Promoting Dispersion Of Graphene And Morphology Evolution Of Polymer Blends By Water-assisted Mixing Extrusion And Their Mechanisms

Melt mixing extrusion is a low-cost and efficient method for preparation of polymer-based nanocomposites.It is a challenge to achieve good exfoliation and dispersion of nano-fillers in polymer matrix for preparation of high-performance polymer-based nanocomposites.For the preparation of poly(vinylidene fluoride)(PVDF)/graphene,polystyrene(PS)/PVDF and polyamide 6(PA6)/PVDF blends and their composites,the screw elements of a twin-screw extruder are assembled based on polymer melt-flow simulation for continuous water-assisted mixing extrusion(WAME).By improving graphene dispersion state,the thermal conductivity and dielectric property of the composites with low graphene content are improved.The effect of water on the rheological properties of polymers is investigated by high pressure rheometer.Combining the microstructure and macro-properties of the composites,the mechanisms of water promoting the dispersion of graphene and morphology evolution of polymer blends during WAME are revealed.According to the results of numerical simulations of polymer melt flow in flow domians formed by diffirent screw configurations,arranging kneading block especially kneading block with a stagger angle of 90?upstream of reverse-screw element can help to increase the melt pressure in the whole flow domian,thereby keeping the injected water liquid during mixng extrusion.Specific capillary number of droplets in the flow domians formed by different screw configurations is calculated,and is used to evaluate the dispersive mixing efficiency of different screw configurations for droplets.The results show that using screw configurations containing kneading blocks can obtain a higher maximum specific capillary ratio,that is,a higher dispersive mixing efficiency.PVDF/graphene oxide(GO)nanocomposites are fabricated for the first time by one-step WAME via injecting GO suspension into PVDF melt.TEM micrographs demonstrate that the GO layers are well exfoliated and dispersed in the PVDF matrix.Well dispersed GO layers increase the crystallinities especially the?-phase of the PVDF due to the special interaction between carbonyl groups at the GO surface and hydrogen atoms in the PVDF.The GO layers are found to reinforce the PVDF matrix,with the addition of 0.6 wt%GO,the Young's modulus and tensile strength of the material are increased by 31.2%and 12.3%,respectively.Predicted aspect ratio of the GO layers using the Halpin-Tsai equation is much larger than that in the PGOm samples,which may be attributed strong interaction between the PVDF chains and GO layers.The mechanism of water promoting the exfoliation and dispersion of GO during WAME is analyzed from interactions among the PVDF chains,GO layers,and water molecules during the WAME.WAME simultaneously promotes the in situ thermal reduction of GO in PVDF matrix.The injected water accelerates the removal of oxygen-containing groups especially C-O group and the transformation of carbon sp~3 into sp~2 bonds of the GO through enhancing the interaction between the PVDF and GO and facilitating the hydrothermal dehydration of GO.The nucleation of crystallites at the PVDF-GO interfaces can help to reduce the interfacial thermal resistivity.Thus,the thermal conductivity of the PVDF/GO nanocomposites prepared with water injection was significantly improved.Compared with the neat PVDF sample,the enhancements in the thermal conductivities are 64.1%and 132.5%for the nanocomposites with low loading of 1.0 wt%prepared without and with water injection,respectively.WAME was used to prepare PS/PVDF-GO blend composites.With the addition of GO,the size of the PVDF phase in the PS/PVDF-GO composites gradually decreases,and the elongation at break of the composites gradually increases.These demonstrate that the GO acts as a compatibilizer in the blend.The compatibilization effect of the GO is promoted by the injected water because it facilitates the migration of the GO layers in the PS phase to the interfaces between the PS and PVDF phases during melt mixing.The compatibilization effect of the GO on the PS/PVDF blend can be explained by good interfacial interactions between the oxygen-containing groups at the GO surface and the molecular chain of PVDF and the?-?stacking between the basal planes of GO layers and the phenyl rings of PS,and the coalescence suppression effect of the PVDF domains due to the steric hindrance.Under high pressure condition with water injection,the complex viscosity,elastic modulus,and loss modulus of PVDF especially PA6 decreases.The decreased complex viscosity can be attributed to the diffusion of water molecules in their melt,leading to increased free volume of their molecular chains and then mobility of them.The droplet size of the dispersed phase PVDF in the PA6/PVDF(50/50)blend prepared by WAME increases,which is attributed to the increases of the viscosity ratio of PVDF to PA and the difference of elastic forces between them in flow.The dispersed phase PA6 in the PA6/PVDF(40/60)blend prepared by WAME is broken into small droplets due to the decreases of the viscosity ratio of PA to PVDF and the difference of elastic force between them in flow.WAME is used to prepare PA6/PVDF(50/50)/expanded graphite(A50F50EG-W)blend composite.The injected water facilitates the PA6 molecular chains to intercalate and exfoliate the EG,leading to increased dielectric constant of the blend composite.Compared with the PA/EG composite with the same EG content(2.5 vol%)prepared by WAME(PA/EG-W),A50F50EG-W blend composite exhibits a higher dielectric constant and maintains a low dielectric loss.At frequency of 100 Hz,the dielectric constant of the blend composite is 33.2,which is almost twice as much as that of the PA/EG composite(18.2).And the dielectric loss is only 0.11.This is mainly due to the fact that,on the one hand,the“volume-removal”effect of the dispersed phase PVDF leads to a higher EG content in the PA phase of the blend than that in the PA/EG composite.So the possibility of forming a micro-capacitance in the A50F50EG-W blend composite increases,leading to significantly increased dielectric constant;on the other hand,the dispersed phase PVDF prevents the directly contact of EG layers in the composites to some extent,thus decreasing the dielectric loss of the A50F50EG-W blend composites.