Effect of water sorption on oxygen-barrier properties of aromatic polyamides

Aromatic polyamides based on poly(m-xylylene adipamide) (MXD-based polyamides) and poly(hexamethylene isophthalamide) (HMD-based polyamides) were examined. Insight into the excellent gas-barrier properties was obtained by the characterization of the effect of water sorption on the thermal transitions, density, refractive index, free-volume hole size, and oxygen-transport properties. Reversing the carbonyl position with respect to the amide nitrogen substantially lowered the oxygen permeability of MXD-based polyamides in comparison with that of HMD-based polyamides by facilitating hydrogen-bond formation. The resulting restriction of conformational changes and segmental motions reduced diffusivity. The primary effect of water sorption was a decrease in the glass-transition temperature (T-g) attributed to plasticization by bound water. No evidence was found to support the idea that sorbed water filled holes of free volume. When the polymer was in the glassy state, the drop in T-g accounted for hydration-dependent changes in the density, refractive index, and free-volume hole size. The correlation of the oxygen solubility with T-g and density confirmed the concept of oxygen sorption as filling holes of excess free volume. In some cases, water sorption produced a glass-to-rubber transition. The onset of rubbery behavior was associated with a minimum in the oxygen permeability. The glass-to-rubber transition also facilitated the crystallization of MXD-based polymers, which complicated the interpretation of oxygen-transport behavior at higher relative humidity. (c) 2005 Wiley Periodicals, Inc.