Importace of The physical Structure of Membranes in Pervaporation.

When hydrophilic dense films made from a series of Polyvinylalcohol (PVA) samples differing by their residual acetyl content are used to dehydrate by pervaporation water-organic solvent mixtures, it turns out that the selectivity decreases while the permeability greatly increases as the acetyl content increases. This behavior was attributed to the decrease in the ability of the polymer to form crystalline regions due to the disorganizing effect of the acetyl groups on the physical structure. Grafting of very small amounts of carboxylic groups of different natures and spatial distributions yields membranes whose swelling and pervaporation properties are explainable mainly by the difference in physical structures. This interpretation was confirmed by the results obtained with membranes of strictly identical chemical structure (100% hydrolyzed PVA) but with controlled crystallinity. Similar behaviors were observed with different water-solvent mixtures.Differences in pervaporation properties were also observed in cellulose acetate membranes when they are annealed in hot water, although no significant crystallinity was observed in the annealed membranes. It can be concluded that the membrane physical structure (i. e. the chain organization in the membranes) has a strong influence on its pervaporation characteristics, and its control makes it possible the preparation of membrane of good performances.