Preparation of high-flux membrane for dye desalination by the stacked swelling strategy of small-molecule side chains

In this study, a high-flux PVDF membrane for dye desalination was prepared using the small-molecule stacked swelling strategy. Firstly, 1,3-bis [tris(hydroxymethyl) methylamino] propane was grafted on the molecular chain of poly(vinylidenefluoride-co-chlorotrifluoroethylene)(PVDF-CTFE) to prepare the non-porous membrane with a thin skin layer by the NIPS method. Then, this non-porous membrane was placed in the glutamate reaction solution to make it undergo the nucleophilic substitution reaction with the C-Cl bond on the main chain of PVDF-CTFE, and then swelling treatment was performed with an alkaline aqueous solution. In this paper, the influence of the swelling effect of small-molecule glutamate with different degrees of reactivity on the membrane pore size and structure was investigated, and the dye/salt separation performance of glutamate-treated swelling membrane was compared with that of the membrane treaded with the larger-molecule tris-propanesulfonic acid. Based on the fact that glutamic acid has small molecular size and steric hindrance, the results show that glutamic acid with different reaction degrees induced single-molecule swelling and multi-molecule stacked swelling, which led to the stepwise increase of membrane pore size. Within a certain degree of reaction, the membrane pore size entered into the pore size platform of different gradients. Therefore, the controllable adjustment of the pore size of the swollen membrane was realized by controlling the degree of reaction of the grafted side chains. The pore size of the multi-molecule stacked swelling membrane of small molecules not only achieved the effect of the single-molecule swelling membrane of larger molecules but also had a greater number of through holes and higher water flux, and the mechanical properties of the small-molecule stacked swelling membrane did not deteriorate significantly. The optimal small-molecule stacked swelling membrane, G12, not only had a high water flux of 99.4 Lm-2h-1bar-1 but also rejected nearly 99 % of the dye and allowed most of the salts (>98 %) to permeate through the membrane when dealing with the mixed solution of direct red 80 and NaCl.