Experimental Observations and Quantitative Predictions of Rheological Properties of Organoclay/Poly(propylene) Nanocomposites at the Startup of Shear Flows

We study the rheological characteristics of nanocomposites containing nano-sized plate like particles in a viscoelastic fluid at the startup of steady state in the simple shear flow mode. The nanocomposites of organoclay-polypropylene with different nanoclay contents were prepared by melt mixing. A rheological equation of state, originally formulated to predict the orientation state and viscoelastic behavior of suspensions of ellipsoidal particles in polymer melts, has been modified to describe the observed phenomena for the nanoclay/poly(propylene) composites. The rotational particle motion and alignment for a group of symmetric ellipsoids with the applied flow field are investigated. Additionally, model calculations of the macroscopic rheological properties for a simple flow case suggest the presence of nano-particles significantly modify the suspended fluid at volume concentrations as low as 0.5%. The model calculations for the startup viscosity are reasonably in agreement with the experimental results at the experimental range covered in this study. At the shear rate of , we observe pronounced stress overshoots at the three nanoclay loadings level tested which are found to be related to the fast alignment of the silicate layers with the shear direction in the polymer melt.