Salt transport of the carboxylated aromatic polyamide: Efforts to elucidate the structure-function relation of polyamide TFC desalination membranes

Five carboxylated polyamides with the number average molecular weight in the range of 15000–17000 Da, are synthesized via low-temperature solution polycondensation to study the effect of carboxyl group (-COOH) content on the carboxylated polyamide salt transport property. The glass transition temperature (Tg) of the carboxylated polyamide increases with the –COOH content due to the formation of more hydrogen bonds, which also leads to a decrease in free volume fraction (FFV). Interestingly, we observe that polymer having the –COOH content of 80%–100% possesses similar water uptake and KW, which provides a special perspective that how the –COOH group influences salt transport by excluding the effect of water content in polymer membranes. Water sorption can disrupt the hydrogen bonding within the –COOH groups, so setting out –COOH groups to bind water, leading to lower free water content and enhanced water/salt sorption selectivity. As the content of –COOH increases, the more negatively charged surface and stronger hydrogen bond interactions result in significant decrease in salt permeability (PS) and salt diffusivity (DS). These results implicate that the introduction of more –COOH groups to the polyamide layer can enhance the water/salt selectivity of TFC membrane without by changing the water sorption.