Free volume and water vapor permeability properties in polyurethane membranes studied by positrons

The effects of the soft segment and temperature on the free volume and gas permeability have been studied by positron annihilation lifetime spectroscopy (PALS) in polyurethane membranes based on polybutylene adipate (PBA), polytetramethylene glycol (PTMG) and polycaprolactone (PCL) as soft segments. The wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) results indicated that PBA-PU and PCL-PU had crystalline region except for PTMG-PU. The PALS results indicated that the free-volume hole size increased rapidly as the crystals melted. The water vapor permeability (WVP) coefficients followed Arrhenius temperature dependence in the molten and rubbery state for all membranes. High free volume gave rise to the negative value of the apparent activation energy of permeation (Ep). Correlations between the free volume and gas transport properties have been discussed. We found that WVP was controlled by not only free volume but also hydrophilicity of polymer.