Capillary pore membranes with grafted diblock copolymers showing reversibly changing ultrafiltration properties with independent response to ions and temperature

• Sequential controlled grafting of zwitterionic and temperature-responsive diblock copolymer in capillary pores.
• Effective pore size responds independently to changes of salt type or concentration and temperature.
• Pronounced changes of dextran sieving as function of the specific combination of the two stimuli.

Track-etched polyethylene terephthalate membranes with a pore size of 110 nm have been modified by a grafted diblock copolymer structure in order to obtain dual-responsive ultrafiltration membranes. The temperature-responsive poly-N-isopropylacrylamide (PNIPAAm) and the ion-responsive poly-N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine (PSPE) were grafted from the pore walls by sequential surface-initiated atom transfer radical polymerization. To achieve a well controlled pore functionalization, the polymerization rate had been adjusted to a very slow chain growth. A successful grafting of PSPE as first block and of PNIPAAm as second block could be demonstrated. To study the temperature and ion responsivity, hydraulic permeability and dextran diffusion experiments at 25 °C and 40 °C with different salt solutions had been carried out. It could be shown that such membranes can change their barrier pore size from a more open to a more closed state in dependency of temperature as well as kind of ions and their concentration in the feed. The grafted layer thickness showed a particularly strong increase in the presence of KClO4. Like expected for a well defined polymer brush, an expanded or collapsed state for each of the individual blocks could be obtained. In presence of KClO4 at 25 °C both blocks are expanded and the pores are in a more closed state. If one of these stimuli is changed, for example when temperature is increased to 40 °C or ions are removed, the block copolymer brushes collapse partially what leads to more open pores. However, if both stimuli are applied together, i.e. if temperature is 40 °C and no ions are in solution, diblock copolymers collapse fully what leads to fully opened pores. It had also been demonstrated that this dual responsivity and the magnitude of the effects depend very strongly on the absolute degree of grafting and the ratio between the two different responsive polymer blocks. Furthermore, the ability of changing effective membrane barrier pore size by ions and temperature has been investigated in more detail by dextran diffusion experiments, and very pronounced and reversible changes of molecular sieving curve and molecular weight cut-off could be obtained.

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