  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 6.56 Wm-1K-1 is achieved for 10 weight % composition sample, representing an
enhancement of ~2770% over pristine polyetherimide (k ~ 0.23 Wm-1K-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
shows 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 detailed understanding of interpenetrating
graphene-polymer structure in the expanded graphite composite. These results
open up novel avenues for achieving high thermal conductivity polymer
composites.
