when X is fluorine or is a hydroxyl group, p is equal to 1, Rf and R'f are acyclic alkyl groups, at least one of said Rf and R'f contains at least 5 carbon atoms, and the sum of the carbon atoms in said Rf and R'f groups is greater than or equal to 7, preferably less than or equal to about 24; preferably the ratio of carbon atoms in said Rf and R'f moieties is at least 2 to 1.