We demonstrate in this work that expanded graphite (EG) can lead to a very large enhancement in thermal conductivity of polyetherimide-graphene and epoxy-graphene nanocomposites prepared via solvent casting technique. A k value of 6.6 W⋅m-1⋅K-1 is achieved for 10 wt% composition sample, representing an enhancement of ~2770% over pristine polyetherimide (k~0.23 W⋅m-1⋅K-1). This extraordinary enhancement in thermal conductivity is shown to be due to a network of continuous graphene sheets over long-length scales, resulting in low thermal contact resistance at bends/turns due to the graphene sheets being covalently bonded at such junctions. Solvent casting offers the advantage of preserving the porous structure of expanded graphite in the composite, resulting in the above highly thermally conductive interpenetrating network of graphene and polymer. Solvent casting also does not break down the expanded graphite particles due to minimal forces involved, allowing for efficient heat transfer over long-length scales, further enhancing overall composite thermal conductivity. Comparisons with a recently introduced effective medium model show a very high value of predicted particle-particle interfacial conductance, providing evidence for efficient interfacial thermal transport in expanded graphite composites. Field emission environmental scanning electron microscopy (FE-ESEM) is used to provide a detailed understanding of the interpenetrating graphene-polymer structure in the expanded graphite composite. These results open up novel avenues for achieving high thermal conductivity polymer composites.
Keywords: effective medium model; expanded graphite; porous; thermal conductivity.