Rheology of polypropylene/clay hybrid materials

The melt-state linear and nonlinear shear rheological properties of hybrid materials of polypropylene and amine-exchanged montmorillonite were studied . The materials were prepared by melt mixing with maleic anhydride function alized polypropylene as the compatibilizer. The clay interlayer spacing las determined by wide-angle X-ray scattering) increased upon melt mixing; how ever, the short-range ordering of the clay layers was preserved. Above inor ganic loadings of 2.0 wt % the hybrid materials exhibited apparent low-freq uency plateaus in the linear viscoelastic moduli. The hybrid storage modulu s was sensitive to the chemistry of the amine exchanged into the clay. The amount of stress overshoot observed in flow reversal experiments was found to be a function of the rest time allowed between the reversal. The transie nt stress in start-up of steady shear scaled with the applied strain. These observations allow features of the polypropylene/montmorillonite hybrid st ructure to be deduced. The transient nonlinear rheology is consistent with an anisometric, non-Brownian structure. These anisometric particulate domai ns are mesoscopic, and internally, they contain multiple, ordered platelets . This mesoscopic structure is itself thermodynamically unstable, because t he rheology indicates that quiescent structural evolution whose origin is n ot Brownian relaxation is observed. The demonstration of the sensitivity of melt-state rheological measurements to interparticle structure and chemist ry of the hybrid materials indicates the potential usefulness of such studi es for the development of new nanocomposite materials.