Programming a Dual-Responsive Switch in Both the Surface and Interior of an Asymmetric Separation Membrane

In order to meet the requirements of complex separation processes, intelligent membranes with diversified adjustable performances are highly desired. Herein, a dual-responsive switch was constructed in both the surface and interior of the poly­(ether sulfone) membranes. Poly­(N-isopropylacrylamide) is introduced in the interior of the membrane via in situ cross-linking polymerization to establish the temperature-responsive system. Then, polydopamine (PDA) is coated on the membrane surface, and the remaining catechol was consumed and oxidized to quinone in order to stabilize the PDA coating during the reduction of Ag+. A poly­(ionic liquid) (PIL) layer is then introduced in the PDA coating to establish the anion-responsive system. The multilevel stimuli-responsive permeability behavior is studied in detail by testing the water flux. The single adjustment range of the membrane permeability is obtained by the control of the anion-responsive system or temperature-responsive system, which is the switch for permeability of the membrane surface and interior, respectively. Different from previous multiresponsive membranes, the anion- and temperature-responsive systems can interact with each other; hence, more adjustment ranges of the permeability are obtained by simultaneously responding to multistimuli (for example, when raising the temperature from 20 to 40 °C, the adjustment ranges were 0–1.7 L/m–2·h–1 and 11–24 L/m–2·h–1 with the counteranions of Cl– and TFSI–). Meanwhile, taking advantage of the cooperative control effect of the multilevel stimuli-responsive systems, the separation performance of the membranes could be controlled in a more flexible way: various separation results for the PEG molecules with different molecular weights are achieved. We hope the proposed approach can bring fresh ideas and potential applications in the fields related to membrane separation.