370 research outputs found
Segmental relaxation in semicrystalline polymers: a mean field model for the distribution of relaxation times in confined regimes
The effect of confinement in the segmental relaxation of polymers is
considered. On the basis of a thermodynamic model we discuss the emerging
relevance of the fast degrees of freedom in stimulating the much slower
segmental relaxation, as an effect of the constraints at the walls of the
amorphous regions. In the case that confinement is due to the presence of
crystalline domains, a quasi-poissonian distribution of local constraining
conditions is derived as a result of thermodynamic equilibrium. This implies
that the average free energy barrier for conformational
rearrangement is of the same order of the dispersion of the barrier heights,
, around . As an example, we apply the results to
the analysis of the -relaxation as observed by dielectric broad band
spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized
from either an isotropic or an oriented glass. It is found that in the latter
case the regions of cooperative rearrangement are significantly larger than in
the former.Comment: 10 pages, 4 figures .ep
Broadband ac conductivity of conductor-polymer composites
The electrical conductivity of a composite model system formed by highly structured carbon black (CB) filled, within an amorphous polymer, poly(ethylene terephtalate) composite is studied. The dc conductivity as a function of CB content follows a scaling law of the type σ∝(p−pc)t yielding for the percolation concentration, pc=0.011 and for the exponent, t=2.17. The analysis of the temperature dependence of the conductivity suggests that for temperatures larger than 45 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through the insulating layer of polymer separating two CB aggregates. At lower temperatures, conductivity becomes temperature independent, which is typical of conventional tunneling. The frequency dependence of the conductivity is also studied between dc and 109 Hz. By the introduction of a shift factor ap, a procedure for the construction of a master curve based on a “time-length equivalence principle” is proposed. Finally, a model is introduced to describe the frequency dependence of the conductivity of CB-filled composites based on the behavior of charge carriers placed in a fractal object
Broadband dielectric spectroscopy of nanocomposites based on PVDF and expanded graphite
International audienceNanocomposites based on poly (vinylidene fluoride) (PVDF) and expanded graphite (EG) were prepared by non-solvent precipitation from solution with different EG concentrations. Films were obtained by compression molding and their structural and dielectric properties studied. From Wide Angle X-ray Scattering (WAXS) experiments, it can be assessed that for all EG concentrations the -crystalline phase of PVDF is the predominant crystalline form. However, for composites with high nanoadditive content, higher than 3 wt.%, the -crystalline phase is also detected. Dielectric spectroscopy results showed that the nanocomposites present both high dielectric constant and electrical conductivity at low percolation threshold
The building stone of the Roman city of Lixus (NW Morocco) : provenance, petrography and petrophysical characterization
Characterization of building material is a key tool to assess deterioration processes and improve potential restoration works of archaeological sites. The aim of this paper is to identify and characterize the building stone used in the construction of the Phoenician-Roman city of Lixus (Larache, Morocco) by means of petrographic and petrophysical technics. Based on the visual analysis of the monuments, three major building stones (i.e., lithotypes) have been identified: (1) Oligocene sandstones, (2) Quaternary sandstones, and (3) Quaternary conglomerates. Based on the analysis of the regional geology and exploitation marks, these three lithotypes have been identified to crop out in the surroundings of Lixus and the quarries, presumably Roman in origin, recognized. The Oligocene sandstone is the primary building stone in Lixus as form and crop out extensively in the Tchemmis hill, at top of which the city is settled. The Quaternary sandstones and conglomerates, which represent nearshore deposits and eolianites, are less abundant as building rocks in Lixus and crop out along the Atlantic coast where form part pf the cliffs close to Larache. Petrographic results indicate that lithotypes differ notably in grain size, ratio of detrital to allochemical components, and the configuration of their porous system. Mechanical analysis show that the Oligocene sandstones are more resistant to compression than the Quaternary sandstones and conglomerates, the latter exhibiting low compressive strength. The Oligocene sandstones, which display scarce porosity and permeability, show a hydric behaviour characterized by a very low degree of absorbing and desorbing water, likely resulting from a poor connectivity of the pore network. Contrary to later lithotype, the Quaternary sandstones and conglomerates, which exhibit very high porosity and permeability, display a hydric behaviour characterized by high degree of both absorbing and desorbing water. This behaviour is attributed to both the low degree of cementation and excellent connectivity of the porous network of the lithotype typical of coastal deposists. Finally, the accelerated artificial aging test they do not show a significant weight loss after twelve cycles of salt crystallization, indicating that the three lithotypes are not vulnerable to sodium sulphate attacks. Results of this study indicate that the good state of conservation of the building rocks of Lixus is linked to intrinsic factors as mineralogy and petrophysical characteristics together with the favourable effect of the climatic condition of the study area
Altered expression of the immunoregulatory ligand-receptor pair CD200-CD200R1 in the brain of Parkinson's disease patients
Neuroinflammation, in which activated microglia are involved, appears to contribute to the development of Parkinson's disease (PD). However, the role of microglial activation and the mechanisms governing this process remain uncertain. We focused on one inhibitory mechanism involved in the control of microglial activation, the microglia inhibitory receptor CD200R1, and its ligand CD200, mainly expressed by neurons. The human CD200R1 gene encodes two membrane-associated and two soluble protein isoforms and the human CD200 gene encodes full-length proteins (CD200full) but also truncated (CD200tr) proteins which act as CD200R1 antagonists. Little is known about their expression in the human brain under pathological conditions. We used human peripheral blood monocytes and monocyte-derived microglia-like cells from control subjects to characterize the expression of the CD200R1 mRNA variants, which showed stimulus-specific responses. We provide evidence of increased CD200R1 (mRNA variants and protein isoforms) and CD200 expression (CD200tr mRNA) in brain tissue of PD patients, mainly in the hippocampus, as well as increased CD200 expression (CD200full and CD200tr mRNAs) in iPSCs-derived dopaminergic neurons generated from skin fibroblasts of PD patients. Our results suggest that CD200-CD200R1 signalling is altered in PD, which may affect the microglial function and constitute a potential target in therapeutic strategies for PD.© 2022. The Author(s)
Quantitative Nanomechanical Properties of Multilayer Films Made of Polysaccharides through Spray Assisted Layer-by-Layer Assembly.
Nanomechanical properties of alginate/chitosan (Alg/Chi) multilayer films, obtained through spray assisted layer-by-layer assembly, were studied by means of PeakForce quantitative nanomechanical mapping atomic force microscopy (PF-QNM AFM). Prepared at two different alginate concentrations (1.0 and 2.5 mg/mL) and a fixed chitosan concentration (1.0 mg/mL), Alg/Chi films have an exponential growth in thickness with a transition to a linear growth toward a plateau by increasing the number of deposited bilayers. Height, elastic modulus, deformation, and adhesion maps were simultaneously recorded depending on the number of deposited bilayers. The elastic modulus of Alg/Chi films was found to be related to the mechanism of growth in contrast to the adhesion and deformation. A comparison of the nanomechanical properties obtained for non-cross-linked and thermally cross-linked Alg/Chi films revealed an increase of the elastic modulus after cross-linking regardless alginate concentration. The incorporation of iron oxide nanoparticles (NPs), during the spray preparation of the films, gave rise to nanocomposite Alg/Chi films with increased elastic moduli with the number of incorporated NPs layers. Deformation maps of the films strongly suggested the presence of empty spaces associated with the method of preparation. Finally, adhesion measurements point out to a significant role of NPs on the increase of the adhesion values found for nanocomposite films.journal article2017 01 092016 12 15importe
Spinodal-assisted crystallization in polymer melts
Recent experiments in some polymer melts quenched below the melting temperature have reported spinodal kinetics in small-angle x-ray scattering before the emergence of a crystalline structure. To explain these observations we propose that the coupling between density and chain conformation induces a liquid-liquid binodal within the equilibrium liquid-crystalline solid coexistence region. A simple phenomenological theory is developed to illustrate this idea, and several experimentally testable consequences are discussed. Shear is shown to enhance the kinetic role of the hidden binodal
Broad-Band Electrical Conductivity of High Density Polyethylene Nanocomposites with Carbon Nanoadditives: Multiwall Carbon Nanotubes and Carbon Nanofibers
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