2.8. Gas Permeability

The gas permeability measurements were performed using a stainless-steel cell with two identical chambers separated by the film. The permeability was evaluated by pressurising the feed chamber up to 0.7 bar with pure gas, namely carbon dioxide (99.998%) or oxygen (99.999%) (Praxair, Maia, Portugal), followed by the measurement of the pressure change in both chambers over time, using pressure transducers (JUMO, Model 404327, Fulda, Germany). The temperature was maintained at 30 °C using a thermostatic bath (Julabo, Model EH, Seelbach, Germany). The permeability was calculated using Equation (6):

where Δp (bar) is the pressure difference between the feed and permeate compartment, P (m⁻² s⁻¹, where 1 barrer = 1 × 10⁻¹⁰ cm³ (STP) cm cm⁻² s⁻¹ cm Hg⁻¹ = 8.3 × 10⁻¹³ m² s⁻¹) is the gas permeability, t (s) is the time, δ (m) is the film thickness, and β (m⁻¹) is the geometric parameter of the cell, as described by Ferreira et al. [2].