Effect of temperature and structure on the free volume and water vapor permeability in hydrophilic polyurethanes

A series of hydrophilic polyurethane films based on polyester polyols poly(butylene adipate) (PBA), poly(ethylene adipate) (PEA) and poly (hexamethylene adipate) (PHA) as soft segments, are characterized by differential scanning calorimetry(DSC) ,wide-angle X-ray diffraction (WAXD) and positron annihilation lifetime spectroscopy (PALS), respectively. The WAXD results indicate hydrophilic PUs have crystalline peaks except PEA-PU. The measurements of free-volume hole property have been carried out over a temperature range of 253-323 K. According to variation of ortho-positronium (o-Ps) annihilation parameters with temperature, we observed that mean free-volume size increased rapidly with vanishing of crystalline region. The permeability coefficients of water vapor measured indicated that water vapor permeability (WVP) increased with the temperature increased in all membranes over 288-313 K. In the polyester-based PUs, PBA-PU has the highest WVP attributed to the highest fractional free volume (f(V)). The effect of chemical structure on WVP and f(V) has been discussed. Experimental results indicated that water vapor permeability may be controlled by not only free volume, but also hydrophilicity of polymer and crystalline region. (C) 2004 Elsevier B.V. All rights reserved.