The Rheology and Microstructure of Carbon Nanotube Suspensions.
Carbon nanotubes (CNTs) are structures of graphitic carbon which can be dispersed in aqueous or polymeric matrices to enhance their mechanical, electronic and thermal properties. It is important to study the microstructure and rheology of such suspensions to improve material processing and product design. This project aims to use rheological data and microscopy techniques to generate a model which successfully predicts the linear and nonlinear viscoelasticity and shear thinning behaviour seen for CNT suspensions. In addition, as part of this project, CNTs were successfully dispersed in an agarose matrix.
CNTs are commonly produced in bundles, held together by attractive van der Waals forces. In order to form a well-dispersed CNT suspension it is necessary to both break apart the CNT bundles and to stabilise the nanotubes in suspension by preventing aggregation.
The most common dispersion techniques for suspensions in a relatively low viscosity medium are sonication (using ultrasound) or mixing under high shear.
|Confocal microscopy images of CNTs suspended in Epoxy after (a) 5 min and (b) 60 min of mixing by sonication.|
The microstructures of multi-walled carbon nanotubes in agarose, epoxy and polypropylene have been imaged using confocal, optical and transmission electron microscopy with varying degrees of success. It can be beneficial to use imaging techniques with differing resolutions so that the full hierarchy of CNT structures can be appreciated.
|Confocal microscopy image of 0.05 wt% CNTs suspended in agarose at (a) low magnification and (b) high magnification|
The figure below shows the linear viscoelastic response of a 5 wt% suspension of CNTs in polypropylene. The plateau in G' and G" at low frequencies is characteristic of the presence of an interconnected network of CNTs. Fitting a 5-mode Maxwell model to the data demonstrated the long relaxation times associated with CNT suspensions.
|Linear viscoelastic behaviour of a 5 wt% suspension of CNTs in polypropylene|
|Relaxation time and modulus values obtained when fitting a five-mode Maxwell model to the frequency sweep data for the 5 wt% suspension of CNTs in polypropylene|