Ubiquity of soft glassy dynamics in polypropylene-clay nanocomposites

The rheology of polypropylene-clay nanocomposites was studied as a function of organoclay loading, degree of exfoliation, and presence of maleic anhydride functionalized polypropylene compatibilizer. Samples exhibit varying degrees of solid-like response in the terminal regime of small-amplitude oscillatory shear (SAOS), certifying that differences in clay silicate delamination were achieved for fixed organoclay loading. Previous work has also demonstrated that mechanically percolated nanocomposites exhibit logarithmically increasing storage modulus with time at low frequency, behavior attributed to the continuous development of a mesoscale organoclay network akin to that observed for colloidal gels. Continuous low frequency SAOS experiments not only affirm such behavior but also reveal that it is ubiquitous to polypropylene-clay nanocomposites, including samples whose organoclay loading and extent of exfoliation place them below the ostensible mechanical percolation threshold. Similar experiments conducted on uncompatibilized samples support the analogy to soft glassy dynamics, whereby van der Waals attractions drive the formation of a heterogeneous, gel-like organoclay network. Intermolecular associations between pendant group functionalities on the compatibilizer have contributed to logarithmic increases in the storage modulus with time for pure maleated samples, but the reduced concentrations of maleated polypropylene present in the materials reported here are shown to not influence the solid-like rheology over time through network formation. Thus, we demonstrate that only organoclay network formation is responsible for the time-dependent rheology in polyolefin nanocomposites.