Anion-exchange membranes (AEMs) have been regarded as an alternative to proton-exchange membranes (PEMs) in energy conversion devices, due to the advantage of saving expensive platinum catalysts. However, neither the molecular design nor the property understanding is sufficient for developing AEMs capable of practical fuel cell applications. It is crucial to thoroughly study the current AEMs in terms of microphase structures and conducting properties. Recently, we developed a new graft type of AEMs by radiation-induced grafting method, where imidazolium and styrene monomers were grafted into poly(ethylene-co-tetrafluoroethylene) (ETFE) base films under a dose of 80 kGy from the ⁶⁰Co γ-ray source (QST, Takasaki), followed by alkylation and ion exchange reactions. These AEMs exhibit well-balanced properties of high ion conductivity (> 100 mS/cm at 80^oC) and good stability, which are believed to be controlled by the microphase structure of the membrane. In this report, we investigate the memrbane structure and reveal the relationship between structure and properties, using small-angle neutron scattering (SANS) method.

Fig. 1 shows typical SANS intensity profiles, I(q), of pristine ETFE films (profile 1), grafted-ETFE membranes (profile 2), dry AEM (profile 3) and AEM equilibrated in D₂O (profile 4) as a function of scattering vector q. The different morphologies which can be deduced from the SANS profiles are also shown in the figure. The experiments confirm the semi-crystalline feature of AEM conserved from the original ETFE base film, which is the key factor for the high membrane mechanical stability, and show the dependence of the structure on hydration. Contrast variation experiments performed on AEM equilibrated in water mixtures of H₂O and D₂O, further show that graft polymers and water consist the hydrophilic ion channels [1]. Since graft polymers distribute randomly in AEM, well-connected hydrophilic ion channels are feasibly formed, which is the key factor for the high membrane conductivity.

Above all, the interplay between the morphology and properties can be characterized as follows: 1) semi-crystalline base film offers good mechanical properties; 2) graft polymers and water form ion channels to promote the conductivity.