Phosphonium-Containing Block Copolymer Anion Exchange Membranes: Effect of Quaternization Level on Bulk and Surface Morphologies at Hydrated and Dehydrated States

Diblock copolymers of isoprene and chloromethylstyrene were synthesized by nitroxide-mediate polymerization. The polychloromethylstyrene block was quaternized with tris(trimethoxyphenyl)phosphine (P(Ph(OMe)(3))(3)) or triphenyl-phosphine (P(Ph)(3)). Membranes with ion-exchange capacity (IEC) ranging from 0.87 to 2.35 mmol/g were prepared. The bulk morphology was investigated via small-angle X-ray scattering and transmission electron microscopy, where hexagonal and body-centered cubic morphologies were observed. The surface morphology was revealed by atomic force microscopy where both parallel and perpendicular alignments were observed. Compared to P(Ph(OMe)(3))(3), P(Ph)(3) exhibits higher quaternization efficiency due to less steric hindrance. Partially quaternized membranes were prepared using P(Ph)(3), and the dependence of domain size on the quaternization level is discussed. The morphologies as a function of relative humidity were investigated. For high IEC membranes, although bulk scattering showed small changes in d-spacing, large changes in domain size on the surface occurred between hydrated and dehydrated conditions. Further investigations of morphological reversibility by humidity cycling showed good recovery between humid and dry conditions, indicating that the expansion and contraction of channels is a reversible process.