Formation of Fractal-like Structure in Organoclay-Based Polypropylene Nanocomposites

We present the structural features of organoclay dispersions in polypropylene melts investigated by shear rheology. Scaling behavior of the nanocomposites linear viscoelastic properties based on apparent yield stress and critical strain measurements enables to assess the fractal dimension <i>d</i>_f of the network formed by clay particles within the matrix. The network structure induces a thixotropic behavior which manifests by solid-like behavior accentuation over time under quiescent conditions and sensitivity to large deformation shear flow. Formation kinetics of the fractal-like network structure at rest is discussed through linear and nonlinear rheological investigations. A two-step process is observed for clay network reorganization over annealing time, with pronounced transition around 10⁴ s. These phenomena, which picture a nonequilibrium state where interparticle attractions favor disorientation of the platelets and network growth, are strongly coupled to the dispersion state of the organoclay within the polymeric matrix.