Entropy‐Driven Selectivity for Chain Scission: Where Macromolecules Cleave

We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high‐temperature (HT) NMR spectroscopy of self‐assembled supramolecular diblock systems as well as temperature‐dependent size‐exclusion chromatography (TD SEC) of covalently bonded Diels–Alder step‐growth polymers.     

Nonlinear dynamics with higher-order modes in lithium niobate waveguide arrays

We present theoretical and experimental studies on nonlinear beam propagation in lithium niobate waveguide arrays utilizing higher-order second harmonic bands. We find that the implementation of the higher-order second harmonic bands leads to a number of new effects. The combined interaction of two second harmonic bands with a propagating fundamental beam can lead to a complete inhibition of nonlinear effects or to the formation of discrete spatial solitons, depending only on the wavelength of the fundamental wave. Furthermore we analyze the properties of discrete solitons, allowing for linear coupling of the second harmonic. Here we predict and demonstrate experimentally a power dependent phase transition of the soliton topology.     

Three-dimensional light bullets in arrays of waveguides

We report the first experimental observation of three-dimensional light bullets, excited by femtosecond pulses in a system featuring quasi-instantaneous cubic nonlinearity and a periodic, transversally modulated refractive index. Stringent evidence of the excitation of light bullets is based on time-gated images and spectra which perfectly match our numerical simulations. Furthermore, we reveal a novel evolution mechanism forcing the light bullets to follow varying dispersion or diffraction conditions, until they leave their existence range and decay.     

Effect of structure on the thermal degradation and flame retardancy of hexolic anhydride based polyesters

Based on the anhydrides like hexolic (5,6,7,8,10,10-hexachloro -3a,4,4a,5,8,8a,9,9a-octahydro-5,8-methanonaphtho-[2,3-c]-furan-1,3-dione), maleic and phthalic and diols like 1,4-butanediol, cis-2-butene-1,4-diol and 2,3-dichloro-2-butene-1,4-diol, a family of polyesters has been synthesized using azeotropic condensation technique. The structural characterizations of the polyesters have been carried out using infra-red, 1^H - and ¹³C- nuclear magnetic resonance spectroscopic methods. The thermal properties of the polyesters have been studied using thermogravimetric technique. The off-line pyrolysis of these materials was done. The qualitative and semi-quantitative analyses of the volatiles as well as the heavy mass fractions of the degradation products were carried out using a gas chromatograph coupled to a mass selective detector (GC-MSD). Thermogravimetric data indicate that the thermal stability and the char residue of the polyester resins decrease in the order 1,4-butanediol based>cis-2-butene-1,4-diol based>2,3-dichloro-2-butene-1,4-diol based polyesters. The GC-MSD data indicate that the amount of flame cooling agents (hexa-, isomeric penta-, tetra- and isomeric tri-chlorocyclopentadienes) produced during the pyrolysis of the polyesters increases in the order 2,3-dichloro-2-butene-1,4-diol based<cis-2-butene-1,4-diol based<1,4-butanediol based polyesters. The trends observed in these two parameters which are contributing factors to the flame retardancy of the polyester materials were suitably explained on the basis of the effect of the structural changes in the diol part of the polyesters on the primary degradation mechanism, the β-chain scission process.     

Partial oxidation of high-boiling hydrocarbon mixtures in the pilot unit

The reason for this investigation into the partial oxidation (POX) of high-boiling hydrocarbons with oxygen in the presence of water vapour was to ensure increased demand on hydrogen, which is essential for the deeper hydrorefining of petroleum oils to ensure better quality of motor fuels. The tests demonstrated the good performance of the pilot plant unit and also the reproducibility of the experiment. The investigation detected a significant impact of water vapour on the selectivity of the partial oxidation of high boiling-point hydrocarbons with regard to the composition of the exhaust gas. In addition, it was noted that the hydrogen content in the reaction product of the partial oxidation of the mixture of visbreaking distillate and residue diluted with pyrolysis fuel oil, achieved 50 %. The content of components in the gaseous product accorded well with a synthesis gas composition produced by the plant unit of the UNIPETROL RPA Co., designed and built under licence from the Shell Petroleum Co.     

Double hump solitary waves in quadratic media with walk-off – domain of existence,stability and decay scenarios

We analytically determine the domain of existence of double hump solitary wave solutions in quadratic media with walk-off. Although a linear stability analysis reveals that these solutions are always unstable the instability gain can be very small for certain parameters. This has important implications for all-optical switching applications. The ultimate decay is prompted by the destruction of an exact phase difference π between the two humps of the fundamental frequency wave and follows different scenarios.     

Analysis of the dispersion relations of nonlinear slab-guided waves

The dispersion relations of TE-polarized nonlinear guided waves (NGW) were investigated in detail. These waves travel along a dielectric slab embedded between two different nonlinear media. A simple relation between the effective nonlinear dielectric constants was established. All dispersion curves exhibit a particular transition point, where one evanescent field maximum changes from a virtual into a real one, and vice versa. Depending on the type of the remaining invariant evanescent field maximum, all NGW's split into two families. The lowest-order membres of every family play a particular role. The power flux supported by a NGW is derived, too.Preliminary results were presented at the XVI^th European Congress of Molecular Spectroscopy, Sofia, Bulgaria (1983)     

Solvent effects in the photolysis of 1,2,3,4-tetrahydro-1-naphthylhydroperoxide. Photocatalytic effect of Co(III) 2,4-pentanedionate

The effect of various solvents on the photoinitiated decomposition of 1,2,3,4-tetrahydro-1-naphthylhydroperoxide (THP) with and without added Co(III) 2,4-pentanedionate has been studied. The conversion of THP, formation of major products and change in the oxidation state of the metal ion have been investigated in different solvents.

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1, 2, 3, 4--1- () Co(III) 2,4-. , .