Water and salt transport properties of zwitterionic poly (arylene ether ketone) for desalination membrane applications

A series of novel poly (arylene ether ketone) with pendant zwitterion groups were designed. Three molecules, 1,4-butane sultone, sodium 2-bromoethanesulphonate and sodium bromoacetate were selected to quaternize with tertiary amine-containing poly (arylene ether ketone) to prepare sulfobetaine poly (arylene ether ketone) (PAEK-SB4 and PAEK-SB2) and carboxylbetaine poly (arylene ether ketone) (PAEK-CB) with controllable content of zwitterion group. The fundamental water and salt transport properties in zwitterionic PAEKs were characterized. Except PAEK-SB2, the water and salt diffusivity and permeability of PAEK-SB4 and PAEK-CB increase with the increase of water sorption, which follows the trend of Yasuda's free volume theory. Salt diffusivity and permeability in PAEK-SB2 were suppressed relative to PAEK-SB4 and PAEK-CB with comparable water sorption, which is due to shorter distance between positively charged group (-N+(CH3)2-) and negatively one (SO3−) in SB2 group leading to stronger self-association interaction between zwitterions. The water/salt permeability selectivity of PAEK-SB2 and PAEK-CB change little with the increase of diffusive water permeability, which break through the trade-off between them. The permeability selectivity of zwitterionic PAEKs is more than one order of magnitude higher than sulfonated polysulfone (BPS), and its orders is PAEK-SB2>PAEK-CB > PAEK-SB4>BPS at the comparable diffusive water permeability. This study suggests that shorter distance between positively charged group and negatively one in zwitterion group benefits water/salt selectivity of zwitterionic PAEKs.