Dense membranes made of poly(vinyl alcohol) (PVA) and poly(N‐vinyl‐2‐pyrrolidone) (PVP) blends of different compositions were studied in gas‐sweeping permeation of a water‐ethanol mixture containing 10 wt.‐% of water. When the PVP content increases, the steady‐state permeation flux and the water content in the permeate show sudden changes at ca. 30–40 wt.‐% of PVP in the membrane: the flux increases drastically, while the water selectivity of the membrane falls to a much lower level. The diffusion coefficients of both water and ethanol, as determined from the permeation flux changes in the transient regime, exhibit a large increase when the PVP content exceeds 30 wt.‐%, and this accounts for the flux increase. The fall in the permeation selectivity is shown to be due to a strong decrease in the sorption selectivity. The threshold of the change corresponds approximately to the PVP content at which PVA crystallites no longer exist in the blend. A further study on the states of water in the blend membranes by differential scanning calorimetry made it possible to relate these changes with a change in the interactions of water with hydrophilic sites in the polymer blend when the PVP content changes.
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Ping et al. (1994) studied this question.
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