Electrophilic aromatic substitution in the curing of brominated poly(isobutylene-co-4-methylstyrene): A mechanistic model study with zinc salts

The crosslinking chemistry of a new elastomer based on brominated poly(isobutylene-co-4-methylstyrene) has been investigated using model compounds. In order to mimic the conditions that prevail within the highly allphatic rubber, the study was carried out in mineral oil using catalysts that are compatible with such low polarity media. Electrophilic aromatic substitution reactions occur in a system consisting of p-isopropylbenzyl bromide and p-isopropyl toluene in the presence of zinc oxide and zinc stearate. The reaction proceeds after a significant induction period while no induction period is seen for a similar reaction with zinc bromide as the catalyst. The in situ formation of reactive species containing Zn? Br bonds appears to be an important step in the overall process. The stoichiometric ratio of zinc salt to benzylic bromide is important. High ratios lead to the accumulation of benzylic stearate or benzylic alcohol in the system which retards the alkylation reaction. The model study emphasizes the importance of stoichiometry of reagents in this reaction and provides insight into the crosslinking mechanism. © 1993 John Wiley & Sons, Inc.     

Improvement of Shack-Hartmann wave-front sensor measurement for extreme adaptive optics

The development of high-performance adaptive optics systems requires the optimization of wave-front sensors (WFSs) working in the high-order correction regime. We propose a new method to improve the wave-front slope estimation of a Shack-Hartmann WFS in such a regime. Based on a detailed analysis of the different errors in the slope estimation with a classical centroid and with the new method, the gain in terms of wave-front-sensing accuracy in both the detector and the photon noise regimes is stressed. This improvement is proposed without major system disruption.     

Nonlinear dynamics of dimers on periodic substrates

We study the dynamics of a dimer moving on a periodic one-dimensional substrate as a function of the initial kinetic energy at zero temperature. The aim is to describe, in a simplified picture, the microscopic dynamics of diatomic molecules on periodic surfaces, which is of importance for thin film formation and crystal growth. We find a complex behaviour, characterized by a variety of dynamical regimes, namely oscillatory, “quasi-diffusive” (chaotic) and drift motion. Parametrically resonant excitations of internal vibrations can be induced both by oscillatory and drift motion of the centre of mass. For weakly bound dimers a chaotic regime is found for a whole range of velocities between two non-chaotic phases at low and high kinetic energy. The chaotic features have been monitored by studying the Lyapunov exponents and the power spectra. Moreover, for a short-range interaction, the dimer can dissociate due to the parametric excitation of the internal motion. Received 8 July 2002 / Received in final form 15 November 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: fusco@sci.kun.nl.     

Closed-loop experimental validation of the spatially filtered Shack-Hartmann concept

An experimental validation of the spatially filtered Shack-Hartmann concept (F-SH) is presented that allows aliasing effects to be decreased. This effect is studied by use of an adaptive optical testbed with a focal plane pinhole in front of the wave-front sensor. First, the ability to close the loop on turbulence is demonstrated. Then the performance gain conferred by the device is quantified and compared with end-to-end simulation results. Another property of the F-SH concept, i.e., the reduction of the waffle modes, is highlighted.     

L 2-lower semicontinuity of functionals of quadratic type

Summary A representation formula for the L ²-lower semicontinuous envelope of a quadratic integral of Calculus of Variations is given. Some particular cases are explicited in the details.This paper has been supported by G.N.A.F.A. (Gruppo Nazionale per l'Analisi Funzionale e le sue Applicazioni).     

Non-linear wave propagation in a layered half space

In this paper we consider the one dimensional propagation of weak discontinuities through a layered half space where quasilinear hyperbolic systems in homogeneous conservative form are involved. The amplitude of reflected and transmitted discontinuities across each layer is determined in terms of the initial conditions. The critical time when the incident wave breaks down is discussed. The results are specialized in the case of periodic layering. Physical examples are considered at the end of the paper.