Exfoliation and yield behavior in nanodispersions of organically modified montmorillonite clay

This work explores the rheological behavior of exfoliated dispersions of montmorillonite clay in xylene. The clay-based nanodispersions elucidate how the incorporation of silicate particles produces networking upon ultrasound induced exfoliation, thus offering useful insight into the reinforcement mechanism in actual polymer nanocomposites. These dispersions exhibit a storage modulus plateau $G′$ ranging from $102$ to over $104 Pa,$ and a yield stress ranging from 4 to over $103 Pa$ as the clay loading increases from 1 to 10 wt %. Creep and recovery measurements show that these nanodispersions possess (a) a viscosity η ranging from 4000 to nearly $108 Pa s,$ (b) a quasisteady state compliance $Je0$ whose reciprocal is comparable to $G′,$ and (c) a retardation time $ηJe0$ over $103 s,$ in agreement with the stress relaxation experiment. Upon the stress-induced sol-gel transition, the dispersion viscosity may drop by a factor of $107.$ Consistent with the yield behavior, the dispersion displays a steady shear viscosity in the controlled rate mode that scales reciprocally with the applied shear rate. Finally, these dispersions suffer from apparent wall slip at a critical stress proportionally lower than the yield stress at different clay loadings. This wall slip is preventable by attaching sandpapers onto the parallel plate surfaces of the flow cell.