A membrane humidifier assembly includes a first flow field plate adapted to facilitate flow of a first gas thereto and a second flow field plate adapted to facilitate flow of a second gas thereto. A polymeric membrane is disposed between the first and second flow fields and adapted to permit transfer of water from the first flow field plate to the second flow field plate. The polymeric membrane includes a polymer having perfluorocyclobutyl groups and a pendant side chain having a protogenic group.