576 research outputs found
Quantum coherence generated by interference-induced state selectiveness
The relations between quantum coherence and quantum interference are
discussed. A general method for generation of quantum coherence through
interference-induced state selection is introduced and then applied to `simple'
atomic systems under two-photon transitions, with applications in quantum
optics and laser cooling.Comment: 17 pages, 7 figures, to be published in Journal of Modern Optics'
special issue on quantum interferenc
Plasticization and Morphology Development in Dynamically Vulcanized Thermoplastic Elastomers
Dynamically vulcanized thermoplastic elastomers constitute one of the main categories among various types of thermoplastic elastomers (TPEs). Due to the commercial importance of this particular group of TPEs, tremendous efforts have been dedicated to improve the understanding and control the phase morphology development. The ultimate goal is to obtain materials with improved physical and mechanical properties. As in other polymeric compounds, the parameters during the mixing stage have a significant influence on the final morphology of dynamically vulcanized blends. Furthermore, the phase morphology and, therefore, the distribution of elastomeric domains in the thermoplastic phase are also strongly dependent on the formulation. This chapter discusses the main important processing factors and, more specifically, highlights the effects of plasticization and curing on the morphology development of dynamically vulcanized thermoplastic elastomer blends. The following text provides fundamental information on how one should take into consideration each parameter affecting the morphology of nonreactive and reactive elastomer/thermoplastic blends
Novel A-B type oscillations in a 2-D electron gas in inhomogenous magnetic fields
We present results from a quantum and semiclassical theoretical study of the
and resistivities of a high mobility 2-D electron gas
in the presence of a dilute random distribution of tubes with magnetic flux
and radius , for arbitrary values of and . We
report on novel Aharonov-Bohm type oscillations in and ,
related to degenerate quantum flux tube resonances, that satisfy the selection
rule , with an integer. We discuss possible
experimental conditions where these oscillations may be observed.Comment: 11 pages REVTE
Caractéristique rhéologique de nanocomposites polypropylène/argile en régime transitoire
National audienceDans cette étude, nous nous intéressons au comportement rhéologique en régime transitoire de mélanges polypropylène/argile organophile. Les mélanges ont été préparés à l'état fondu, à l'aide d'un mélangeur interne et d'une extrudeuse bivis co-rotative. L'évolution du comportement rhéologique lors du démarrage a été étudiée en cisaillement simple aller-retour sur un rhéomètre à géométrie plan-plan. On observe un pic de contrainte au démarrage, ainsi qu'un effet de restructuration, qui se traduit par l'augmentation de l'amplitude du pic après un temps de repos. Le pic observé est différent suivant la morphologie des nanocomposites, notamment le niveau d'exfoliation. Deux modèles, basés sur deux phénomènes physiques différents, ont été étudiés. Le premier est un modèle thixotrope qui relie les propriétés rhéologiques aux mécanismes de création et de rupture des flocs. Le deuxième est un modèle généralement utilisé pour décrire le comportement rhéologique des polymère chargés de fibres courtes. Le modèle thixotrope prédit assez bien l'amplitude du pic de contrainte en fonction du temps de repos. Cependant, l'évolution de la contrainte au démarrage est moins bien modélisée. Le modèle de suspensions de fibres décrit mieux l'évolution de cette dernière, mais ne prend pas en compte l'effet de restructuration observé
The structural amphiphilicity of cellulose nanocrystals characterized from their cohesion parameters
Cellulose nanocrystals (CNCs), usually considered as isotropically polar
nanoparticles, are sheet-like crystalline assemblies of cellulose chains. Here,
we link the anisotropy of the CNC structure to an amphiphilic behavior in
suspension. The Hansen solubility parameters (HSP: δD; δP; δH) of woodbased
H2SO4-hydrolyzed CNCs were measured from sedimentation tests in a
wide set of 59 solvents and binary mixtures. Two sets of cohesion parameters
corresponding to a polar surface (18.1; 20.4; 15.3) ± (0.5; 0.5; 0.4) MPa1/2
and to a mildly non-polar one (17.4; 4.8; 6.5) ± (0.3; 0.5; 0.6) MPa1/2 were
determined, with respective solubility radii of 7.8 and 2.1MPa1/2. The polar
sphere is thought to correspond to the (110)&(110) surfaces of cellulose
Iβ nanocrystals, while the smaller non-polar sphere is coherent with the exposure
of (200) surfaces. The HSP graph provides new insights on the amphiphilic
nature of CNCs and a mapping of their chemical affnity for solvents
and polymer matrices
Ultrasonication of spray- and freeze-dried cellulose nanocrystals in water
The structural and rheological properties of aqueous suspensions of spray-dried cellulose nanocrystals (CNCs) were investigated and compared to those of freeze-dried. The cellulose nanocrystals were obtained from sulfuric acid hydrolysis of wood pulp. Ultrasonication was used to disperse cellulose nanocrystals in Milli-Q water and the power applied during ultrasonication was shown to be the controlling parameter for their dispersion, more than total energy. Dynamic light scattering measurements showed a decrease of the average hydrodynamic diameter down to the same limiting value, i.e. ∼75 nm, for both spray and freeze-dried cellulose nanocrystals. Since the same maximum dispersion state was reached for both CNC types, it indicated that the spray drying process did not limit dispersion, provided that sufficient ultrasonication was provided. Moreover, no desulfation occurred during ultrasonication at ambient temperature. Strong ultrasonication also caused a decrease of intrinsic viscosity, along with an increase in maximum packing concentration. These properties were correlated to agglomerates break-up, which released both ions and water in suspension. The ionic strength increase may lead to a thinner electrostatic double layer surrounding the cellulose nanocrystals, reducing their apparent concentration
Spectral properties on a circle with a singularity
We investigate the spectral and symmetry properties of a quantum particle
moving on a circle with a pointlike singularity (or point interaction). We find
that, within the U(2) family of the quantum mechanically allowed distinct
singularities, a U(1) equivalence (of duality-type) exists, and accordingly the
space of distinct spectra is U(1) x [SU(2)/U(1)], topologically a filled torus.
We explore the relationship of special subfamilies of the U(2) family to
corresponding symmetries, and identify the singularities that admit an N = 2
supersymmetry. Subfamilies that are distinguished in the spectral properties or
the WKB exactness are also pointed out. The spectral and symmetry properties
are also studied in the context of the circle with two singularities, which
provides a useful scheme to discuss the symmetry properties on a general basis.Comment: TeX, 26 pages. v2: one reference added and two update
The apparent structural hydrophobicity of cellulose nanocrystals
The Teas graph of wood-based sulfuric acid-hydrolyzed cellulose nanocrystals (CNCs) was plotted
based on sedimentation tests in a set of 25 common solvents. Comparisons with those of sucrose and dextran,
taken as equivalents for cellobiose (cellulose repeating unit) and amorphous cellulose, respectively, highlighted
the amphiphilic nature of CNCs. In the absence of any chemical arguments, the hydrophobic behavior displayed
is thought to be caused by the exposition of (200) lattice planes at the CNC surface. This apparent structural
hydrophobicity may be exploited to achieve the dispersion of CNCs in some mildly-non polar matrices such as
poly(ethylene glycol) and poly(lactic acid). The Teas graph is a useful tool to predict the dispersibility potential
of CNCs and to select a proper solvent for nanocomposite preparatio
Tailoring cellulose nanocrystals rheological behavior in aqueous suspensions through surface functionalization with polyethyleneimine
The present paper reports the surface modification of commercially available cellulose nanocrystals
(CNCs) using polyethyleneimine (PEI) by means of non-covalent electrostatic interaction between the
negatively charged sulfate groups of CNCs and positively charged amine functionalities of PEI. The
modification, carried out in an aqueous medium, results in a stable CNC-PEI suspension with no phase
separation that exhibit interesting rheological behavior due to bridging-type inter-particle interactions.
The Newtonian 3% (w/w) CNC suspension evolves into a non-Newtonian gel system after
modification with PEI with a consequent increase of almost three decades in complex viscosity. Preshearing
of the 3% (w/w) CNC-PEI suspension resulted in the loss of the linear viscoelastic properties
with increasing shear rate, as would be expected from the breaking of the inter-particle network.
However, the system gradually re-established the inter-particle network in less than an hour to give the
original rheological parameters. The effect of PEI on the rheological properties was attributed to the
physical adsorption of PEI chains on the CNC particles, examined by dynamic light scattering (DLS),
zeta potential, X-ray photoelectron spectroscopy (XPS), elemental analyses, and isothermal adsorption
studies. The modified CNC-PEI particles did not show any significant change in the particle
morphology compared to the unmodified CNCs, as observed from transmission electron microscope
(TEM) images
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