Structure and properties of heterogeneous and homogeneous ion-exchange membranes subjected to ageing in sodium hypochlorite

The effects of sodium hypochlorite (NaClO) on ion-exchange membranes were carefully investigated and discussed with the aim to assess one of their possible causes of ageing. A thorough comparison was carried out for the degradation of ion-exchange membranes with different structures, i.e. homogeneous and heterogeneous ones, as well as with different functional groups, i.e. anionic (bearing 3CH2N+R3 groups) and cationic (bearing image groups) ones. An artificial ageing protocol was implemented, which consisted in immersing four types of membranes, homogeneous AMX-SB and CMX-SB as well as heterogeneous, MA-41 and MK-40 in aqueous NaClO solutions at constant pH and concentration for different times over a 700 h period. The physico-chemical, structural, and mechanical properties of each sample were investigated, before and after ageing, by means of complementary analytical techniques, namely conductivity, ion-exchange capacity, water uptake, and thickness measurements, as well as SEM, ATR-FTIR, TGA, and tensile strength tests. The results demonstrated that ageing mechanisms were different for anion-exchange membranes and cation-exchange membranes; however, ageing was similar among homogeneous and heterogeneous membranes of the same type (anionic or cationic). Sodium hypochlorite provoked a degradation of the quaternary ammonium sites of anion-exchange membranes and chain scission of the poly(styrene-co-divinylbenzene) backbone from anion and cation-exchange membranes through chain radical oxidation. No significant degradation was found for the polymer binder of membranes investigated in this paper.