## 2 Materials and Methods
### Experimental Methods  
A picoammeter (Keithley 2400, Keithley Instruments Inc., Beaverton, OR, USA) was used to measure the electrical conductivity of the mono-and bi-filler polymer composites. Before measurements the samples were coated with a silver coating. Silver coating doesn't influence measurements, since it is measured the bulk conductivity while the sample is between two electrodes and silver coating is on both sides of the disc samples (with a diameter of 16 mm and thickness 3 mm). The edges of the samples are thick enough (3 mm) and they are not covered with the silver paint. That is why there is no contact between two silver-painted surfaces. During the electrical measurements, the resistance of the material in ohm was obtained. Electrical conductivity was calculated using the following Equation (1): \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Composition Code (wt.\%)** & **GNP Content** & **MWCNT Content** & **PLA Content** & **Name** \\  & **(wt.\%)** & **(wt.\%)** & **(wt.\%)** & **Name** \\ \hline PLA & - & - & 100 & referent \\ PLA/1.5\% GNP & 1.5 & - & 98.5 & mono-filler \\ PLA/3\% GNP & 3 & - & 97 & mono-filler \\ PLA/6\% GNP & 6 & - & 94 & mono-filler \\ PLA/1.5\% MWCNT & - & 1.5 & 98.5 & mono-filler \\ PLA/3\% MWCNT & - & 3 & 97 & mono-filler \\ PLA/6\% MWCNT & - & 6 & 94 & mono-filler \\ PLA/1.5\% GNP/1.5\% MWCNT & 1.5 & 1.5 & 97 & bi-filler \\ PLA/1.5\% GNP/4.5\% MWCNT & 1.5 & 4.5 & 94 & bi-filler \\ PLA/3\% GNP/3\% MWCNT & 3 & 3 & 94 & bi-filler \\ PLA/4.5\% GNP/1.5\% MWCNT & 4.5 & 1.5 & 94 & bi-filler \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The mono-filler and bi-filler composites. \[k=\frac{L}{R\cdot S} \tag{1}\]  
where \(k\) is electrical conductivity (S/m); \(L\) is a thickness of the sample (m); S is the area of the circle (\(\tau\)\({}^{2}\)). A Hot Disk 2500 thermal constant analyzer (Hot Disk Inc., Goteborg, Sweden) was used to measure the thermal conductivity of the GNP and MWCNTs polymer composites through a transient plane source method. Before measurements, the samples were polished using an abrasive paper in order to obtain very flat surfaces. The measurements were performed with specimens of sizes 16 mm diameter and about 3 mm thickness by putting the sensor (3 mm diameter) between two similar slabs of material. The sensor supplied a heat pulse of 0.01 W for 40 s to the sample at room temperature and the associated change in temperature was recorded. The thermal conductive parameters including thermal conductivity, thermal diffusivity, and specific heat of the samples were measured. Bright field transmission electron microscopy (TEM, FEI Company, Hillsboro, OR, USA) analysis was performed by using FEI TECNAI G12 Spirit-Twin (LaB6 source) instrument equipped with a FEI Eagle-4k CCD camera and operating with an acceleration voltage of 120 kV. The analysis was performed on sections obtained at room temperature by using a Leica EM UC6/FC6 ultramicrotome. The sections were placed on 400 mesh copper grids. Scanning electron microscopy (SEM, FEI Company, Hillsboro, OR, USA) measurements were performed using FEI QUANTA 200F scanning electron microscope at different magnifications. SEM characterizations of the nanocomposite blends were done using brittle fractured composite surface prepared using liquid nitrogen. The sample surface was covered with gold-palladium (Au/Pd) using EMITECH K575X. The Raman spectra of carbonaceous components were obtained using a Raman Confocal Spectrometer (WITec Alpha 300 R, WITec Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) with a 50\(\times\) objective lens, grating of 600 grooves/mm and excitation laser wavelength of 532 nm. Laser beam aperture of 1.2 micrometres and measured laser power of 3 mW. Raman spectroscopy is an efficient technique to characterize graphene in terms of quality, structural integrity and number of layers.