Polymer rigidification in graphene based nanocomposites: Gas barrier effects and free volume reduction

The gas barrier properties of polymer nanocomposite membranes made of Few Layer Graphene nanoplatelets dispersed in amine-modified epoxy resins were studied by gas phase permeation analysis with H2, N2 and CO2. The gas permeability of the nanocomposites decreases with increasing filler content. Filler addition does not change the gas selectivityof the pure epoxy membrane. Positron Annihilation Lifetime Spectroscopy analysis indicates that, by increasing the filler content, the free volume structure does not change but the fractional free volume decreases. Gas transport results were explained by the formation of a constrained polymer region of thickness lp at the filler-matrix interface. The lp parameter was evaluated from the reduction of the fractional free volume of the nanocomposite samples with respect to that of the epoxy matrix. The average value of lp, 20 nm, permits to reproduce quantitatively the experimental data at all examined temperatures, filler concentration and test gas.