Nanoindentation of annealed Naf ion/sulfonated graphene oxide nanocomposite membranes for the measurement of mechanical properties

The functionalized graphene oxides (GOs) are advanced nanofillers to improve the mechanical properties of the nanocomposite membranes, which can be further enhanced by a simple thermal annealing. Here, we report the preparation and annealing of Nafion/sulfonated GO (N/sG) nanocomposite membranes for the improvement of mechanical properties. In particular, the influence of thermal annealing on the mechanical behaviors of the N/sG composite membranes was investigated by the nanoindentation. The macro- and microscopically uniform dispersion of sGO in the polymer matrices arising from the favorable interactions between them could effectively improve the modulus of Nafion by up to 24.3% (from 0.70 GPa to 0.87 GPa at the loading of 1.0 wt% of sGO) while preserving the hardness. Both the modulus and hardness of the composite membranes were further enhanced by ~27% (from 0.70 GPa to 0.89 GPa) and ~6.6% (from 62 MPa to 65 MPa), respectively, after the thermal annealing at 160 °C. This annealing-induced improvement results from the reorientation of crystalline domain in the composite membranes. These findings provide a new insight to improve the mechanical behaviors of the nanocomposite membranes by means of the incorporation of the functionalized GOs and the treatment of thermal annealing.