2.2.1. Water Uptake and Thickness Swelling

Water uptake and thickness swelling were determined by monitoring the weight and thickness change of the sPSU membrane under dry and wet states [19]. After drying the membrane in an oven at 60 °C for 24 h, the weight (m_dry) and thickness (Th_dry) were measured. Then, the samples were immersed in distilled water for 24 h at 20 °C and for 2 h at higher temperatures (from 40 to 100 °C each 20 °C). Thereafter, the membranes were picked out, rapidly dried with a paper tissue to eliminate surface water droplets and their mass (m_wet) and thickness (Th_wet) quickly measured. The procedure has been repeated five times for each sample, with an error of circa 2%. Accordingly, the water uptake (wu, %) was calculated as:

while the thickness swelling (S_Th, %) was determined by the following equation: