Mechanisms of moisture sorption in barrier polymers used in food packaging: Amorphous polyamide vs. high-barrier ethylene-vinyl alcohol copolymer studied by vibrational spectroscopy

The nature of the association between sorbed water and two high-barrier hydrophilic polymers used in oxygen-sensitive food packaging, and exhibiting opposite oxygen barrier behavior in the presence of moisture, has been studied by FT-Raman and FT-Infrared spectroscopy. The polymers considered in this work were an ethylene-vinyl alcohol copolymer with superior oxygen barrier properties (32 mol-% of ethylene EVOH) and an amorphous polyamide (aPA). The results revealed that for the latter glassy amorphous polymer, water molecules associate with the C=O and N-H groups of the ca. 10% free amide moieties, being the excess sorbed water self-associated in clusters; thus, moisture sorption does not appear to disrupt the originally present hydrogen-bonded amide groups. This, unusual behavior leads to an overall increase in the extension of the hydrogen-bonding, which may help explain the lower oxygen permeability displayed by the aPA with increasing relative humidity on the basis of the known free-volume competing mechanism. Differently, water sorption appears to progressively disrupt the strong polymer self-association present in the very efficient high-barrier semicrystalline EVOH material by hydrogen-bonding to hydroxyl groups, hence leading to the well-known highly plasticized rubbery structure with much lower intermolecular cohesion and oxygen barrier.