WVP tests were conducted by the ASTM method E with certain
modifications [33]. Each film sample was sealed over a circular
opening of 0.00185 m2 in a permeation cell that was subsequently
stored at 20 1C in a desiccator. To maintain a 75% RH gradient
across the film, anhydrous silica (0% RHc) was placed inside the cell
and a saturated NaCl solution (75% RH) in the desiccator. The RH
therefore was always lower inside the cell than outside, and the
water-vapor transport was accordingly determined from the
weight gain of the permeation cell. When steady-state conditions
were reached (after about 1 h), eight weight measurements were
made over a period of 7 h. Changes in the weight of the cell were
recorded and plotted as a function of time. The slope of each line
was calculated by linear regression (Origin Pro 8.5 software) and
the water-vapor-transmission rate was calculated from the slope
(g H2O s1
) divided by the cell area (m2
). WVP (g H2O/Pa s m) was
calculated as