Electrical conductivity of segregated network polymer nanocomposites
Kim, Yeon Seok
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A set of experiments was designed and performed to gain a fundamental understanding of various aspects of the segregated network concept. The electrical and mechanical properties of composites made from commercial latex and carbon black are compared with another composite made from a polymer solution. The percolation threshold of the emulsion-based composite is nearly one order of magnitude lower than that of the solution-based composite. The segregated network composite also shows significant improvement in both electrical and mechanical properties with low carbon black loading, while the solution-based composite achieves its maximum enhancement at higher carbon black loading (~25wt%). The effect of the particle size ratio between the polymer particle and the filler was also studied. In order to create a composite with an extremely large particle size ratio (> 80,000), layer-by-layer assembly was used to coat large polyethylene particles with the carbon black. Hyper-branched polyethylenimine was covalently grafted to the surface of polyethylene to promote the film growth. The resulting composite has a percolation threshold below 0.1 wt%, which is the lowest percolation threshold ever reported for a carbon-filled composite. Theoretical predictions suggest that the actual percolation threshold may be lower than 0.002 wt%. Finally, the effect of the emulsion polymer modulus on the segregated network was studied. Monodispersed emulsions with the different glass transition temperature were used as the matrix. The composites made using the emulsion with higher modulus show lower percolation threshold and higher conductivity. Higher modulus causes tighter packing of carbon black between the polymer particles. When the drying temperature was increased to 80?C, the percolation thresholds became closer between some systems because their moduli were very close. This work suggests modulus is a variable that can be used to tailor percolation threshold and electrical conductivity, along with polymer particle size.