CLUSTERING OF SOLVENTS IN MEMBRANES AND ITS INFLUENCE ON MEMBRANE-TRANSPORT PROPERTIES

Solvent clustering in membranes, if it occurs, could modify the sorpti on and diffusion behaviors of polymer membranes and therefore their tr ansport properties. Solvent cluster formations in hydrophilic poly(vin yl alcohol) (PVA), in hydrophobic poly(dimethylsiloxane) (PDMS) and in termediate poly(vinyl acetate) (PVAc) membranes were studied by infrar ed spectroscopy, differential scanning calorimetry, X-ray scattering t echniques, and by visual observations. A water saturated PVAc membrane contains non-freezable (bound) and liquid-like water molecules in equ ivalent amounts (ca. 1.6 wt%); membrane infrared studies during the so rption process indicated that water penetrates first in the membrane a s monomeric molecules which aggregate later to form clusters. In PVA m embranes, water molecules can exist under three states, i.e. non-freez able, freezable bound and liquid-like states, depending on the water c ontent in the membrane. The cluster formation seems to affect the memb rane selectivity towards water and the water and ethanol diffusivities in PVA membranes. In PDMS membranes, solvent clusters were evidenced by infrared spectroscopy and visual observations. The clustering of al cohols in PDMS makes the sorption behavior of these solvents in the me mbrane deviate from the Flory-type behavior observed for other solvent s: the sorbed amount increases drastically with the surrounding-solven t activity in the high activity range, but the solvent diffusivity dec reases.