RGO@polyHIPE Composite Foams:porous Structure Regulation And Piezoresistive Property Investigation

The foam-structured piezoresistive material has the advantages of light weight,softness,breathable and large specific surface area.This type of piezoresistive materials has been widely studied in the recent years.There are some results that the properties of the foam-structured piezoresistive materials were closely related to their porous structure.However,these materials are mostly prepared by the "solid template method".Due to the limited ability to adjust pore structure,the current researches on the relationship between "porous structure" and "compressive performance" of composite materials is rather limited.The paper uses a "high internal phase emulsion(HIPE)template method" to obtain a series of composite materials with different pore structures by changing the pH value of the internal phase,GO concentration,GO sheets size and shear strength.The formation mechanism of pore structure and the influence of pore structure on the mechanical properties,electrical properties and piezoresistive properties of the materials were investigated.The following research results were obtained:1.The interfacial activity of GO increases with the decrease of pH value,but even if the pH value is as low as 3,the interfacial activity is still much lower than that of Span 80.Therefore,the W/O interface is mainly stabilized by Span 80 even in the presence of GO.Therefore,the interfacial tension is almost unaffected by the pH value and the amount of GO.2.At low pH,GO tends to aggregate in NaCl solution,and the size of the aggregate increases with the decrease of pH value,the increase of GO sheet size,the decrease of shear strength and the increase of GO concentration.In W/O HIPE,these aggregates together with Span 80 stable water droplets would act as an emulsion template.Based on the above properties of GO,combined with the changes of pH value,GO dosage,shear strength and GO sheet size,the size and distribution of HIPE droplets can be adjusted,and the pore structure of rGO@polyHIPE composite foam is designed.Moreover,the pore structure is similar to the morphology of the droplets in the HIPE before polymerization.3.When the amount of GO is 0.2wt%,the pH value of the aqueous phase of the prepared HIPE is controlled within the range of 5 to 7,and the composite foam having a hierarchical structure can be obtained after HIPE polymerization.In the composite foam,the small holes surrounding the large holes can effectively transfer the load and avoid stress concentration,so the modulus of the structural foam is 40%higher than that of the non-hierarchical foam.In addition to mechanical properties,the electrical conductivity of composite foams is also closely related to the pore structure.When the pH value is gradually increased from 3 to 8,the distribution density of rGO in the conductive network decreases,and the material conductivity decreases from 5.51×10-5 S/m to 2.06×10-6 S/m.After testing the piezoresistive performance of the material,it was found that the material prepared at pH=5 had the best piezoresistive performance,the highest strain sensitivity and stress sensitivity,1.12 and 0.43 kPa-1,respectively.The strain response range and stress response range of the foam were 0～60%and 0～207.5 kPa,thanks to the hierarchical hole structure to transfer loads more effectively and build new conductive paths.4.The pore structure of the composite foam can also be regulated by GO concentration.When the pH value is 6 and the GO concentration is between 0.12wt%and 0.26wt%,the composite foam also exhibits a hierarchical structure,and the modulus is also significantly better than the non-hierarchical composite foam prepared by the GO concentration of 0wt%and 0.28wt%.When the GO concentration is increased from 0.12wt%to 0.28%by weight,the electrical conductivity of the composite foam increases from 1.28×10-6 S/m to 4.51 ×10-5 S/m.After testing the piezoresistive properties of the composite foam,it was found that the foam had the highest sensitivity at a GO concentration of 0.26 wt%,since this GO concentration was close to a percolation threshold of 0.253 wt%.The foam has a strain response interval of 0 to 60 and a maximum sensitivity of 1.5.Its stress response range can reach 0-227.0kPa,and the stress sensitivity is 0.83kPa-1.