  In this work, we have comprehensively studied the effect of auxiliary
intercalating agents on the thermal conductivity of expanded graphite (EG)
polymer composites. We report an ultra-high enhancement of 4030% in thermal
conductivity of polyetherimide/graphene nanocomposite (k = 9.5 Wm-1K-1)
prepared through the use of EG with hydrogen peroxide (H2O2) as an auxiliary
intercalating agent at 10 wt% compositions (k of pure polyetherimide ~ 0.23
Wm-1K-1). This ultra-high thermal conductivity value is found to be due to an
EG-mediated interconnected graphene network throughout the composite,
establishing a percolative environment that enables highly efficient thermal
transport in the composite. Comparative studies were also performed using
sodium chlorate (NaClO3) as an auxiliary intercalating agent. At 10 wt%
composition, NaClO3 intercalated EG was found to lead to a smaller enhancement
of 2190% in k of composite. Detailed characterization performed to elucidate
this advantage, revealed that hydrogen peroxide led to primarily edge oxidation
of graphene sheets within EG, leaving the basal plane intact, thus preserving
the ultra-high in-plane thermal conductivity of ~ 2000 Wm-1K-1. Sodium
chlorate, on the other hand, led to a higher degree of oxidation, with a large
number of oxygen groups on the basal plane of graphene, dramatically lowering
its in-plane thermal conductivity. Thermal diffusivity of H2O2 prepared EG
paper was measured to be 9.5 mm2/s while that of NaClO3 case was measured to be
6.7 mm2/s, thus directly confirming the beneficial impact of H2O2 on k of
graphene itself. This study is the first to address the role of intercalating
agents on k of expanded graphite/polymer composites and has led to the
discovery of H2O2 as an effective intercalating agent for achieving ultra-high
thermal conductivity values.

Materials Science