Morphological alterations induced by temperature and humidity in ethylene-vinyl alcohol copolymers

The morphologies of a number of high barrier ethylene-vinyl alcohol food packaging films with different ethylene contents have been evaluated by simultaneous WAXS/SAXS, FT-IR, DSC, and Raman spectroscopy. This rather descriptive pioneering study was aimed at the understanding of the morphological changes that occur in these polymers as a result of temperature, humidity, and combination of temperature and humidity treatments. From the results, the temperature effect was, as expected, found to improve polymer crystalline morphology, leading to a higher, denser, and more stable crystallinity. Lower ethylene content copolymers underwent partial solid-solid phase transition toward a more thermodynamically stable monoclinic morphology upon sufficient annealing. On the other hand, moisture sorption was found to result in melting of ill-defined crystals, particularly for the lowest ethylene content copolymers. This water sorption-induced crystal melting process has not been reported before and was seen to be largely suppressed by enhancing crystal stability. Combined temperature and humidity effects, as those for instance generated in retorting autoclaves, were found to dramatically deteriorate the polymer crystallinity, irrespective of initial crystal robustness. By making use of simultaneous time-resolved WAXS/ SAXS experiments during in-situ retorting of a water-saturated EVOH copolymer with 32 mol % of ethylene, it was found that heated moisture weakened very readily the polymer crystalline morphology, which melted around 80 degreesC below its actual melting point.