A proton exchange membrane (PEM) with an ion exchange capacity of not less than 1 molar equivalent per kilogram and less than 20% water swelling is provided. The PEM includes a polymer having a polyphosphazene backbone with a polyaromatic functional group linked to the polyphosphazene as a polyaromatic side chain, a non-polyaromatic functional group linked to the polyphosphazene as a non-polyaromatic side chain, and an acidic functional group linked to the non-polyaromatic side chain. The polyaromatic functional group linked to the polyphosphazene provides for increased thermal and chemical stability, excellent ionic conductivities and low water swelling. The mole fraction of polyaromatic functional groups linked to the polyphosphazene backbone is between 0.05 and 0.60.