Vinyl ether end‐groups in poly(ethylene glycol)s from the Na2CO3‐promoted degradation of 1,3‐dioxolan‐2‐one polymers

The Na₂CO₃‐promoted polymerization of 1,3‐dioxolan‐2‐one (I) to afford poly(ethylene glycol) III was reinvestigated. The reaction appeared to involve a nucleophilic attack against the carbonyl and methylene groups of I to afford poly(carbonate) II with poly(ethylene glycol) linkages and ethylene oxide IV as a side product (10–22%). As the reaction progressed, poly(carbonate) II decreased and poly(ethylene glycol) III increased. Under some conditions, poly(ethylene glycol)s V and VI with vinyl ether terminal groups were formed unexpectedly. The formation of unsaturated products during the polymerization of I/EO (ethylene oxide) has not been reported in the literature. We believe that vinyl ethers were formed from the degradation of poly(carbonate)s and were accompanied by a reduction in molecular weight. The structures of vinyl ethers V and VI were confirmed by hydrogenation of the double bond into the ethyl ether group in VII and VIII, respectively. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 152–160, 2000     

On the Interaction of Turbulent Shear Layers with Harmonic Perturbations

The problem of coherent perturbations in a turbulent shear layer is considered for the purpose of developing a mathematical model based on a triple decomposition that extracts the coherent components of random fluctuations. The governing equations for the mean and the coherent parts of flow are derived, assuming the eddy-viscosity equivalence for the random part of flow, and solved by iterations to provide a coupled solution of the problem as a whole. Calculations agree well with experimental data in the upstream part of the layer where the mean–coherent flow interaction is the most important. In this region, the interaction changes the mean flow velocity distribution in such a manner that the neutral stability curve is shifted upstream relative to its position in the undisturbed layer and the perturbation intensity decreases further downstream. Experiments show that the coherent waves suppress the turbulent Reynolds stress production downstream of this region, but the model fails to predict the layer spreading correctly probably due to an inadequate turbulence closure of the mean flow. For the case of a turbulent mixing layer, we suggest a new closure relation that takes into account this coherent-random interaction.     

Thermodynamics and kinetics of initial gas phase reactions in chemical vapor deposition of titanium nitride. Theoretical study of TiCl4 ammonolysis

Thermodynamic equilibrium and kinetics of the gas‐phase reaction between TiCl₄ and NH₃ have been studied computationally using results from recent quantum mechanical calculations of titanium tetrachloride ammonolysis. 1 These calculations were based upon the transition state theory for the direct reactions and RRKM theory for the reactions proceeding via intermediate complex. Rate constants for the barrierless reactions were expressed through the thermodynamic characteristics of the reagents and products using a semiempirical variational method. The kinetic simulation of the gas‐phase steps of CVD was performed within a model of a well‐stirred reactor at temperatures 300–1200 K and residence times between 0.1–2 s. At temperatures below 450 K formation of donor–acceptor complexes between TiCl₄ and NH₃ is the dominating process. At higher temperatures sequential direct ammonolysis takes place. At typical LPCVD conditions the only product of the first step of ammonolysis, TiCl₃NH₂, is formed in substantial amount. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1366–1376, 2001     

Sharpening of carbon nanocone tips during plasma-enhanced chemical vapor growth

In situ tip sharpening of vertically aligned carbon nanocones (VACNCs) was demonstrated. VACNCs were synthesized on patterned catalyst dots of 100 nm in diameter using dc plasma-enhanced chemical vapor deposition. The VACNC tip diameter was found to decrease with growth time. This enables synthesis of ultra-sharp VACNCs even for relatively large catalyst dot sizes, which is quite important for practical applications. We also find that for a given set of growth parameters the diameter of the initially formed catalyst nanoparticle determines the maximum length of the growing VACNC. The mechanism of VACNC growth and sharpening is discussed.     

Many-body energy localization transition in periodically driven systems

According to the second law of thermodynamics the total entropy of a system is increased during almost any dynamical process. The positivity of the specific heat implies that the entropy increase is associated with heating. This is generally true both at the single particle level, like in the Fermi acceleration mechanism of charged particles reflected by magnetic mirrors, and for complex systems in everyday devices. Notable exceptions are known in noninteracting systems of particles moving in periodic potentials. Here the phenomenon of dynamical localization can prevent heating beyond certain threshold. The dynamical localization is known to occur both at classical (Fermi–Ulam model) and at quantum levels (kicked rotor). However, it was believed that driven ergodic systems will always heat without bound. Here, on the contrary, we report strong evidence of dynamical localization transition in both classical and quantum periodically driven ergodic systems in the thermodynamic limit. This phenomenon is reminiscent of many-body localization in energy space.     

Water dynamics in silica nanopores: the self-intermediate scattering functions

The dynamics of water molecules confined in approximately cylindrical silica nanopores is investigated using molecular simulation. The model systems are pores of diameter varying between 20 and 40 ? containing water at room temperature and at full hydration, prepared using grand canonical Monte Carlo simulation. Water dynamics in these systems is studied via molecular dynamics simulation. The results of the basic characterization of these systems have been reported in A. A. Milischuk and B. M. Ladanyi [J. Chem. Phys. 135, 174709 (2011)]. The main focus of the present study is the self-intermediate scattering function (ISF), F(S)(Q, t), of water hydrogens, the observable in quasi-elastic neutron scattering experiments. We investigate how F(S)(Q, t) depends on the pore diameter, the direction and magnitude of the momentum transfer Q, and the proximity of water molecules to the silica surface. We also study the contributions to F(S)(Q, t) from rotational and translational motions of water molecules and the extent of rotation-translation coupling present in F(S)(Q, t). We find that F(S)(Q, t) depends strongly on the pore diameter and that this dependence is due mainly to the contributions to the ISF from water translational motion and can be attributed to the decreased mobility of water molecules near the silica surface. The relaxation rate depends on the direction of Q and is faster for Q in the axial than in the radial direction. As the magnitude of Q increases, this difference diminishes but does not disappear. We find that its source is mainly the anisotropy in translational diffusion at low Q and in molecular reorientation at higher Q values.     

Nonparametric denoising of signals with unknown local structure, I: Oracle inequalities

We consider the problem of pointwise estimation of multi-dimensional signals s, from noisy observations (y_τ) on the regular grid . Our focus is on the adaptive estimation in the case when the signal can be well recovered using a (hypothetical) linear filter, which can depend on the unknown signal itself. The basic setting of the problem we address here can be summarized as follows: suppose that the signal s is “well-filtered”, i.e. there exists an adapted time-invariant linear filter with the coefficients which vanish outside the “cube” {0,…,T}^d which recovers s₀ from observations with small mean-squared error. We suppose that we do not know the filter q^*, although, we do know that such a filter exists. We give partial answers to the following questions:

– is it possible to construct an adaptive estimator of the value s₀, which relies upon observations and recovers s₀ with basically the same estimation error as the unknown filter ?

– how rich is the family of well-filtered (in the above sense) signals?

We show that the answer to the first question is affirmative and provide a numerically efficient construction of a nonlinear adaptive filter. Further, we establish a simple calculus of “well-filtered” signals, and show that their family is quite large: it contains, for instance, sampled smooth signals, sampled modulated smooth signals and sampled harmonic functions.

Electronic excitations in ZnWO4 and ZnxNi1?xWO4 (x = 0.1 ? 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO₄ as well as the Zn _x Ni_1−x WO₄ solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO₆]⁶⁻ anions, becomes modulated by the optical absorption of Ni²⁺ ions in the Zn _x Ni_1−x WO₄ solid solutions; and (ii) no significant change in the excitation spectra of Zn_0.9Ni_0.1WO₄ is observed compared to pure ZnWO₄. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO₄.