Ion exchange membranes are commonly used in many applications such as water desalination and wastewater nutrient recovery. The permselectivity of ion exchange membranes is one of its most critical properties, but often must be balanced with adequate permeability. High permeation of negative ions coupled with low permeation of positive ions is desirable in anion exchange membranes (AEMs). However, separation of these oppositely charged ions tends to suffer as the concentration of the feed solution increases, thereby limiting the range of processable feed solutions. In this work, a robust AEM was synthesized with a polychlorostyrene-b-polycyclooctene-b-polychlorostyrene ABA triblock polymer quaternized with methylpiperidine. By adjusting the block length of polychlorostyrene, different sized water channels are formed within the AEM which result in highly tunable anion selectivity. Small angle x-ray scattering (SAXS) and TEM confirm the size of these water channels while permeability and sorption tests elucidate the influence of these different morphologies on separation ability. The AEM also has low water uptake for increased stability and mechanical strength, which is vital in terms of extending the membrane lifetime. With this robust material, AEMs may enable the desalination of highly saline waters such as seawater and enable more economic wastewater nutrient recovery.