pH-induced hysteretic gating of track-etched polycarbonate membranes: swelling/deswelling behavior of polyelectrolyte multilayers in confined geometry.
pH-induced hysteretic gating of track-etched polycarbonate membranes (TEPC) has been achieved by depositing layer-by-layer assembled polyelectrolyte multilayers comprising poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) at a high pH condition (pH > 9.0). Scanning electron microscopy and transmission electron microscopy showed that the average bilayer thickness of multilayers was greater within the cylindrical pores of track-etched polycarbonate membranes compared to the multilayers on planar substrates (e.g., Si wafers and the face of TEPC membranes). Swelling/deswelling properties of multilayers and gating properties of the multilayer-modified TEPC membranes were studied by measuring the flux of pH-adjusted deionized water. Large discontinuous changes in the transmembrane flux were observed, indicating that the multilayers within the cylindrical pores of TEPC membranes exhibit the discontinuous swelling/deswelling behavior observed previously for planar systems. The degree of swelling as estimated by simple models, however, showed that (PAH/PSS) multilayers in the confined geometry swelled to smaller extents compared to the same multilayers on planar substrates under the same conditions. Multilayer-modified membranes showed reversible gating properties as the pH condition of feed solution was alternated between pH 2.5 and 10.5. In situ atomic force microscopy (AFM) was used to visualize the closing of the pores as a function of time. The hysteretic gating property of the multilayer-modified TEPC membrane was utilized to achieve either a "closed" or "open" state at one pH condition depending on the pretreatment history, thereby enabling either the retention or passage of high-molecular weight polymers by varying the membrane pretreatment condition.