Effect of functional groups on the agglomeration of graphene in nanocomposites

A base wash procedure was used to partially remove the oxygen functional groups from the graphene oxide (GO) flakes to prepare the base-washed GO (BwGO). It is found that the base wash treatment does not alter the physical properties (size, thickness etc.) of the GO significantly but the chemical composition is changed. Nanocomposites were prepared by incorporating the BwGO flakes obtained in a poly(vinyl alcohol) (PVA) matrix. It was found the storage modulus of the nanocomposites is enhanced from 4.4 GPa to 6.5 GPa with 5 wt% of BwGO. This is in agreement with the micromechanical estimation obtained by using Raman spectroscopy that follows the interfacial stress transfer from the matrix to the BwGO fillers. The lower effective modulus of BwGO than GO as calculated using the classical 'rule of mixtures' is due to the loss of functional groups on GO that serve as a surfactant to prevent the flakes from re-agglomerating. An agglomeration factor ηa is therefore proposed and a concept of 'effective volume fraction' is introduced to quantify and evaluate the level of agglomeration of fillers in nanocomposites, which can be otherwise difficult to visualize optically. It is found that the removal of the functional groups causes the flakes to re-agglomerate, and reduces the 'effective volume fraction' by about 10-20%.