The rheology and modeling of chemically treated carbon nanotubes suspensions

This paper reports recent experimental findings and rheological modeling on chemically treated single-walled carbon nanotubes (CNTs) suspended within an epoxy resin. When a CNT suspension was subject to a steady shear flow, it exhibited a shear-thinning characteristic, which was subsequently modeled by a Fokker-Planck (FP) based orientation model. The model assumes that the shear flow aligns CNT in the flow direction, but there are events such as Brownian motion and tube-tube interaction trying to randomize the orientation. In the FP orientation model, randomizing events were modeled with an appropriate rotary diffusion coefficient (D-r) and the shear-thinning behavior was explained in terms of progressive alignment of CNTs toward the shear direction. In terms of linear viscoelasticity (LVE), small-amplitude oscillatory measurements revealed mild elasticity for semidilute treated CNT suspensions. The exact origin for this elasticity is not clear and both tube-tube interaction and bending/stretching of CNTs have been proposed by other authors as possible origins. It is, however, clear from the current modeling that the experimental evolution of storage modulus (G') cannot be described using a single-mode Maxwell model or simple Brownian rod modeling. In this paper, experimental LVE data of the treated CNT suspensions were fitted using the FP orientation model with an "effective diffusion coefficient" term and an empirical relation was subsequently identified for the effective diffusion term. Intuitively, chemical treatment has created a weakly interconnected network of CNT and it is believed that the mild elasticity originated from this weak network as well as other randomizing events (Brownian motion and tube-tube hydrodynamic interaction). Finally, step strain experiments confirmed the presence of a weak network at small strains, which at large strains was found to be destroyed. Incorporation of a strain softening factor allowed for the formulation of a self-consistent FP based orientation model describing both the steady shear and LVE responses of treated CNT suspensions. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3093105]