Polymer crystallization dynamics,as reflected by differential scanning calorimetry. Part 2: Numerical simulations

With the aid of a model for the kinetics of polymer crystallization, as put forward in previous publications, the shape of DSC-curves and their position on the temperature scale were simulated for various conditions of heat transfer in the apparatus. It turns out that the outcome is very dependent on the assumptions made with respect to these heat transfer conditions. For the ideal condition — no temperature differences between sample, pan and furnace — an invariable shape is predicted for the DSC-curves. They only shift to lower temperatures with increasing cooling rates. For more realistic conditions, the curves not only shift but become broader and their maxima decrease. They show a more familiar appearance. These calculations are very involved, however, A simple balance equation is shown to yield equivalent results, if a dimensionless characteristic number like the Nusselt number remains considerably smaller than one. This number contains an effective heat transfer coefficient between sample and furnace which, surprisingly, should not be too high. Apparently, the heat capacity of the pan does not play an important role under these conditions. This is investigated in Appendix II. Appendix I describes the procedure of the numerical simulations.     

Characterization of the Effects of Vanadium Traps in Cracking Catalysts by Imaging Secondary Ion Mass Spectrometry and Microactivity Test

The vanadium trapping effect of Mg and La containing additives in cracking catalyst contaminated with 2300 ppm Ni and 4700 ppm V has been analyzed by microactivity test (MAT) and imaging secondary ion mass spectrometry (SIMS). The results of SIMS imaging are consistent with cracking activity data and show that the La/spinel is a superior vanadium trap for the fluid catalytic cracking of hydrocarbons (FCC) operation. La/spinel serves as a dual function additive for both vanadium trap and SO_x removal. The optimum amount of La/spinel added to the cracking catalyst is about 15% by weight. This results in an increased catalytic activity, an increase in gasoline yield, and a decrease in coke and gas factors. The MgAl₂O₄ phase of Mg/Al₂O₃ additive is found capable of trapping vanadium while its MgO of Mg/Al₂O₃ phase can migrate to zeolite particles (the active component of the FCC catalyst) that, in turn, causes a decline in the activity of that catalyst.     

Link between the kinetic‐ and exchange‐energy functionals in the generalized gradient approximation

An approximate kinetic‐energy functional of the generalized gradient approximation form was derived following the “conjointness conjecture” of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange‐energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed‐shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic‐energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Thermal Degradation of Polychloroprene Rubber Under Isothermal and Non-Isothermal Conditions

The results of a kinetic investigation of the non-isothermal degradation and the isothermal degradation of the residual deformation under constant deflection for polychloroprene rubber are presented. A relationship was derived between the degree of non-isothermal conversion and the value of the investigated mechanical property. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microscopic Models for Proton Transfer in Water and Strongly Hydrogen-Bonded Complexes with a Single-Well Proton Potential

A new mechanism and formalism for proton transfer in donor-acceptor complexes with long hydrogen bonds introduced recently [1], is applied to a proton transfer in liquid water. Structural diffusion of hydroxonium ions is regarded as totally adiabatic process, with synchronous hindered translation of two closest water molecules to and from the reaction complex as crucial steps. The water molecules induce a gated shift of the proton from the donor to the acceptor in the double-well potential with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. The short-range and long-range proton transfer as structural diffusion of Zundel complexes is also considered. The theoretical formalism is illustrated with the use of Morse, exponential, and harmonic molecular potentials. This approach is extended to proton transfer in strongly hydrogen-bonded donor-acceptor complexes. In contrast to the above model [1], the short hydrogen bond between the donor and acceptor moieties, however, completely erodes the barrier along the proton transfer mode. This introduces some physical pattern differences from proton transfer reactions in truly double-well potentials with a finite proton transfer barrier at the transition configuration with respect to the environmental nuclear coordinates. The differences apply particularly to the origin of the kinetic isotope effect. We discuss explicitly details of the excess proton conductivity in aqueous solution, but the concepts and formalism apply broadly to acid-base reactions, proton conduction channels, and other strongly hydrogen-bonded O- and N-proton donor-acceptor systems.     

New Temperature Integral Approximate Formula for Non-isothermal Kinetic Analysis

A new approximate formula for temperature integral is proposed. The linear dependence of the new fomula on x has been established. Combining this linear dependence and integration-by-parts, new equation for the evaluation of kinetic parameters has been obtained from the above dependence. The validity of this equation has been tested with data from numerical calculating. And its deviation from the values calculated by Simpson's numerical integrating was discussed. Compared with several published approximate formulae, this new one is much superior to all other approximations and is the most suitable solution for the evaluation of kinetic parameters from TG experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic parameters of urease immobilized on modified acrylonitrile copolymer membranes in the presence and absence of Cu(II) ions

Poly(acrylonitrile-methylmethacrylate-sodium vinylsulfonate) membranes were subjected to seven different chemical modifications and the amount of the newly formed groups was measured for each membrane. Urease was then covalently immobilized onto the modified membranes and the amount of bound protein was determined. The kinetic parameters V(max) and K(m) of the immobilized urease were studied under static and dynamic conditions. Results showed that the rate of the enzyme reaction was higher for the membranes modified with NH(2)OH . H(2)SO(4), NH(2)NH(2) . H(2)SO(4), NaOH + EDA and NaOH + GA + EDA. It was confirmed that the reaction rate, measured under dynamic conditions, was higher than that one determined under static conditions. The influence of Cu(II) ions, as inhibitors, on the enzyme reaction kinetics (V(i) and K(i)) was also investigated. It turned out that the most sensitive membranes towards Cu(II) were those modified with NH(2)NH(2) . H(2)SO(4), NaOH + EDA and H(2)O(2). The results initiated further investigations on the influence of other heavy metal ions (Cd(II), Zn(II), Ni(II) and Pb(II)) over urease bound to a NH(2)OH . H(2)SO(4)-modified membrane. It was found that the inhibition effect of the heavy metal ions over immobilized urease decreases in the order: Cu(II) > Cd(II) > Zn(II) > Ni(II) > Pb(II). [Diagram: see text]     

Transport properties of the Lorentz gas: Fourier's law

We investigate the stationary nonequilibrium (heat transporting) states of the Lorentz gas. This is a gas of classical point particles moving in a region gL containing also fixed (hard sphere) scatterers of radiusR. The stationary state considered is obtained by imposing stochastic boundary conditions at the top and bottom of , i.e., a particle hitting one of these walls comes off with a velocity distribution corresponding to temperaturesT ₁ andT ₂ respectively,T ₁ <T ₂. Letting be the average density of the randomly distributed scatterers we show that in the Boltzmann-Grad limit,,R 0 with the mean free path fixed, the stationary distribution of the Lorentz gas converges in theL ¹-norm to the stationary distribution of the corresponding linear Boltzmann equation with the same boundary conditions. In particular, the steady state heat flow in the Lorentz gas converges to that of the linear Boltzmann equation, which is known to behave as (T ₂-T ₁)/L for largeL, whereL is the distance from the bottom to the top wall: i.e., Fourier's law of heat conduction is valid in the limit. The heat flow converges even in probability. Generalizations of our result for scatterers with a smooth potential as well as the related diffusion problem are discussed.Research supported in part by NSF Grant no. Phy 77-22302.On leave of absence from the Fachbereich Physik der Universität, München. Work supported by a DFG fellowship.     

Electron correlation effects on the shape of the Kohn–Sham molecular orbital

Even systems in which strong electron correlation effects are present, such as the large near-degeneracy correlation in a dissociating electron pair bond exemplified by stretched H₂, are represented in the Kohn–Sham (KS) model of non-interacting electrons by a determinantal wavefunction built from the KS molecular orbitals. As a contribution to the discussion on the status and meaning of the KS orbitals we investigate, for the prototype system of H₂ at large bond distance, and also for a one-dimensional molecular model, how the electron correlation effects show up in the shape of the KS σ _g orbital. KS orbitals φ^HL and φ^FCI obtained from the correlated Heitler-London and full configuration interaction wavefunctions are compared to the orbital φ^LCAO, the traditional linear combination of atomic orbitals (LCAO) form of the (approximate) Hartree-Fock orbital. Electron correlation manifests itself in an essentially non-LCAO structure of the KS orbitals φ^HL and φ^FCI around the bond midpoint, which shows up particularly clearly in the Laplacian of the KS orbital. There are corresponding features in the kinetic energy density t _s of the KS system (a well around the bond midpoint) and in the one-electron KS potential v _s (a peak). The KS features are lacking in the Hartree-Fock orbital, in a minimal LCAO approximation as well as in the exact one. Received: 11 December 1996 / Accepted: 10 January 1997     

An efficient synthesis of the orally-active GpIIb/IIIa antagonist FR184764

An efficient synthesis of the orally-active GpIIb/IIIa antagonist FR184764 was achieved. The key intermediate, an optically active ethynyl β-amino ester, was synthesized efficiently by utilizing a lipase catalyzed kinetic resolution step.