Anion exchange membranes composed of a poly(2,6-dimethyl-1,4-phenylene oxide) random copolymer functionalized with a bulky phosphonium cation

A random copolymer, tris(2,4,6-trimethoxyphenyl) phosphonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO–TPQP) was cast from three different solvents: dimethyl sulfoxide (DMSO), ethyl lactate, or a 41:59vol% mixture of DMSO and ethyl lactate. Solvents were selected via analysis of the Hansen solubility parameters to vary the phase separation of the polymer in the films. An optimized mixture of DMSO and ethyl lactate chosen for film fabrication and this film was contrasted with films cast from the neat constituent solvents. Atomic force microscopy identified domains from nanometer to tens of nanometer sizes, while the light microscopy showed features on the order of micron. SAXS revealed a cation scattering peak with a d-spacing from 7 to 15Å. Trends in conductivity and water diffusion for the membranes vary depending on the solvent from which they are cast. The mixed solvent cast membrane shows a linear Arrhenius behavior indicating fully dissociated cationic/anionic groups, and has the highest bromide conductivity of 3mS/cm at 95% RH, 90°C. The ethyl lactate cast membrane shows a linear Arrhenius relation in conductivity, but a Vogel–Tamman–Fulcher behavior in its water self-diffusion. While water increases bromide dissociation, water and bromide transport in these films seems to be decoupled. This is particularly true for the film cast from ethyl lactate. [Display omitted] •Solvents were selected by Hansen solubility parameter.•Solvents impact membrane morphology, water uptake and conductivity.•Membrane with less tortuosity has high water diffusion coefficient.•Optimized solvents fully dissociated cation anion pairs and has highest conductivity.