Hydrophilic silica additives for disulfonated poly(arylene ether sulfone) random copolymer membranes

Abstract

Hydrophilic silica (SiO2) nanoparticles (average size = 7 nm), which act as inorganic acids at low pH (<2), were added together with a PEO-PPO-PEO triblock copolymer dispersant to a random disulfonated poly(arylene ether sulfone) copolymer in the potassium salt (-SO3 K--(+)) form in order to control permeation and rejection characteristics of the homopolymer. The dispersants (shell) absorbed on the surface of SiO2 nanoparticles (core) formed a distinctive core-shell structure. The PEO units located at the outside of the dispersant formed complexes with -SO3 K--(+) groups of BPS-20 via ion-dipole interactions. These interactions induced a compatible binary system following the Flory-Fox equation associated with glass transition temperature (T-g) depression and prevented extraction of the water-soluble dispersant even under aqueous conditions. The ion-dipole interaction, combined with hydrogen bonding between SiO2 and the dispersant, caused SiO2 nanoparticles to be well distributed within the BPS-20 matrix up to a limit of 1 wt.% of SiO2 and minimized the formation of non-selective cavities within the matrix's hydrophilic water channels. The resulting BPS-20_SiO2 nanocomposites showed improved salt rejection and reduced ionic conductivity. These trends are analogous to those of disulfonated copolymer systems, with polar groups creating hydrogen bonding or acid-base complexation with -SO3 K--(+) groups in BPS copolymers. Well dispersed SiO2 nanoparticles in highly water-permeable desalination membranes are expected to result in an increase in salt rejection but very little change in water permeability. The addition of nanoparticles to desalination membranes may offset the permeability-selectivity tradeoff observed in polymer membranes. Above 1 wt.%, SiO2 nanoparticles increased both the interchain distance between polymer chains and the water uptake. However, the increased hydrophilicity due to high SiO2 content did not yield improved water permeation of the nanocomposite membranes. The SiO2 nanoparticles acted as barriers, hindering water passage (restrictive diffusion) and lowering water permeability. Meanwhile, acidic hydroxyl groups (OH2+) on the SiO2 surface in the sulfonate matrix led to improved ionic conductivity, but NaCl rejection capability decreased because the concentration of -SO3 K--(+) was diluted by highly absorbed water molecules, resulting in weakened Donnan exclusion. The mechanical properties and chlorine resistance of all BPS-20_SiO2 nanocomposites were comparable to those of BPS-20. (C) 2012 Elsevier B. V. All rights reserved.