What is the flow activation volume of entangled polymer melts?

We evaluate the flow activation volume in polymer melts of isotactic polypropylene and atactic polystyrene with step-shear experiments at different melt temperatures. The melt is initially sheared with constant shear rate until the attainment of a melt state with nearly constant viscosity. Perturbations to this experiment, involving shear steps in short-time intervals with decreasing rates, are induced next. Measurements of the shear stress value at each shear rate step allow the evaluation of an experimental (apparent) flow activation volume. The true flow activation volume is evaluated by extrapolating the experimental data to infinite shear stress values. The value obtained is larger than the physical volume of the chain and agrees with the volume of a tube confining chains with a molecular weight between M _n and M _w. Besides supporting the validity of tube model, experiments based on this protocol may be used on model polymer samples, in composites with nanoparticles and in polymer blends to access the validity of mechanisms considered by flow models.     

What is the mole?

The mole is a difficult concept. Surveys have shown that even many teachers do not have a proper understanding of it. To help to meet this problem, the SI/IUPAC formulation of the mole is carefully presented and explained. New SI proposals are also briefly discussed.     

How does solvent molecular size affect the microscopic structure in polymer solutions?

Monte Carlo simulation has been used to investigate the effects of linear solvent molecular size on polymer chain conformation in solutions. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains (R(g)) is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of R(g) on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules.     

What is the role of the interfacial interaction in the slow relaxation of nanometer-thick polymer melts on a solid surface?

At the nanoscale and interfaces, the relaxation behavior of polymer melts, which affects the polymer's long-term performance in many important applications, is very different from that in the bulk. The role of polymer-substrate interfacial interaction, which does not have a bulk counterpart, has not been fully understood to date. In this study, the relaxation of nanometer-thick perfluoropolyether melts on a silicon wafer has been investigated by water contact angle measurement. The polymer-substrate interactions have been systematically changed by tailoring the polymer structure to clarify the effect of the interfacial interaction. The experimental results show that (1) when there is attractive interaction at the interface, some polymers are anchored to the substrate and others are free, (2) the attractive interfacial interaction drives the free polymers to relax at the interface, and (3) the relaxation is much slower than in the bulk, which has been attributed to the low mobility of the anchored polymer chains and the motional cooperativity between anchored and free polymer chains in the nanometer-thick films.     

Native Mass Spectrometry: What is in the Name?

Electrospray ionization mass spectrometry (ESI-MS) is nowadays one of the cornerstones of biomolecular mass spectrometry and proteomics. Advances in sample preparation and mass analyzers have enabled researchers to extract much more information from biological samples than just the molecular weight. In particular, relevant for structural biology, noncovalent protein–protein and protein–ligand complexes can now also be analyzed by MS. For these types of analyses, assemblies need to be retained in their native quaternary state in the gas phase. This initial small niche of biomolecular mass spectrometry, nowadays often referred to as “native MS,” has come to maturation over the last two decades, with dozens of laboratories using it to study mostly protein assemblies, but also DNA and RNA-protein assemblies, with the goal to define structure–function relationships. In this perspective, we describe the origins of and (re)define the term native MS, portraying in detail what we meant by “native MS,” when the term was coined and also describing what it does (according to us) not entail. Additionally, we describe a few examples highlighting what native MS is, showing its successes to date while illustrating the wide scope this technology has in solving complex biological questions.

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What is the future of electrophoresis in large-scale genomic sequencing?

Although a finished human genome reference sequence is now available, the ability to sequence large, complex genomes remains critically important for researchers in the biological sciences, and in particular, continued human genomic sequence determination will ultimately help to realize the promise of medical care tailored to an individual's unique genetic identity. Many new technologies are being developed to decrease the costs and to dramatically increase the data acquisition rate of such sequencing projects. These new sequencing approaches include Sanger reaction-based technologies that have electrophoresis as the final separation step as well as those that use completely novel, nonelectrophoretic methods to generate sequence data. In this review, we discuss the various advances in sequencing technologies and evaluate the current limitations of novel methods that currently preclude their complete acceptance in large-scale sequencing projects. Our primary goal is to analyze and predict the continuing role of electrophoresis in large-scale DNA sequencing, both in the near and longer term.     

What is the mechanism of oriented crystal growth on rubbed polymer substrates? Topography vs epitaxy

From the oriented growth of p-nitroaniline crystals on various rubbed polymer substrates, the orientation mechanism was found to basically rely upon the formation of intermolecular interactions at the crystal/polymer interface. The observations also show that the surface topography only plays a minor role in the oriented nucleation.     

What is the structure of the RecA-DNA filament?

The bacterial RecA protein has been a model system for understanding how a protein can catalyze homologous genetic recombination. RecA-like proteins have now been characterized from many organisms, from bacteriophage to humans. Some of the RecA-like proteins, including human RAD51, appear to function as helical filaments formed on DNA. However, we currently have high resolution structures of inactive forms of the protein, and low resolution structures of the active complexes formed by RecA-like proteins on DNA in the presence of ATP or ATP analogs. Within a crystal of the E. coli RecA protein, a helical polymer exists, and it has been widely assumed that this polymer is quite similar to the active helical filament formed on DNA. Recent developments have suggested that this may not be the case.     

To What Extent is FAIMS Beneficial in the Analysis of Proteins?

High field asymmetric waveform ion mobility spectrometry (FAIMS), also known as differential ion mobility spectrometry, is emerging as a tool for biomolecular analysis. In this article, the benefits and limitations of FAIMS for protein analysis are discussed. The principles and mechanisms of FAIMS separation of ions are described, and the differences between FAIMS and conventional ion mobility spectrometry are detailed. Protein analysis is considered from both the top-down (intact proteins) and the bottom-up (proteolytic peptides) perspective. The roles of FAIMS in the analysis of complex mixtures of multiple intact proteins and in the analysis of multiple conformers of a single protein are assessed. Similarly, the application of FAIMS in proteomics and targeted analysis of peptides are considered.

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What is an element? What is the periodic table? And what does quantum mechanics contribute to the question?

This article considers two important traditions concerning the chemical elements. The first is the meaning of the term “element” including the distinctions between element as basic substance, as simple substance and as combined simple substance. In addition to briefly tracing the historical development of these distinctions, I make comments on the recent attempts to clarify the fundamental notion of element as basic substance for which I believe the term “element” is best reserved. This discussion has focused on the writings of Fritz Paneth which are here analyzed from a new perspective. The other tradition concerns the reduction of chemistry to quantum mechanics and an understanding of chemical elements through their microscopic components such as protons, neutrons and electrons. I claim that the use of electronic configurations has still not yet settled the question of the placement of several elements and discuss an alternative criterion based on maximizing triads of elements. I also point out another possible limitation to the reductive approach, namely the failure, up to now, to obtain a derivation of the Madelung rule. Mention is made of some recent similarity studies which could be used to clarify the nature of ‘elements’. Although it has been suggested that the notion of element as basic substance should be considered in terms of fundamental particles like protons and electrons, I resist this move and conclude that the quantum mechanical tradition has not had much impact on the question of what is an element which remains an essentially philosophical issue.     

What is an atom in a molecule?

The derivation of the Hirshfeld atoms in molecules from information theory is clarified. The importance for chemistry of the concept of atoms in molecules (AIM) is stressed, and it is argued that this concept, while highly useful, constitutes a noumenon in the sense of Kant.     

Prediction of the phase structure in polymer blends: what is needed for success?

Rules for prediction of the phase structure in immiscible polymer blends from the knowledge of their composition, component properties and the flow field in a mixing or processing device are discussed. The reliability of qualitative prediction of the dependence of phase structure on system parameters is used as a criterion of plausibility of the rules. No general reliable rule for prediction of the phase structure type (continuity of phases) is available in the literature. Dependence of the droplet break‐up frequency on its size, contribution of simultaneous collisions of three or more droplets to coalescence and the effect of complex flow field on coalescence must be better described for a reliable qualitative prediction of the dependence of the droplet size on the system parameters.     

Transition state variation in the menshutkin reaction?

Data for reaction of CH₃I and CD₃I with substituted pyridines were previously argued to indicate a variant transition state as rate of reaction was varied, but the fact that both sets of results follow LFER's may be taken as evidence against such a conclusion.     

Is the HCCS radical linear in the excited state?

The A (2)Pi-X (2)Pi 415 nm band system of the linear HCCS radical has been known since 1978, but the vibronic structure in this complex spectrum, which has both spin-orbit and Renner-Teller complications, has never been satisfactorily assigned, despite serious experimental and theoretical efforts. In a further attempt to understand the spectrum, we have studied the laser-induced fluorescence spectra of jet-cooled HCCS and DCCS, produced from thiophene precursors using the discharge jet technique. The 0(0) (0) bands of HCCS and DCCS have been rotationally analyzed, providing precise ground and excited state spin-orbit splittings. The energy levels of the v(')=0 (2)Pi(3/2) component of DCCS are found to be perturbed by a very low-lying (2)Sigma vibronic level, indicating that the HCC bending mode Renner-Teller effect is much larger than predicted by ab initio calculations with a linear excited state geometry. With this observation, the vibronic bands in the spectra of both isotopomers have been consistently assigned for the first time. Model calculations show that the large Renner-Teller effect and substantially different HCCS and DCCS excited state zero-point spin-orbit splittings can be explained with the assumption of a quasilinear excited state geometry.     

Which molecularly imprinted polymer is better?

Molecularly imprinted polymers (MIP) have been successfully synthesized toward many different compounds in the last decades. The mechanistic details of selective binding at binding sites are not yet well understood. For this reason the characterization of MIP binding has been mostly phenomenological and this makes the transfer of results between different laboratories or between different types of applications difficult. In this paper we analyze the relationship between different types of characterization like isotherms, binding site models, chromatographic k and α values, etc. as they relate to different applications like HPLC, solid phase extraction (SPE), binding assays, batch extraction and sensors. It is shown that α values determined by elution chromatography depend on seemingly irrelevant factors as the length and diameter of the column, respectively. The determination of distribution ratios or partition coefficients is proposed as an easily understandable and useful quantity in the characterization of novel MIPs. Data used for the characterization of a MIP should be transferable between different applications but the qualification of MIPs as better or worse will depend on the application in case.     

Supramolecular chemistry: What is the commercialisation plan?

Opportunities for commercialisation along with personal views are provided in this essay.     

What is the mechanism of soap film entrainment?

Classical Frankel's law describes the formation of soap films and their evolution upon pulling, a model situation of film dynamics in foams (formation, rheology, and destabilization). With the purpose of relating film pulling to foam dynamics, we have built a new setup able to give an instantaneous measurement of film thickness, thus allowing us to determine film thickness profile during pulling. We found that only the lower part of the film is of uniform thickness and follows Frankel's law, provided the entrainment velocity is small. We show that this is due to confinement effects: there is not enough surfactant in the bulk to fully cover the newly created surfaces which results in immobile film surfaces. At large velocities, surfaces become mobile and then Frankel's law breaks down, leading to a faster drainage and thus to a nonstationary thickness at the bottom of the film. These findings should help in understanding the large dispersion of previous experimental data reported during the last 40 years and clarifying the pulling phenomenon of thin liquid films.     

What is the acidity of a plutonium solution?

The determination of the pH of a plutonium solution has traditionally depended on an electrode or a titration in the presence of a complexing agent. A new approach uses the equilibrium distribution of the Pu oxidation states to estimate the hydrogen ion concentration. The method is used to estimate the equilibrium constant of the first hydrolysis reaction of tetravalent plutonium.