relationship inherent in such an approach of restrictive noncommutativity, we derive, for a given temperature T  , an upper bound on the parameter of spacetime noncommutativity _ΛNC∼|θ|^−1/2${\Lambda }_{\mathrm{NC}}\sim {|\theta |}^{-1/2}$. Considering such epochs in the very early universe which are expected to reflect spacetime noncommutativity to a quite degree, like the reheating stage after inflation, or believable pre-inflation radiation-dominated epochs, the best limits on _ΛNC${\Lambda }_{\mathrm{NC}}$ are obtained. We also demonstrate how the nature and size of the thermal system (for instance, the Hubble distance versus the future event horizon) can affect our bounds.     

Re‐evaluation of distillation and comparison with HNO3 leaching/solvent extraction for isolation of methylmercury compounds from sediment/soil samples

Distillation was re‐evaluated for the formation of artifacts arising from increasing naturally occurring mercury(II) concentrations, as opposed to previous identification of artifacts by spiking standard mercury(II) into samples. Naturally occurring mercury(II) concentrations lower than 2 µg g^?1 were found not to affect methylmercury (MeHg) results. However, when the natural concentrations of mercury(II) were greater than 2 µg g^?1, in contrast to standard mercury(II) spiked in samples, the MeHg concentrations measured were found to decrease (not increase) with increasing naturally occurring mercury(II) concentrations. This indicated that standard mercury(II) spiked in samples behaved differently from naturally occurring mercury(II) in the formation of MeHg artifacts during distillation. As a result, spiking standard mercury(II) into samples to identify the formation of MeHg artifacts is not adequate. It is difficult to explain why high naturally occurring mercury(II) suppresses MeHg measurements during distillation. In comparison with HNO₃ leaching/solvent extraction (and other existing techniques), distillation was found to generate results comparable for samples containing less than 2 µg g^?1 mercury(II). The HNO₃ leaching/solvent extraction showed significant advantages over other procedures, as this technique generated the highest recoveries with good precision for all samples analyzed, and the results were found to be independent of mercury(II) concentrations for both naturally occurring and spiked standard mercury(II). Thus, except for samples from high mercury‐contaminated fields, distillation is still a good choice. Both the positive bias (possibly caused by artifact formation of MeHg) and the negative bias (due to incomplete leaching, back‐adsorption, and/or decomposition of MeHg) were investigated. Geologically, physically, and chemically different samples were used for the investigation. Copyright © 2004 John Wiley & Sons, Ltd.     

Production of hair intercomparison materials for use in population monitoring programmes for mercury and methylmercury exposure

Ten kilograms of hair obtained from India were used as the basis for two intercomparison materials, one with natural low levels of mercury and methylmercury, and one with an elevated level of methylmercury. The latter was produced by labeling the hair with a solution containing methylmercury. To convert the hair into homogeneous powders, cryogenic milling was utilized. 70% of the final material passed through a 0.075 mm sieve. Subsequent studies were carried out to establish the homogeneity of the materials and the stability of the methylmercury label. The materials will be distributed in an international intercomparison, the results from which will be used to obtain recommended values for total mercury and methylmercury.     

Preliminary study of the effects of some physical parameters on the stability of methylmercury in biological samples.

The effects of storage conditions (long-term storage of wet samples in a deep-freeze or thermal cycling), freeze-drying and gamma-irradiation at 1 and 5 Mrad on the stability of methylmercury in some biological samples were investigated. Methylmercury was determined by volatilization separation followed by gas chromatography and by ion-exchange separation of inorganic and organic species followed by measurement by cold vapour atomic absorption spectrometry (CVAAS). Total mercury was determined by CVAAS. Biological samples studied included fish and shellfish tissues, human hair and blood samples and appropriate reference materials. From the preliminary results obtained it can be concluded that fresh and dried fish muscle and fish certified reference materials show good stability with time and against temperature cycling. Shellfish and blood should not be repeatedly frozen and unfrozen otherwise possible losses of methylmercury can occur. Losses of methylmercury of up to 30% from wet mussels occurred on prolonged storage in a deep-freeze. Gamma-irradiation reduced the methylmercury content of the fish and shellfish only for hake (Merluccius merluccius). Further experiments should be carried out to confirm this and to investigate if this effect is species dependent. Apparent losses of methylmercury on freeze-drying of blood need to be reconfirmed on further samples.     

Apparent Molar Volume and Apparent Molar Expansibility of Rubidium, Cesium, and Ammonium Cyclohexylsulfamate in Aqueous Solution

Summary. The apparent molar volume of rubidium, caesium, and ammonium cyclohexylsulfamate was determined from the density data of their aqueous solutions at 293.15, 298.15, 303.15, 313.15, 323.15, and 333.15 K. From the apparent molar volume, determined at various temperatures, the apparent molar expansibility was calculated. The limiting apparent molar volume and apparent molar expansibility were evaluated and apportioned into their ionic components. The limiting partial molar ionic volumes and expansibilities are discussed in terms of the various effects of the ion in solution on the structure of water. It was shown that the limiting partial molar ionic expansibilities of the alkali-metal cations increase with their ionic radii. The coefficients of thermal expansion of the investigated solutions at 298.15 K were calculated and are presented graphically together with some alkali-metal cyclohexylsulfamates and tetramethylammonium cyclohexylsulfamate. The densities of the investigated solutions can be adequately represented by an equation derived by Redlich.     

Certification of total mercury and methylmercury concentrations in mussel homogenate (Mytilus edulis) reference material, IAEA-142

Due to the increased demand for new reference materials certified for total and methylmercury (MeHg) a sample of mussel homogenate (IAEA-142) has been prepared. Thirteen experienced laboratories reported results for total Hg of which 9 laboratories also reported results for MeHg content. Laboratories reporting MeHg results used various isolation techniques (solvent extraction, saponification, acid leaching, ion-exchange separation, and distillation) and detection systems (cold vapour atomic absorption spectrometry (CV AAS), cold vapour atomic fluorescence spectrometry (CV AFS), gas chromatography with electron capture detector (GC/ECD) and HPLC with CV AAS detector). In the case of total Hg, most of the laboratories used acid digestion, only two used alkaline dissolution, followed either by CV AAS or CV AFS. One laboratory used neutron activation analyses with radiochemical separation. The data received were in good agreement. The value for total Hg was certified to be 126 ng/g, with a 95% confidence interval from 119 to 132 ng/g. For MeHg the certified value of 47 ng/g expressed as Hg was assigned, with a 95% confidence interval from 43 to 51 ng/g. Stability testing has shown that both total and MeHg are stable if samples are stored in a dry and dark place at room temperature. The sample is now available as a certified reference material and is, in particular, useful for quality control measurements of Hg and MeHg in mussel samples at low concentration levels. Received: 24 September 1996 / Revised: 20 November 1996 / Accepted: 8 December 1996