Chlorine stability of fully aromatic and mixed aromatic?aliphatic polyamide thin film composite membranes

Abstract

Composite membranes containing a poly(m-phenylenediamine-trimesoamide) – poly(MPD-TMC), a fully aromatic polyamide – or poly-(1,3?cyclohexanebis?methylamine-trimesoamide) – poly(CHMA-TMC), a mixed aromatic-aliphatic polyamide – barrier layer were prepared and were exposed to different concentrations of NaOCl solution containing 2000 ppm NaCl. An increase in water flux from 78 to 103 litres/m² h and a decrease in salt rejection from 94% to 91.3% were observed for poly(MPD-TMC) membranes, and a decrease in both water flux (73 to 36 litres/m² h) and NaCl rejection from (77.9% to 38.5%) for poly(CHMA?TMC) membranes were observed upon exposure to NaOCl solution for 24?360 ppmh. The observed results for the poly(MPD?TMC) membrane may be attributed to the conversion of the amide N?H group to a N?Cl group, followed by polymer degradation upon exposure to NaOCl solution. The decrease in both flux and rejection in the poly(CHMA-TMC) membrane may be due to the conversion of the hydrogen bonding amide N?H group to the stable N?Cl group in polyamide chain, which does not undergo decomposition as it contains aliphatic diamine. The infrared (IR) spectra of chlorine-exposed poly(CHMA-TMC) membranes have shown a clear split in both the amide-I (C=O stretch) band at 1645 cm−1 and the amide-II (C?N?H) band at 1543 cm−1, whereas a decrease in the intensities of the amide-I and amide-II bands was observed for poly(MPD-TMC) membranes.