Characterization Of Flow-induced Structures In Carbon Nanotube Suspensions

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 semidilute 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 was enhanced 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 noncentral 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 flow reversal 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.