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• W1610728681 abstract "Les nanotubes de carbone sont des nanoparticules fibreuses presentes dans de nombreuses applications. Grâce a leur importante surface specifique apparente, leur densite de courant electrique elevee, leur stabilite thermique et leurs excellentes proprietes mecaniques, les nanotubes de carbone sont utilises pour ameliorer les proprietes physiques des matrices polymeriques.Les proprietes macroscopiques des suspensions proviennent de leurs proprietes a l’echelle du micron et du submicron. La structure des suspensions peut etre facilement influencee par de nombreux parametres tels que les forces de cisaillement externes, la concentration de la suspension, la temperature, les caracteristiques des particules, etc. L’etude de la structure de la suspension represente un defi majeur et devient un sujet d’interet grandissant pour de nombreux chercheurs.Dans cette etude, la structure d’une suspension modele de nanotubes de carbone dispersee dans une resine epoxy est etudiee en utilisant un ensemble de methodes rheologiques, les theories fractales et de mise a l’echelle ainsi qu’un modele structural thixotropique. L’effet de l’histoire d’ecoulement sur les proprietes viscoelastiques lineaires des suspensions et l’evolution de la structure lors de l’arret du cisaillement ont ete investigues pour une large gamme de vitesse de pre-cisaillement, de concentrations et de temperatures. Les resultats de ces analyses sont les suivants.L’effet de l’histoire d’ecoulement s’est avere plus prononce pour les suspensions diluees ou semi-diluees. Les faibles vitesses de pre-cisaillement entrainent un plus grand nombre d’enchevetrements entre les particules. Cela resulte de la reduction du seuil de percolation rheologique. Pour les suspensions diluees ou semi-diluees, apres l’arret du cisaillement, lesvidifferentes structures metastables formees ont pu etre distinguees par leurs differents modules de conservation qui sont inversement relies a la vitesse de pre-cisaillement. Pour les suspensions concentrees, les structures metastables formees ont donne des modules de conservation equivalents quelque soit le pre-cisaillement applique. Il a ete montre que la vitesse de formation de ces structures metastables augmentait avec l’augmentation de la concentration. De plus, pour les concentrations faibles et intermediaires, la vitesse de formation des structures metastables decroit lorsque l’on augmente la vitesse de pre-cisaillement applique, alors que pour les concentrations elevees, le pre-cisaillement n’a pas d’influence sur la formation des structures.Il a ete trouve que le module elastique des structures metastables formees est relie aux vitesses du pre-cisaillement appliquees selon une loi de puissance; les parametres de celle-ci dependent fortement de la concentration. Cette correlation est applicable aux resultats a cisaillements constants pour les suspensions formant une courbe maitresse sur une large echelle de concentrations, au-dessus et en-dessous du point de gelification. Cela illustre egalement l’importance du module de conservation des structures metastables comme un parametre representant l’evolution de la structure. A la lumiere des theories fractales et de mise a l’echelle, il apparait dans cette recherche que les suspensions de nanotubes etudiees sont classees comme des suspensions a floculation lente pour lesquelles l’elasticite des structures provient a la fois des liaisons inter- et intra-flocs. De plus, le potentiel d’interaction des suspensions est une combinaison des composants centraux et non-centraux. La faible sensibilite de la dimension fractale des suspensions a l’histoire d’ecoulement est en accord avec le module de conservation constant des structures metastables, qui est faiblement influence par la vitesse de pre-cisaillement proche et au-dessus du point de gelification.viiEtant donne que les forces de cisaillement perturbent l’etat de dispersion et d’enchevetrement des particules, cela peut causer la formation de structures sous ecoulement ou la distorsion des structures dependant de la concentration et de la vitesse du pre-cisaillement appliquee. En comparant le module de conservation des suspensions sans pre-cisaillement et celui des structures metastables apres pre-cisaillement a differentes vitesses, une vitesse de pre-cisaillement critique a ete determinee pour les concentrations faibles et intermediaires au-dessus desquelles des enchevetrements de nanotubes se sont rompus; cela reduit leur elasticite et provoque la formation incomplete de la structure au repos. L’evolution de la structure est en accord – qualitativement – avec les predictions d’un modele structural thixotropique.A la difference de nombreuses suspensions de fibres et de nano-composites, le mouvement Brownien est un mecanisme influencant la formation de la structure de la suspension de nanotubes de carbone modele en l’absence d’ecoulement. Cette conclusion a pu etre formulee grâce a une analyse quantitative de la vitesse de formation de la structure selon differentes temperatures conjointement a la reponse de la suspension en demarrage dans des directions opposees.----------Carbon nanotubes (CNTs) are fibre-like nano-particles with many different applications. Due to their high specific surface area, high electric current density, thermal stability and excellent mechanical properties, they are used to reinforce physical properties of polymer matrices.The macroscopic properties of suspensions are inherited from their properties at micron and sub-micron scales. The suspensions structure can be easily influenced by many parameters such as the extent of external shear forces, the suspension concentration, temperature, the particles specifications, etc. This makes the study of the suspension structure a very challenging task and has been the subject of interest to many researchers.In this thesis, the structure of a model carbon nanotube suspension dispersed in an epoxy is studied by employing a set of rheological methods, scaling and fractal theories and a structural thixotropic model. The effect of flow history on linear viscoelastic properties of suspensions and the evolution of structure upon cessation of shear flow has been studied over a wide range of pre-shearing rates, concentration and temperature. The results of these analyses are as follows.The effect of flow history is more pronounced on the suspensions structure in dilute and semi-dilute concentration regimes. By pre-shearing at low rates, more inter-particle entanglements were induced, which resulted in reduction of rheological percolation thresholds. After cessation of shear flow, for dilute and semi-dilute suspensions, the formed metastable structures were distinguishable by different storage moduli, which were inversely related to the rate of pre-shearing. However, for the concentrated suspensions, the formed metastable structures had an approximately equal storage modulus regardless of the rate of the applied pre-shearing. It was shown that the rate of formation of these metastable structures wasixenhanced by increasing concentration. Furthermore, the rate of structure build-up decreased by increasing the applied pre-shear rate in low and intermediate concentrations, while it remained almost intact with respect to the pre-shearing rate at high concentrations.It was found that the elastic modulus of the formed metastable structures scaled with the applied pre-shear rate in a power-law form, the parameters of which strongly depended on the concentration. As a result, scaling the steady shear results of the suspensions using this correlation formed a master curve over a wide range of concentrations below and above the gel point; this illustrated the importance of the storage modulus of metastable structures as a parameter, which represented the parameters involved in the evolution of structure. The conducted research in the light of scaling and fractal theories revealed the fact that the model CNT suspensions under investigation was classified as slowly flocculating suspensions in which the elasticity of structures originated from both the inter- and intra-floc links. Moreover, the interaction potential of the suspensions was a combination of central and non-central components. The less sensitivity of the fractal dimension of the suspensions to the flow history was in agreement with the invariant storage modulus of the metastable structures, which was barely influenced by the rate of pre-shearing near and above the gel point.Since application of shear forces disturbed the state of dispersion and particle entanglements, it may cause formation of some flow-induced structures or distortion of structures depending on the concentration regime and the rate of the applied pre-shearing. By comparing the storage modulus of the suspensions without pre-shearing and the one for the metastable structures after pre-shearing at various rates, a critical pre-shear rate was found at low and intermediate concentrations above which some nanotube entanglements broke down; this reduced their elasticity and resulted in the incomplete structure build-up at rest during transient flowxreversal measurements. The structural evolution that has been explained so far was shown to be in qualitative agreement with the predictions of a structural thixotropic model.Unlike many fiber suspensions and nano-composites, the Brownian motion was an influential mechanism in structure build-up of the carbon nanotube model suspensions in the absence of flow. This was concluded by a quantitative analysis of the rate of the structural build-up under the variation of temperature in conjunction with the extent of structure reconstruction at rest in a set of transient stress growth measurements in opposite directions." @default.
• W1610728681 created "2016-06-24" @default.
• W1610728681 creator A5014146599 @default.
• W1610728681 date "2011-09-01" @default.
• W1610728681 modified "2023-09-27" @default.
• W1610728681 title "Characterization of Flow-Induced Structures in Carbon Nanotube Suspensions" @default.
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