Neutron depolarization and transmission by a nickel single crystal near Tc

We report the preliminary results of a series of neutron transmission experiments on a nickel single crystal. A marked and complicated temperature dependence of the two neutron polarization states of the transmitted beam is observed.     

Structure and properties of a photosensitive polyimide: Effect of photosensitive group

The photosensitive poly(p-phenylene biphenylteracarboximide) (BPDA-PDA) precursor was synthesized by attaching photocross-linkable 2-(dimethylamino)ethyl methacrylate (DMAEM) monomer to its poly(amic acid) through acid/base complexation. The polyimide thin films were prepared by a conventional cast/softbake/thermal imidization process from the photosensitive precursors with various concentrations of DMAEM. The structure and properties of the polyimide films were investigated by small-angle and wide-angle x-ray scattering, refractive indices and birefringence analysis, residual stress and relaxation analysis, stress-strain analysis, and dynamic mechanical thermal analysis. In comparison with the polyimide film from the poly(amic acid), the films, which were imidized from the photosensitive precursors, exhibited a better molecular order and microstructure; however, they exhibited less molecular orientation in the film plane. Despite the enhancement in both the molecular order and microstructure, the film properties (i.e., mechanical properties, thermal expansion, residual stress, optical properties, dielectric constant, and water sorption) degraded overall due to both the decrease in molecular in-plane orientation and the formation of microvoids caused by the bulky photosensitive group during thermal imidization. That is, on one hand, the PSPI precursor formation provides an advantageous, direct patternability to the BPDA-PDA precursor, and on the other hand, it results in degraded properties to the resulting polyimide film. © 1995 John Wiley & Sons, Inc.     

The conformation of succinic acid in aqueous solution studied by 1h and 13c nmr

The vicinal H-H coupling constants of succinic acid were obtained as a function of pH and related to its conformation. The syn-clinal arrangement of the two CO₂H groups appears as more stable than the anti-periplanar conformation at variance with previous work but in agreement with recent data on similar molecules. The vicinal¹³CO₂H-¹H coupling constant was also obtained and is found to support that conclusion. The analysis was extended to solvents of variable dielectric constant and no significant effect was found.     

Effect of an intense laser field on the magnon damping in free carrier magnetic semiconductors

The influence of an intense radiation fieldE₀ on the magnon damping in free carrier magnetic semiconductors is discussed. It is shown that the effect of the intense field is to give a drift velocity v₀ = eE₀/mΩ to the carriers such that when v₀ exceeds the phase velocity of the spin waves the electrons are more able to emit magnons than they absorb and as a consequence the magnon population grows with time.     

Amplification of optical phonons by a radiation field in semiconductors under strong magnetic fields

The effect of a strong magnetic field on the optical phonon damping in an electron-phonon system is discussed. It is found that as the laser frequency approaches the electron cyclotron frequency the optical phonon population may become unstable.     

Two connected inverse spectral transforms

We establish a transformation which connects the potentials of the one-dimensional Dirac and Klein-Gordon operators. This transformation links the solutions of the nonlinear evolution equations solvable by means of the two inverse spectral transforms which use the Dirac and Klein-Gordon direct and inverse spectral problems.     

Computer simulation of shock waves in the completely asymmetric simple exclusion process

We study the evolution of the completely asymmetric simple exclusion process in one dimension, with particles moving only to the right, for initial configurations corresponding to average density _– ( ₊) left (right) of the origin, _{– +}. The microscopic shock position is identified by introducing a second-class particle. Results indicate that the shock profile is stable, and that the distribution as seen from the shock positionN(t) tends, as time increases, to a limiting distribution, which is locally close to an equilibrium distribution far from the shock. Moreover , withV=1– _–– ₊, as predicted, and the dispersion ofN(t), ²(t), behaves linearly, for not too small values of ₊– _–, i.e., , whereS is equal, up to a scaling factor, to the valueS _WA predicted in the weakly asymmetric case. For ₊= _– we find agreement with the conjecture .Dedicated to the memory of Paola Calderoni.     

Site selectivity in the reaction of 2-formyl-2H-azirine-N-arylimines with diphenylketene

2-Formyl-2H-azirine-N-arylimines 1a-c react with diphenylketene 2 to afford 2H-2-azirinyl-2-azetidi-nones 3a-c and N-aryldiphenylacetamides 4a-c by reaction at the exo-imine group. Derivatives containing the bulkier methyl group at the 2-position, 1d-f , produce only 4a-c.     

Polymerization of ethylene using a high-activity Ziegler–Natta catalyst. II. Molecular weight regulation

This article describes studies on the variables that regulate the molecular weight in ethylene polymerization using a highly active Ziegler–Natta catalyst with hydrogen for molecular weight control. The dependence of the degree of polymerization on the concentration of catalyst, cocatalyst, monomer, partial pressure of hydrogen, and temperature has been established. The rate constant for chain transfer with cocatalyst has been evaluated. © 1993 John Wiley & Sons, Inc.