A foam comprised of a copolymer comprising one or more vinylidene aromatic monomers and one or more unsaturated acids, the copolymer having about 0.01 to about 15.0 percent by weight of the one or more unsaturated acids wherein the acid groups are pendant from the copolymer and a metal salt, metal oxide or combination thereof, the metal having a valence of at least 2, wherein the copolymer is crosslinked through ionic bonds between the unsaturated acids and the metal of the metal salt or metal oxide and the foam contains decomposition residue of the metal salt.