Structure dependence of water vapor permeation in polymer nanocomposite membranes investigated by positron annihilation lifetime spectroscopy

Incorporation of nanoplatelets is known to be effectively in the reduction of gas permeation of polymer membranes, which has been typically attributed to the torturous path effect. However, the measured water vapor transmission rates (WVTR) from the poly(vinyl alcohol) (PVA)/graphene oxide (GO) nanocomposite membranes are found to be smaller than the theoretical values calculated by the Bharadwaj model based on the torturous path effect. This discrepancy is indicative of the existence of other structural factors responsible for the WVTR that are rarely discussed in literature. In this work, the positron annihilation lifetime spectroscopy (PALS), a widely recognized method for characterization of the atomic-scale microstructure of polymers, is employed herein to study the free volume of the PVA/GO composite membranes. The free volume size of the composite membrane decreases with increasing GO contents, and the fractional free volume (FFV) decreases with the increase of annealing temperature. Strong dependence of the WVTR of PVA/GO on the FFV and crystallinity has been revealed, indicating that, along with the nanoplatelets-induced torturous path effect, they are important factors affecting the water vapor permeation of polymer nanocomposite membranes.