Solvent effects on the permeability of membrane-supported gels

The hydrodynamic (solvent) permeability of porous membranes filled with cross-linked polyacrylamide (PA) was measured as a function of pore size, surface chemistry, and solvent quality. The gel was formed by first soaking the membrane in the solution containing the acrylamide monomer, cross-linking agent, and initiator, and then polymerizing in situ using thermal activation. The water permeability of the gel-filled membranes at a fixed polymer volume fraction within the pores was insensitive to the pore diameter over the range 0.09-0.48 mum. There was no measurable effect of two different surface chemistries, indicating that the gel was trapped inside the porous matrix rather than covalently bonded to the internal membrane surfaces. The most important result was that there were large variations of the permeability when the quality of the solvent was made poorer by adding different alcohols to water. Going from a good solvent for PA (water) to poorer solvents (increasing alcohol concentration), the permeability increased. For example, the hydrodynamic permeability of the membrane-supported gels increased 100 times at alcohol concentrations for which bulk gel collapsed by a factor of 7 in volume. The large changes in permeability of the membrane-supported gels were reversible when the alcohol content of the solvent was reduced; the gel's permeability in pure water was always recovered to within +/-15% after exposing the gel to alcohol-rich solutions. These results, along with results of partitioning and filtration experiments using colloidal particles, support the conjecture that a membrane-supported gel undergoes microsyneresis (coarsening of the microstructure) when exposed to a poor solvent but does not collapse as a bulk gel would.