Effects Of Processing Flow Field On Morphology And Properties Of Polypropylene/Graphene Nanoplatelets Nanocomposites

The discovery of graphene has opened up a vast area of researchand nowadays graphene applications are developed in manyfields.Graphene has remarkable properties,such as high thermal conductivity,superior mechanical properties and excellent electrical properties.Compared with carbon nanotubes,graphene has a higher surface-to-volume ratio and a lower production cost.Therefore,graphene is one of the most promising nanofillers for polymers for enhancing the mechanical,electrical and thermal properties.However,as for other nanofillers,the mainchallenge in manufacturing polymer/graphene nanocomposites bymelt blending is related to the exfoliating process and the control offinal dispersion of graphene into the polymer matrix.Namely,it is very difficult to obtain well-exfoliated and well-dispersed nanocomposite structures.Therefore,this paper discusses the effects of both materials structure and processing method on the properties of polypropylene(PP)/graphene nanoplatelets(GNPs)composites.Using FESEM.XRD?DSC?TG?FTIR and the measurement results of electrica,thermal and rheology properties,combing with the classical calculation model and finite element analysis to find out the relationship among processing-structure-properties.For better analyzing the effects of processing flow field on the morphology development and dispersion mechanism of polypropylene/graphene nanoplatelets composites,this paper primarily discuss GNPs sizes' effects on conductive network formation,as the GNPs sizes would be changed during extrusion processing.The specific contents are as follows:(1)Electrical percolating composites of polypropylene,(PP)filled with five different graphene nanoplatelet(GNPs)fillers,and their hybrid systems were prepared using melt blending.The effect of GNPs size and their hybrid system on the conductive network formation is investigated.The formation of a conductive network can be affected by the structure and morphology of GNPs of different sizes.The GNPs with a larger diameter and thinner thickness are beneficial to produce a conductive network.The conductivity of the PP/GNPs composite depends on the aspect ratio of the GNPs when the content exceeds the percolation threshold.However,when the GNPs content is near the percolation threshold,both diameter and dispersion of the GNPs can affect the conductivity significantly,and electron-tunneling theory should be taken in account.The highest electrical conductivity was obtained for a PP/large-diameter-GNPs/medium-diameter-GNPs hybrid-system.To explain the hybrid system,an"island-bridge”-structured conductive-network is proposed.The better conducting network may be due to scattered“islands" that connect with each other via a long"bridge".This bridge links the islands for better charge-transport across the GNPs and the obstruction of PP matrix,which enables the formation of a better conducting network.Even though GNPs with small diameter show perfect dispersion,they contribute less to the formation of a conductive network.(2)Four different screw configurations were designed to prepare polypropylene(PP)nanocomposites reinforced with graphene nanoplatelets(GNPs).The relationship between the shear field,the morphology evolution of GNPs in PP and the properties of PP/GNPs nanocomposites were investigated.Finite element method software,POLYFOLW,was used to quantify the shear flow field in the four screw configurations.The morphology development was analyzed by field-emission SEM and optical transmission microscopy.The dispersion mechanism of GNPs in the PP matrix under the effect of a shear field was simulated showing that left-handed kneading elements resulted into the longest residence time and the addition of mixing elements resulted in the highest shear stress.Calculation results showed that during the extrusion process,large GNPs agglomerates(above 4O?m)tend to be exfoliated into smaller ones by two dispersion mechanisms corresponding to rupture and erosion.Since the shear stress provided by the twin-screw extruder configuration was limited,small agglomerates(below 40?m)would exfoliate to thin layers only by following an erosion mechanism.The erosion mechanism and GNPs migration resulted to be mainly affected by the residence time.By increasing the residence time,nanocomposites resulted to be better dispersed and the particle size distribution was more homogeneous.Moreover,we found that the more the GNPs agglomerates thickness can be reduced by exfoliation,the more connection can established within the GNPs network.The improvement of screw configurations resulting in the better dispersion of GNPs would increase the electrical(from 10-12 up to 10-4 S/m when the GNPs content was 6wt.%)and thermal conductivities and to significantly reduce the percolation threshold of electric conduction.(3)A ultrasound-assisted extrusion system was added to melt extrusion process to prepare polypropylene(PP)nanocomposites reinforced with graphene nanoplatelets(GNPs).The relationships among the ultrasound vibration,exfoliation and dispersion morphology of GNPs in PP matrix,the crystallinity and the macroscopic properties of nanocomposites were investigated.The properties measurement results showed that the present of ultrasound vibrations increased the conductive properties,decreased the apparent viscosity and crystallinity of PP/GNPs nanocomposites.FESEM results revealed that the ultrasound vibration increased the exfoliation and dispersion of GNPs in PP matrix.This morphology was benefit for forming electrical and thermal network,therefore the electrical conductivity and thermal conductivity of PP/GNPs nanocomposites were increased.But the powerful vibration that provided by 300W ultrasound power would reduce the diameter of GNPs,then reduce its conductive properties.FTIR and TGA results showed that ultrasound vibration had less effect on the chemical bond and the degradation of PP/GNPs nanocomposites.