Conductive fillers such as graphene are able to increase the electrical conductivity in polymer composite systems. Beyond a certain concentration called the electrical percolation threshold, graphene particles can form interconnected 3D percolated network and thus leading to a sudden rise in the conductivity of the composites [1]. In this context, this work aims to highlight for the first time the differences in terms of the microstructure of polymer blend composite systems based on polylactic acid (PLA 2003D, Nature Works) and polycaprolactone (PCL CapaTM 6800 , Perstorp) that are filled with 10 wt.% of graphene nanoplatelets (GNP-Grade M5, XG Sciences) and their influence on the electrical properties. The polymer composites were prepared using the melt blending technique via a mini twin-screw extruder. The polymer proportions were varied (the percentage of PLA was increased from 30 wt.% to 80 wt.% in the polymer total weight percentage). 3D printing and compression moulding techniques were used to manufacture the samples for the conductivity tests and the microstructural analysis by scanning electron microscopy (SEM). The SEM image (Figure 1.a) is related to PLA30/PCL70/10 wt.% GNP compression moulded composite in which the PLA nodules (brighter phase) are dispersed in the PCL (darker phase) that contains all the GNPs. The same sea-island morphology was obatined for the 3D printed sample. And from the electrical conductivity measurement tests, this formulation showed inferior electrical performance as compared to PLA60/PCL40/10 wt.% GNP composite (Figure 1.b). The latter possesses superior conductivity due to the presence of a co-continuous structure of PLA and PCL phases in addition to the selective localization of the graphene in the PCL phase. This phenomenon is related to the existence of a double percolation threshold that exists in the case of immiscible polymer blend composites which contain filler whose preference is to one polymer phase rather than the other [2]. References [1] Marsden, A.J.; Papageorgiou, D.G.; Valles, C.; Liscio, A.; Palermo, V.; Bissett, M.A.; Young, R.J.; Kinloch, I.A.; Electrical percolation in graphene-polymer composites. 2D Materials 2018, 5, 1-34. [2] Zhang, K.; Yu, H.O.; Shi, Y.D.; Chen, Y.F.; Zeng, J.B.; Guo, J.; Wang, B.; Guo, Z.; Wang, M.; Morphological regulation improved electrical conductivity and electromagnetic interference shielding in poly(L-lactide)/poly(ε-caprolactone)/carbon nanotube nanocomposites via constructing stereocomplex crystallites. Journal of Materials Chemistry C 2017, 5, 2807-2817.