Conductive Channels in Anion Conducting Multiblock Copolymers: Different Number and Type of Head-Groups

Abstract

The conductivity and physical properties of the ion conductive channels in a multiblock copoly(arylene ether) polymer was quantitatively measured as a function of (i) number of tethered head-groups per polymer repeat unit and (ii) type of cation head-groups. Multiblock copoly(arylene ether)s with 1, 2, 3 and 4 alkyl chain tethered quaternary ammonium head-groups on each hydrophilic repeat unit were synthesized via polycondesation, Friedel-Crafts, and reduction reactions, Figure 1. The effect of the number of ionic groups on the ion exchange capacity (IEC), morphology and properties of the anion exchange membranes was investigated. As the ionic group density increased, both the ionic conductivity and water uptake of the membranes increased. The 3-tether membrane formed the most efficient ionic channels as determined by the highest ionic conductivity/IEC. The 2, 3, and 4-tether (per hydrophilic repeat unit) membranes had 9 to 10 bound waters of hydration per ionic group pair, however, the amount of unbound, freezable waters increased with the number of ion pairs per polymer repeat unit. The unbound, freezable water was unproductive and led to lower ionic conductivity/IEC ratio, such as in the 4-tether membrane. The optimum membrane had three head-groups per hydrophilic repeat unit (X3Y5-3): 130.6 mS/cm ionic conductivity at 80oC, 58.3% water uptake, IEC of 1.83 meq/g. All the membranes showed acceptable thermal stability and alkaline stability in 1 M KOH at 60oC for 1000 h. As the number of head-groups per hydrophilic repeat unit increased, the mechanical strength of the membranes decreased. The two-tether (per hydrophilic repeat unit) block copolymer was used to compare three previously reported head-groups: quaternary trimethylammonium (TMA), quinuclidium (ABCO), and tris(2,4,6-trimethoxyphenyl)phosphonium (TTMPP) cations in terms of size of the conducting channels, ionic conductivity of the mobile hydroxide ion, mechanical properties, quantity of productive and unproductive water, and chemical stability of the membrane in base. The interdomain spacing acquired from small angle X-ray scattering showed that multiblock copolymers with larger cations formed larger ion conduction channels in the membrane. However, the larger cations resulted in lower ion exchange capacity (IEC) even though the polymer backbone and tether arrangements were identical. TMA was the most stable cation after exposure to 1 M NaOH at 60oC for 20 days. ABCO had a lower number of bound water molecules and a 22% loss in ion conductivity after treatment in 1 M NaOH at 60oC for 20 days due to the higher hydroxide ion concentration in the ion conductive blocks. Membranes with TMA head-groups also had the best mechanical properties, while the TTMPP head-group was the poorest. Thus, the alkyl trimethylammonium head-group is the best suited cation head-group among those studied with this backbone. In addition, two membrane preparation methods were compared. Quaternization before membrane casting had better morphology and membrane properties than quaternization after membrane casting. The presence of the cation head-groups assists in phase segregation. Figure 1