Dispersion of graphite nanoplates during melt mixing

Graphite nanoplates (GnPs) are exfoliated graphite particles in the form of thin flakes—with large surface areas and nanosized thicknesses— that tend to agglomerate because of van der Waals interactions. GnPs are commonly used to reinforce composite materials. It is expected that maximum composite performance levels should occur when GnP agglomerates are dispersed within the polymer matrix as primary 2D particles and they have good interfacial adhesion. Interfacial bonding between the GnPs and the polymer can be improved by tailored chemical functionalization of the graphite surfaces.1–3 Dispersion of GnP agglomerates in polymers, however, is complex. Melt compounding is an economically attractive process that is used successfully in industrial applications to prepare polymer composites. Previous studies of melt compounding with carbon nanofibers4 and carbon nanotubes5, 6 used a variety of mixing equipment (e.g., extruders, internal mixers, and prototype mixers). These investigations demonstrated that the intensity and type of hydrodynamic stresses, residence time, and interfacial adhesion play a major part in nanoparticle dispersion. Although the dispersion mechanism of carbon nanotubes in polymers is well studied, the equivalent GnP literature is still mostly focused on composite properties and applications.7 Few studies concerning the dispersion of GnPs through melt mixing methods have so far been reported. We have monitored the progression of dispersion for ‘as-received’ (i.e., pristine) and functionalized GnPs within a polypropylene (PP) matrix to investigate this complex process.8 Our prototype small-scale continuous mixer, which we used for our experiments, is shown in Figure 1. With our device we create repetitive converging/diverging flow sequences along its length. As such, a strong extensional stress component is generated. We are also able to precisely control flow rate and temperature.4, 5 Once the experiments are completed, we can quickly dismantle the equipment so that the material samples at the various axial locations can be collected for subsequent characterization. We obtained the functionalized graphite nanoplates (F-GnPs) through Figure 1. Schematic diagram of the prototype small-scale continuous mixer. : Diameter.