One of the most important physical factors related to the thermal conductivity of composites filled with graphene nanoplatelets (GNPs) is the dimensions of the GNPs, that is, their lateral size and thickness. In this study, we reveal the relationship between the thermal conductivity of polymer composites and the realistic size of GNP fillers within the polymer composites (measured using three-dimensional (3D) non-destructive micro X-ray CT analysis) while minimizing the effects of the physical parameters other than size. A larger lateral size and thickness of the GNPs increased the likelihood of the matrix-bonded interface being reduced, resulting in an effective improvement in the thermal conductivity and in the heat dissipation ability of the composites. The thermal conductivity was improved by up to 121% according to the filler size; the highest bulk and in-plane thermal conductivity values of the composites filled with 20 wt% GNPs were 1.8 and 7.3 W/m·K, respectively. The bulk and in-plane thermal conductivity values increased by 650 and 2,942%, respectively, when compared to the thermal conductivity values of the polymer matrix employed (0.24 W/m·K).

The drying process of graphene-polymer composites fabricated by solution-processingfor excellent dispersion is time consuming and suffers from a restacking problem.Here, we have developed an innovative method to fabricate polymer compositeswith well dispersed graphene particles in the matrix resin by using solventfree powder mixing and in-situ polymerization of a low viscosity oligomerresin. We also prepared composites filled with up to 20 wt% of grapheneparticles by the solvent free process while maintaining a high degree of dispersion.The electrical conductivity of the composite, one of the most significantproperties affected by the dispersion, was consistent with the theoreticallyobtained effective electrical conductivity based on the mean field micromechanicalanalysis with the Mori-Tanaka model assuming ideal dispersion. It can be confirmedby looking at the statistical results of the filler-to-filler distance obtainedfrom the digital processing of the fracture surface images that the variousoxygenated functional groups of graphene oxide can help improve the dispersionof the filler and that the introduction of large phenyl groups to the graphenebasal plane has a positive effect on the dispersion.