Piezoresistive behavior of polymer reinforced by expanded graphite

In this study, piezoresistive properties of expanded graphite reinforced polypropylene modified by maleic anhydride were investigated. Expanded graphite as a nanoscale filler dispersed in a polymer matrix was explored in the last few years to examine the novel properties such as dielectric properties and electrical conductivity. It was shown that the change in electrical conductivity was consistent with the classical percolation theory whereby a threshold existed for transition from an insulator to a semi-conductor. Most piezoelectric materials are ceramic based. Little is understood on the piezoresistive behavior of polymer nanocomposites. The expanded graphite reinforced maleated PP in this study were prepared using a melt-compounding method. The percolation threshold of the composite conducting films at room temperature was found to be as low as 0.4 wt%. It was also shown that at a compressive stress less than 10 MPa, the electrical resistivity decreased fairly rapidly with an increase in compressive stress. Thereafter the resistivity reached a steady state and then slightly increased with the increase in the stress exceeding 30 MPa. The electrical response as a function of time under constant stress was investigated experimentally. The results showed that the electrical resistivity decreased significantly with time under high mechanical stress while it increased slightly at a compressive stress of 31 MPa. The potential mechanisms of piezoresistivity is qualitatively discussed in light of the observed experimental data.