Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium.

Control of equilibrium and non-equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (Tg ), both Tg and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network

Fretting of CoCrMo and Ti6Al4V Alloys in Modular Prostheses

Implantation of a total hip replacements (THR) is an effective intervention in the management of arthritis. Modularity at the taper junction of THR was introduced in order to improve the ease with which the surgeon could modify the length of the taper section and the overall length of the replacement. Cobalt chromium (Co–28Cr–6Mo) and titanium (Ti–6Al–4V) alloys are the most commonly used materials for the device. This study investigates the fretting behaviour of both CoCr–CoCr and CoCr–Ti couplings and analyses their damage mechanisms. A reciprocating tribometer ball on plate fretting contact was instrumented with in situ electrochemistry to characterise the damage inflicted by tribocorrosion on the two couplings. Fretting displacements amplitudes of 10, 25 and 50 mm at an initial contact pressure of 1 GPa were assessed. The results reveal larger metallic volume loss from the CoCr–CoCr alloy compared to the CoCr–Ti alloy, and the open circuit potential indicates a depassivation of the protective oxide layer at displacement amplitudes .25 mm. In conclusion, the damage mechanisms of CoCr–CoCr and CoCr–Ti fretting contacts were identified to be wear and fatigue dominated mechanisms respectively

Static Friction between Elastic Solids due to Random Asperities

Several workers have established that the Larkin domains for two three dimensional nonmetallic elastic solids in contact with each other at a disordered interface are enormously large. This implies that there should be negligible static friction per unit area in the macroscopic solid limit. The present work argues that the fluctuations in the heights of the random asperities at the interface that occur in the Greenwood-Williamson model can account for static friction.Comment: Contains some improvements in the treatment of the subjec

Simulations of the Static Friction Due to Adsorbed Molecules

The static friction between crystalline surfaces separated by a molecularly thin layer of adsorbed molecules is calculated using molecular dynamics simulations. These molecules naturally lead to a finite static friction that is consistent with macroscopic friction laws. Crystalline alignment, sliding direction, and the number of adsorbed molecules are not controlled in most experiments and are shown to have little effect on the friction. Temperature, molecular geometry and interaction potentials can have larger effects on friction. The observed trends in friction can be understood in terms of a simple hard sphere model.Comment: 13 pages, 13 figure

On the driven Frenkel-Kontorova model: II. Chaotic sliding and nonequilibrium melting and freezing

The dynamical behavior of a weakly damped harmonic chain in a spatially periodic potential (Frenkel-Kontorova model) under the subject of an external force is investigated. We show that the chain can be in a spatio-temporally chaotic state called fluid-sliding state. This is proven by calculating correlation functions and Lyapunov spectra. An effective temperature is attributed to the fluid-sliding state. Even though the velocity fluctuations are Gaussian distributed, the fluid-sliding state is clearly not in equilibrium because the equipartition theorem is violated. We also study the transition between frozen states (stationary solutions) and=7F molten states (fluid-sliding states). The transition is similar to a first-order phase transition, and it shows hysteresis. The depinning-pinning transition (freezing) is a nucleation process. The frozen state contains usually two domains of different particle densities. The pinning-depinning transition (melting) is caused by saddle-node bifurcations of the stationary states. It depends on the history. Melting is accompanied by precursors, called micro-slips, which reconfigurate the chain locally. Even though we investigate the dynamics at zero temperature, the behavior of the Frenkel-Kontorova model is qualitatively similar to the behavior of similar models at nonzero temperature.Comment: Written in RevTeX, 13 figures in PostScript, appears in PR

Charge separation: From the topology of molecular electronic transitions to the dye/semiconductor interfacial energetics and kinetics

Charge separation properties, that is the ability of a chromophore, or a chromophore/semiconductor interface, to separate charges upon light absorption, are crucial characteristics for an efficient photovoltaic device. Starting from this concept, we devote the first part of this book chapter to the topological analysis of molecular electronic transitions induced by photon capture. Such analysis can be either qualitative or quantitative, and is presented here in the framework of the reduced density matrix theory applied to single-reference, multiconfigurational excited states. The qualitative strategies are separated into density-based and wave function-based approaches, while the quantitative methods reported here for analysing the photoinduced charge transfer nature are either fragment-based, global or statistical. In the second part of this chapter we extend the analysis to dye-sensitized metal oxide surface models, discussing interfacial charge separation, energetics and electron injection kinetics from the dye excited state to the semiconductor conduction band states

A regularized nonnegative canonical polyadic decomposition algorithm with preprocessing for 3D fluorescence spectroscopy

International audienceWe consider blind source separation in chemical analysis focussing on the 3D fluorescence spectroscopy framework. We present an alternative method to process the Fluorescence Excitation-Emission Matrices (FEEM): first, a preprocessing is applied to eliminate the Raman and Rayleigh scattering peaks that clutter the FEEM. To improve its robustness versus possible improper settings, we suggest to associate the classical Zepp's method with a morphological image filtering technique. Then, in a second stage, the Canonical Polyadic (CP or Cande-comp/Parafac) decomposition of a nonnegative 3-way array has to be computed. In the fluorescence spectroscopy context, the constituent vectors of the loading matrices should be nonnegative (since standing for spectra and concentrations). Thus, we suggest a new NonNegative third order CP decomposition algorithm (NNCP) based on a non linear conjugate gradient optimisation algorithm with regularization terms and periodic restarts. Computer simulations performed on real experimental data are provided to enlighten the effectiveness and robustness of the whole processing chain and to validate the approach

Developing pathways for oral care in elders: evidence‐based interventions for dental caries prevention in dentate elders

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102639/1/ger12081.pd

Phosphorus poisoning during wet oxidation of methane over Pd@CeO2/graphite model catalysts

10siThe influence of phosphorus and water on methane catalytic combustion was studied over Pd@CeO2 model catalysts supported on graphite, designed to be suitable for X-ray Photoelectron Spectroscopy/Synchrotron Radiation Photoelectron Spectroscopy (XPS/SRPES) analysis. In the absence of P, the catalyst was active for the methane oxidation reaction, although introduction of 15% H2O to the reaction mixture did cause reversible deactivation. In the presence of P, both thermal and chemical aging treatments resulted in partial loss of activity due to morphological transformation of the catalyst, as revealed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis. At 600 °C the combined presence of PO43− and water vapor caused a rapid, irreversible deactivation of the catalyst. XPS/SRPES analysis, combined with operando X-ray Absorption Near Edge Structure (XANES) and AFM measurements, indicated that water induces severe aggregation of CeO2 nanoparticles, exposure of CePO4 on the outer layer of the aggregates and incorporation of the catalytic-active Pd nanoparticles into the bulk. This demonstrates a temperature-activated process for P-poisoning of oxidation catalysts in which water vapor plays a crucial role.partially_openembargoed_20171009Monai, Matteo; Montini, Tiziano; Melchionna, Michele; Duchoň, Tomáš; Kúš, Peter; Tsud, Nataliya; Prince, Kevin C.; Matolin, Vladimir; Gorte, Raymond J.; Fornasiero, PaoloMonai, Matteo; Montini, Tiziano; Melchionna, Michele; Duchoň, Tomáš; Kúš, Peter; Tsud, Nataliya; Prince, Kevin C.; Matolin, Vladimir; Gorte, Raymond J.; Fornasiero, Paol

The Cytotoxic Necrotizing Factor of Yersinia pseudotuberculosis (CNFy) is Carried on Extracellular Membrane Vesicles to Host Cells

In this study we show Yersinia pseudotuberculosis secretes membrane vesicles (MVs) that contain different proteins and virulence factors depending on the strain. Although MVs from Y. pseudotuberculosis YPIII and ATCC 29833 had many proteins in common (68.8% of all the proteins identified), those located in the outer membrane fraction differed significantly. For instance, the MVs from Y. pseudotuberculosis YPIII harbored numerous Yersinia outer proteins (Yops) while they were absent in the ATCC 29833 MVs. Another virulence factor found solely in the YPIII MVs was the cytotoxic necrotizing factor (CNFy), a toxin that leads to multinucleation of host cells. The ability of YPIII MVs to transport this toxin and its activity to host cells was verified using HeLa cells, which responded in a dose-dependent manner; nearly 70% of the culture was multinucleated after addition of 5 mu g/ml of the purified YPIII MVs. In contrast, less than 10% were multinucleated when the ATCC 29833 MVs were added. Semi-quantification of CNFy within the YPIII MVs found this toxin is present at concentrations of 5 -10 ng per mu g of total MV protein, a concentration that accounts for the cellular responses see