Nonlinear molecular weight dependency in polystyrene and its nanocomposites: deciphering anomalous gas permeation selectivity in water vapor-oxygen-hydrogen barrier systems

Abstract Polystyrene (PS) and its nanocomposites have shown considerable potential as selective barriers for gas permeation, yet the influence of molecular weight (MW) on their gas transport behavior remains insufficiently understood. In this study, three PS samples with ascending MWs (PS1–PS3) and their reduced graphene oxide (rGO) nanocomposites (PS1-G–PS3-G) were prepared via hot pressing. Notably, the water vapor transmission rate (WVTR) exhibited a non-monotonic dependence on MW, increasing from 1.50 g/m2·day (PS1) to 1.59 g/m2·day (PS2), then decreasing to 1.27 g/m2·day (PS3). A similar trend was observed in the nanocomposites, which displayed lower WVTR values (0.54, 0.61, and 0.48 g/m2·day, respectively). In contrast, the oxygen transmission rate (OTR) rose sharply with MW – from 509.41 g/m2·day (PS1) to 1,119.8 g/m2·day (PS2) and further to 4,586.11 g/m2·day (PS3) – while rGO incorporation substantially suppressed oxygen permeability (40.72, 58.09, and 138.32 g/m2·day, respectively). Hydrogen transmission rates (HTR) varied only slightly among the PS samples (830, 842, and 907 cm2/(m2·day·0.1 MPa), and the addition of rGO induced minimal changes. Among all samples, PS1 and PS1-G exhibited the most favorable barrier selectivity, combining strong water and oxygen resistance with retained hydrogen permeability. These results demonstrate that MW plays a decisive role in governing gas barrier selectivity and provide design insights for optimizing polymer nanocomposites in advanced barrier applications.