Effect of compounding method and processing conditions on the electrical response of exfoliated graphite nanoplatelet/polylactic acid nanocomposite films

The focus of this study is to understand the effect of compounding method and polymer physical properties on the electrical response of exfoliated graphite nanoplatelet (GNP)/polylactic acid (PLA) nanocomposite films. Two compounding methods were employed: (i) melt mixing, followed by compression molding, and (ii) solution mixing once the polymer was dissolved in chloroform, followed by solution casting. The physical properties of the polymer, namely the crystallization characteristics were altered using two different cooling rates during compression molding: (i) slow cooling and (ii) fast cooling. The microstructure of the films was examined using scanning electron microscopy. Thermal properties, crystallization behavior, and electrical behavior were determined as a function of the GNP content, compounding method, and cooling rate using differential scanning calorimetry, X-ray diffraction, and impedance spectroscopy, respectively. It was concluded that the SC films had the lowest percolation threshold between 1 and 5 wt% of GNP, followed by the solution-cast films with percolation threshold between 5 and 8 wt% of GNP. The FC films did not percolate until a GNP content of 15 wt% was used. The vast differences in percolation thresholds are attributed to the differences in polymer matrix crystallinity and composite microstructure both in terms of microporosity and GNP dispersion/distribution within the polymer due to the different cooling rates and different compounding methods employed.