In one embodiment, the copolymer is of a vinyl imidazolium salt of the formula ##STR12## wherein X is an anion and R is a hydrogen atom or an alkyl group, typically with from 1 to about 8 carbon atoms and preferably with from 1 to about 3 carbon atoms, and vinyl pyrrolidinone, of the formula ##STR13## wherein the copolymer is of the general formula ##STR14## wherein X is any suitable or desired an