Nanofiltration properties of asymmetric membranes prepared by phase inversion of sulfonated nitro-polyphenylsulfone

Abstract

This study is a systematic investigation of preparation and characteristics of membranes, made from sulfonated nitro-polyphenylsulfone (SPPS-NO2) with different degrees of sulfonation and prepared by non-solvent–induced phase separation in different immersion baths (deionized water, 0.1 M HCl, or 1 M NaCl).Following nitration, the PPS-NO2 was sulfonated with different amounts of chlorosulfonic acid to achieve polymers with different ion exchange capacities (IECs), from 0 to 2.2 meq/g.The cross-sectional membrane morphology changed from porous to dense following sulfonation as seen by scanning electron microscopy. The phase inversion of the polymer solution was studied using the cloud-point method, light microscopy, and Langmuir isotherm. It was found that the liquid-liquid demixing of the polymer solution changed due to the increase in the IEC. This was mainly attributed to an increase in polymer miscibility in the aqueous non-solvent bath with an increasing degree of sulfonation. In addition, the phase-inversion properties of the ionic polymers were influenced by changing the aqueous composition of the immersion baths, probably due to partial protonation of the ionic sulfonic groups (in the case of HCl as the non-solvent) or to solvation effect (when NaCl was the non-solvent).Changing the morphology and of the membrane influenced its performance. Increasing the IEC increased the salt rejection and decreased flux. However, the permeability was improved without a large loss of selectivity by changing the non-solvent to aqueous NaCl. In addition, the new membrane showed high chlorine resistance due to the addition of a nitro group to the polymer backbone, and high acid resistance property.