Rheology of multiwall carbon nanotube suspensions

Abstract

Carbon nanotubes have exhibited unusually large changes in selective physical and mechanical properties when added to polymers or polymer composites in small quantities. To understand their rheological behavior and processibility, we mixed multiwalled carbon nanotubes (MWNT) in epoxy and created suspensions of different dispersion qualities, MWNT aspect ratios, concentrations, suspension network structures, and MWNT orientation states. Their rheological properties were measured with a cone and plate rheometer. It was found that as MWNT dispersion quality improved or their network connections, the aspect ratio or concentration increased, the MWNTs interactions became stronger as indicated by a higher storage modulus G′, complex viscosity ∣η*∣, and steady shear viscosity η. It was found that suspensions which contained a mixture of separated MWNT along with small MWNT aggregates exhibited G′ that was independent of frequency suggesting solidlike behavior. This frequency sweep method could be used to characterize the network structure state for well dispersed nanotube suspensions. The steady shear viscosity η was found to be more sensitive to MWNT separation than G′ and ∣η*∣. All suspensions exhibited shear thinning behavior and nanotubes showed statistically significant alignment in the direction of the shear flow. Aligned MWNT suspensions not only displayed lower η than randomly orientated MWNT suspensions, but also had lower G′ and ∣η*∣ values. Transmission electron microscope (TEM) images of cured suspensions and a recently developed capillary method was used to validate the MWNTs suspension state and its microstructure.