Polymer Eva-Oh Membrane with Improved Water/Gas Separation Performance: Influence of Vac/Voh Repeating Units Ratio on Membrane Physical Chemical Properties

New hydrolyzed ethylene vinyl acetate (EVA) (EVA-OH) membranes were synthesized via the EVA partial hydrolysis reaction for the use in the water/gas separation field. The reaction conditions of the partial hydrolysis were optimized in terms of the EVA-OH chemical structure by using 1D and 2D NMR spectroscopy. It has been shown that a change of the EVA-OH chemical composition (i.e. the ratio between vinyl alcohol (VOH) and vinyl acetate (VAc) repeating units) has a significant influence on the membrane crystallinity due to a different nature of the VOH and VAc repeating units. The VAc groups act as a “soft” (i.e. amorphous) segmental unit owing to the low organization of the polymer chains, whereas the VOH groups operate as a “rigid” (i.e. semi-crystalline) unit because of the formation of the VOH crystals via the interchain interactions (i.e. hydrogen bonding between free alcohol groups). Thus, the noticeable change of the EVA-OH glass transition temperature Tg is observed depending on the hydrolysis degree – the Tg value of the EVA-OH 1 membrane (i.e. with 44 wt% of VAc) is ⁓ −9 °C, while for the EVA-OH 5 membrane (i.e. with 9 wt% of VAc) it equals to ⁓ 19 °C. Such a change of the membrane morphology allowed us to optimize the mechanical and transport properties of EVA-OH membranes. The presence of the “rigid” VOH repeating units positively influences the membrane tensile stiffness and resistance, thereby preventing the membrane adhesive properties. Consequently, the EVA-OH membrane tensile stiffness, stress at break, and elongation are varied in the range of 2.5–204 MPa, 4–45 MPa, and 1450-600%, respectively, depending on the VOH content. Besides, the amorphous VAc segmental units generate a higher permeation flux (i.e. higher water/gas permeability), whereas the semi-crystalline VOH repeating units ensure the membrane barrier properties and improve its selectivity. In addition, EVA-OH polymer membranes demonstrate a promising CO2 separation performance in terms of the gas selectivity. The best EVA-OH membrane shows the selectivity coefficient α(CO2/N2) of ⁓ 75 and α(CO2/O2) of ⁓ 13.