Water effect on the morphology of EVOH copolymers

The effect of water on the morphology of four ethylene vinyl alcohol copoly mers (EVOH) with different ethylene contents was studied by differential sc anning calorimetry (DSC). EVOH film samples equilibrated in controlled atmo spheres at different relative humidities (RK) and 23 degrees C were analyze d. Under dry conditions, the glass transition temperature (T-g) was unaffec ted by copolymer ethylene content. As RH increases, T-g decreases. It seems that the presence of water within the polymer matrix results in plasticiza tion of the polymer. T-g varies from around 50 degrees C (dry) to below roo m temperature. EVOH copolymers are glassy polymers when dry and rubbery pol ymers at high RHs. Fox and Gordon-Taylor's equations well describe T-g depl etion at low water uptake, although severe water gain results in a consider able T-g decrease, which is not predicted by these theories. Melting temper ature, T-m, and enthalpy, Delta H-m, were also analyzed. When dry, T-m, dec reases as ethylene content increases. No significant water effect was found on either T-m or Delta H-m. Hence, crystallinity seems to be unaffected by water presence. (C) 1999 John Wiley & Sons, Inc.