A sodium/zinc mixed ionomer comprises carboxylate groups and a combination of counterions that consists essentially of sodium cations and zinc cations. The sodium/zinc mixed ionomer is the neutralization product of a precursor acid copolymer. The precursor acid copolymer comprises copolymerized units of an α-olefin and an α,β-ethylenically unsaturated carboxylic acid, and it has a melt flow rate of about 10 to about 4000 g/10 min. In addition, the precursor acid copolymer, when neutralized to a level of about 40% to about 90%, and when comprising counterions that consist essentially of sodium ions, produces a sodium ionomer that has a freeze enthalpy that is not detectable or that is less than about 3.0 j/g, when determined by differential scanning calorimetry. Further provided are articles comprising or prepared from the sodium/zinc mixed ionomer.