wherein x is an integer of 1 or 2; when x is 1, Rf is selected from the group consisting of linear or branched perfluoroalkyl groups having from 2 to about 8 carbon atoms, perfluorocycloalkyl-containing perfluoroalkyl groups having from 5 to about 8 carbon atoms, and perfluorocycloalkyl groups having from 4 to about 8 carbon atoms; when x is 2, Rf is selected from the group consisting of linear or