Mechanism of thermal degradation of urea-formaldehyde polycondensates

The thermal degradation to 500°C of urea-formaldehyde polycondensate occurs in four successive steps. In each step, partial volatilisation takes place while the polymer undergoes chemical modification to give progressively more stable structures.Below 200°C methylene ether bridges are transformed into methylene bridges and branching and crosslinking reactions occur with maximum rates at 125°C and 165°C, respectively. Above 200°C radicals formed by chain scission induce the formation of cyclic structures in the polymer which undergoes extensive fragmentation above 300°C. Water, formaldehyde, carbon monoxide and dioxide, methane, ammonia, monomethylamine and trimethylamine are the gaseous products evolved.By combining data on the chemical modifications and gases evolved in each step, reaction mechanisms are proposed.     

Effects of water and electric field on atomic oxygen adsorption on Pt–Co alloys

Density functional theory is used to study the effect of atomic oxygen adsorption at various coverages with and without the presence of water on ordered and Pt-segregated PtCo surfaces. The strength of O adsorption, as well as surface reconstruction effects due to the adsorbate are strongly influenced by the presence of the oxygen-philic transition metal on the surface or subsurface. At high O coverage, buckling of the Co atom on PtCo surfaces is much smaller than that of Pt on Pt(1 1 1) surfaces, and buckling of Pt atoms on Pt-skin surfaces is negligible. Also, the effect of an electric field perpendicular to the surface on adsorbed water and atomic oxygen is investigated. Spontaneous water dissociation is not found on the ordered and segregated alloy surfaces within the entire applied electric field range (−0.51 to 0.51 V/Å). Water changes orientation under strong negative fields, switching from a metal–O to a metal–H interaction, and the effect is much more pronounced in the low-coordination sites of cluster models.     

An effective solution for a real cutting stock problem in manufacturing plastic rolls

We confront a practical cutting stock problem from a production plant of plastic rolls. The problem is a variant of the well-known one dimensional cutting stock, with particular constraints and optimization criteria defined by the experts of the company. We start by giving a problem formulation in which optimization criteria have been considered in linear hierarchy according to expert preferences, and then propose a heuristic solution based on a GRASP algorithm. The generation phase of this algorithm solves a simplified version which is rather similar to the conventional one dimensional cutting stock. To do that, we propose a Sequential Heuristic Randomized Procedure (SHRP). Then in the repairing phase, the solution of the simplified problem is transformed into a solution to the real problem. For experimental study we have chosen a set of problem instances of com-mon use to compare SHRP with another recent approach. Also, we show by means of examples, how our approach works over instances taken from the real production process. All authors are supported by MEC-FEDER Grant TIN2007-67466-C02-01 and by contract CN-05-127 of the University of Oviedo and the company ERVISA, and by FICYT under grant BP04-021.     

Spacer-mediated synthesis of contra-thermodynamic spiroacetals: stereoselective synthesis of C2-symmetric difructose dianhydrides

The xylylene moiety (ortho, meta, and para) was employed as a rigid tether in the spacer-mediated synthesis of difructose dianhydrides (DFAs), a unique class of bis-spiroacetal derivatives present in food products. The synthetic methodology exploits the suitability of triflic acid to promote spirocyclization in organic solvents under irreversible reaction conditions, using anomeric isopropylidene fructose derivatives as precursors. Advantage was taken of the strong dependence of the conformational properties of DFAs on the relative configuration of the spiroketal centers. Highly stereoselective syntheses of the contra-thermodynamic difructofuranose and difructopyranose diastereomers, namely the C2-symmetric derivatives having the beta-configuration at both anomeric centers, have been accomplished by judicious choice of the xylylene positional isomer and of the linking position to the fructose building blocks. Interestingly, the rigid spacer concept has also been implemented to favor intermolecular processes leading to higher macrocyclic architectures that incorporate the bis-spiro fructodisaccharide subunit.     

A sufficient condition for kernel perfectness of a digraph in terms of semikernels modulo F

A kernel of a directed graph is a set of vertices which is both independent and absorbent. And a digraph is said to be kernel perfect if and only if any induced subdigraph has a kernel. Given a set of arcs F , a semikernel S modulo F is an independent set such that if some Sz-arc is not in F , then there exists a zS-arc. A sufficient condition on the digraph is given in terms of semikernel modulo F in order to guarantee that a digraph is kernel perfect. To do that we give a characterization of kernel perfectness which is a generalization of a previous result given by Neumann-Lara [Seminúcleos de una digrfica. Anales del Instituto de Matemticas 2, Universidad Nacional Autónoma de México, 1971]. And moreover, we show by means of an example that our result is independent of previous known sufficient conditions.     

Study of the mechanism of intumescence in fire retardant polymers: Part II—Mechanism of action in polypropylene-ammonium polyphosphate-pentaerythritol mixtures

It is shown that, by the addition of a typical intumescent mixture of ammonium polyphosphate and pentaerythritol to polypropylene, the mechanism of intumescence which develops on heating is not significantly affected by dispersion of the intumescent mixture in the polymer. On the other hand, in these mixtures, polypropylene seems to evolve, by thermal degradation, a smaller amount of flammable products than when it is heated alone. The ammonium polyphosphate-pentaerythritol additive is shown to induce fire retardant characteristics in polypropylene by means of a ‘condensed phase’ mechanism.     

Diameter vulnerability of GC graphs

Concern over fault tolerance in the design of interconnection networks has stimulated interest in finding large graphs with maximum degree Δ and diameter D such that the subgraphs obtained by deleting any set of s vertices have diameter at most D′, this value being close to D or even equal to it. This is the so-called (Δ,D,D′,s)-problem. The purpose of this work has been to study this problem for s=1 on some families of generalized compound graphs. These graphs were designed by Gómez (Ars Combin. 29-B (1990) 33) as a contribution to the (Δ,D)-problem, that is, to the construction of graphs having maximum degree Δ, diameter D and an order large enough. When approaching the mentioned problem in these graphs, we realized that each of them could be redefined as a compound graph, the main graph being the underlying graph of a certain iterated line digraph. In fact, this new characterization has been the key point to prove in a suitable way that the graphs belonging to these families are solutions to the (Δ,D,D+1,1)-problem.     

Theoretical study on the properties of linear and cyclic amides in gas phase and water solution

The structural and energetic properties of a group of selected amides, of well-known importance for the design of efficient clathrate inhibitors, are calculated with Hartree-Fock and density functional theory, B3LYP, theoretical levels, and a 6-311++g** basis set in the gas phase and a water solution. The conformational behavior of the molecules is studied through the scanning of the torsional potential energy surfaces and by the analysis of the differences in the energetic and structural properties between the isomers. The properties of the amides in water solution are determined within a self-consistent reaction field approach with a polarizable continuum model that allows the calculation of the different contributions to the free energy of solvation. The calculated barriers to rotation are in good agreement with the available experimental data and the comparison of the gas and water results shows the strong effect of the solute polarization. The properties of different amide-water complexes are calculated and compared with available experimental information.     

Intercalation ofn-alkylamines into α-titanium phosphate from aqueous solutions

Journal of Inclusion Phenomena and Macrocyclic Chemistry - The intercalation reactions betweenn-alkylamines and α-titanium phosphate in aqueous media have been investigated. The compounds with...