Polarographische Untersuchungen zur Polymerisation konjugiert ungesättigter Ketone, 1. Mitt.: Die säurekatalysierte Polymerisation von Methylvinylketon in Methanol

Zusammenfassung Die polrographischen Kurven konjugiert ungesättigter Ketone — im vorliegenden Fall von Methylvinylketon (MVK) — zeigen in saueren Lösungen zeitliche Veränderungen als Folge einer Polymerisation. Die Wirkung der Zusätze von Protonendonatoren (starke und schwache Säuren) zu methanol. Lösungen vonMVK auf Kurvengestalt und-lage sowie das zeitliche Absinken der stufen gestattet den Polymerisations-mechanismus (Start-, Folge- und Abbruchreaktion) zu formulieren. Die polarographische Methode erweist sich zur Verfolgung und Aufklärung von derartigen Polymerisationsreaktionen, bei denen die Monomeren Reduktionsstufen zeigen, als geeignet.

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Polarographic investigation of the polymerization of unsaturated ketones, I: The acid catalyzed polymerization of methyl vinyl ketone in methanolic solution

Polarographic curves of conjugated unsaturated ketones — in the present case methylvinylketone (MVK) — in acid solutions change with time due to polymerization.The effect of proton donors (strong and weak acids) to methanolic solutions ofMVK on the shape and position as well as on the rate of decrease of the wave height allow to formulate the polymerization-mechanism (initiation-, propagation-and termination-reaction). The polarographic method proves to be suitable to observe and clarify polymerization reactions, if the monomer species show reduction-waves.

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Mit 5 Abbildungen     

Coupled nonlinear effects in modeling field emission from CNTs

Field emission from carbon nanotubes (CNTs) is a complex process involving a range of physical effects and phenomena. In this paper, we systematically develop a multiphysics model to describe this process. We integrate the model numerically to estimate the output current from a CNT based field emission device. Numerical simulations have been able to capture the transients in current as observed in actual experiments. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Constitutive modeling of shape memory alloy wire with non-local rate kinetics

A constitutive modeling approach for shape memory alloy (SMA) wire by taking into account the microstructural phase inhomogeneity and the associated solid–solid phase transformation kinetics is reported in this paper. The approach is applicable to general thermomechanical loading. Characterization of various scales in the non-local rate sensitive kinetics is the main focus of this paper. Design of SMA materials and actuators not only involve an optimal exploitation of the hysteresis loops during loading–unloading, but also accounts for fatigue and training cycle identifications. For a successful design of SMA integrated actuator systems, it is essential to include the microstructural inhomogeneity effects and the loading rate dependence of the martensitic evolution, since these factors play predominant role in fatigue. In the proposed formulation, the evolution of new phase is assumed according to Weibull distribution. Fourier transformation and finite difference methods are applied to arrive at the analytical form of two important scaling parameters. The ratio of these scaling parameters is of the order of 10⁶ for stress-free temperature-induced transformation and 10⁴ for stress-induced transformation. These scaling parameters are used in order to study the effect of microstructural variation on the thermo-mechanical force and interface driving force. It is observed that the interface driving force is significant during the evolution. Increase in the slopes of the transformation start and end regions in the stress–strain hysteresis loop is observed for mechanical loading with higher rates.      

Provably secure dynamic identity-based three-factor password authentication scheme using extended chaotic maps

With the aim of guaranteeing secure communication through public networks, three-factor password authentication (TF-PWA) scheme plays a key role in many internet applications. Since in TF-PWA scheme, the communicating entities can mutually authenticate each other and generate a shared session key, which will be used for secure exchange of messages in succeeding communication among them. As a result, the TF-PWA schemes gain enormous consideration in recent years. More recently, due to light-weight features of the extended chaotic map, it is also extensively applied in designing of public key encryption, key agreement, image encryption, S-box, hash function, digital signature, password authentication, etc. The aim of this paper was to design a dynamic identity-based three-factor password authentication scheme using extended chaotic map (ECM-TF-PWA) in the random oracle model. The proposed scheme is provably secure based on the intractability assumption of chaotic map-based Diffie–Hellman problem. The informal security analysis gives the evidence that our scheme protects all attacks and provides functionality attributes that are needed in a three-factor authentication system. Besides, the performance discussion shows that our scheme performs better than others in respect of computation and communication cost.     

Effects of quenching on the morphology and crystal structure of ZnO nanostructures

Zinc oxide nanostructures were prepared by a simple wet chemical procedure using zinc acetate and sodium hydroxide as precursors. The process was subjected to quenching treatment and the effect of the treatment on the formation of the nanostructures was studied using atomic force and scanning electron microscopies. The change in crystal structure of the nanostructures due to quenching was studied using an X-ray diffractometry that established that physical and structural properties of the nanostructures were largely influenced by the quenching treatment.     

Low-lying Resonance State of 15 C: Application of Supersymmetric Quantum Mechanics

Calculation of the low-lying resonance state of ¹⁵C is performed using a two-body model (¹⁴C+n) with the application of Supersymmetric Quantum Mechanics (SSQM). The effective two-body potential presents a shallow well followed by a low and wide barrier. Large spatial extension of the halo state causes loss of accuracy due to numerical difficulties in calculating low-lying resonances. Use of SSQM permits one to construct an isospectral potential with a deep well followed by a high barrier which can effectively trap the system and hence facilitates the numerical calculation of resonance states more accurately than the original shallow potential. This method gives quite accurate result for the resonance energy of the ${\frac{3}{2}^{+}}$ state of ¹⁵C. Calculated width of this state also agrees fairly well within the experimental error bar.     

Fluorescence sensing of theobromine by simple 2,6-diamino-pyridine and the novel cyclic chair-like hydrogen-bonded tetramer of its diacetyl derivative

2,6-Diaminopyridine is found to be a simple fluorescent sensor for theobromine and its diacetyl derivative 2 also effectively binds theobromine. The receptor-binding sites are based on the co-operative hydrogen-bonding abilities of secondary amides. An unprecedented hydrogen-bonded self-organised cyclic tetrameric supramolecular network is shown for one such small molecule 2 containing one heterocyclic ring in contrast to the binuclear substrates like guanine or pterin which usually form cyclic tetrameric structures.     

Observation of a universal aggregation mechanism and a possible phase transition in Au sputtered by swift heavy ions

Two exponents delta for the size distribution of n-atom clusters, Y(n) approximately n{-delta}, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation. For small clusters, below 12.5 nm in size, delta has been found to be 3/2, which can be rationalized as occurring from a steady state aggregation process with size independent aggregation. For larger clusters, a delta value of 7/2 is suggested, which might come from a dynamical transition to another steady state where aggregation and evaporation rates are size dependent. In the present case, the observed decay exponents do not support any possibility of a thermodynamic liquid-gas-type phase transition taking place, resulting in cluster formation.     

2-Amino-4-methylpyrimidine: A simple supramolecular scaffold for carboxylic acid complexes both in solid and solution states

Studies concentrating on hydrogen bonding interactions between 2-amino-4-methylpyrimidine (AMPY) with selected dicarboxylic acids have been investigated in the solid state. Two pyrimidinium-dicarboxylate organic salts with stoichiometry 1:1 [AMPY: maleic acid] and 2:1 [AMPY:d(+)-malic acid] have been prepared and characterized by X-ray crystallographic analysis. Maleic and d(+)-malic acids were found to exhibit unique supramolecular polymeric structures with AMPY involving proton transfer to the specific ring nitrogen of AMPY. As an extension, AMPY has been used to build a receptor structure 1. Receptor 1 shows binding of dicarboxylic acids in CH₃CN and represents a marginal selectivity with d(+)-malic acid. Binding interactions were investigated by UV-vis and fluorescence studies.