Poly(methyl methacrylate)-block-polysulfone-poly(methyl methacrylate) Triblock Copolymer-Modified Support Layers for Forward Osmosis Membranes

In this work, poly(methyl methacrylate)-block-polysulfone-poly(methyl methacrylate) (PMMA-b-PSf-b-PMMA) as an amphiphilic block copolymer, which is appropriate for support layer modification of forward osmosis (FO) membranes, was prepared via an atom transfer radical polymerization (ATRP) method using a bromo-terminated PSf as a macroinitiator. First, the Br-terminated PSf macroinitiator was synthesized by esterification of the phenolic groups with α-halo-ester. Second, the PSf-Br macroinitiator was used to start the ATRP process of the MMA monomer, resulting in a PMMA-b-PSf-b-PMMA block copolymer. This block copolymer was combined with PSf during the phase inversion process. Using the PMMA-b-PSf-b-PMMA-modified support layers, we constructed thin film composite (TFC) membranes by interfacial polymerization. The effect of the PMMA-b-PSf-b-PMMA block copolymer content on the porosity, morphology, hydrophilicity, and polyamide layer (PA) properties was carefully studied using several characterizations. The results showed that the support layer characteristics including porosity, hydrophilicity, and pure water flux were considerably improved after PMMA-b-PSf-b-PMMA incorporation. In addition, the best performing TFC20 (modified with 20 wt % PMMA-b-PSf-b-PMMA) membrane exhibits a satisfactory FO performance: a high water flux of 34.09/17.52 LMH (pressure-retarded osmosis (PRO)/FO configuration) and a minimum specific reverse salt flux of 0.09/0.1 g/L (PRO/FO configuration) using DI water as the feed solution and 1 M NaCl as the draw solution. Furthermore, the water permeability was significantly improved with an optimal addition of the PMMA-b-PSf-b-PMMA block copolymer, which should be related to an increase in the PA layer roughness. Meanwhile, the structural parameter (S) value severely declined to 625.2 μm for TFC20 from 1925.6 μm for the control TFC. Therefore, the modification of the PSf substrate with the PMMA-b-PSf-b-PMMA copolymer is an effective method to alleviate the internal concentration polarization during the FO process.