The Effect of Agglomeration on the Electrical Percolation of Polyimide/Graphene Nanocomposites

Electrically conductive polymer/graphene nanocomposites are multipurpose in nature and have a wide range of potential applications in electronics and electrical engineering. The conductivity of these composites depends on a range of parameters such as the choice of polymer matrix, graphene filler type, the processing methodology and the quality of dispersion. The optimization of the dispersion process of graphene in the polymer matrix is a major challenge in the development of nanocomposites and their integration into industrial applications. This paper investigates the efficacy of centrifugation as a means of isolating smaller graphene agglomerates from the supernatant after initially dispersing the material in polyamic acid using high shear mixing. The relationship between the centrifugation time and agglomerate size and its effect on the electrical percolation of the polymer nanocomposite is elucidated. A set of polyimide films prepared using varying centrifugation times and graphene nanoflake concentrations were prepared by thermal imidization and electrically characterized to assess the impact of the structure of the nanocomposite on the electrical percolation threshold. Graphene nanoflakes agglomerates were reduced in size by increasing the centrifugation time and this led to a decrease in the electrical conductivity of the final films. The electrical conductivity of polyimide graphene (PI/G) nanocomposites consistently increased with increasing filler concentration. PI/G prepared using 2 min centrifugation times exhibited an electrical conductivity of 2.7 × 10
^-3
S/m, while the corresponding PI/G prepared using 5min of centrifugation had an electrical conductivity of 0.16 × 10
^-3
S/m. The percolation phenomena were clearly observed for all nanocomposites as the conductivity rose from ~10
^-13
to ~10
^-3
S/m, while the percolation thresholds varied depending on the centrifugation time (agglomeration density).