Stimuli-responsive gating membranes responding to temperature, pH, salt concentration and anion species

Quadri-stimuli-responsive gating membranes with hierarchically structured gates, which are able to respond to temperature, pH, salt concentration and anion species, are successfully developed. The diblock poly(N-isopropylacrylamide)-block-poly(methacrylic acid) (PNM) polymeric chains, which act as hierarchically structured responsive gates in the membrane pores, are grafted onto the membrane pore surfaces by using the surface initiated atom transfer radical polymerization (SI-ATRP) method. The thermo-responsive and pH-responsive gating performances of the PNM-grafted membranes are resulted directly from the poly(N-isopropylacrylamide) (PN) and poly(methacrylic acid) (PM) segments in the grafted PNM gates respectively. The prepared PNM-grafted membrane pores tend to “open” as increasing the environmental temperature across the lower critical solution temperature (LCST) or as decreasing the pH value of the surrounding solution across the pKa, and vice versa. The salt-concentration- and anion-species-responsive gating performances of the PNM-grafted membranes are obtained by adopting the Hofmeister effect or salt effect on the swelling/shrinking behaviors of grafted PNM polymeric gates. If the proportion of PN segments in the grafted PNM gates is dominated, the membrane pores tend to “open” as increasing the salt concentration or changing the salt from sodium chloride to sodium sulfate in the buffer solution; on the contrary, if the proportion of PM segments is dominated, the membrane pores tend to “close” as increasing the salt concentration or changing the salt from sodium chloride to sodium sulfate. The results provide valuable guidance for developing multiple-stimuli-responsive gating membranes for diverse applications including chemical valves and sensors.