Facilitating supercritical CO2 assisted exfoliation of graphene nanoplatelets with the polymer matrix

Abstract

The currently mass-produced graphene materials are typically graphene nanoplatelets (GnPs) with multiple-layer structures the properties of which are not comparable to those of mono-layer or few-layer graphene. In this work, high concentrations of commercial GnPs (15 wt% with potential to be higher) were effectively exfoliated in polypropylene (PP)/GnP nanocomposites using supercritical CO2 (sc-CO2) in a foaming process. It was shown that the exfoliation of GnPs was increased with an increased expansion ratio in the foaming window of PP/GnP (159–163 °C). However, a high degree of GnP exfoliation can be maintained at elevated temperatures (e.g., 200 °C) with minimized expansion ratios (<1.3), indicating gas expansion inside GnPs is the main mechanism for graphene delamination during foaming. The electrical conductivity of the solid PP/15 wt%-GnP was increased by almost 5 orders of magnitude after sc-CO2 treatment. The foaming pressure was shown to have a critical influence on GnP exfoliation, and the pressure threshold to delaminate graphene layers was significantly reduced in the presence of PP matrix, as compared to GnP powder. The results were attributed to PP chain penetration into GnP under sc-CO2, and the prevention of rapid loss of intercalating CO2 by the PP molecules surrounding the GnPs during the pressure release step. This study opens up a new potential avenue for developing highly exfoliated and versatile polymer/graphene solid nanocomposites using supercritical fluid treatment.