Short-Time Elasticity of Polymer Melts: Tobolsky Conjecture and Heterogeneous Local Stiffness

An extended Molecular-Dynamics study of the short-time "glassy" elasticity exhibited by a polymer melt of linear fully-flexible chains above the glass transition is presented. The focus is on the infinite-frequency shear modulus $G_\infty$ manifested in the picosecond time scale and the relaxed plateau $G_p$ reached at later times and terminated by the structural relaxation. The local stiffness of the interactions with the first neighbours of each monomer exhibits marked distribution with average value given by $G_\infty$. In particular, the neighbourhood of the end monomers of each chain are softer than the inner monomers, so that $G_\infty$ increases with the chain length. $G_p$ is not affected by the chain length and is largely set by the non-bonding interactions, thus confirming for polymer melts the conjecture formulated by Tobolsky for glassy polymers.Comment: 18 pages, 6 figure

Cage rattling does not correlate with the local geometry in molecular liquids

Molecular-dynamics simulations of a liquid of short linear molecules have been performed to investigate the correlation between the particle dynamics in the cage of the neighbors and the local geometry. The latter is characterized in terms of the size and the asphericity of the Voronoi polyhedra. The correlation is found to be poor. In particular, in spite of the different Voronoi volume around the end and the inner monomers of a molecule, all the monomers exhibit coinciding displacement distribution when they are caged (as well as at longer times during the structural relaxation). It is concluded that the fast dynamics during the cage trapping is a non-local collective process involving monomers beyond the nearest neighbours.Comment: 15 pages, 6 figure

New Insights into Food Fermentation

: Food fermentation has been used for thousands of years for food preservation [...]

THE CORPORATE GOVERNANCE DRIVERS: WHAT RELATIONS WITH PERFORMANCE AND RISK? EMPIRICAL EVIDENCE FROM ITALIAN CONTEXT

Considering a sample of Italian firms and defining a good Governance index (gGI), we investigated if there is a relation between the gGI, the performance and the default risk and which governance determinants are most responsible of these effects. To deepen the analysis, the aforementioned relations are also observed by comparing family and non-family firms and the companies more or less active in M&A. We found that the Corporate Governance quality presents some correlations with performance and risk. The non-family companies are better structured, showing a positive correlation between some Corporate Governance drivers and performance and Z-score. Furthermore, the “well-advised” firms in external strategies are able to obtain a better correlation with performance and also a good relation with Z-scor

Competition of the connectivity with the local and the global order in polymer melts and crystals

The competition between the connectivity and the local or global order in model fully-flexible chain molecules is investigated by molecular-dynamics simulations. States with both missing (melts) and high (crystal) global order are considered. Local order is characterized within the first coordination shell (FCS) of a tagged monomer and found to be lower than in atomic systems in both melt and crystal. The role played by the bonds linking the tagged monomer to FCS monomers (radial bonds), and the bonds linking two FCS monomers (shell bonds) is investigated. The detailed analysis in terms of Steinhardt's orientation order parameters Q_l (l = 2 - 10) reveals that increasing the number of shell bonds decreases the FCS order in both melt and crystal. Differently, the FCS arrangements organize the radial bonds. Even if the molecular chains are fully flexible, the distribution of the angle formed by adjacent radial bonds exhibits sharp contributions at the characteristic angles {\theta} = 70{\deg}, 122{\deg}, 180{\deg}. The fractions of adjacent radial bonds with {\theta} = 122{\deg}, 180{\deg} are enhanced by the global order of the crystal, whereas the fraction with 70{\deg} < {\theta} < 110{\deg} is nearly unaffected by the crystallization. Kink defects, i.e. large lateral displacements of the chains, are evidenced in the crystalline state.Comment: J. Chem. Phys. in pres

Towards Micromechanical Sensors with (La,Sr)MnO3 Epitaxial Films☆

Abstract The rich spectrum of functionalities exhibited by oxide thin films is an appealing feature for the development of micro and nanomechanical devices [1,2] . MEMS made of heterostructures of crystalline oxide materials having targeted physical properties may be applied as sensors having different integrated functionalities. In this work, we explore the feasibility of manganite thin film based epitaxial MEMS for magnetic micromechanical sensing. We investigate the electromechanical properties of LSMO freestanding structures for future applications in the field of micromechanical magnetic sensors

Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex-situ tapes

Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H*, the upper critical field Hc2 and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H* values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, although at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes.Comment: 26 pages, 9 figures, submitted to J. Appl. Phy

Cage effect in supercooled molecular liquids: Local anisotropies and collective solid-like response

Both local geometry and collective extended excitations drive the moves of a particle in the cage of its neighbours in dense liquids. The strength of their influence is investigated by the molecular dynamics simulations of a supercooled liquid of fully flexible trimers with semirigid or rigid bonds. The rattling in the cage is investigated on different length scales. First, the rattling anisotropy due to local order is characterized by two order parameters sensing the monomers succeeding or failing to escape from the cage. Then the collective response of the surroundings excited by the monomer-monomer collisions is considered. The collective response is initially restricted to the nearest neighbours of the colliding particle by a Voronoi analysis revealing elastic contributions. Then the long-range excitation of the farthest neighbours is scrutinised by searching spatially extended correlations between the simultaneously fast displacements of the caged particle and the surroundings. It is found that the longitudinal component has stronger spatial modulation than the transverse one with a wavelength of about one particle diameter, in close resemblance with experimental findings on colloids. It is concluded that the cage rattling is largely affected by solid-like extended modes

Structural properties of adsorbent phyllosilicates rule the entrapping ability of intercalated iron-phenanthroline complex towards thiols

The interaction of volatile organic sulfur derivatives, such as 1-heptanethiol (C7H16S), with clay minerals treated with a μ-oxo Fe3+-phenanthroline 1:1 complex results strongly affected by crystal chemical properties of pristine mineral phases. In particular, two sepiolite clays with different structural features demonstrated significantly different ability to immobilize the Fe3+-phenanthroline complex at two pH values (pH = 5.4 and pH = 2.3). The most effective binding was obtained with sepiolite with higher structural disorder at pH 5.4. Accordingly, the resulting hybrid material showed also the greatest efficiency in removal of thiol in gas phase. A direct correlation can be established between the adsorption of the Fe3+-phenanthroline complex and the gas binding process at room temperature. In fact, 1-heptanethiol entrapping occurs via redox reactions between Fe3+ and a first thiol molecule to give the reduced Fe2+-phenanthroline complex and disulfide, followed by the binding of further thiols to the reduced metal centre. The extremely high amount of thiol immobilized by the hybrid material also suggests the co-presence of a catalytic mechanism that guarantees the reoxidation of Fe+2 to Fe+3 and the restoration of redox reactions with thiol. Investigation and conclusions were supported by the several experimental techniques: elemental analysis, X-ray powder diffraction analyses, UV–Vis measurements, FT-IR and NMR spectroscopies, thermogravimetric analyses