Anion-exchange membranes containing diamines: preparation and stability in alkaline solution

Anion-exchange membranes (AEM) are prepared from chloromethylated polysulfone and a number of diamine compounds. The properties of the new AEM including the water content, ion-exchange capacity, permselectivity and area resistance are thoroughly studied. By varying the amount of diamine into the casting polymer solution, the properties of the membranes can be optimized. Generally for diamines with small aliphatic length of the alkanoic chain, excess of diamine is required to produce AEM with low electrical resistance and high permselectivity. The properties of the new AEM are also tested after exposure for several days into 2 M NaOH, at 40 °C. Generally, this exposure causes reduction of the membrane's ion-exchange capacity resulting in an increase of its electrical resistance and decrease of its water content and permselectivity. The degradation of the membranes depends on the length of the alkanoic chain in the diamine units, i.e. slower degradation rate is observed for membranes with longer alkanoic residues. Therefore, the membranes containing the N,N,N′,N′-tetramethylhexanediammonium (TMHDA) groups are the most stable under the studied alkaline environment. The AEM, however, containing N,N,N′,N′-tetramethylethylenediammonium (TMEDA) and bis-N,N,N′,N′-tetramethylbutanediammonium (TMBDA) groups decompose significantly.