Research on carbon nanofiber-enhanced polypropylene composites has shown that electrical conductivity peaks when the nanofibers are heat treated to around 1500o C for all volume fractions above 5.5 volume % nanofibers.
The electrical conductivity and mechanical properties of polypropylene composites based on carbon nanofibers that had been heat treated at different temperatures were determined. The results show that the gradual conversion of the exterior layers of the poorly crystallized CVD carbon initially present on the fiber surface impacts both the mechanical properties and the electrical transport properties.
Top left graph: Tensile strength of composites as a function of nanofiber heat treatment temperature. Polypropylene samples with no reinforcing nanofibers average around 30 MPa tensile strength. There is a marginal increase up to 1500oC and then a decrease. Indicating that nanofibers heat-treated to 1500oC give optimum tensile properties.
Top right graph: Tensile modulus of composites fabricated from nanofibers heat-treated at different temperatures. The tensile modulus of unreinforced polypropylene for an average of 6 samples is 1.25 GPa.
Bottom left graph: Intrinsic conductivity of carbon nanofibers compressed in an air filled die at room temperature. Electrical conductivity peaks around 1500oC for all volume fractions above 5.5 volume % nanofibers. Very low volume fractions suffer from fiber-fiber contact problems.
Bottom right graph: Conductivities of heat-treated carbon nanofiber/polypropylene composites. The conductivity values are lower than those measured in the intrinsic conductivity experiment since the polymer makes fiber to fiber electrical contact more difficult.