Effect of functionalization on thermal conductivities of graphene/epoxy composites

Functionalization is widely used to improve the dispersion of graphene in a polymer matrix when fabricating composites. However, there is no consensus on whether functionalization of graphene enhances thermal conductivities (TCs) of composites. Herein, we discover that although functionalization is effective in improving TCs of composites when the lateral size of graphene is small, the opposite is true if the graphene size is large. Such complication arises from the fact that while functionalization increases the graphene-polymer interface thermal conductance, it deteriorates graphene's intrinsic TC. A critical size theory based on the effective medium approach is proposed to identify the predominant factors determining TCs of composites and reconcile the inconsistency observed in experiments regarding the effect of functionalization. The improved interface conductance by functionalization dominates the TCs of composites, i.e. 'interface dominant', when the graphene size is smaller than the critical value of a few micrometers, making functionalization necessary. However, when the graphene size exceeds the critical value, the inherently high in-plane TC of pristine graphene becomes more important, i.e. 'filler dominant', thereby effecting functionalization unnecessary. These findings offer a general criterion in determining whether functionalization should be applied for highly conductive composites for various thermal management applications.