Water-vapor permeability of hydrophilic polyurethanes studied by positron annihilation

Positron annihilation lifetime spectroscopy was used to study the effect of temperature and content of PEG on the free volume size, distribution and water-vapor permeability (WVP) for a series of polyester-based polyurethane films based on poly(butylene adipate) (PBA) as soft segments. The PALS results indicated that the mean volume of free-volume increased and the distribution of free-volume broadened with the temperature increased. The variation rate of free-volume hole size as a function of temperature has an abrupt increase, which is attributed to the crystalline region melting. In all membranes, water-vapor permeability coefficient increased with the increase of temperature. PBA-20 has almost the same values of WVP as PBA-10 and PBA-15. More PEG groups added exert no significant effects on the water-vapor permeability, because there are two opposite effects of PEG groups on WVP. The experimental results indicated that WVP could be controlled by not only free volume, but also hydrophilicity of polymer.