Environmental stimuli-responsive membranes, or smart membranes, benefited from their environmental-sensitive permeability/selectivity, are attractive for applications in controlled drug delivery, biological/chemical separation, sensors, tissue engineering, and so on. The fabrication of environmental stimuli-responsive membranes mainly has two approaches: modification of existing porous membrane substrates by graft polymerization, and fabrication of membranes by modifying the membrane-forming polymers. Although the first approach could utilize most of existing membrane materials, it has such shortcomings as decrease of membrane pores and consequent flux decline, poor reproducibility, sophisticated membrane pretreatment or special equipment. Such shortcomings keep the fabrication of stimuli-responsive membranes by the first approach still far away from industrial implementation. The latter approach, imparting membrane materials with responsiveness before membrane fabrication with phase inversion method, is still hindered by the limited and yet uncontrollable modification methods (ozone and ray treatments). Controllable manufacture of stable stimuli-responsive membranes by this approach on large scale is still difficult.
pH-responsive poly(ether sulfone) (PES) composite membranes blended with amphiphilic polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymers are fabricated through simple phase inversion in water. The anchor block PS has a lower surface tension than that of PES, which is beneficial to anchor the PS block near the membrane pore surfaces. Modification of membrane matrix surface including membrane surface and pore surface by PAA chains is achieved by surface-segregation of PS-b-PAA copolymers, and confirmed by water contact angle measurement on membrane surface and confocal laser scanning microscopy characterization of membrane cross-section. The effects of the blended PS-b-PAA copolymers on the microstructures and pH-responsive characteristics of the PES composite membranes are investigated systematically. The hydraulic permeation results show that the pH responsive characteristics of composite membranes blended with PS-b-PAA copolymers are proven to be significant, fast, reversible and durable with pH switching between 3 and 8. FITC-dextran sieving experiments of a PS-b-PAA blended composite membrane at ambient pH values of 3 and 8 show 28% and 90% rejection of 70 kDa solute, respectively. At pH =3 (< pKa), the membrane pores are in “open” state because of the shrunken conformation of the PAA chains on the pore surfaces; in contrast, at pH=8 (> pKa), the membrane pores are in “closed” state due to the swollen conformation of the PAA chains. The results provide valuable guidance for fabricating stimuli-responsive membranes by blending amphiphilic block copolymers.