The WVTR was determined according to the ASTM E96-E96M-16 standard (cup test). TQC Sheen permeability cups (10 cm 2 surface area) were used to perform the WVTR measurement. The permeability cup consists of a cup, a seal ring, and a cover ring. A thin layer of wax was applied to the cup ledge and to the seal ring between which the film was placed to seal the cup. The PEFA films, being too small for the cup opening, were masked with an Aluminium Mocon Ink Mask and conditioned for 48 h in a controlled humidity room at 21 °C and 50 ± 3 % RH before testing. The standard presents two testing methods: the desiccant method where anhydrous calcium chloride (CaCl2) is placed inside the cup to generate a 0 % RH, and the water method where water is poured inside the cup to generate a 100 % RH. By placing the cups in the controlled humidity room, we tested the PEFA film according to both methods, thus exposing the films first to a 50 -0 % RH gradient and then to a 100 -50 % RH gradient. The water uptake from the CaCl2 or the water loss toward the environment was monitored by measuring the cup weight 10 times over 48 h. The WVTR is computed as: