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 10(2) to over 10(4) Pa, and a yield stress ranging from 4 to over 10(3) Pa as the clay loading increases from 1 to 10 wt %. Creep and recovery measurements show that these nanodispersions possess (a) a viscosity eta ranging from 4000 to nearly 10(8) Pas, (b) a quasisteady state compliance J(e)(0) whose reciprocal is comparable to G', and (c) a retardation time etaJ(e)(0) over 10(3) s, in agreement with the stress relaxation experiment. Upon the stress-induced sol-gel transition, the dispersion viscosity may drop by a factor of 10(7). 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. (C) 2003 The Society of Rheology. [DOI: 10. 1122/1.1545074].