Foams and method of forming foams of ionomers of copolymers of vinylidene aromatic monomer and unsaturated compounds with acid groups

A foaming composition comprising:

• (a) a copolymer of 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 that form acid groups pendant from the copolymer;
• (b) a metal salt, metal oxide or combination thereof, the metal salt and metal oxide each having a metal and the metal having a valence of at least 2;
• and,
• (c) a blowing agent, wherein the foaming composition, after melt blending the copolymer, the blowing agent and metal salt, metal oxide or combination thereof, forms a crosslinked polymer crosslinked by reversible ionic bonds and the crosslinked polymer has a melt viscosity at 0.314 s−1 angular frequency that is at least 10 percent higher than the copolymer when measured at 200° C. and has a viscosity at 628 s−1 angular frequency that is essentially the same as the copolymer, and the metal having a valence of 2 is of the metal salt and the metal is present as metal ions to equivalents of anions of the acid groups pendant from the copolymer at an equivalents ratio of 200:1 to 1:200.