Thermal gradient of in-flight polymer particles during cold spraying

International audienceThe manufacture of polymer coatings via the cold-spray process remains challenging owing to the viscoelastic-viscoplastic behavior exhibited by polymers. One crucial step to improve cold-spray polymer coating is to determine the particles' thermal history during their flight from inside the nozzle to their impact on the substrate. In this study, we propose estimating the velocity and temperature of an isolated polymer particle traveling through a nozzle with a sharp change in its cross-section. The preliminary results show that the geometric discontinuity constricts the flow, thereby increasing the particle velocity. Moreover, a significant thermal gradient is expected inside the particle, which in turn leads to a gradient of mechanical properties of the polymeric particle during impact

Electric fields for the tailoring of polymeric nonocomposites with carbonaceous filler particles

We report on the application of electric fields for the alignment and network formation of carbonaceous fillers in an epoxy resin thermoset during the curing. The structuring of the conductivity distribution by means of inhomogeneous electric fields in a carbon black filled epoxy resin will be shown. The use of homogeneous electric fields induces a preferential orientation and chain formation of fibrous filler particles. This will be demonstrated for dispersions of carbon nanofibres in epoxy resin in an alternating electric field. The resultant anisotropy of the electrical properties will be discussed

Dynamic mechanical relaxation of cross-linked styrene-butadiene polymers containing free chains: Possibility of reptation

cited By 5International audienceThe structure of styrene-butadiene rubbers (SBRs) consists of chains attached to cross-links of relatively small number density. These rubbers show an α-relaxation process that is attributed to segmental motions of the attached chains. To investigate how an SBR's rubber-like behavior would change when free chains interpenetrate its structure, we prepared one SBR of mol wt 120 × 103containing 3-4% free chains of identical mol wt, and studied its dynamic mechanical relaxation. For 0.1 Hz frequency measurements, the α-relaxation peak appeared at 210 K, and a second relaxation peak, which we refer to as αR-relaxation, at 300 K. A third relaxation of small magnitude was resolved at temperatures between the αR- and the α-relaxation peaks, and its origin was discussed in relation to other studies. Spectral features of the αR-and the α-relaxations were found similar. We investigated the merits of the time-temperature superposition of the α- and of the αR-relaxations. The αRrelaxation vanished when the free chains were extracted from the SBR by swelling-extraction procedure. The features of α-relaxation can be interpreted as segmental motions of the SBR, but the features of the αR-relaxation is be interpretable only in terms of tube model for diffusion of free chains in a lightly cross-linked structure. We estimated the mol wt of these free chains and the mass between entanglements, and then used these quantities to calculate, by using the Doi-Edwards formalism, the relaxation time of the αR-process at 293 K. This relaxation time agrees with the experimental value. The αR-relaxation seems consistent with the tube model for diffusion of free chains in the cross-linked structure of SBR. © 2014 Elsevier Ltd. All rights reserved

Influence of annealing treatments on the essential work of fracture of biaxially drawn poly(ethylene terephthalate)

cited By 3International audienceThe biaxial sequential stretching process of poly(ethylene terephthalate) produces films with a fibrillar microstructure in which fibrils are parallel to the transverse extrusion direction. The mechanical properties of such films are strongly anisotropic due to both the orientation of crystallites and the properties of the intrafibrillar and interfibrillar amorphous phases. The idea is to modulate the properties of the amorphous phase without altering the fibrillar structure by annealing treatments. The morphology (crystallinity and orientation of the crystalline phase) of the annealed films is characterized and their mechanical properties (tensile tests and essential work of fracture) are tested in the longitudinal direction (parallel to the micro fibrils) and in the transverse direction (perpendicular to the micro fibrils). The crystalline phase orientation is the key parameter governing modulus anisotropy. Concerning crack propagation, annealing treatments lead to opposite evolution of the specific essential work of fracture parameter (w e) in the longitudinal and transverse directions. Thus, it is possible to erase fracture propagation anisotropy through an adequate annealing treatment. © 2012 Society of Chemical Industry

Different dynamic behaviors of the dissociation and recombination reactions in a model calculation of polyethylene by first-principles steered molecular dynamics simulation

cited By 0International audienceWe investigate the different dynamics of the stress-induced dissociation and recombination reactions in a model of polyethylene by a first-principles molecular dynamics simulation at the B3LYP/6-31g(d) level. The dissociation under external forces acting on the chemical reaction site at 300 K follows the same pathway as the one calculated by the static first-principles method because it has a similar activation barrier to that of the static first-principles calculation. On the other hand, in the recombination process, thermal fluctuations causes collisions between hydrogen atoms at the chain ends. Furthermore, when external forces do not directly act on the chemical reaction site, two different dissociation processes are observed. On the other hand, recombination process is not observed due to rarely contact of the radical carbon. These results indicate that dissociation and recombination dynamics are very different, showing the importance of the dynamic calculation. © 2015 Elsevier B.V. All rights reserved

Carbon black filled thermoset with controlled microstructure for electrical applications

This paper describes recent developments in carbon-black epoxy composites. Firstly it is observed that AC and electrical properties versus volume fraction of carbon black are in manifest disagreement with predictions of the statistical percolation theory both for real and imaginary parts of the conductivity. They exhibited, in particular, an abrupt variation at a very low fraction of carbon black. The onset of conduction in these composites arose from an precipitous clustering of the conductive particles rather than a “conventional” percolation transition in random insulator-conductor mixture. A simple model based on electrostatic repulsion between electrically charged carbon black particles accounts qualitatively for this singular insulator to conductor transition. Secondly, it is shown that the final microstructure could be largely modified by application of a static electric field on the composite during the curing. The final conductance of the composites can by this way be customized within several orders of magnitude. In the application point of view this method could directly be used to derive heterogeneous composites with controlled electrical properties from homogeneous compounds

Nucleation and nonisothermal crystallization kinetics in cross-linked polyethylene/zinc oxide nanocomposites

cited By 8International audienceIn the present study organic-inorganic hybrid nanocomposites of cross-linked polyethylene (XLPE) with 0, 2, 5 and 10 wt% of trimethoxyoctyl- silane surface treated ZnO nanoparticles were prepared by melt mixing. Non-isothermal crystallization kinetics is examined in detail to reveal the crystallization characteristics of the cross-linked system (XLPE) in the presence of nanomaterials (ZnO) as the dispersed phase. Based on the diffusion controlled growth theory, all the nanocomposites of the present system, exhibit a constant nucleation rate or the growth of nanometer aggregates that constitutes nuclei, with an increasing nucleation rate. Non-isothermal crystallization kinetic parameters and the theoretical estimation of nucleation activity certify the nucleating capability of ZnO nanomaterials in the cross-linked continuous phase of XLPE. The experimental results confirmed that, even at very fast cooling rates, the promising role of nanoparticles for nucleation is able to compensate for the negative effect of fast cooling. © 2014 the Partner Organisations

Modeling of Anisotropic Magneto-Rheological Elastomers for Mechanical to Electrical Energy Conversion

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