Percolation behavior of electromagnetic interference shielding in polymer/multi-walled carbon nanotube nanocomposites

Percolation behavior of electrical conduction that is related to the formation of conductive networks has been well demonstrated in conductive polymer composites (CPC). However, the electromagnetic interference (EMI) shielding of CPC is normally very low at the percolation threshold due to the different mechanism. Here, we first predicted the percolation behavior of EMI shielding existing in CPC because the EMI shielding effectiveness (SE) was mainly dependent on the conductivity of CPC if assuming the constantly magnetic permeability. In practice, we also found the percolation behavior of EMI shielding in the multi-walled carbon nanotube (MWCNT) filled poly(-lactide) (PLLA) with good dispersion and isotactic polypropylene (iPP) with poor dispersion. Furthermore, the percolation threshold of EMI shielding was higher than the percolation threshold of electrical conduction. The percolation thresholds of EMI shielding and electrical conduction were 2.00 and 0.40 vol% for PLLA/MWCNT nanocomposites, and 5.40 and 1.40 vol% for iPP/MWCNT nanocomposites, respectively. The SEM and TEM image shows the sparse and dense MWCNT network near the percolation threshold of electrical conduction and EMI shielding, respectively, indicating that the sparse network of MWCNT can create high electrical conductivity as long as the formation of conductive paths. However, the high performance EMI shielding requires dense network of MWCNT.