Preparation and characterization of multiwalled carbon nanotube dispersions in polypropylene: Melt mixing versus solid-state shear pulverization

Dispersions of multiwalled carbon nanotubes (MWNT) in polypropylene (PP) were prepared via conventional melt batch mixing and solid-state shear pulverization. The properties and structure of each system were assessed via linear viscoelasticity, electrical conductivity, PP crystallization kinetics, dynamic mechanical analysis, scanning electron microscopy, and small angle X-ray scattering. Increasing either the duration or the intensity of melt mixing leads to higher degrees of dispersion of MWNT in PP, although at the cost of substantial melt degradation of PP for long mixing times. Samples prepared by pulverization exhibit faster crystallization kinetics and higher mechanical stiffness than the melt blended samples, but in contrast show no measurable low frequency elastic plateau in melt rheology, and lower electrical conductivity than melt-mixed samples. X-ray scattering demonstrates that neither sample has uniform dispersion down to the single MWNT level. The results illustrate that subtle differences in the size and distribution of nanotube clusters lead to differences in the nanotube networks with strong impact on bulk properties. The results also highlight distinctions between conductive networks and load transfer networks and demonstrate that a complete and comparative picture of dispersion cannot be determined by simple indirect property measurements.