INVESTIGATION OF POLY(VINYL ALCOHOL) POLY(N-VINYL-2-PYRROLIDONE) BLENDS .3. PERMEATION PROPERTIES OF POLYMER BLEND MEMBRANES

Dense membranes made of poly(vinyl alcohol) (PVA) and poly(N-vinyl-2-p yrrolidone) (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 fl ux and the water content in the permeate show sudden changes at ca. 30 - 40 wt.-% of PVP in the membrane: the flux increases drastically, wh ile the water selectivity of the membrane falls to a much lower level. The diffusion coefficients of both water and ethanol, as determined f rom the permeation flux changes in the transient regime, exhibit a lar ge increase when the PVP content exceeds 30 wt.-%, and this accounts f or the flux increase. The fall in the permeation selectivity is shown to be due to a strong decrease in the sorption selectivity. The thresh old of the change corresponds approximately to the PVP content at whic h PVA crystallites no longer exist in the blend. A further study on th e states of water in the blend membranes by differential scanning calo rimetry 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.